radar altimetry of mercury: a preliminary analysis · journal of geophysical research, vol. 91, no....

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 91, NO. B1, PAGES 385-401, JANUARY 10, 1986

Radar Altimetry of Mercury' A Preliminary Analysis

J. K. HARMON AND D. B. CAMPBELL

National Astronomy and Ionosphere Center, Arecibo, Puerto Rico

D. L. BINDSCHADLER AND J. W. HEAD

Department of Geological Sciences, Brown University, Providence, Rhode Island

I. I. SHAPIRO

Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts

Measurements of Mercurian topography based on Arecibo radar observations are presented. The data, which were obtained from 1978 to 1984, cover much of the equatorial zone of Mercury between 12øN and 5øS latitude. Over thirty continuous altitude profiles were obtained, each spanning from 20 to 90 degrees of longitude at a resolution of 0.15 ø (longitude) by 2.5 ø (latitude). Radar depths for large craters support previous indications from imagery that Mercurian craters are shallower than lunar craters of the same size. One very large (800 km) impact basin shows some distinct topographic structure, although its relative shallowness suggests postimpact modification by isostatic relaxation or volcanic filling. The plains of Tir Planitia appear topographically smooth to the radar. These plains extend well into the hemisphere not imaged by Mariner 10, possibly forming part of a large annulus of smooth plains around Caloris Basin. The circum-Caloris smooth plains are strongly down-bowed, indicating subsidence under a load. This and other similarities to lunar maria suggest a volcanic origin for these plains. Additional areas of topographically smooth terrain have been found in both the imaged and unimaged hemispheres. Several ridges, scarps, and fault zones have been identified in the altimetry. Three mapped arcuate scarps show heights of about 700 m and cross-sectional widths of about 70 km. One of these features is clearly a ridge, while the other two scarps have a more ridge-like appearance than is suggested by images. One large-scale topographic drop of 3 km correlates well with a mapped system of faults and intracrater scarps. The equatorial zone of Mercury shows 7 km of maximum relief, although the typical elevation difference between highlands and lowlands is closer to 3 km. Three major highland areas are found, the largest two of which are roughly antipodal and aligned within about 10 ø of the "hot poles" of Mercury. The unimaged hemisphere, possessing both large craters and topographically smooth areas, does not appear to be markedly different in its topography from the imaged hemisphere. No evidence has been found for another Caloris-type impact structure in the unimaged hemisphere.

1. INTRODUCTION

A program of 2380 MHz radar observations of Mercury has been conducted at Arecibo Observatory since 1978. These observations have yielded an extensive set of altitude measurements over the equatorial zone of the planet. In this paper we present the altimetry results and discuss some of the possible implications for Mercurian geology.

The earliest information on Mercurian topography was provided by radar-ranging observations [Smith et al., 1970; Ingalls and Rainville, 1972]. These measurements showed that the range of altitudes on Mercury was somewhat less than had been measured for the equatorial zones of Venus and Mars. The most detailed of the Mercury radar measurements, and the last to be published, were by Zohar and Goldstein [1974]. Their data showed "hills and valleys" with 1-2 km relief and provided the first evidence of craters.

With the Mariner 10 spacecraft encounters of 1974-1975 came the first detailed look at the surface of Mercury. The Mariner 10 photographic images, which covered 45% of the planet at 0.1-4 km resolution, revealed a heavily cratered, lunar-like surface [Murray et al., 1974; Trask and Guest, 1975]. The images suggested tectonics dominated by compres- sive forces as manifested in the unique Mercurian scarp system. While these results were consistent with the relatively

Copyright 1986 by the American Geophysical Union.

Paper number 5B5488. 0148-0227/86/005 B- 5488 $05.00

subdued topography measured by earth-based radar, Mariner 10 carried no altimeter and quantitative altimetry was limited to shadow measurements of high-relief features such as craters and scarps [Gault et al., 1975; Strom et al., 1975] and a few photoclinometry results [e.g., Hapke et al., 1975]. In addition, many of the Mariner 10 images were obtained at unfavorable illumination angles and one entire hemisphere (the dark side at the times of encounter) was not imaged at all.

It was clear, then, that useful earth-based radar work on Mercury remained to be done and a regular program of radar- ranging observations of the planet was undertaken at Arecibo. In the following section we present a brief discussion of the observations and the data reduction to altitude profiles. This discussion is followed by a display of the altitude profiles irl a form which is intended to be convenient for the reader wishing to make comparisons with the U.S. Geological Survey shaded- relief and geologic maps and the Mariner 10 images. In the latter half of the paper we point out some of the more interesting features of the Arecibo altimetry, and discuss them in the context of the geological interpretations which have accumu- lated in the decade since the Mariner 10 encounters.

2. DATA ACQUISITION AND REDUCTION

Observations and Planet Coverage

The observations were made with the 2380 MHz (12.6 cm wavelength) radar on the 305-m-diameter telescope at the Are- cibo Observatory in Puerto Rico. The specifications of the

385

386 HARMON ET AL.' RADAR ALTIMETRY OF MERCURY

TABLE 1. Radar System Characteristics

Parameter Value or Type

Frequency Transmit power Antenna gain System temperature Transmit waveform Polarization

Sampling interval Frequency resolution Integration time

2380 MHz (12.6 cm wavelength) 400 kW 71 dB 35ø-90øK

phase-coded CW (4/•s baud-length) right circular (transmit) left circular (receive) 2 0.061 or 0.122 Hz 10-20 min

observing system and some relevant observational parameters are listed in Table 1. The measurements were made in the

so-called "delay-Doppler" mode. Delay (altitude) discrimination was achieved by phase-coding the transmitted signal, and Doppler (longitude) discrimination was obtained from Fourier analysis of the coherently detected echo. The delay- Doppler data were analyzed in such a way as to yield a profile of altitude along the subradar track.

Mercury's 7 ø orbital inclination to the ecliptic renders a _+ 12 ø equatorial band accessible to earth-based radar alti-

metry. Achieving extensive coverage over even this very restricted latitude band requires making observations at a large number of orbital aspects which, in turn, requires that observations be spread out over several years. The data presented in this paper were derived from approximately 150 days of observations spread over the six-year period 1978-1984. The location tracks of the Arecibo altitude profiles on Mercury's surface are mapped in Figure 1.

Altitude Measurements

The first step in the data analysis was to produce an altitude profile for each observing "run." One to three observing runs were made on a given day; a run consisted of a transmit and a receive period, each equal in duration to the round-trip light-travel time (10-20 min). The decoded echo was Fourier analyzed to yield a delay-Doppler array for each 8.4 or 16.8 s coherence interval, giving a frequency resolution of 0.06 or 0.12 Hz or, equivalently, longitude resolutions of about 0.07 ø- 0.15 ø . These arrays were then summed incoherently over the full receive period to give a single delay-Doppler array for each run. Numerically computed echo power-versus-delay

8

4

0 •

560 ' ' ' ' ' ' 9'0 ' ' 270 180 0

Longitude (øw) Fig. 1. Location tracks on Mercury for the Arecibo radar alti-

metry profiles (1978-1984). The radar resolution cell actually extends over 2.5 ø of latitude centered on the indicated track (see text). Note that the latitude scale is exaggerated; the Arecibo coverage is confined to a near-equatorial band.

