The lopsided topography of Mars, as detailed by the Mars Orbital Laser Altimiter (MOLA) instrument aboard NASA's Mars Global Surveyor spacecraft Blue areas are lowlands, while the hghlands are mostly red.
Credit: NASA Goddard Space Flight Centre
PARIS: For nearly 30 years, space scientists have wrestled with one of the greatest enigmas in the Solar System: why does Mars have two faces?
Pictures sent back by the U.S. Viking landers in the late 1970s unveiled Mars's northern hemisphere as an enormous lowland basin, where - or so it was suspected - a mighty ocean may have raged.
But Mars's southern hemisphere is abruptly, bizarrely different. It comprises rugged, crater-pitted highlands, whose altitude is up to 8,000 metres greater than the north.
Two rival theories have evolved to explain the planet's Janus-like nature. One explanation was that around 3.8 billion years ago, volcanic forces within the planet's bowels caused a massive upwelling on the Martian surface.
The huge load caused the planet to tip over so that the beer-belly bulge could ride more comfortably at its equator, and the titanic stresses of this shift thrusted up swathes of land.
The contrasting theory, first sketched in 1984, is that the northern basin was the result of a giant asteroid which collided with the infant Mars.
But, critics retort, this could hardly be the case, for the basin is oval rather than circular and the rough-edged 'crater rim' lies at very different altitudes.
In a paper appearing today in the British journal Nature, American researchers at the U.S. space agency NAS, the Massachusetts Institute of Technology (MIT) in Boston and the University of California at Santa Cruz believe they now have convincing evidence that the impact theory is solid.
Elliptical craters do exist elsewhere, such as the South Pole-Aitken basin on the Moon, and are caused by objects that smash into a planet at an angle, they say.
And, they argue, volcanic eruptions at the fringe of the Martian basin distorted the elevations, which explains the strange differences in the rim's height.
Using data from two U.S. scouts, the Mars Reconnaissance Orbiter and the Mars Global Surveyor, the team reconstructed the elevations before the volcanoes burst into life, and a magnificent elliptical crater - the greatest yet seen in the Solar System - came into view.
"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 Francis Nimmo, associate professor of Earth and planetary sciences at University of California at Santa Cruz and first author of one of the papers.
Jeffrey Andrews-Hanna of MIT agreed. "The match between a perfect ellipse and the traced boundary line between the two topographic regions was startling," he said. "And in addition to the elliptical boundary of the basin, there are also signs along part of the rim of a possible second, outer ring - a typical characteristic of large impact basins."
"An impact is really the only mechanism that can produce these large-scale elliptical depressions, these holes in the ground," he added.
Another study appearing in the same issue, led by graduate student Margarita Marinova and Oded Aharonson, an associate professor of planetary science at the California Institute of Technology in Pasadena, calculates the size and speed of the impacting object. "The dichotomy is arguably the oldest feature on Mars," notes Oded Aharonson, associate professor of planetary science at Caltech and an author of the study. The feature arose more than four billion years ago, before the rest of the planet's complex geologic history was superimposed.
After running 500 simulations combining various energies, velocities, and impact angles through a powerful supercomputer cluster, the researchers narrowed in on a 'sweet spot' -a range of single-impact parameters that would make exactly the type of crater found on Mars.
The favoured simulation suggests an impact energy of around 1,029 joules - equivalent to 100 billion gigatons of TNT. The asteroid, or impactor, would have hit Mars at an angle between 30 and 60 degrees while traveling at 6 km to 10 km per second. By combining these factors, the researchers calculated that the projectile was roughly 1,600 km to 2,700 km across.
The new work adds to a body of evidence that has built over the past two decades about how the Solar System today was shaped by a vicious bombardment by space rubble.
One idea is that the youthful Earth was once hit by a Mars-sized planet, which smashed a piece of our crust into space. Captured and then smoothed by Earth's gravitational pull, this material evolved into the Moon, according to this hypothesis.
"The early Solar System was a very dangerous place to be a planet," says Andrews-Hanna. "But without those impacts, we wouldn't have the planets as we know them today."
