Saturn’s Auroras and Polar Atmosphere from Cassini UVIS Wayne Pryor Robert West Kris Larsen Ian Stewart Larry Esposito Joshua Colwell William McClintock Alain Jouchoux Don Shemansky Joseph Ajello Candice Hansen John Clarke Jacques Gustin Denis Grodent Jean-Claude Gerard Kevin Baines Pierre Drossart Amy Simon-Miller Presented at Fall AGU, 2006

Abstract Cassini'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 nm and FUV data from 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 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 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 spectroscopy EUV channel nm FUV channel nm 64 spatial x 1024 spectral pixels Spectral imaging is done by spacecraft slews Saturn’s emissions: H Lyman-  and H 2 bands from auroras and dayglow. Reflected sunlight spectrum: Rayleigh scattering in H 2 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 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, :20-16:5817:53-18:33 19:29-20:0921:04-21:45 22:40-23:21

EUV aurora data Feb 17, 2005 (black) Model w/o self-absorption (dashed brown) Model fit (solid brown) for H 2 = 5x10 20 cm -2 & T=500K Å Wavelength ( Å ngstroms) Relative Intensity

June 21 (Day 172), :30-14:30 “EUVFUV” from 35 Rs N-S-N UVIS scan Slit E-W Auroral oval imaged twice Images deconvolved Blue H 2, H emission Orange reflected sunlight Aurora changes over ~1 hour Oval 70-75S

Saturn Day 172, 2005 Limb-fitting puts auroral oval in S range

Polar Dark Spot (2005 day 172) ratio 1st/2nd 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 ~2.7x cm 2 Model at left has C 2 H 2 mixing ratio=10 -6 off spot, 5x10 -6 on spot, with C 6 H 6 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 ratio 37S 1.1x x S 2.2x x10 -13

Recent Cassini Camera S Polar Image Intense storm at the site of an unusual UV spectrum

Summer 2006 FUV H 2 band night-side data (707 spectra, days ): sub s/c lat=0 (rings suppressed!) N S \/ N/S flip SNSN Spectra 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

Planned Campaign 2007 days UVIS_038SA_GLOAURTRI002_VIMS through GLOAURTRI031_VIMS A 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 UVIS_038SA_GLOAURTRI002_VIMS Cassini Range 26 R s, subspacecraft lat. 45°N

UVIS Saturn Conclusions A full Saturn auroral oval has been imaged by UVIS Auroral brightness varies by ~3 Auroral modeling with self-absorption in H 2 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, , Esposito, L. W., et al., UVIS shows an active Saturnian system, Science, 307, , 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, , Pryor, W. R., et al., Cassini UVIS Observations of Jupiter’s Auroral Variability, Icarus 178, , 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.