TABLE 2. Locations of Mercury Radar Profiles' 1978-1984

Number Longitude Latitude

1 4ø-27øW 10.7ø-9.9øN 2 5ø-42øW 3.8ø-4.1 øN 3 6ø-54øW 2.0ø-3.2øN 4 21ø-61øW 0.5øS-0.4øN

5 24ø-69øW 10.3ø-8.3øN 6 26ø-56øW 1.8ø-1.4øS

7 37ø-69øW 9.4ø-7.7øN 8 43 ø-71 øw 5.0ø-4.3 øS 9 66ø-85øW 3.5ø-4.1 øN

10 71ø-97øW 7.2ø-6.5øN 11 97ø-129øW 6.5ø-5.7øN 12 104ø-126øW 5.5ø-5.3øN 13 106ø-120 øw 6.00-5.3 ON 14 109ø-138øW 3.6ø-4.8øN 15 112ø-148øW 5.1ø-6.5øN 16 113ø-141 øW 0.1 øS-0.7øN 17 122ø-140øW 4.5ø-4.5øS 18 127ø-169øW 5.9ø-5.3øN 19 137ø-224øW 2.0øS-0.2øN 20 150ø-197øW 5.3ø-7.4øN 21 158ø-241øW 6.9ø-10.3øN 22 174ø-209øW 4.4ø-5.9øN

23 182ø-218øW 2.0ø-3.4øN 24 216ø-259øW 4.3ø-5.3øN 25 246ø-269øW 0.7ø-0.1 øS

26 261 ø-330øW 7.9ø-8.9øN 27 261ø-291øW 5.7ø-6.2øN 28 279ø-338øW 10.4ø-10.8øN 29 283ø-308øW 2.1ø-2.7øN 30 287ø-309øW 0.3ø-0.8øN

31 291ø-319øW 11.6ø-11.8øN 32 320ø-339øW 1.0ø-l.3øN

33 328ø-360øW 0.1ø-l.2øN 34 338ø-359øW 11.4ø-10.9øN

templates were then fit to the run-averaged delay-Doppler array to determine the time delay to the leading edge of the planet at each Doppler frequency. From these delays were subtracted the computed time delays to a reference sphere of radius 2439 km. The resultant residual delays were then ex- pressed as altitudes relative to the reference sphere. Using the Doppler frequencies to provide an effective longitude discrimination, the final result for each run was an altitude profile along a roughly east-west linear track extending approximately 7 ø of longitude to either side of the subradar point. This is the same analysis technique used by Ingalls and Rain- ville [1972] and Shapiro et al. [1972], and the reader is re- ferred to these papers for a more detailed explanation.

By making several runs on the same day and observing on contiguous days (Mercury rotates by about 5 ø per day), we achieved extensive overlap among the individual profiles. The final step in the analysis consisted of collating overlapping profiles and averaging them over 0.15 ø longitude bins to produce a single "composite" profile. Each composite profile spans from 20 ø to 90 ø of longitude and consists of data taken from as many as 11 days of observing. A list of the composite Mercury profiles from 1978-1984 is given in Table 2.

The surface resolution ("radar footprint") of the altitude measurements is approximately 0.15 ø x 2.5 ø (6.4 x 106 km). The 0.15 ø longitude bin size is roughly equal to the coarsest resolution of the individual profiles and is larger than the maximum longitude smear from planet rotation over an observing run. The latitude resolution is much coarser, as each altitude data point can be influenced by echoes arising from anywhere within roughly a degree to the north or south of the subradar track. Interpretation problems resulting from scat-

HARMON ET AL..' RADAR ALTIMETRY OF MERCURY ...

..

Lt5 .;.

-.t0

-0

387

Ye t0 7

0.4 ........ Hlø: -0.5

I i ] ! i I 1 70 60 50 40 50 20 I0

Longi!ude (øw) Fig. 2a. (Top) USGS shaded-relief map of the H-6 (Kuiper) quadrangle (0-72øW longitude), with dark lines to indicate

the subradar tracks. (Bottom) The corresponding altitude profiles for H-6. The vertical scale is relative altitude only. The zero-altitude datum (defined by a 2439.0-km-radius reference sphere) is indicated by horizontal lines. Numbers with the profiles indicate latitude (southern latitudes are negative). Abbreviations denote identified features: As, Asvaghosa; Ch, Chaikovskij; Do, Donne; Ha, Handel; Ho, Homer; Ru, Rudaki; SM, Santa Maria Rupes; Th, Thakur; Ye, Yeats.

tering inhomogeneities or altitude gradients within the resolution cell will be treated, where appropriate, in later sections.

Reduction of the radar data to reliable absolute altitudes

requires making self-consistent, a posteriori corrections to the altitude profiles using a refined center-of-mass ephemeris for the planet. This has been done for the data obtained from 1978 through 1982, and the profiles from this period are displayed as altitudes relative to a datum level given by the 2439-km-radius reference sphere about the planet's center-of- mass point. This correction has not as yet been made to the data from 1983-1984. Nevertheless we have included these

more recent profiles (displayed without reference datum) as they are quite suitable for showing topographic relief on non- global scales.

All of the altimetry data presented in this paper is available in magnetic tape form upon request. Requests should be sent

to J. K. Harmon, Arecibo Observatory, P.O. Box 995, Are- cibo, Puerto Rico 00613.

3. DATA PRESENTATION

The composite altitude profiles are displayed in Figures 2a-2e. Portions of a few of the profiles listed in Table 2 have been deleted where there is redundant coverage. The profiles are stacked by latitude, with arbitrary vertical shifts made to preserve clarity. The zero-altitude datum (2439 km reference sphere) is indicated for those profiles obtained prior to 1983 (see previous section).

The data in Figures 2a-2e are plotted in 72 ø longitude sections corresponding to the five U.S. Geological Survey (USGS) equatorial quadrangles of Mercury. Three of these, the H-6 (Kuiper), H-7 (Beethoven), and H-8 (Tolstoj) quadran-

388 HARMON ET AL..' RADAR ALTIMETRY OF MERCURY

5.7 6.5 7 • ,

,- 63

F:: 5.5

2.0

-4.5 t t ............ I ..........

140 150 t20 110 t00 90 80

Longitude (øw) Fig. 2b. (Top) USGS shaded-relief map of the H-? (Beethoven) quadrangle (72ø-144øW longitude), with subradar

tracks indicated. (Bottom) The altitude profiles for H-7. The display format follows that of Figure 2a (see caption). Ly, Lysippus; Th, Theophanes.

gles, are available in 1: 5,000,000-scale shaded-relief maps published by the U.S. Geological Survey [1976, 1977a, b] and reproduced in the Atlas of Mercury [Davies et al., 1978]. The relevant portions of these shaded-relief maps are included in Figures 2a-2c with the subradar tracks superimposed. The H-6 and H-8 quadrangles are also available in USGS geologic maps [Schaber and McCauley, 1980; De Hon et al., 1981l, and we refer to these maps in the discussion sections. The western portion of the H-8 quadrangle and the whole of the H-9 and H-10 quadrangles cover that portion of Mercury which was not imaged by Mariner 10 (the "unimaged hemisphere").

Since the USGS shaded-relief maps are the standard reference maps, careful checks were made for consistency between the Arecibo and USGS coordinate grids. The longitudes of features in the Arecibo altitude profiles agree very closely with their corresponding locations on the H-7 and H-8 quadrangles, whereas a small (0.4 ø) longitude shift was required for the

profiles in Figure 2a in order to line them up with features on the H-6 quadrangle (see the appendix). Uncertainties in Mer- cury's pole position admit potential latitude grid errors of about a degree (see the appendix). The large N-S dimension of the radar footprint and the possibility of inhomogeneous surface scattering properties makes latitude errors harder to identify than longitude errors. Although it may be possible to use the altimetry data to place tighter constraints on Mercury's pole position, such an analysis is beyond the scope of this paper.

We have used the rms data scatter within the 0.15 ø averaging bin as an empirical measure of the random error of each altitude estimate. These errors are typically between 0.05 and 0.2 km, but can be as much as 1 km. The error bars have been

omitted from the profiles in Figures 2a-2e for reasons of clarity, but they are shown in the "detail" figures accompanying sections 4, 5, and 6.

HARMON ET AL..' RADAR ALTIMETRY OF MERCURY 389

210 200 190 180 1:70 I60 150

,i Longitude (øw) Fig. 2c. (Top) USGS shaded-relief map of the H-8 (Tolstoj) quadrangle (144ø-216øW longitude), with subradar tracks

indicated. Note that the Mariner 10 terminator is at 190øW, with terrain to the west unimaged. (Bottom) The altitude profiles for H-8. The display format follows that of Figure 2a (see caption). Mo, Mozart; TP, Tir Planitia; Ty, Tyagaraja; Ze, Zeami.

