The weathermen of Mars

Researchers at the Laboratory, the University of Arizona Lunar Planetary Laboratory and Cornell University's Center for Radiophysics and Space Research have discovered further evidence for the possible existence of a changing, and perhaps predictable, Martian climate.

At this week's American Geophysical Union meeting in San Francisco, scientists will unveil thermal, epithermal, and fast neutron data gathered from February through November 2002 by the neutron spectrometer subsystem aboard NASA's Mars Odyssey spacecraft.

The mapping of the Odyssey data indicates deposits of hydrogen in large areas centered on Arabia Terra and 180 degrees east longitude near the equator. The thermal and fast flux data also indicates that these deposits are buried below a shallow layer of water-poor Martian soil.

According to Bill Feldman of Space and Atmospheric Sciences (NIS-1) and Los Alamos' principal investigator on the project, this spatial distribution of hydrogen deposits cannot be explained by the general north-south latitude gradient of water vapor that is in the present Martian atmosphere, thereby requiring different climatic conditions in the relatively recent past.

Maps of thermal, epithermal and fast neutrons for the northern Martian latitudes - northward of +45 degrees - also were studied to determine the time variation of carbon dioxide frost at latitudes during late winter-mid summer. The data collected between February and November 2002, were broken into 16, roughly two-week intervals of time. The edge of the carbon dioxide frost cap is seen to steadily recede during the period, revealing subsurface deposits of water-rich soil.

Previously, Los Alamos' neutron spectrometer had mapped the Martian surface while it was summer in the south and winter in the north. That data revealed the extent to which the northern and southern polar caps are covered by a thick layer of carbon dioxide, or dry ice. During winter, the carbon dioxide layers extend from the poles to within about 60 degrees of the equator because the dry ice frost settles out of the atmosphere when temperatures fall about 186 degrees below zero Fahrenheit. During the warmer summer the carbon dioxide layer evaporates completely in the north but remains as a thick cover of the residual polar cap in the south.

Thermal neutrons are low energy neutrons that are in thermal contact with the soil. Epithermal neutrons are intermediate-energy neutrons that are scattering down in energy after bouncing off of the soil material. Fast neutrons are the highest energy neutrons produced in the interaction between very high energy galactic cosmic rays and the soil.

The Jet Propulsion Laboratory, a division of the California Institute of Technology, manages the Mars Odyssey mission for NASA's Office of Space Science. Investigators at Arizona State University in Tempe, the University of Arizona in Tucson and NASA's Johnson Space Center in Houston, operate the science instruments. Additional science partners are located at the Russian Aviation and Space Agency and at the Laboratory. Lockheed Martin Astronautics in Denver is the prime contractor for the project and developed and built the orbiter. Mission operations are conducted jointly from Lockheed Martin and JPL.

-- Todd Hanson

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