Mars’ Southern Hemisphere: Influences of the Great Impact Basins on Extra- Tropical Weather and the Water Cycle

Introduction: Large-scale, extratropical weather disturbances (i.e., highand low-pressure systems or “transient synoptic-period baroclinic/barotropic waves”) are critical components of the global (i.e., general) circulation of a rapidly rotating, differentially heated, shallow atmosphere such as on Earth and Mars. Critically, these wave-like disturbances act as agents for the transport of heat and momentum between low and high latitudes of the planet, and co-incidingly, for the transport of tracers within the atmosphere (e.g., aerosols such as water vapor/ice and dust for Mars). Measurements provided by the Mars Global Surveyor (MGS) thermal emission spectrometer (TES) instrument and the Radio Science (RS) occultation investigation have provided, for the first time, unequivocal evidence that the middle and high latitudes of Mars’ southern hemisphere (SH) experiences large-scale, traveling weather systems [1–3]. Prior to these atmospheric measurements and traveling-wave analyses, a variety of atmospheric circulation model investigations [4–6] postulated that such large-scale extratropical disturbances ought to be present in this hemisphere. In addition, over a similar seasonal span (e.g., NH late autumn/SH late autumn; NH winter/SH winter; etc), such theoretical and atmospheric modeling studies, and the MGS/TES and RS observations, have indicated that the vigor of the SH transient waves is much less intense and the depth scales shallower, compared to their northern hemisphere (NH) counterparts. Although temperature amplitudes can be O(2–10×) weaker, mostly similar wave mode scales (wavenumbers 1-3) and periods (e.g., 2–4 days at low levels; 6–20+ days at upper levels) are found for the southern extratropical waves. In some ways similar to the northern hemisphere, during late autumn and winter, the southern extratropics exhibits a dominant “storm track” (or “storm zone”), where the intensity of, and associated atmospheric variability with, the traveling weather disturbances is the most intense and regionalized geographically. At low atmospheric levels in the SH extratropics (e.g., below a 1-2 scale heights), the variability is greatest in the western hemisphere, roughly between the Tharsis highlands and the prime meridian [1–3]. The SH storm zone appears more intense during SH late winter and early spring [1–3]. Fundamental to assessing the present climate of Mars is forming an understanding of its water cycle via spacecraft observations, theoretical studies and modeling. Recent spacecraft missions (e.g., Mars Express (MEX), Mars Reconnaissance Orbiter (MRO)) have provided new measurements critical to key components of this cycle, in particular, both seasonal and interannual variations in atmospheric water column abundance, water Fig. 1: South polar stereographic projection (latitudes > 55S) of CRISM multispectral (MSP) observations obtained during early spring (Ls = 183.5 − 191.0) of CO2 and H2O near-surface during MY 28 (February through July 2007): pure CO2 ice (red); pure H2O ice (dark blue); mix of CO2/H2O ice (cyan); no ice (gray). (Courtesy Adrian Brown, SETI Institute)