4. CRATERS AND BASINS

Much of the structure in the radar altimetric profiles can be identified with specific features on the USGS maps. The radar signatures of some of the more prominent named craters and basins are indicated in Figures 2a-2c. The depths and shapes of several of the larger craters and basins can be ascertained from Arecibo data, and one very large impact basin has been studied intensively.

Crater Depths

Shadow measurements from Mariner 10 images have yielded depth estimates for Mercurian craters with diameters in the range 1-170 km [Gault et al., 1975; Malin and Dzurisin, 1977, 1978]. These studies have revealed a strong similarity between crater depth/diameter relations for Mercury and the moon,

although large Mercurian craters tend to be shallower than lunar craters of comparable diameter. Both planets show a flattening or turnover of the depth-versus-diameter curve for diameters greater than 10-20 km.

Radar altimetry offers an alternative crater depth measure- ment technique, one which is not restricted to structures near the terminator in Mariner 10 images. However, the large size of the radar footprint limits reliable depth estimates to craters larger than about 40 km in diameter, well past the turnover point in the depth/diameter relation. Accuracies of 15-20% have been claimed for the shadow-derived depths [Malin and Dzurisin, 1978] and comparable or better accuracies are likely for the radar-derived depths of large craters such as Mozart and Handel (see Figures 2a, 2c, and 6). For smaller craters the radar depth estimates will be less reliable due to the coarse latitude resolution. There will be a tendency to underestimate


i i i

• 10.4 I

_

_

I I I 9.3

10.3 ••• •'•

4.5 5.7 5.5

0.2

280 270 260 250 240 230 220 ,

Longitude (øw) Fig. 2d. Altitude profiles for the H-9 quadrangle (216ø-288øW

longitude). The display format follows that of Figure 2a (see caption). This entire quadrangle lies in the unimaged hemisphere.

crater rim heights if the rim crest fills only a small fraction of the area of the radar resolution cell. There may also be a corresponding tendency to underestimate crater floor depths, although the intrinsically high radar cross sections of crater floors should mitigate this effect. Finally, a bias can be intro- duced if the crater floor is offset to the north or south of the

subradar track, in which case the measured depth will include a contribution from the planet's spherical figure. This bias amounts to 0.37 km for a 1 ø latitude offset, and increases as the square of the offset. The most striking example of this effect is the anomalous 6 km drop seen at 143øW, 1.8øS (Figure 2b). This is apparently a signature of the crater Theop- hanes, which lies 2.5 ø south of the nominal subradar track. This is an extreme case, however, and in most cases we would expect the upward depth bias due to planet curvature to be about 10% or less. We have elected not to correct for the

curvature bias because of (1) the potential errors in subradar latitude associated with uncertainties in the true pole position, and (2) the likelihood that this bias will be offset to some extent by underestimation of the rim height.

In Table 3 are listed the radar-derived depth estimates for 23 craters ranging from 40 to 260 km in diameter. Only craters in the imaged hemisphere of Mercury are included.

560

10.9

O9 1.5 • •,1.0

I I I I

550 540 530 520

8.9 h,. • FI•

310 300 290

Longitude (øw) Fig. 2e. Altitude profiles for the H-10 quadrangle (288ø-360øW

longitude). The display format follows that of Figure 2a (see caption). This entire quadrangle lies in the unimaged hemisphere.

TABLE 3. Mercury Crater Depth Estimates

Name Diameter, Depth,

Position km km Class

Mozart Handel

Chaikovskij Lysippus Zeami Rudaki Thakur

Tyagaraja Yeats

Lu Hsun

Asvaghosa

190.6øW, 8.0øN 260 2.9 1 34.0øW, 3.9øN 155 3.1 1 50.5øW, 8.0øN 145 2.4 2

133.0øW, 1.3øN 130 1.8 2 147.6øW, 2.5øS 123 2.4 1 54.5øW, 3.5øS 115 1.8 2 63.8øW, 2.7øS 105 2.3 2

148.9øW, 4.3øN 104 4.1 1 34.9øW, 9.7øN 100 2.0 1 82.1øW, 7.3øN 100 1.4 1 59.9øW, 3.5øS 99 2.5 2 24.0øW, 0.1 oS 98 3.1 1 21.2øW, 10.7øN 86 2.0 1

129.9øW, 0.1øN 75 2.1 1 132.8øW, 6.3øN 72 1.8 2 55.8øW, 3.7øS 65 2.7 1

121.9øW, 5.4øN 58 1.8 2 117.8øW, 6.0øN 55 1.8 1 119.0øW, 0.2øN 51 2.0 2 132.7øW, 4.0øN 50 2.5 2 115.7øW, 5.2øN 48 1.8 2 110.5øW, 5.5øN 41 1.2 2 113.5øW, 6.5øN 40 1.9 2

Degradational states for these craters were obtained from the geologic maps of the H-6 and H-8 quadrangles •Schaber and McCauley, 1980; De Honet al., 1981] and were estimated for the H-7 quadrangle from Mariner 10 images and the USGS shaded-relief map of the quadrangle. Following the treatment of Malin and Dzurisin F1977], we have grouped the craters into "fresh" or class 1 (USGS classes C3-C5) and "degraded" or class 2 (USGS classes C•-C2) and have plotted their depths as a function of diameter (Figure 3). Included in Figure 3 are the Mariner 10 shadow-derived depth data of Malin and Dzu- risin [1977]; their power-law relation for fresh Mercurian craters is plotted along with an envelope encompassing their range of values for both fresh and degraded craters. Also shown in Figure 3 is the power-law depth/diameter relation

i i • i !

iooo

Diameter (km)

Fig. 3. Depth versus diameter for fresh class 1 (triangles) and degraded class 2 (crosses) Mercurian craters as measured by radar altimetry (Table 3). The dotted line shows the approximate range of shadow-derived depth values for fresh and degraded Mercurian craters l,Malin and Dzurisin, 1977]. The straight lines are the fitted power-law c•epth/diameter relations for fresh craters on the moon l,Pike, 1974] and Mercury rMalin and Dzurisin, 1977].

HARMON ET AL.: RADAR ALTIMETRY OF MERCURY

1" ' Basin •'• .................... :,: .... .Homer: '.o 4 /'- --....-4

- - '' '

ß E]ecta

..•-•

'EL)

..--•.

1

-,50 ::

:i Longitude (øw)

45 40 35 30 25

Fig. 4. Altitude profiles selected from the H-6 quadrangle (see Figure 2a) showing topography across Homer Basin and an unnamed basin to the west. Vertical bars indicate the + 1 standard deviation altitude errors. Broken lines indicate the approximate locations of the basin rims as discerned from Mariner 10 images and USGS maps. Arrows denote the approximate locations of the inner (I) and outer (O) basin rings of Homer. The location of the rim/ejecta from Titian Crater in the floor of the western basin is shown. The subradar tracks for these profiles are shown on the USGS shaded-relief map at the top.

391

for fresh lunar craters [Pike, 1974]. The radar-derived depths fall well within the range of depths found by Malin and Dzuris- in [1977], although our average depth for fresh craters is 17% less than their 2.9 km depth for 100-km-diameter fresh craters. Unfortunately, the two sets of depth estimates (radar and shadow) are disjoint; no craters were measured by both meth- ods and thus it is impossible to establish reliably whether there is a systematic disagreement between the two techniques. It is clear, however, that most of the radar depths of fresh Mercurian craters are significantly lower than the 4.2 km depth typical of 100-km-diameter fresh lunar craters, which lends support to the assertion that large craters on Mercury are shallower than their lunar counterparts.

Radar profiles in the unimaged hemisphere of Mercury

(Figures 2d, 2e) show several features that are obviously of- impact origin. Examples include structures at 306øW, 11.7øN and at 355øW, 11.0øN, as well as a possible peak-ring basin located at 279øW, 8.3øN. The depths of these craters are in the 2-3 km range, which is consistent with the depths of craters in the imaged hemisphere.

