Measurements of Mars' gravity field made using the Mars Global Surveyor have produced the first high-resolution global maps of the planet's uneven gravity. The observations, which show gravity features that do not match the profile suggested by martian terrain, hint at two very different histories for the planet's northern and southern hemispheres.
The global surveyor's radio science team recently completed measurements of the strength of the gravity field above some 2,500 points around the martian globe. The measurements offer a much more complete picture of Mars' gravity than had been developed using data from observations of the Mariner 9 spacecraft, and the two Viking orbiters, which were launched in the 1970s.
Most of the earlier data was distributed over the northern hemisphere, with almost nothing known about the gravitation in the south, said Leonard Tyler, leader of the global surveyor's radio science team.
"In many ways it's like seeing the planet for the first time, it's just so striking, how different it is," Tyler said.
Preliminary conclusions from the gravity measurements are published in the October 1 issue of the journal Science. The primary author of the paper is David Smith, of NASA's Goddard Space Flight Center, who collaborated with other experts, including Tyler.
The results are intriguing scientists because Mars' relatively smooth northern hemisphere -- which averages several kilometers lower in elevation than the cratered, uneven south -- is gravitationally very rough. Conversely, the southern hemisphere, with it's rough, irregular topography is gravitationally very smooth.
"That probably says something about the relative strength and relative age of the northern hemisphere and the southern hemisphere," said Maria Zuber, a geophysicist at the Massachusetts Institute of Technology. "It would certainly argue for the northern hemisphere going through a drastically different history than the southern, that the subsurface is quite different."
Measuring gravity tells scientists a great deal about what lies beneath surface of a planet. By comparing the gravity and the topography, they can learn about the strength of the outer surface -- how it behaves mechanically. Whether the crust is rigid or elastic reveals how thick it is. In the end, this indicates how hot or cold the planet was earlier in its history.
"Mars has had a pretty vigorous early history," said Zuber. "Trying to figure out how much heat the planet was dumping out early in its evolution bears on whether it ever had a runaway greenhouse -- and how warm, and what it might have been, and how long water might have been able to persist on the planet. So this is a way to analyze that quantitatively."
While all that might eventually be possible, scientists have not had a chance to do the full analysis, David Smith said. Smith leads the team of scientists in charge of the Mars Global Surveyor's laser altimeter, the instrument responsible for the detailed topography maps of Mars.
"We are providing just a piece of this jigsaw puzzle and it will be a little while before we understand quite why this difference [between the south and the north] is occurring," Smith said. "These results are so new, in a sense -- they're observational results -- and we honestly do not understand them at the moment."
The entire set of data from the Global Surveyor's gravity measurements will be released to scientists within the next week.
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