Massive impact scars Mars

Scientists have identified what could be the site of the largest impact structure in the Solar System — an elliptical-shaped basin on Mars that is four times larger than any other known impact crater. They have also provided new insight on the origin of the two differently shaped hemispheres of Mars: one hemisphere is lower and has a thinner crust than the other. This feature of the topography of Mars has puzzled scientists for decades.

Reporting in this week’s Nature, three papers by the scientists provide fresh evidence that there was a massive, planetary-wide impact on Mars more than 4 billion years ago that formed the vast impact structure and also caused the ‘hemispheric dichotomy’ of the planet.

The striking difference in topography between the smooth plains of the northern hemisphere and the rugged southern highlands, which are about 4 km higher than the northern plains and have a crust about 25 km thicker, has been one of the most prominent, yet unexplained, features of Mars.

Two scenarios have been proposed to explain this difference in the hemispheres: an impact of a large asteroid or comet; or large-scale convective flows in the mantle of Mars. But until now scientists have not yet had enough evidence to support either theory. The problem is further complicated with 30% of the boundary being buried under more recent lava flows from the Tharsis volcanic province, the largest and perhaps longest-lived volcanic region in the Solar System, which formed about 3.8 million years ago.

In one paper, Jeffrey Andrews-Hanna from the Massachusetts Institute of Technology and his colleagues map the dichotomy boundary more accurately than before by using gravity observations to remove the area contributed by the Tharsis volcanic bulge. They discover that the full extent of the boundary accurately fits the ellipse, which they name the ‘Borealis basin’. More than 10,000 km long and 8,500 km wide, this elliptical crater was most likely formed by an oblique impact event.

In a second paper, Margarita Marinova from the California Institute of Technology and colleagues investigate the conditions under which a large impact event could have produced the observed crustal features on Mars without being destroyed completely. Using three-dimensional simulations, they test various impact angles, energies and velocities, finding that a low-angle impact of between 30–60° produces melt that is largely contained and does not erase the evidence of the impact.

Finally, Francis Nimmo from the University of California and colleagues develop a series of high-resolution two-dimensional models to study the behavior of the Martian crust during an impact. They propose that a large impact, apart from excavating a crustal cavity of the correct size, would explain two additional observations: crustal disruption for the hemisphere where the impact occurred, which they suggest is responsible for the observed decline in magnetic field strength of the opposite hemisphere; and the melt from the impact forming the northern lowlands crust.

Commenting in a related ‘News and Views’ article, Walter Kiefer at the Lunar and Planetary Institute in Houston notes that the three papers collectively strengthen the plausibility of the giant-impact model for the hemispheric dichotomy on Mars, but they do not rule out mantle convection as the primary cause. He then outlines ways to further test the impact model.