Crater and Basin Structure

Despite the subduing effect of the finite radar resolution cell, most of the crater profiles exhibit some evidence of upraised rims. Rim heights of 0.5-1.0 km are typical, while the fresh craters Tyagaraja and Zeami display asymmetric rims rising as much as 1.5 km above the surrounding terrain (see Figure 2c). Mozart, the largest fresh crater within our radar coverage,


has an asymmetric rim structure which is probably influenced strongly by the underlying large-scale topography. There is, however, a distinct break in slope in the radar profiles south and east of Mozart (see Figure 6) which correlates well with the edge of Mozart's continuous ejecta blanket as mapped by $chaber and McCauley [1980]. Here the radar profiles indicate a thickness of about 1 km for the rim/ejecta deposits at a distance of 1.2 crater radii from the rim crest.

Interior structure can be discerned for some of the larger craters and basins. For example, the craters Asvaghosa and Yeats both show hints of central peak topography (Figure 2a). One of the profiles across Mozart (Figures 2c and 6) shows some interior structure which may be indicative of an inner ring, the existence of which is also hinted at by the USGS H-8 shaded-relief map. Mozart appears to have a somewhat bowl- shaped floor (Figures 2c and 6, uppermost profile), in contrast to the usual assumption that larger craters and basins are flat-floored structures. The large crater in the unimaged hemisphere at 279øW, 8.5øN (Figure 2d), though comparable in size to Mozart, shows shallower walls broken by terrace features suggestive of an inner ring. This structure is likely to be a peak-ring basin of the same class as the basins Renoir and Rodin.

The peak-ring structure within Homer Basin (Figure 4) shows up only as rather subdued breaks in slope on the eastern side of the basin and as a 600-m-high interior peak in the topography on the western side. Homer shares its western rim with a larger, older basin to the west. This unnamed basin is centered at 44øW, 2.1øS and was included along with Mozart and Homer in a survey of large craters and basins by Schaber et al. [1977]. Superposition of ejecta from Homer as well as its highly degraded appearance imply that this basin is quite old. The basin is 2.2 km deep at its lowest elevation (measured from the rim crest). This is shallower than the 2.8 km depth measured for Homer, although it is likely that the apparent depth of Homer is enhanced by the fact that it lies astride a regional west-facing downslope. Unlike Homer, the basin at 44øW shows no obvious topographic or morphological ring. In addition to the superposition of ejecta from Homer, the interior of this basin has been modified by the impact that formed Titian Crater (see Figure 4) and by extensive smooth plains formation [De Honet al., 1981]. Radar profiles place the topographic rim of the unnamed basin near the narrow trough which De Hon et al. [1981] map as a crater chain. The topographic rise from the basin floor begins well to the east of this feature, however, and the trough itself is apparently too shallow or too narrow to yield an identifiable radar signature.

There are several cases where crater profiles are influenced or distorted by underlying topography. For example, the prominent rim of the crater ̂ svaghosa (Figure 2a and 10) is probably accentuated by the fact that it was excavated from a topographically high region formed by the combined ejecta of several large impact craters [De Hon et al., 1981] and lies adjacent to two topographically lower plains regions to the east and west. Craters Handel and Yeats and Homer Basin are

tilted, apparently due to a significant west-facing regional slope (see section 6), as are the craters Rudaki and Zeami (Figures 2a and 2c). In addition, the interior of the crater Yeats displays a topographic high that may be the signature of an intracrater scarp, although there may also be a contribution from the central peak.

Large Basins

The apparent dearth of large impact structures (diameter greater than 400 km) on Mercury relative to lunar abundances has been discussed by Schaber et al. [1977]. They suggest that

this is a result of' either a lower basin production rate or a higher degree of isostatic relaxation than has operated on the moon. Others have discussed the lack of identifiable multiring structures on Mercury compared to the moon or Mars and have emphasized the role that crustal characteristics may have played in the formation of very large impact structures on the terrestrial planets [Wood and Head, 1976; Strom, 1979]. More recently, Spudis and Strobell [1984] claim to have identified a number of degraded multiring structures based on careful photogeologic mapping of massifs and massif chains, arcuate segments of ridges and scarps, rejuvenated crater rims, and regions of anomalously high topography. Topographic information provided by radar allows us to examine and evaluate three large impact structures discussed in these works.

The one large basin identified in both the Schaber et al. [1977] and Spudis and Strobell [1984] surveys for which we have extensive radar coverage is that which is bisected by the equator in the western portion of the H-7 quadrangle. Schaber et al. [1977] list it as an 839-km-diameter, single-rimmed basin centered on 130øW, 1.8øN. It is the second largest Mer- curian basin in their survey after Caloris. Spudis and Strobell [1984] identify this same basin (which they name "Mena- Theophanes") as a multiring structure centered on 129øW, IøS, with concentric rings of diameters 260, 475, 770, and 1200 km. In Figure 5 we show four of the altitude profiles across the basin on an expanded scale (see also Figure 2b) along with markers indicating the approximate basin edges as given by the USGS shaded-relief map and Schaber et al. [1977]. The profiles show that the topography across this basin is complex and strongly latitude-dependent. The basin floor has been significantly altered by postbasin impacts; several have left craters more than 70 km in diameter. Other portions of the basin floor appear topographically smooth, possibly indicating a smooth plains fill. The southernmost profile at 4.5øS is the simplest of the four profiles in Figure 5. It shows 1.2 km of rather smooth, down-bowed relief, an upraised rim in the east, and a western rim which corresponds to a scarp visible in Mariner 10 imagery. Another profile at 2øS (Figure 2b; profile not shown in Figure 5) shows a rise-up from the basin floor just north of Theophanes Crater which could correspond to the outer basin ring of Spudis and Strobell [1984]. The most northerly profile in Figure 5 shows a very prominent basin rim in the NW along with two smooth, down-bowed sections of basin floor in the NW and NE. The two northern profiles in Figure 5 show very little topographic expression across the NE rim of the basin, whereas the eastern portion of the third profile (that nearest the equator) does show some structure which may be basin-related. The radar data show that, overall, the interior of the basin is not significantly lower than the level of the adjacent terrain. This suggests that the basin has been severely modified by postimpact processes such as isostatic relaxation, impact cratering, and volcanic filling. In addition, the interior of the basin may have experienced some localized subsidence due to the emplacement of smooth plains. Although there are some apparent correspondences between topography and the locations of concentric basin rings identified by Spudis and Strobell [1984], the radar data do not offer unambiguous support for the argument that this basin is a multiring structure.

Two other basins identified by Spudis and Strobell [1984] are also covered by altimetry profiles. The first of these is a four-ring, 1250-km-diameter structure centered on 168øW, 6øN; the second is a five-ring, 1500-km-diameter structure centered on 4øW, 10øN [Spudis and Strobe!l, 1984]. In neither case do we find any long-wavelength topographic signature in the radar data indicative of a basin. While some smaller-scale


I

150

t I I I t

0.7

-4.5 _

i 1 140 130 t20 I0

Longitude (øw) Fig. 5. Altitude profiles selected from the H-7 quadrangle (see Figure 2b) showing topography across the large basin

centered at 130øW, 1.8øN [Schaber et al., 1977]. Vertical bars indicate the q- 1 standard deviation altitude errors. Brackets denote the approximate locations of the basin rim as given by the USGS shaded-relief map and Schaber et al. [1977]. See text for discussion. The subradar tracks for these profiles are shown on the USGS shaded-relief map at the top.

features in the altimetry may correspond to remnant ring structures associated with these basins, no such identifications could be made unambiguously. We stress, however, that our radar data, with its limited coverage and resolution, is inferior to images for the identification of remnant ring structures associated with highly degraded basins.

