The thermal structure, dust loading, and meridional transport in the Martian atmosphere during late southern summer

Citation

Santee, Michelle (1993) The thermal structure, dust loading, and meridional transport in the Martian atmosphere during late southern summer. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/5qzc-4127. https://resolver.caltech.edu/CaltechTHESIS:01092013-102520560

Abstract

This thesis consists of two papers, both of which investigate the state of the Martian atmosphere during a relatively clear period in late southern summer. The first paper describes a new technique for the simultaneous retrieval of atmospheric temperatures and dust abundances from thermal emission spectra. The second paper describes a diagnostic stream function model which is used with the temperature and dust results of the first paper to solve for the meridional and vertical components of the diabatic circulation simultaneously. The abstracts for the two papers are reproduced below.

PAPER I:

The temperature structure and dust loading of the Martian atmosphere are investigated using thermal emission spectra recorded by the Mariner 9 infrared interferometer spectrometer (IRIS). The analysis is restricted to a subset of the IRIS data consisting of approximately 2400 spectra in a 12-day period extending from L_s = 343° to L_s = 348°, corresponding to late southern summer on Mars. Simultaneous retrieval of the vertical distribution of both atmospheric temperature and dust optical depth is accomplished through an iterative procedure which is performed on each spectrum. The inclusion of dust opacity in the retrieval algorithm causes the retrieved temperatures to change by more than 20 K in some atmospheric layers. The largest column-integrated 9 µm dust optical depths (~ 0.4) occur over the equatorial regions. The highest atmospheric temperatures (> 260 K) are found at low altitudes near the sub-solar latitude (~ 6° s), while the coldest temperatures (˂ 150 K) are found at levels near 1.0 mbar over the winter pole. A comparison of temperature maps for 2 PM and 2 AM indicates diurnal temperature variations as large as 80 K at low altitudes near the sub- solar latitude, whereas diurnal temperature changes at pressures less than 1.0 mbar are typically about 10 K. Both dayside and nightside temperatures above about 0.1 mbar (~ 40 km) are warmer over the winter (north) polar region than over the equator or the summer (south) polar region. This thermal structure suggests the existence of a net zonally-averaged meridional circulation with rising motion at low latitudes, poleward flow at altitudes above 40 km, and subsidence over the poles. Because a meridional circulation transports atmospheric constituents as well as heat, it has significant implications for the net flux of dust and water into the polar regions.

PAPER II:

The circulation of the Martian atmosphere during late southern summer is calculated diagnostically from the observed atmospheric temperature distribution. We use global maps of temperature and dust optical depth (~ 0-60 km) retrieved from a subset of the Mariner 9 IRIS thermal emission spectra spanning L_s = 343°- 348° [Santee and Crisp, 1992]. This thermal structure is characterized by a reversed meridional temperature gradient (temperatures increasing poleward) at altitudes above about 40 km. Zonal- mean zonal winds are derived from the zonally-averaged temperatures assuming gradient wind balance and zero surface zonal wind. Both hemispheres have intense mid- latitude westerly jets (with velocities of 80- 90 m/s near 50 km); in the southern tropics the winds are strongly easterly (with velocities of 100 m/s near 50 km). The north-south atmospheric transport includes contributions from the zonal- mean meridional circulation and large-scale waves. Their net effect can be approximated by the diabatic circulation, which is that circulation needed to maintain the observed temperature distribution (warm winter pole, cool tropics) in the presence of the radiative forcing. A radiative transfer model [Crisp, 1990] which accounts for absorption, emission and multiple scattering by particles and non- grey gases is used to compute the solar heating and thermal cooling rates from diurnal averages of the retrieved IRlS temperature and dust distributions. At pressures below 4 mbar, there are large net heating rates (up to 8 K/ day) in the equatorial region and large net cooling rates (up to 20 K/ day) in the polar regions. These net heating rates are used in a diagnostic stream function model which solves for the meridional and vertical components of the diabatic circulation simultaneously. We find a two-cell circulation, with upwelling over the equator(~ 1.5 cm/s), poleward motion in both hemispheres(~ 2 m/s), and subsidence over the poles (1-2 cm/ s). This circulation is sufficiently vigorous that the meridional transport time scale is ~ 13 days. Vertical transport is primarily advective in nature, except in the high-altitude winter polar regions, where diffusive processes dominate. Water vapor desorbed from the low-latitude regolith during late northern winter/ early northern spring may be transported upward by the ascending branch of this circulation, where it would be transported poleward by the high-altitude meridional winds. This process could provide a high-altitude source of water vapor for the polar hood.

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