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 comprises rugged, crater-pitted highlands, whose altitude is up to 8,000 metres (26,000 feet) 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 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.

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 on Thursday in the British weekly journal Nature, alongside two other studies.

One study calculates that the impacting object must have been about 2,000 kilometres (1,200 miles) across -- making it bigger than Pluto -- and struck at an angle of 45 degrees.

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."

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