The largest prominent impact structure on Mercury is Ca- loris Basin. The southern edge of the basin (Caloris Montes) extends down to 14øN latitude, 5 ø north of the nearest subradar track. Although the Arecibo profiles do not cross the basin proper, the profiles in the southern environs of Caloris are very distinctive and probably reflect topographic structure related to the event that formed Caloris and to postimpact processes. Radar profiles in this region (see Figures 2c and 6)

provide strong evidence for a large expanse of downwarped smooth plains extending well into the unimaged hemisphere; this may, in fact, be part of an irregular annulus of smooth plains entirely surrounding Caloris Basin (see section 5). The radar data also show that the region south of Caloris is one of the highest regions in the equatorial zone of Mercury in terms of absolute altitude (see section 7). The photoclinometry results of Hapke et al. [1975] suggest that the center of Caloris is at least 7 km below the eastern edge of the basin. Thus it is likely that the inner edge of the smooth plains annulus is higher than the center of Caloris. Clearly, the distinctive radar signature of a basin like Caloris would be easily recognized if such a basin lay within roughly 45 ø of the equator in the unimaged portion of the planet.


8.5 T _ Mozart

œ/•ta i Smooth PIo,ns I', CalørloSndFl ecta_ Mozart/., •;,.• • Smooth Pla,ns

'! I

.... I I Plains i

T I

•9o •ao •o •o

Longitude (øw) Fig. 6. Altitude profiles from the H-8 quadrangle (see Figure 2c)

showing topography in the environs of Tir Planitia. Vertical bars indicate the + 1 standard deviation altitude errors. Broken lines

denote the approximate boundaries of major geological terrain types as given by the geologic map of Schaber and McCauley [1980]. The vertical broken line (T-T) indicates the location of the Mariner 10 western terminator.

5. SMOOTH PLAINS

Smooth plains constitute the youngest of the major geologic units on the surface of Mercury [Trask and Guest, 1975]. Although the smooth plains of Mercury bear some resem- blance to lunar maria, their mode of emplacement remains uncertain. It has been suggested that the smooth plains are impact melt and/or ejecta from large basins [Wilhelms, 1976; Oberbeck et al., 1977•, although there appears to be somewhat more evidence supporting a volcanic origin [Strom et al., 1975; Trask and Strom, 1976; Cintala et al., 1977].

Comparison of the Arecibo radar profiles with geologic maps of Mercury shows a strong correspondence between areas mapped as smooth plains on the basis of Mariner 10 imagery [Trask and Guest, 1975; $chaber and McCauley, 1980; De Honet al., 1981] and those areas which have a smooth appearance in the radar altimetry. The largest mapped expanses of smooth plains near the equator occur in the H-8 quadrangle [$chaber and McCauley, 1980], and our profiles in this quadrangle (Figure 2c) have a generally smoother appearance than do profiles in the H-6 and H-7 quadrangles (Figures 2a and 2b).

We obtained a quantitative measure of the "roughness" at any given point along a radar profile by fitting a least squares line to the 5 ø longitude segment (210 km E-W) centered on that point, and then assigning the rms altitude variation about that line as the "roughness" at that center point. The moving linear fits effectively high-pass filter the profiles, providing a measure of "topographic roughness" characteristic of structure on the scale of large craters. The combined roughness estimates are shown on the map in Figure 7. From comparisons of Figure 7 with geologic maps and images, we find that areas of low roughness values correlate well with mapped expanses of smooth plains or with intercrater plains areas with low densities of large craters. We also find that smooth plains areas tend to yield the narrowest radar scattering functions, which indicates that the relative smoothness also holds at the

smaller scales (less than 100 m) dominating the quasispecular scattering characteristics. This has been corroborated by recent quasispecular scattering analyses by Clark et al. r1984].

While such scattering studies are obviously of interest, we will restrict the remainder of our discussion to inferences of smoothness based solely on the altimetry data.

The rms roughness values in Figure 7 range from about 0.1 to 1 km. The heaviest concentration of smooth profiles extends from 160 ø to 240øW longitude. The smooth area between 160øW and 180øW coincides with the smooth plains of Tir Planitia (Figure 6). Figure 7 shows a local rise in the roughness at 190øW due to the crater Mozart, and then another large expanse of relatively smooth terrain extending west of Mozart and well into the unimaged hemisphere. These smooth plains are probably continuous with Tir Planitia to the south of Mozart [see $chaber and McCauley, 1980]; to- gether, these two regions probably constitute a single enor- mous expanse of smooth plains.

These results lend support to the notion that there exists an annular region of smooth plains surrounding Caloris [Strom et al., 1975; Dzurisin, 1976, 1978; Melosh and Dzurisin, 1978a; McKinnon, 1979]. Additional support is provided by the large-scale topography of the region south of Caloris. The profiles across Tir Planitia have a distinct concave appearance (Figures 2c and 6) and show that Tir Planitia has a floor approximately 1 km below the terrain to the east. The con- cavity of profiles west of Mozart is even more striking; here the floor is as much as 2.5 km below the surrounding terrain. The Tir Planitia depression has its lowest altitude at 178øW, whereas the western depression is lowest at 208øW. This would place the longitudinal bisector of the plains annulus at 193øW longitude, close to the apparent longitudinal bisector of Caloris Basin. In Figure 8 we show the mapped distribution of smooth plains around Caloris and have indicated the approximate extent of the smooth plains identified by radar in the unimaged part of the planet. Although the distribution is irregular, there is a clear tendency for regions of smooth plains to fall within approximately one basin diameter of the rim of Caloris. A final piece of evidence for the continuation of the smooth plains into the unimaged hemisphere is provided in Figure 9. This albedo map of Mercury [Murray et al., 1972], which was constructed from telescopic observations, shows semiannular features around Caloris which correspond well with the smooth plains in the imaged hemisphere as well as with the radar-inferred smooth plains on the unimaged side. Additional photographic imaging of the region west of the Mariner 10 terminator is required, however, to establish whether the annular plains do, in fact, completely surround Caloris.

12

-4

• I I I I I [ I [ I I CI •

360 270 180 90 0

Longitude (øw) Fig. 7. Profile location map of Mercury (see Figure 1) with verti-

cal shading added to indicate topographic roughness in km r.m.s. (see text for the roughness definition). The trace at the bottom gives the roughness as a function of longitude, averaged over all latitudes. Note the large expanse of low roughness covering the central longitudes. As in Figure 1, the latitude scale is exaggerated.

HARMON ET AL.' RADAR ALTIMETRY OF MERCURY 395

Fig. 8. Distribution of smooth plains (shaded) in and around Caloris Basin, adapted from a figure by Trask and Guest [1975]. The crosshatching indicates the probable extension of the smooth plains terrain into the unimaged hemisphere as indicated by the radar altimetry (see text). Also indicated are the locations of Tir Planitia, Budh Planitia, and the Mozart crater and ejecta deposits (labelled Mo).

The most plausible explanation for the distinctive down- bowed topography over Tir Planitia and the plains west of Mozart is that there has been subsidence in these areas in

response to an emplaced load. The down-bowed section of profiles on Tir Planitia corresponds closely to the location of the undivided smooth plains region mapped by Schaber and McCauley [1980], whereas the higher, slightly rougher area to the east was mapped as intercrater plains and mixed smooth plains/Caloris ejecta terrain (see Figure 6). The distinctive concave shape of the topography and the abundance of mare-like ridges within Tir Planitia noted by Strom et al. [1975] and Dzurisin [1978] are consistent with a lithospheric loading and flexure process analogous to the emplacement and defor- mation of lunar maria [Solomon and Head, 1979, 1980].

The radar altimetry results appear to strengthen the analogy between the Mercurian smooth plains and lunar maria and, hence, to provide indirect support for a volcanic origin for the smooth plains. Dzurisin [1976], who supports the volcanic theory, suggested that Tir Planitia may have been filled with magma withdrawn from beneath Caloris and that this sapping process may have resulted in the subsidence and ridge formation in the Caloris Basin floor. It has been further suggested [McKinnon, 1979] that the subsequent subsidence of the emplaced plains outside of Caloris may have induced the final episode of uplift and graben formation in the floor of Caloris.

