observational quests for mercury’s exosphere

20 November 2006 Boston Mercury Observation Workshop

Observational Questsfor Mercury’sExosphere

Ann L. SpragueLunar and Planetary Laboratory

University of ArizonaTucson, AZ 85721

[email protected]


Constituent Reference Column Abundance (atoms per cm2)

Hydrogen (H) Broadfoot et al. 1976 ~5x1010

Helium (He) Broadfoot et al. 1976 ~2x1013

Oxygen (O) Broadfoot et al. 1976 ~7x1012

Sodium (Na) Potter and Morgan 1985

~2x1012

Potassium (K) Potter and Morgan 1986

~1x1010

Calcium (Ca) Bida et al. 2000 ~1x107

Mercury’s Known Exospheric Species

Quest #1 What else?


Sprague et al. 1995

Predict S in Mercury’s Exosphere

From:

meteoritescometssulfides in regolith

Test this hypothesis withMESSENGER


Na and K

• Association with surface features• Diurnal wind• Distribution with true anomaly• Multiple release mechanisms• Relationship to the magnetosphere

Quest #2 Relationships to other physical parameters


Sprague Kozlowski Hunten 1990

Potassium


The radar image thatshook the world ofplanetary science!!!

Ice at the polesOf Mercury!!!

Or…is it Sulfur?

Or…is it cold iron and titanium free silicate pyroclastics?

Or…is it somethingelse?

Slade et al. 1992, Harmon and Slade 1992, Butler et al. 1993

Quest #3 Relationship of exosphere to regions ofhigh radar albedo and coherent backscatter


A point to be made………..


fresh crust

might be a source


Kuiper-Murasaki Crater Complex with seeing smear


Radar and Visible bright Spot BGeometry not quite right to show Spot A rotating aroundfrom the west.

Amateurs are imaging the surface of Mercury and finding high albedo regions

Na and K observations over these regions to look for sources would be useful

Image courtesy of Steve Massey


Image courtesy Frank Melillo, Holtsville, NY.

700nm


a. 55N, 27S; 6E, 354W

Spot B and Spot A

b. Same

c. Same,

also Kuiper-Murasaki

d. Spots B and A

e. Same

f. Same

g. Caloris Basin

h. 65N, 205E; 155W

0, 235E,125W

9S, 255E, 105W

i. 35S, 287-317E

43-73W

Filter Imaging andSpectroscopy (Na)

Sprague et al. 1998


Quest #4 What source for Ca and O ??


Not observed over the

Surface like Na and K

Bida et al. 2000

Killen et al. 2004

Radiance of emission~ 2.5 kRay

High radial velocity (-2 km/s)indicates high energy


Test the hypothesis that exospheric O and Caare of solar wind origin.

At Mercury the Ca/O ~ 2.0 10-3 with a factor of 2 uncertainty in the O measurement (Broadfoot et al. 1976); there is small variability in the Ca measurements.

The ratio of Ca/O in the solar wind is ~ 1-4 10-3 (Wurz et al. 2001, 2003).

The similarity of the Ca/O ratio above the limb of Mercury (~ 2.0 10-3) to that of the solar wind, compels serious consideration that the source of Ca and O in Mercury's exosphere is originally the Sun via the delivery of O+6 and Ca+11 in the solar wind.

Koehn and Sprague, In Press, PSS, 2006


Model impact distribution of Ca+11 on the surface of Mercury for specific solar wind conditions Southern hemisphere impact sites dominate Color bar--the number of parcels impacting per site Each parcel represents ~1017 Ca ions Similar behavior, with more pronounced preference for

southern hemisphere impacts is seen for O+6

Mid- latitudeenhancements


Summary: Quests

Quest #1 What other constituents?

Quest #4 What is source for Ca and O?

Quest #2 Relationship to other physical parameters?

Quest #3 Relationship to regions ofhigh radar albedo, coherent backscatter,

and high visible albedo?


Ann says “Sorry I couldn’tbe with you in Boston”

Thank You

observational quests for mercury’s exosphere

Documents

source of ca

exospheric o

surface of mercury

delivery of o

ca measurements

regions ofhigh radar

high visible albedo

w spot b