Massive impact on Mars supported by computer simulations
BY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: June 26, 2008

Dramatic differences between the topography of the northern and southern hemispheres of Mars, which have been explained by many different theories including that of a giant asteroid impact, could finally be supported by computer simulations of such a planet sculpting collision.

The so-called hemispheric dichotomy has been well documented by Mars-faring spacecraft, revealing Mars as a planet of two halves with relatively young low-lying plains in the north, and comparatively old, cratered, highlands in the south. The crust in the south is also much thicker, and exhibits unusual magnetic anomalies that are notably absent in the northern hemisphere.

"Two main explanations have been proposed for the hemispheric dichotomy: either some kind of internal process that changed one half of the planet, or a big impact hitting one side of it," says Francis Nimmo of the University of California, Santa Cruz (UCSC). "The impact would have to be big enough to blast the crust off half of the planet, but not so big that it melts everything. We showed that you really can form the dichotomy that way."

The top image shows the false colour view of Mars' topography. The blue to green colours represent the lowest topography in the northern hemisphere, while reds indicate the older, cratered highlands. Of particular note is the Hellas impact basin (dark purple oval in the southern hemisphere) and the Tharsis bulge containing Olympus Mons towards the right central region of this image. The bottom image shows a cross section illustrating the relative heights of the southern hemisphere (right) sloping down to the northern hemisphere (left). Images: NASA/Goddard Space Flight Centre Scientific Visualisation Studio.

The model used by Nimmo's group calculated the effects of an impact in two dimensions. The other study, led by Margarita Marinova of Caltech, used a different model to calculate impacts in three dimensions, and reached the same conclusion. Most impact events occur at an angle and Marinova ’s group found that the impact of a body roughly one-half to two-thirds the size of the Moon striking at an angle of 30-60 degrees would have caused the observed dichotomy.

"The two approaches are very complementary; putting them together gives you a complete picture," says Nimmo. "The two-dimensional model provides high resolution, but you can only look at vertical impacts. The three-dimensional model allows non-vertical impacts, but the resolution is lower so you can't track what happens to the crust."

According to Nimmo's analysis, shock waves from the impact would travel through the planet and disrupt the crust on the other side, causing changes in the magnetic field recorded there. The predicted changes are consistent with observations of magnetic anomalies in the southern hemisphere.

In addition, new crust that formed in the northern lowlands would be derived from deep mantle rock melted by the impact and should have significantly different characteristics from the southern hemisphere crust. Some of these differences may be confirmed by studying Martian meteorites that have been ejected from the red planet and collected on the Earth. However, we know very little about the differences in compositions of rocks from the northern and southern hemispheres.

“There are apparent differences in the spectral signatures of the northern and southern hemispheres, but that is only telling us about the top few microns of material, and may just be caused by dust, which tends to get blown around,” Nimmo tells Astronomy Now. “Our model predicts that there should be differences. Future measurements, especially compositional measurements of the southern highlands, would provide a useful test of our hypothesis.”

The study also suggests that the impact occurred around the same time as the impact on Earth that created the Moon. “This is based primarily on a geochemical argument,” says Nimmo. “If the giant impact was the cause of a major episode of melting, which is apparently recorded in the geochemistry of Martian meteorites, then the date of that melting event is roughly 60-100 million years after the formation of the Solar System. That's about the same time as the formation of the Moon.” Nimmo also tells Astronomy Now that because only a very small fraction of Martian material would have been thrown off the planet completely, this would not have been enough to form a moon like our own. And if the impact had been much larger, then like the ancient Earth, the whole planet may have been completely melted, preventing any north-south dichotomy to form at all.

"This is how planets finish their business of formation," concludes Erik Asphaug, a member of Marinova's team from UCSC. "They collide with other bodies of comparable size in gargantuan collisions. The last of those big collisions defines the planet."