Although the plains region southeast of Caloris is quite smooth, some distinct topographic features can be identified in radar profiles (see Figure 6). Some of these features have been identified as ridges on the basis of comparisons of altimetry with Mariner 10 images and maps. The smallest and most numerous of these are mare-like ridges [Strom et al., 1975], 100 to 200 m high and located in the low plains. Two large asymmetric ridges bound the low plains of Tir; the first rises 350 m above the mixed smooth plains/Caloris ejecta to the east and runs roughly north-south along 174øW, while the second rises about 1 km above the edge of the Mozart ejecta to the west at about 183øW. Both of these boundary ridges run approximately north-south over a distance of at least 400 km. Just to the east of the low plains, two topographic highs appear in the northernmost profile in Figures 2c and 6. Lo- cated at approximately 170øW and 172øW at 7.2øN, these features correspond to mapped deposits of Caloris ejecta [Schaber and McCauley, 1980], perhaps implying embayment of ejecta by smooth plains material.

The expanse of smooth plains west of Mozart contains topographic highs that are similar to those seen in profiles of Tir Planitia. The most prominent of these is a 1.1-km-high feature at 209øW, 9øN, which appears to extend at least as far south as 5.9øN (Figure 2c).

A relatively smooth section of radar profile extends from 220 ø to 240øW at 10øN (Figures 2d and 7), west of the pro-

396 HARMON ET AL.: RADAR ALTIMETRY OF MERCURY

360

:.

+ '30:

270

+ 30

360 270 S 90 0 Fi•. 9. Albedo map oœ Mercury œ•om telescopic observations. Note the scmiannula• œ½atu•½s ccm½•½d on 190øW that

closely •½s½mb1½ the distribution oœ smooth plains su•oundin8 the Calofis Basin. Fi8u•c œ•om M•rray ½• aL [1972]. Rcpfimcd by permission oœ Academic

posed circum-Caloris annulus. This section may be smooth plains or, by analogy with the terrain southeast of Tir Planitia, it could be a region of intercrater plains bounding the smooth plains to the east. The flat section of plains from 230ø-240øW at 10øN (Figure 2d) is of interest in that it is known to be a region of high thermal inertia [Chase et al., 1976], a possible indicator that a high percentage of the surface is covered by rock or rock outcroppings. A swath of thermal data extending from 200øW, 18øN to 245øW, 7øN (much of which presumably includes the smooth plains annulus) shows significantly higher thermal inertias than does the terrain further west and south

(250øW to 340øW), suggesting a soil of higher density, a greater exposure of consolidated rock, or both [Chase et al., 1976].

Although the largest expanse of smooth terrain is in the region south of Caloris, radar profiles and Figure 7 indicate several other areas of comparably low roughness. Two of these areas are in the unimaged hemisphere at 285ø-297øW, 8ø-11øN, and 330ø-352øW, liøN (Figures 2d and 2e). A few smaller smooth regions can be seen in the imaged hemisphere. One area east of Asvaghosa Crater actually consists of two plains regions separated by a ridge (Figures 2a and 10). These plains have been mapped as smooth plains by De Hon et al. [1981]. Smooth, concave areas are also seen at the northern and southern portions of a large basin in the H-7 quadrangle (see Figure 5). The concave shape of these areas could be the result of basin excavation, but it is more likely to be the result of subsidence (see section 4).

6. SCARPS AND FAULT ZONES

Scarps are the most distinctive and widespread tectonic features on the surface of Mercury. As defined by Dzurisin [1976, 1978-1, planimetrically arcuate scarps have been generally accepted as being thrust faults caused by crustal shortening and compression associated with core shrinkage i-Strom et al., 1975l, lithospheric cooling [Solomon, 1976], and/or tidal despinning [Melosh and Dzurisin, 1978bl. Shadow measurements of these features indicate that they range from 500 to 1000 m

in height [Dzurisin, 1978], although a height of 3 km was reported for a portion of Discovery Scarp [Strom et al., 1975].

Three of the arcuate scarps in the tectonic maps of Dzurisin [1976, 1978] and Strom et al. [1975] have been identified in radar profiles. All three scarps are located in the same region of the H-6 quadrangle (Figure 2a); we display them on an expanded scale in Figure 10. These scarps are delineated by shadow and, as they are near the Mariner 10 eastern terminator (illumination from the west), they have been mapped as east-facing dips. The first of these scarps is Santa Maria Rupes, which runs south of the crater Asvaghosa and is crossed by a radar profile at 19øW, 3.9øN (Figure 10). The profile shows an asymmetric ridge-like feature 700 m high and 70 km wide in cross section. The maximum slope of the ridge faces east and corresponds to the mapped position of the scarp [De Hon et al., 1981]. The central cleft is probably a small 25-km-diameter impact crater.

The second of these features runs to the NW of the crater

Donne. It is crossed by a radar profile at 15øW in the southern profile of Figure 10, about 4 ø east of Santa Maria Rupes. It consists of a complex double-peaked ridge with its maximum slope facing east and corresponding to the location of the mapped scarp [De Hon et al., 1981]. The western peak is lower and broader than the eastern peak, but its west-facing slope does appear highlighted in Mariner 10 imagery. The maximum height and cross-sectional width of this double- peaked feature are comparable with those of Santa Maria Rupes (see Figure 10).

The third "scarp" is located in the upper profile in Figure 10 at 13øW, 10.3øN, due east of Asvaghosa Crater. Roughly 750 m high and 70 km wide, it shows a more symmetric and rounded profile than the other two features. The shadow thrown by this feature is very prominent due to its proximity to the Mariner 10 terminator and, again, coincides with the eastern edge of the radar feature. The radar profile of this feature is much more characteristic of a "ridge" than a "scarp," casting doubt as to whether it is, in fact, a thrust fault. The picture is less clear for the other two "scarps" (in the lower


l I tlJ .......... t111 .... '1 I ti1t ttll ........ . I ' I ,J ,,I I t I I 1 I

Asvaghos 'a

4.0

Santa Maria Rupes / 3.8

30 25

t0.7

20 15 I0 5 0

Longitude (øw) Fig. 10. Altitude profiles selected from the H-6 quadrangle (see Figure 2a) showing topography across the three

mapped arcuate scarps discussed in the text. Vertical bars indicate the_ 1 standard deviation altitude errors. Arrows (S) denote the locations of the downthrown (shadow) sides of the scarps as determined from the USGS geologic and shaded-relief maps and Mariner 10 images. The subradar tracks for the two profiles are shown on the Mariner 10 image at the top. The shorter dark line on the right side of the image is an image flaw.

profile in Figure 10); both show significant west-facing down- slopes but are asymmetric, with maximum dip on the eastern (shadow) sides. In any event, the radar results suggest that care must be taken in the identification of scarps when using images obtained at only one illumination angle.

The most distinctive large-scale topographic feature on the H-6 quadrangle (Figure 2a) is a 3 km drop in mean elevation that occurs between 30øW and 40øW longitude. The most impressive part of this drop is at 37.5øW longitude in the two profiles at 2.7øN and 4.0øN latitude, just west of Handel Crater. Here most of the 3 km drop occurs within 70 km (1.5 ø of longitude), although some of this west-facing slope is taken up across Handel itself. To the north, a somewhat shallower

slope occurs across and to the west of Yeats Crater (10øN). To the south, the high eastern rim and asymmetric profile of Homer Basin (Figures 2a and 4) suggest that the basin also straddles this west-facing slope. Inspection of Mariner 10 images and the geologic map of the H-6 quadrangle [-De Hon et al., 1981] shows that this regional slope occurs in an area with several west-facing intracrater fault scarps. These mapped scarps [De Honet al., 1981] transect the craters Yeats and Sinan, the crater just NW of Handel, and a crater NW of Yeats. The crater NW of Handel does, in fact, show a radar signature in the form of two narrow topographic lows near the west rim of Handel at 37øW and 38øW, 4øN, centered on the region of maximum slope (Figure 2a). Mariner 10 images


20 /

10-

-I0 6O

Sinan

• Crater r,m

! Fault scarp Mercator Project•on .• Ridge

Alttmetry 0 200 .... groundtrack k•lometers

contour,, elev(•tion in km

I•/ Yea•t,• • 0 / . ./9 . e... • .......... ....

