The Two Faces of Mars: Mystery Unmasked?

Author:  Yangguang Ou
Institution:  Florida State University
Date:  July 2008

Recent findings published in Nature might be the answer to the long-asked question of the discrepancy between the Martian hemispheres. Mars, the fourth planet from the sun, has two distinct hemispheres. Its northern half, a lowland region that could have been the site for an enormous ocean, is approximately two miles on average below its southern counterpart in terms of elevation. This almost 50-50 split of the Martian surface has intrigued many scientists as to its formation. The Volcanic Eruption theory and the Space Impact theory are two competing explanations long established as possible answers to this question. Recent studies are providing evidence in support for the latter proposal.

Both of the theories have been established for some time. The Space Impact theory explains that the discrepancy between the elevation of the Martian hemispheres was a result of a devastating collision with a space object, such as an asteroid, approximately 4 billion years ago. The Volcanic Eruption theory, on the other hand, explains that the discrepancy is not due to external influences, but internal geologic activities within the planet. It relates the cause of the discrepancy to an enormous volcanic eruption on Mars approximately 3.8 billion years ago.

Both of the theories have had significant support from the scientific community. However, researchers at the University of California, Santa Cruz, led by Francis Nimmo, have produced computer simulations to see if a space impact could be the potential culprit for the lowland north. These simulations show that an asteroid approximately two-thirds the size of our moon could have caused this geological formation on Mars if its impact angle was between 30 to 60 degrees. "It's a very old idea, but nobody had done the numerical calculations to see what would happen when a big asteroid hits Mars," said Nimmo.

These simulations have been supported by recent data sent by the Reconnaissance Orbiter and the Global Surveyor, two multifunctional spacecraft designed by NASA and the Jet Propulsion Laboratory (JPL) to orbit Mars. The data revealed that the crust in the northern hemisphere is much thinner compared to that of the southern hemisphere, which could have resulted from a tremendous collision that removed half of the top crust. In addition, there are magnetic deviations in the south that are not present in the north, which could be explained by the collision shock waves sent through the planet that disrupted the other crust on the other side, causing the magnetic abnormalities recorded.

The findings by Nimmo and his team have been accepted by Oded Aharonson, an associate professor at the California Institute of Technology (Caltech). "The dichotomy is arguably the oldest feature on Mars," explained Dr. Aharonson that the Martian geologic discrepancy has been dated to approximately four billion years ago, a time of planet formation as well as a space impact on earth that might have given rise to our moon. A third study, also published in Nature, led by Jeffrey Andrews-Hanna and Maria Zuber of the Massachusetts Institute of Technology (MIT) in Cambridge, and Bruce Banerdt of Nasa's Jet Propulsion Laboratory (JPL) in California, also provided impact evidence using gravitational and topographical data.

Written by Yangguang Ou

Reviewed by Pooja Ghatalia, Muhammed Ziadh

Published by Pooja Ghatalia.