The Annual Heat Balance of the Martian Polar Caps from Viking Observations

Citation

Paige, David Abbey, Jr. (1985) The Annual Heat Balance of the Martian Polar Caps from Viking Observations. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/DPB3-P589. https://resolver.caltech.edu/CaltechTHESIS:04162010-092926816

Abstract

This thesis presents the first measurements of the annual heat budgets of the polar caps of Mars from spacecraft observations. The primary motivation for this work is to understand why seasonal CO₂ frost deposits at the north pole of Mars disappear in the summer, whereas seasonal CO₂ deposits near the south pole do not. This behavior is not expected to first order because both Martian poles receive the same total amount of sunlight at the top of the atmosphere over the course of a year. Understanding why the Martian north and south polar caps behave in an asymmetric fashion is important because the vapor pressures of permanent polar CO₂ deposits determine the planet-wide surface pressures of CO₂ gas, which is the dominant constituent of the Martian atmosphere.

Annual radiation budgets for the core regions of the north and south polar caps are determined from solar reflectance and infrared emission observations obtained by the Infrared Thermal Mappers (IRTMs) aboard the two Viking orbiters. The results show that the absence of CO₂ frost at the north pole during summer is primarily due to an asymmetry in the rates of CO₂ frost sublimation at surface in the north and south during spring. Further analysis traces this difference to seasonal frost reflectivities being approximately 20% lower in the north than in the south during late spring. It is shown that seasonal frost deposits at the poles demonstrate a remarkable tendency to not become darker when contaminated with dust, and to become brighter with increasing rates of solar illumination. Since peak solar illumination rates are presently higher at the south pole than the north pole because of the large eccentricity of Mars' orbit, the tendency for the frost to become brighter with increasing rates of solar illumination explains the asymmetry. The tendency for the frost to not become darker when contaminated by dust explains why the seasonal behavior of the Martian polar caps is highly repeatable from year to year despite interannual variations in the occurrence of Martian global dust storms.

It is suggested that the unexpected properties of Martian seasonal frost deposits revealed by the results of this study are caused by the tendency for dust particles within the frost to absorb solar radiation, sublimate the solid CO₂ that supports them and sink into the frost. If the current behavior of Martian CO₂ frost is an indicator to past events, then interactions between dust and frost have exerted a powerful stabilizing influence on the Martian climate system.

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