.............

....

50 40 30 20

Longitude (Ow) Fig. ll. Schematic map of the central portion of the H-6 quadrangle covering the region where radar altimetry

indicates a large west-facing downslope. The topographic contours are shown based on radar altimetric profiles (subradar tracks shown as dotted lines). The indicated ridges and intracrater scarps are from the geologic map of De Honet al. [1981]. Note that although no expression of faulting (e.g., scarps or ridges) is seen south of Handel, the trend of the topography remains the same. Note also the narrow plateau east of Rudaki Crater. Abbreviations are of crater names; Ru, Rudaki; Ch, Chaikovskij; Ti, Titian.

show some hints of continuation of the faults outside the cra-

ters. The indistinct nature of the presumed faults outside the craters may reflect a difference in material properties between the floor of a crater and the plains and ejecta external to a crater.

The major topographic features in the area of the 3 km drop are shown in Figure 11. As can be seen in this schematic, it appears that the scarps in this region are part of a single system of faults that trends north-south in the northern half of the H-6 quadrangle and is associated with the 3 km drop in mean elevation seen in radar altimetry. This system of topographic features does not correlate well with any of the basin rings of Spudis and Strobell [1984] and any curvature of this structural trend is very slight. Thus, it seems unlikely that this fault system is related to any extremely large impact structure.

Another west-facing slope with a 2 km drop can be seen in two radar profiles at 8øW longitude, 2.0øN and 3.8øN latitude (Figure 2a). The high side of this slope is the highest absolute altitude that we have measured for the planet (Figure 12). Unfortunately, it lies too close to the Mariner 10 eastern terminator to allow for comparison with images and appears only at the very edge of the radar profiles.

A 1.5-2 km west-facing slope occurs at approximately 52øW at latitudes extending from 4.8øS to the equator. It apparently defines the western side of a rather narrow plateau of intercrater highlands [De Hon et al., 1981] separating the basin centered at 44øW, 2.1øS from the lowlands west of Rudaki Crater (see Figure 11). There is no obvious connection between this feature and the large fault zone to the northeast. No obvious structural or topographic features can be seen in the altimetry or Mariner l0 derived images or maps that might show a

connection between the two. Detailed comparisons with images are difficult in this area, however, due to the high Mariner 10 illumination angles at this longitude.

An escarpment can be seen in the radar profile across Zeami Crater on the eastern edge of the H-8 quadrangle (Figure 2c). The eastern wall of Zeami drops 3.3 km to the crater floor, whereas the west rim climbs only 2 km before dropping back to an altitude 3 km below the level of the terrain to the east of the crater. The geologic map of $chaber

-2-

-•-

-• f I I I I I I I I I .560 270 180 90 0

Longitude (øw) Fig. 12. Combined plot of all Arecibo radar profiles of Mercury

from 1978-1982, showing absolute altitudes relative to the 2439.0-km- radius reference sphere (denoted by the central horizontal line). Lati- tudes of profiles on this figure range from 11.8øN to 5.0øS.

HARMON ET AL.' RADAR ALTIMETRY OF MERCURY 399

and McCauley [1980] shows extensive faulting on the eastern wall of Zeami; similarities have been noted between Zeami and certain "floor-fractured" craters on the moon [Schultz, 1977' Head et al., 1981]. It seems likely that the interior of Zeami has been influenced by the underlying topography of the area and that a large escarpment or steep slope was present before the event that excavated Zeami. It is possible that the Zeami escarpment/slope may be an extension of the system of faults and lineaments which trend NE of Tolstoj Basin [Schaber and McCauley, 1980]. Given the proximity of Zeami to the Mena-Theophanes Basin (see section 4), however, basin rim structure must be considered a possible con- tributing or alternative explanation for the Zeami escarpment.

7. EQUATORIAL GLOBAL TOPOGRAPHY

To illustrate the equatorial topography on the global scale, we have plotted all of the composite radar profiles from 1978- 1982 on a 0-360 ø longitude scale in Figure 12. The altitude scale is absolute; the zero-altitude datum is defined by the 2439.0-km-radius reference sphere (see section 2). Figure 13 shows a histogram giving the distribution of altitudes from Figure 12.

The mean of the altitudes in Figure 12 is+0.7 km (2439.7 km mean radius), which is consistent with the 2439 ___ 1 km mean equatorial radius measured by Ash et al. [1971] from planetary radar observations. Also, there is good spot agreement with the 2439.5 _+ 1 km radius measured at 295.3øW, 1.1øN from radio occultations [Fjeldbo et al., 1976]. The zero- altitude datum corresponds to the typical elevation of Mercu- rian lowlands (see Figure 12 and Figures 2a-2e) and is close to the "most probable" altitude (+0.3 km) as given by the peak of the histogram (Figure 13).

The extreme range of Mercurian altitudes is 7 km (Figures 12 and 13) as measured from the lowest crater floors to the high plateau near the Mariner 10 eastern terminator. The elevation difference between Mercurian highlands and lowlands is typically about 3 km, which corresponds to the approximate equivalent width of the altitude distribution in Figure 13. For comparison, the moon has approximately 10 km of peak-to- peak relief as measured by Apollo laser altimetry [Kaula et al., 1974]. Some of this relief is due to the roughly 2.5 km offset between the moon's center-of-mass and center-of-figure, since the topographic datum used by Kaula et al. [1974] was based on a sphere about the center-of-mass. Altimetry results from the Lunar Sounder Experiment show about 7 km of relief if crater floors are excluded and if the topographic datum is based on a 1738 km sphere about the center-of-figure [Brown et al., 1974].

Radar altimetry for Mercury shows two major topographic highs in the equatorial zone of the planet. The first extends across the eastern terminator in Mariner 10 images from 350øW to 35øW and resembles a plateau with an abrupt drop- off on its western side. This drop in elevation is associated with an extensive system of faults (see section 6). The second major high area covers a broad region south of Caloris Basin between 160øW and 240øW. Caloris itself is approximately bisected by the Mariner 10 western terminator at 190øW longitude. This high area contains two local topographic lows centered on 180øW and 210øW which correspond to areas of smooth plains. A third, less extensive highlands area can be seen in the more northerly profiles near 310øW in the unimaged hemisphere (Figures 2e and 12).

The fact that Mercury is in a precise 3'2 spin-orbit resonance indicates that the long axis of the planet's dynamical figure is aligned with the perihelion subsolar points at 0øW

el_> _

_

_

[ 1

-5 0 5

,,qlt•tude (kin)

Fig. 13. Histogram of the Mercurian altitudes shown in Figure 12. The histogram is normalized by the number of altitude data points within each altitude bin, not by area. The zero-altitude datum corresponds to a 2439.0-km-radius reference sphere. The highest and lowest altitudes measured are indicated by arrows.

and 180øW longitude. The Arecibo results (Figure 12) show that Mercury's topographic figure is roughly aligned with its dynamical figure, although the two large bulges appear to align somewhat better along a 10ø-190øW longitude axis. Goldreich and Peale [1966] have shown that a difference between the equatorial moments-of-inertia of only about 0.01% for an ellipsoidal figure would likely ensure a high probability of capture into the 3:2 resonance state. This corresponds to variations in an ellipsoidal dynamical figure of about 100 m. It is then conceivable that the dynamical figure of Mercury could be dominated by a long-wavelength component of uncompensated topography associated with the observed bulges.

