This proposal is to make STIS spectroscopic observations of the H Ly alpha emission line profile from Mars to determine the fraction of hot hydrogen in the martian corona. Prior ACS/SBC images of the hydrogen corona have shown its large extent, and found it to be more highly variable than previously thought. The variation in the extent of the corona appears to be dependent on the water vapor abundance in the lower atmosphere, which would have profound implications for the escape of water over the history of Mars. The next step is to isolate the fraction of H atoms in the corona that are superthermal, which can be done using STIS E140H and a long narrow aperture (52x0.2), providing 0.025 nm resolution. These observations are the next step is addressing the escape of hydrogen and water from Mars, and extending the loss rate into the past.

This proposal is for STIS E140H 52x0.2 observations with the aperture E/W across Mars near opposition (Feb/March 2012). With STIS E140H and the 52x0.5 aperture the FWHM resolution at Ly α is .0057 nm, and the count rate is 0.4 counts/sec – kR per arc sec2. We propose the narrower 52x0.2 aperture to increase the spectral resolution to ~ .0025 nm, but this will also decrease the count rate to 0.16 cts/sec-kR in the same aperture length. This spectral resolution corresponds to a 500 K Voigt profile, easily sufficient to separate the ~300 K thermal component broadened by multiple scattering (Fig. 2) from a hot (~ 1000 K) component. These observations will concentrate on the H line, ~ 6 kR on the disc, giving 38 counts per 0.2 arc sec resolution element in a 1000 sec exposure. To detect a hot component down to ~ 200 R at 3σ, we will need a total 8000 sec exposure time (3 orbits), and we can then bin the data along the aperture as needed to improve the S/N on the broad emission from the hot atoms in the wings of the line. The geocoronal H Ly α emission will contaminate one wing of the martian line. We propose to split the observations into 2 visits, one pre-opposition and one post-opposition, using the LOS orbital speeds of the Earth and Mars to Doppler shift the geocoronal line by 2 spectral resolution elements (.005 nm or 12 km/sec) in each direction, then average the spectra. Each visit will consist of 3 orbits observing Mars and 1 orbit observing the sky background a few arc min from Mars (to aid in the background subtraction). The roll angle will be loosely constrained to have the aperture E/W across Mars. Mars’ motion across the sky generally makes it possible to locate good guide stars within any multi-week observing window, and we have shown in GO 8658 that bright object protection and counting rate limits will not hinder the observations of Mars. The long-slit mode of STIS E140H has been used many times for planetary observations, and has been well calibrated in our earlier programs (mainly for observations of Jupiter). The total program is for 8 HST orbits. SOLAR SYSTEM 12 false 0.0 0.0 false false false false AVAILABLE SOLAR SYSTEM Long-aperture spectra centered on Mars, w/roll angle to place aperture close to E/W across disk - final roll angle will depend on time of year of observation. PLANET Planet Mars STD=MARS F-LINE(1400)=3+/-1e-14, F-CONT(1800)=1+/-5e-13 EARTH SOLAR SYSTEM offset 5 arc min from Mars for sky background measurement PLANET Offset Planet Mars STD=MARS TYPE=POS_ANGLE,RAD=300,ANG=10,REF=NORTH only Lyman alpha emission from sky EARTH SOLAR SYSTEM offset 5 arc min from Mars for sky background measurement PLANET Offset Planet Mars STD=MARS TYPE=POS_ANGLE,RAD=300,ANG=30,REF=NORTH only Lyman alpha emission from sky EARTH STIS/FUV-MAMA Spectroscopic No No E140H 1234 STIS/FUV-MAMA Imaging No No 25MAMA STIS/FUV-MAMA Spectroscopic No No E140H 1234 The scheduling data was generated by CASM 19.3. CASM was run in Phase II mode. 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