Viking observation of water vapor on Mars: Revision from up-to-date spectroscopy and atmospheric models
Résumé
A reanalysis of the Mars Atmospheric Water Detector (MAWD, Viking 1 and 2 orbiters) Planetary Data System (PDS) dataset (Jakosky and Farmer 1979) is presented taking into account a new spectroscopic database and improved atmospheric model assumptions. Starting from HITRAN 2004 edition and later (Rothman et al., 2005), the number of lines in the 1.38-μm band has been significantly increased, and their parameters have been modified. The implication of this new spectroscopic data and atmospheric model based on Martian Climate Database (MCD, Forget et al., 1999) gives a significant impact on the H2O retrieval: the total H2O abundance after the reanalysis has decreased twofold in all seasons and most of geographic locations. Overall decrease is associated with larger cumulative strength of the band in HITRAN 2004; low saturation height of water profiles imposed by MCD significantly contributes to decrease of summer polar maximum. Revised MAWD data are compared with later H2O measurements on Mars Global Surveyor (MGS) and Mars Express (MEX). A good agreement with SPICAM/MEX near-IR (1.38-μm band) measurements is found. However, both sets of near-IR measurements are systematically below TES/MGS results obtained in thermal infrared, with a factor of 1.5-2.0. This difference might be associated with remaining ambiguity of the near IR spectral data, and of line broadening in CO2 in both spectral ranges. The reanalyzed MAWD data are in much better agreement with later measurements suggesting more homogeneous, and significantly dryer water cycle on Mars, with no signature of change between the Viking epoch (MY 12-14) and MGS-Mars Express measurements (MY 24-29).