An alternative explantion for the 3:2 spin-orbit resonance is that there is a lunar-like mascon associated with the smooth

plains in or around Caloris. An early suggestion was made by Murray et al. [1974] that the mascon was located in the interior plains of Caloris itself. This was later challenged [Dzu- risin, 1976; Melosh and Dzurisin, 1978a] on the basis that a mascon is inconsistent with evidence that the final episode in the tectonic history of Caloris was uplift of the basin floor. Strom [1979] points out, however, that this argument against a Caloris mascon presumes that the final uplift was isostatic. Melosh and Dzurisin [1978a], offering an alternative to a Ca- loris mascon, argued that a positive gravity anomaly associated with 400 m of uncompensated material in a 1300-km- wide circum-Caloris smooth plains annulus would suffice to control the planet's dynamical figure. McKinnon [1979] claims that such an annular ring-load would be "substantially compensated" due to its large size, but that some stresses would be set up which would inhibit complete compensation. The apparent similarity between the circum-Caloris smooth plains and lunar maria, strengthened somewhat by the radar evidence for subsidence, establishes a circum-Caloris mascon as a plausible hypothesis. However, the available data (radar and imaging) are insufficient to determine how much of the smooth plains material remains uncompensated.

8. CONCLUSIONS

The Arecibo radar observations of Mercury provide information on the morphology of surface features with horizontal dimensions ranging from the 50 km scales characteristic of


scarps, ridges and moderate-sized craters, to the 1000-2000 km scales of large impact basins and major upland regions. When used in tandem with Mariner 10-derived images and maps, radar altimetry has allowed us to better constrain and more closely examine previous interpretations of the geology of the planet.

Crater depth measurements appear to confirm earlier findings from shadow measurements [Gault et al., 1975; Malin and Dzurisin, 1977], although the slightly shallower radar depths warrant further study. Both radar and shadow measurements suggest that Mercurian craters are shallower, on the average, than lunar craters. Only one large (800 km) basin has been found to show a radar altimetric signature, the overall shallowness of which suggests significant modification by isostatic relaxation or volcanic filling.

Altimetry over the southern environs of Caloris Basin indicates that the circum-Caloris smooth plains extend well into the unimaged hemisphere. Both imaged and unimaged sections of these plains are strongly down-bowed, suggesting that there has been subsidence under the load of the smooth plains fill. This and other similarities to lunar maria offer evidence, albeit circumstantial, in favor of a volcanic origin for the smooth plains around Caloris. Although the Arecibo data are consistent with the hypothesis that there is an irregular annulus of smooth plains surrounding Caloris Basin, new photographic images of the region to the west of the Mariner 10 terminator will be required to establish this definitively.

Some of the most important results of this study bear upon the tectonic evolution of Mercury. We have obtained altimetric profiles across three features mapped as arcuate scarps. One of these features appears to be a ridge rather than a scarp while the other two features, though showing some cross- sectional asymmetry consistent with thrust-faulting, have profiles which are more ridge-like than is suggested by images. These results should be factored into models that account for

such structures as compressional tectonic features. Altimetry has allowed us to document the existence of at least one large fault zone associated with 3 km of reliefi This fault zone also

appears to be intimately connected with one of the two broad topographic bulges in the equatorial zone of Mercury. These bulges rise roughly 3 km above the mean equatorial radius and are aligned along an axis within about 10 ø of the so-called "hot poles" of Mercury. These bulges present an interesting alternative to the hypothesis that the smooth plains in or around Caloris control the dynamical figure of Mercury. The question of whether their origin is due to tidal stresses or to some endogenic process is crucial to understanding the tectonic history of the planet.

Radar observations of the equatorial zone of the unimaged hemisphere reveal uplands and lowlands, a number of large craters, and some topographically smooth areas. The topography of this hemisphere shows no marked differences with re- spect to the imaged hemisphere and no evidence has been found for the existence of another Caloris-type impact structure.

The discussion we have included in this paper is intended to be descriptive and preliminary. There are several areas which warrant more intensive study. Further geophysical modelling of the Caloris region should include the radar topography as a constraint. It is clear, however, that a reasonably complete understanding of this complex and interesting region of Mer- cury will require an orbiting spacecraft with altimetric, gravi- metric, and imaging capabilities. The Arecibo altimetry has provided some new information on the topography of ridges, scarps and faults; more detailed interpretation of these find-

ings is in order. Finally, we note that the possibilities for study of the topography of Mercury with earth-based radar have not been exhausted, and continued observations should be made of the unimaged hemisphere and of areas in the imaged hemisphere deemed interesting on the basis of existing data.

APPENDIX' THE MERCURIAN COORDINATE GRID

Since the spatial resolution of the Arecibo radar altimetry is comparable with uncertainties or inconsistencies in the carto- graphic grid systems, an assessment of the grid systems is in order.

Longitude Coordinate

The USGS shaded-relief map of the H-6 quadrangle adopts the longitude reference used in the Mariner 10 cartography [Davies and Batson, 1975], in which the 20 ø longitude meridian is defined as passing through the small reference crater Hun Kal. Davies and Batson [1975] state that the IAU 0 ø longitude meridian (defined by referencing to the subsolar meridian on January 1, 1950) is at 359.5 ø ___ 0.5 • on the Mar- iner 10 grid. Comparisons of Arecibo altimetry features with the USGS H-6 quadrangle showed the Arecibo longitudes to be systematically larger by approximately 0.4 ø . This difference implies that the 0 ø longitude reference used in the Arecibo altimetry reduction programs is at approximately 359.6øW in the Mariner 10 (Hun Kal) system, which is consistent with the Davies and Batson [1975] estimate of the position of the IAU meridian. Comparisons of Arecibo longitudes with longitudes tabulated in the Astronomical Almanac [U.S. Naval Observa- tory, 1984], which uses the 1979 IAU system, shows precise agreement to within 0.01 ø

A position check of the longitudes of altimetry features with their locations on the USGS H-7 and H-8 shaded-relief maps showed no systematic discrepancies such as that found in comparisons with the USGS H-6 map. This contrast implies that there is an inherent misregistration of the USGS H-7 and H-8 maps relative to H-6. A check of the common boundary of the H-6 and H-7 relief maps does, in fact, show such a mismatch. These findings were confirmed by spot checks between the USGS relief maps and the Mercury Control Net [Davies and Katayama, 1976]. This check showed precise agreement between USGS and Control Net longitudes for the H-6 quadrangle, but that the USGS longitudes were approximately 0.5 ø larger than Control Net longitudes for the H-7 and H-8 quadrangles.

Since the USGS shaded-relief maps provide the best means of interpreting the radar altimetry, it was decided to adjust the Arecibo profiles to conform to the USGS longitudes. Accord- ingly, the Arecibo profiles on the H-6 quadrangle (Figure 2a) were shifted east by 0.4 ø, whereas no shift was made to profiles on the other quandrangles.

Latitude Coordinate

Both the Arecibo and USGS coordinate systems follow the IAU convention placing the Mercurian spin axis normal to the orbital plane. The best estimate of the pole position from Mariner 10 data is offset by 2 ø from the IAU pole, with an error oval encompassing the IAU pole [Klaasen, 1976]. In- voking some dynamical studies by Peale [1969], Klaasen con- cluded that the spin axis was most likely aligned within approximately 1 ø of the orbit normal. This, then, admits the possibility of errors in subradar latitude of the order of a degree.

HARMON ET AL.: RADAR ALTIMETRY OF MERCURY 401

Acknowledgments. We are grateful to the staff of Arecibo Observa- tory for their assistance in various aspects of this project. In particu- lar, we would like to thank R. Velez, T. Crespo, A Hine, and G. Giles. We would also like to thank T. Forni and J. Chandler for their

assistance in the ephemeris generation. One of us (I.I.S.) gratefully acknowledges support from National Science Foundation grant PHY-8243330. D.L.B. wishes to thank the staff and administration of

Arecibo Observatory for their help and for support during his stay. A portion of this work was supported by NASA grant NGR-40-002-116 from the Planetary Geology Program to J.W.H. The National Astronomy and Ionosphere Center (Arecibo Observatory) is operated by Cornell University under contract with the National Science Foundation and with support from the National Aeronautics and Space Administration.

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