13 March 2008

Why is Mars Lopsided?

Posted by Ryan Anderson


Take a look at this topographic map of Mars. The first thing that most people notice is that the northern hemisphere is mostly lower elevation (blue), and the southern hemisphere is mostly higher elevation (red). Nobody knows why. This “dichotomy” is one of the biggest questions in Mars science, and there were several talks yesterday afternoon trying to explain it.

There are two main types of theory to explain the Mars crustal dichotomy. One suggests that one or more giant impacts blasted away the crust in the north and left it lower than the south. The other theory relies on internal processes, such as motion in the mantle of Mars. It is possible to get a huge blob of less dense mantle to rise up in a “plume” that can change the thickness of the crust.

Herb Frey gave a talk suggesting that, based on the impact basins that we can see, the crust in the north must have been thinner even before the impacts occurred. He looked at two nearly identical basins, one in the north and one in the south, and showed that the crust under the northern basin was just too thin to be purely due to the basin formation. It must have been thin even before the impact that caused the basin.

How do we know how thick the crust is? It’s tricky. The general idea is that you watch how your spacecraft orbit the planet very very carefully. If the planet is a little more or less dense than average in a certain location, then it will change the gravity slightly, and therefore affect the orbit. With a bunch of assumptions about the density of the crust and mantle, you can come up with estimates of crust thickness.

Take a look at the topographic map of Mars again. The second thing that you’ll probably notice is a big blob of high elevation right on the equator between the southern highlands and the northern lowlands. This is the Tharsis volcanic bulge (you can see four huge shield volcanoes showing up as the very highest elevations).

Shijie Zhong gave an interesting presentation attempting to explain both the Tharsis bulge and the north-south dichotomy. He showed that, when you have really thick crust floating on the mantle, it forms a thick “keel” of rock that is more viscous than the mantle. This keel can interact with a large, upwelling mantle plume and actually can cause the planet’s crust to move as a solid shell! Zhong suggested that a large mantle plume may have formed the extra-thick southern crust, and then interacted with it, shifting the entire crust of Mars with respect to the plume until the plume was at the equator. Once the plume was beneath thinner crust, it could cause massive volcanism and form the Tharsis bulge.

Finally, Jeff Andrews-Hanna gave a really interesting talk suggesting that the northern lowlands are due to one huge impact very early in the history of Mars. To do this, he had to “remove” the Tharsis bulge. He did this by looking at crust thickness estimates. Unlike most of the crust on Mars which “floats” on the mantle, Tharsis is supported by the strength of the crust nearby. It’s similar to the way that you can have a stone bridge spanning a river with no support directly under the center of the arch, as long as the foundation on either side of the river is strong enough. Tharsis basically has no roots, it stays at high elevation because Mars has a thick crust that can hold it up. By understanding which parts of the crust are floating on deep “roots” and which parts are being supported, Andrews-Hanna made a map of the “roots” of the Martian crust. This map essentially removes Tharsis!

With Tharsis removed, you can trace the boundary between the lowlands and highlands all the way around Mars. Andrews-Hanna showed that this boundary matches an oval or elliptical shape that could be explained by a monstrous impact of a ~2000 km diameter object at a 45 degree angle. He even suggested that a region called Arabia terra may be the remains of a multi-ring structure in this ancient monster basin. Very cool stuff!

None of these talks are the final word on the origin of Tharsis or the north-south dichotomy, but they represent important steps in the right direction.