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RESPONSE OF MARTIAN GROUND ICE TO ORBIT-INDUCED CLIMATE CHANGE. M. A. Chamber- lain1 and W. V. Boynton2, 1Planetary Science Institute (1700 E Fort Lowell Rd Suite 106, Tucson, AZ, [email protected] du, 2Lunar and Planetary Laboratory (University of Arizona, Tucson).
We use a 1-D thermal model to estimate the extent tent, both of which modify the ice distribution. Over of near-surface ground ice on Mars for past and present the precession cycle, the limit of ground ice in the epochs that is stable to diffusion with the atmosphere. northern hemisphere varies ~14º in latitude while the The model is largely based on that used by [1] and pre- limit in the south only varies ~6º. dicts a ground ice distribution for the present epoch We estimate the distribution of ground ice for that is very similar to previous models (e.g. [1] and obliquities from 10º to 45º. Previous estimates of [2]), which in turn are consistent with the observed hy- ground ice at different obliquities were made by [7] drogen gamma ray flux [3]. We then apply our model who suggested that ground ice would be stable every- to past epochs to estimate where ground ice was stable where on Mars at obliquities of 32º and higher. We at these times and speculate about potential correla- found that ground ice is not as extensive at high obliq- tions with observed landforms. uities and does not become globally stable at any Model: Similar to [1], the model we developed al- obliquities we test. Ground ice in our models is less lows thermal properties of the ground to vary with extensive because of lower atmospheric water contents depth. The attenuation of insolation through the at- and the vapor depletion scheme used in our model. mosphere, its thermal radiation and sensible heat ex- change with the surface are calculated in a similar way to models of [1] and [4]. A new component we include in the model is a va- por depletion scheme that reduces the average water vapor density of the near-surface atmosphere, relative to the water content of the atmospheric column. Cold nighttime temperatures at the surface deplete water va- por in the lowest part of the atmosphere. We estimate the water content of the martian at- mosphere with a simple model that calculates the water carrying capacity of the atmosphere over a surface with properties of exposed ice. This reproduces the Figure 1 The extent of ground ice for different present water content of the martian atmosphere and obliquities; ice is stable from the pole to the limit of estimates water contents at other obliquities that are the color corresponding to an obliquity in each hemi- similar to GCM results ([5] and [6]). sphere. Results: We calculate the extent of ground ice on Mars in its present orbit and obliquity with different at- Implications: Finding that ground ice does not be- mospheric water contents. We find that ice does not come globally extensive is consistent with the extent of become stable at low latitudes with our models, even ground-ice related landforms, such as dissected mantle with high water content because the vapor depletion al- and terrain softening, that are not found at low lati- gorithm does not allow the atmosphere to become su- tudes. Also, the extents of various styles of mantle per saturated. cover (as mapped by [8]) appear to coincide with limits We estimate the response of the ground-ice distri- of stable ice for the present epoch and low obliquity bution to both precession of the planet and changes in epochs. obliquity. Obliquity is expected to have a more signif- References: [1] Mellon M. T. et al. (1993) Icarus, icant effect on climate and ground ice than precession, 169, 324-340. [2] Schorghofer N. and Aharonson O. though over the past few hundred thousand years (2005) JGR., 110, E05003. [3] Boynton W. V. (2002) obliquity has been relatively constant. The position of Science, 297, 81-85. [4] Haberle R. M. and Jakosky B. perihelion relative to the rotation axis may induce M. (1991) Icarus, 90, 187-204. [5] Mischna M. A. et changes in the extent of ground ice that are of the same al. (2003), JGR, 108, 16-1. [6] Mischna M. A. and order as the changes in ice extent due to the small Richardson M. I. (2005) Geophys. Res. Letters, 32, obliquity changes in this time. L03201. [7] Mellon M. T. and Jakosky B. M. (1995) As the planet precesses, there is a slight change in JGR, 100, 11781-11789. [8] Milliken R. E. and Mus- th surface temperature and in the atmospheric water con- tard J. F. (2003) 6 Mars Conf., Abstract 3240.