Computer Science
Scientific paper
Jul 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991hst..prop.2393b&link_type=abstract
HST Proposal ID #2393
Computer Science
Hst Proposal Id #2393
Scientific paper
It is proposed to measure with HRS the D/H ratio of Lyman alpha intensities from the visible disks of Venus and Mars in order to have a key clue on the evolution of water on these two planets. Whereas the D/H ratio for Earth is 1.6x10-4, indicating no substantial water escape since origin, one single measurement (through in situ mass spectrometry) for Venus indicated a ratio of 1.6 x 10-2 (enrichment 100). However, IUE La observations pushed to IUE ultimate capabilities failed to show the D La emission at 1.5 x 10^-2 of the H La emission, implying a D/H radio significantly smaller than previously reported (factor 8). This important finding needs to be confirmed with a positive detection at a lower level. On Mars, HDO has been detected, showing an enrichment of about 6 in the lower atmosphere. HST observation in the upper atmosphere would bring strong constraints on differentiation and escape of D probably valid for both planets. Even with the Earth's ratio of 1.6x 10^-4, D La can be detected both on Venus and Mars with HST/HRS. The two lines D and H are separated by 0.33 A and well resolved with HRS Echelle A. The D/H ratio in the bulk lower atmosphere transfers into a different D/H La emission ratio because of atmospheric processes, different solar excitation rates, and radiative transfer. All these effects require modellings which are well mastered by the proposers, with computer codes used in
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