Saturn’s Auroras and Polar Atmosphere from Cassini UVIS

Wayne PryorRobert WestKris LarsenIan StewartLarry EspositoJoshua ColwellWilliam McClintockAlain JouchouxDon ShemanskyJoseph AjelloCandice HansenJohn ClarkeJacques GustinDenis GrodentJean-Claude GerardKevin BainesPierre DrossartAmy Simon-MillerPresented at Fall AGU, 2006

AbstractCassini's Ultraviolet Imaging Spectrograph (UVIS) has completed two years of study of Saturn's atmosphere and auroras. Two long-slit spectral channels are used to obtain EUV data from 56.3-118.2 nm and FUV data from 111.5-191.3 nm. 64 spatial pixels along each slit are combined with slit motion to construct spectral images of Saturn, with sufficient spatial resolution to image Saturn's auroral oval when Cassini leaves Saturn's equatorial plane. We will present new images and time-series data from summer 2006. Detailed spectral models of molecular hydrogen auroral emissions that include hydrocarbon absorption and hydrogen self-absorption have now been compared to UVIS data. We are analyzing a UV spectral feature detected in an auroral oval image from 2005. The feature is an absorption feature concentrated inside the oval, at wavelengths dominated by reflected sunlight and acetylene absorption. The absorption feature appears as a broad absorption "scoop". One plausible molecule that has a similar absorption feature is benzene, which has a cross-section some 500 times larger than acetylene in this spectral region. Thus UVIS is sensitive to small quantities of benzene. Enhanced polar benzene has been previously observed at Jupiter and can be generated in coupled photochemical/auroral models. We will explore the uniqueness of this interpretation, and compare the inferred benzene abundances to results from complementary Cassini CIRS infrared observations. Additional out of the equatorial plane UVIS Saturn data planned for the coming months will improve the signal- to-noise ratio and spatial resolution on the auroral ovals and their interior. Coordinated observations with Cassini VIMS and Hubble Space

Telescope are being scheduled for 2007.

UVIS and VIMS are on the Cassini Orbiter

UVIS long-slit spectroscopyEUV channel 56.3-118. 2 nm

FUV channel 111.5-191.3 nm

64 spatial x 1024 spectral pixels

Spectral imaging is done

by spacecraft slews

Saturn’s emissions:

H Lyman- and H2 bands

from auroras and dayglow.

Reflected sunlight spectrum:

Rayleigh scattering in H2

and acetylene absorption bands

Saturn Auroral Spectrum

Hubble Auroral Campaign Feb 17, 2005

• J. Clarke, J.-C. Gerard PI’s:

• Day-side S auroras by HST ACS in UV (5 orbits)

• Night-side N auroras by Cassini VIMS and UVIS

• Cassini VIMS_003SA_THRCYLMAP001_UVIS_FOV

• Started 2005-048 T23:08:00 GMT, ran 8 h 22 m

• Cassini at ~800,000 km range

• Aurora was weak that day

HST Campaign ACS Images: Feb 17, 2005

16:20-16:58 17:53-18:33

19:29-20:09 21:04-21:45

22:40-23:21

EUV aurora data Feb 17, 2005 (black) Model w/o self-absorption (dashed brown)

Model fit (solid brown) for H2 = 5x1020 cm-2 & T=500K

Wavelength (ÅÅngstroms)

Relative Intensity

June 21 (Day 172), 2005 03:30-14:30

“EUVFUV” from 35 Rs• N-S-N UVIS scan• Slit E-W• Auroral oval imaged

twice• Images deconvolved

• Blue H2, H emission

• Orange reflected sunlight

• Aurora changes over ~1 hour

• Oval 70-75S

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Saturn Day 172, 2005

• Limb-fitting puts auroral oval in 70-75 S range

Polar Dark Spot (2005 day 172)

1738-1816 1816-1894 ratio 1st/2nd 1349-1426 wavelengths (A)

Spot in 1st image is gone at longer wavelengths (2nd image)

Localized small hydrocarbons? (spectrum is noisy)

Polar convergence & downwelling?

UVIS polar benzene feature?Pantos et al., 1978: broad benzene feature

peaks at cross-section of ~2.7x10-16 cm2 Model at left has C2H2 mixing ratio=10-6 off spot, 5x10-6 on spot, with C6H6 of 4x10-9 on spot

Cassini CIRS sees enhanced benzene at 14.8 m at 80S (no 90S report yet) column above 10 mb (cm-2) 10 mb mixing ratio37S 1.1x1014 0.98x10-13

80S 2.2x1014 8.73x10-13

Recent Cassini Camera S Polar Image

• Intense storm at the site of an unusual UV spectrum

Summer 2006 FUV H2 band night-side data (707 spectra, days 147-174): sub s/c lat=0 (rings

suppressed!)

N

S

\/ N/S flip

S

N

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Spectra 457-460 show grazing near- occultation

Recent UVIS N Auroral Image: 2006 Day 303

• Emission on a complete oval• Generally brighter pre-

midnight• Emission spot visible inside

the oval after midnight

• Sub-spacecraft lat. 45 N• Range 22.6 Rs

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Planned Campaign 2007 days 38-40UVIS_038SA_GLOAURTRI002_VIMS

through GLOAURTRI031_VIMSA nice chance for UVIS/VIMS to resolve the oval and polar haze!

While HST observes Saturn 5 orbits day 38, 1 orbit day 39, 1 orbit day 40

• Cassini Range 26 Rs, subspacecraft lat. 45°N

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UVIS Saturn ConclusionsA full Saturn auroral oval has been imaged by UVIS

Auroral brightness varies by ~3

Auroral modeling with self-absorption in H2 and hydrocarbon absorption has been performed

Emissions inside the oval are seen too.

Small south polar dark spot (enhanced acetylene, and maybe benzene?) seen in reflected sunlight near 1750 ± 75 Å and not other wavelengths

CIRS confirms benzene at high latitudes on Saturn (but lower mixing ratios)

UV feature is in the polar storm in the recent Cassini press release!

-Still need longer exposures on dayside pole to improve the S/N

UVIS Auroral References• Esposito, L. W., et al., The Cassini Ultraviolet Imaging Spectrograph Investigation.

Space Sci. Reviews, 115, 299-361, 2004.

• Esposito, L. W., et al., UVIS shows an active Saturnian system, Science, 307, 1251-1255, 2005.

• Ajello, J. M., et al., The Cassini Campaign Observations of the Jupiter Aurora by the Ultraviolet Imaging Spectrograph and the Space Telescope Imaging Spectrograph, Icarus 178, 327-345, 2005.

• Pryor, W. R., et al., Cassini UVIS Observations of Jupiter’s Auroral Variability, Icarus 178, 312-326, 2005.

• Nichols, J. D., et al., Response of Jupiter’s UV auroras to interplanetary conditions as observed by the Hubble Space Telescope during the Cassini fly-by campaign, Annales Geophysicae, in press, 2006.