A New Understanding Martian Water Ice
8 Oct 2002
(Source: American Astronomical Society - Division for Planetary Sciences)
DPS Press Release
Embargoed until 11:30 AM, Tue., Oct. 8, 2002
Mars north polar cap found to have water ice under dry ice Measurements made between February and September 2002 by a neutron spectrometer aboard NASA's Mars Odyssey spacecraft have provided a more detailed understanding of the Martian north polar cap during late winter and early spring.
Maps of the polar cap during this period of time show that the winter cap, which is composed of frozen carbon dioxide (dry ice), is steadily shrinking with time. However, as it shrinks, it leaves behind a ring of terrain that is both lower than expected in temperature and less efficient in its production of neutrons detectable by the neutron spectrometer. Dr. William Feldman of Los Alamos National Laboratories in New Mexico and his colleagues interpret these results as evidence for a buried water-ice-rich permanent terrain that underlies the thick seasonal frost cap composed of dry ice.
In contrast, the intensity of neutrons near the center of the frost cap reaches a maximum well after the cap has begun to shrink in response to solar heating of the atmosphere above the north polar. Computer modeling of this behavior indicates that this variation is consistent with a frost thickness at the center of the cap that at first grows and then shrinks with time. Detailed seasonal weather models for Mars predict a behavior that closely matches this thickening of the cap as the edges recede poleward.
However, the models also predict larger numbers of neutrons near the center of the cap during the time of maximum cap thickness than were observed by the Odyssey neutron spectrometer. The discrepancy suggests that the polar atmosphere overlying the north-polar cap at that time is significantly enriched in nitrogen, which is a strong absorber of neutrons.
Feldman and his colleagues estimate that the amount of nitrogen enrichment needed to account for the difference (more than about 7% by mass) is consistent with depletion of carbon dioxide from the polar atmosphere as it freezes out to form the carbon dioxide ice cap, followed by replenishment of the atmosphere by gas from more equatorial latitudes that contain the normal abundance (2.7% by mass) of nitrogen.
Contact information: William Feldman
Los Alamos National Laboratory
Mail Stop D466
Los Alamos, NM 87545