Under the surface of Mars, the planet's two hemispheres could conceal a temperature difference of several hundred degrees.
The contrast between north and south is already obvious at the surface. The north is mainly made up of vast low-lying plains. The south is higher, more rugged and covered with craters, with a generally thicker crust. A study published in Nature now indicates that this divide extends deep into the planet.
Two global topographic views of Mars produced using the MOLA laser altimeter aboard Mars Global Surveyor. The colors indicate elevation.
Credit: NASA/JPL
The researchers obviously did not place a thermometer in the Martian mantle. They used the trajectories of three spacecraft: Mars Global Surveyor, Mars Odyssey and Mars Reconnaissance Orbiter. Sixteen years of radio measurements made it possible to track tiny variations in their speed, caused by changes in Mars's gravitational field.
To understand this, Mars is not affected in exactly the same way by the Sun's gravitational pull throughout the year. Its orbit is slightly elongated and its axis is tilted. The planet therefore deforms very slightly over the course of the seasons. The way it responds to this attraction depends in particular on the rigidity of rocks deep below the surface.
Yet the measured variations in gravity do not match those expected if Mars's interior were identical everywhere. The calculations indicate a difference in rigidity of more than 20% in the mantle, the large rocky layer between the crust and the core. This difference broadly follows the visible divide between the two hemispheres.
Model of Mars's internal structure as inferred by the present study.
Temperature provides the main explanation favored by the researchers. Their model indicates that the mantle beneath the southern highlands would be approximately 200 to 400 °C hotter than the mantle beneath the northern plains. It could also contain slightly more iron.
How could such a difference still exist today? One possibility comes from the thick crust in the south, which may have limited heat loss for billions of years. Slow movements of hot rocks in the mantle could also play a role. Mars's ancient history, including a possible giant impact, is also still being studied.
This heat could help explain other observations. Waves from some Martian earthquakes appear to lose more energy in the south, which is consistent with hotter rocks. The researchers also indicate that a high temperature could allow small amounts of molten rock—and therefore magma—to exist locally deep below the surface.
Future measurements could distinguish between these possibilities. In particular, the researchers propose using electromagnetic observations to search for possible magma pockets beneath the highlands. Even more precise gravity maps could also reveal whether molten material rises and then becomes trapped in the thick southern crust.