Scientists solve Martian riddle
2008-06-26 14:38
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 US 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 consists of rugged, crater-pitted highlands, whose altitude is up to 8 000m 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 up-welling on the Martian surface.
Tip over
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 space rock which whacked into 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.
Researchers at Nasa and the Massachusetts Institute of Technology (MIT) believe they 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.
Perfect ellipse
Using data from two US 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.
"The match between a perfect ellipse and the traced boundary line between the two topographic regions was startling," says MIT's Jeffrey Andrews-Hanna.
"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."
He declares: "An impact is really the only mechanism that can produce these large-scale elliptical depressions, these holes in the ground."
The paper appears in the British weekly journal Nature on Thursday, alongside two other studies.