May 27, 1988
Scientists are getting more glimpses of this warmer, wetter past on Mars while Global Surveyor circles the planet in a temporary 11.6-hour elliptical orbit. Findings from data gathered during the early portions of this hiatus in the mission's orbital aerobraking campaign are being presented today at the spring meeting of the American Geophysical Union in Boston.
Among many results, the Thermal Emission Spectrometer instrument team, led by Dr. Philip Christensen of Arizona State University, Tempe, has discovered the first clear evidence of an ancient hydrothermal system. This finding implies that water was stable at or near the surface and that a thicker atmosphere existed in Mars' early history.
Measurements from the spectrometer show a remarkable accumulation of the mineral hematite, well-crystallized grains of ferric (iron) oxide that typically originate from thermal activity and standing bodies of water. This deposit is localized near the Martian equator, in an area approximately 300 miles (500 kilometers) in diameter.
Fine-grained hematite, with tiny particles no larger than specks of dust, generally forms by the weathering of iron-bearing minerals during oxidation, or rusting, which can occur in an atmosphere at low temperatures. The material has been previously detected on Mars in more dispersed concentrations and is widely thought to be an important component of the materials that give Mars its red color. The presence of a singular deposit of hematite on Mars is intriguing, however, because it typically forms by crystal growth from hot, iron-rich fluids.
Meanwhile, the Mars Orbiter Laser Altimeter instrument is giving mission scientists their first three-dimensional views of the planet's north polar ice cap. Principal Investigator Dr. David Smith of NASA's Goddard Space Flight Center, Greenbelt, MD, and his team have been using the laser altimeter to obtain more than 50,000 measurements of the topography of the polar cap in order to calculate its thickness, and learn more about related seasonal and climatic changes.
These initial profiles have revealed an often striking surface topology of canyons and spiral troughs in the water and carbon dioxide ice that can reach depths as great as 3,600 feet below the surface. Many of the larger and deeper troughs display a staircase structure, which may ultimately be correlated with seasonal layering of ice and dust observed by NASA's Viking mission orbiters in the late 1970s.
The laser data also have shown that large areas of the ice cap are extremely smooth, with elevations that vary only a few feet over many miles. At 86.3 degrees north, the highest latitude yet sampled, the cap achieves an elevation of 6,600 to 7,900 feet (1.25 to 1.5 miles or 2-2.5 kilometers) over the surrounding terrain. The laser measurements are accurate to approximately one foot (30 centimeters) in the vertical dimension.
In June, the ice cap's thickness will reach a maximum during the peak of the northern winter season. Thickness measurements from April will be compared to those that will be taken in June, contributing to a greater understanding of the Martian polar cap's formation and evolution.
In addition, the Global Surveyor accelerometer team, led by Dr. Gerald Keating of George Washington University, Washington, DC, has discovered two enormous bulges in the upper atmosphere of Mars in the northern hemisphere, on opposite sides of the planet near 90 degrees east latitude and 90 degrees west longitude. These bulges rotate with the planet, causing variations of nearly a factor of two in atmospheric pressure, and systematic variations in the altitude of a given constant pressure of about 12,000 feet (four kilometers).
Additional information about these findings and other exciting new results from the Mars Global Surveyor mission is available at the following Internet sites:
http://mars.jpl.nasa.gov/mgs/
http://emma.la.asu.edu
http://ltpwww.gsfc.nasa.gov/tharsis/mola.html
After a month-long period during which the Sun was between Earth and Mars and thus degraded communications with Global Surveyor, the spacecraft has resumed taking scientific data in its temporary elliptical orbit. In September, it will once again begin dipping into the upper atmosphere of Mars each orbit in a process called aerobraking. The drag from this procedure will allow the spacecraft to reach a low circular orbit and begin its primary two-year global mapping mission starting in March 1999.
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