Article Main Body

Mars' north polar cap

Image Credit: NASA/JPL/Arizona State University, R. Luk

Slight variations in the orbits of three NASA spacecraft orbiting Mars have allowed researchers to produce a gravity map of the Red Planet, indicating that it has a liquid outer core.

Sixteen years of data from Mars Global Surveyor, Mars Odyssey and Mars Reconnaissance Orbiter have been combined to map the gravitational field of Mars – essentailly the distribution of less dense and more dense rock in the subsurface. The results show higher gravity regions such as Tharsis, with its volcanoes, surrounded by lower gravity regions, thought to be a result of lithosphere flexure arising from the load of the volcanoes. As the volcanic edifices grew, they loaded the lithosphere and it bent and perhaps fractured. The much improved map confirmed suggestions that Mars has a liquid outer core and gave better measurements of martian tides. They also add to the interest of the marked boundary between Mars' northern lowlands and southern highlands. In places, the boundary is matched by a low gravity anomaly; this had been suggested as a consequence of butred channels directing water into the notherns 'sea' in Mars' ancient past, but the greater detail of this map makes that idea seem unikely. 

The data also allowed the seasonal growth of the polar caps to be tracked and quantified in much greater detail by MRO. Some 3 or 4 trillion tons of carbon dioxide freeze out of the atmosphere each winter, forming the polar cps, such as the one shown here  at Chasma Borealis. This is between 12 and 16% of the carbon dioxide in the atmosphere – a sign of the seasonal extremes found on Mars. 

You can also watch a NASA Goddard video about this map.