Video: An impact by an object more than half as wide as Mercury is thought to have created a crater that covers most of Mars’s northern hemisphere (Courtesy of S Lombeyda/ M Marinova/ O Aharonson)
EVERY scar tells a story, yet a huge gash on Mars has long proven very hard to read. Now a peek beneath the planet’s surface reveals that the scar is the largest known impact structure in the solar system – gouged out by a collision that reshaped the Red Planet.
The surface of Mars’s northern hemisphere lies about 6 kilometres lower than that of the southern hemisphere. This has greatly influenced the planet’s evolution, says Jeffrey Andrews-Hanna of the Massachusetts Institute of Technology.
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The northern hemisphere’s thinner crust means more magma has been able to push to the surface to fuel volcanism, and the difference in altitude meant that ancient outbursts of liquid water tended to flow from south to north.
It also means that atmospheric pressure on northern surfaces is higher than in the south, encouraging winds there to scour the surface more than in the opposite hemisphere. As a result, it seems likely that more dust has been blown from north to south than the other way.
A prime theory to explain how this global asymmetry came about is that a huge impact blasted away much of the northern hemisphere’s crust. The prime suspect in the hunt for clues to such a collision had been an extensive structure in the northern hemisphere, thought to have formed about 4.4 billion years ago.
Strange shape
This timing fits with other evidence that numerous large projectiles were careening through the inner solar system at this time, such as the Mars-sized planet that walloped the primordial Earth and formed our Moon, yet left no trace on Earth.
“The timing fits – numerous large projectiles were careening through the inner solar system 4.4 billion years ago”
However, the Mars impact theory has been undermined by the structure’s irregular, kidney shape, since an impact would have punched out a circle or ellipse.
To examine the suspected crater in more detail, Andrews-Hanna and colleagues analysed variations in the strength of gravity above the Martian surface using NASA’s Mars Reconnaissance Orbiter. This revealed that the structure was actually a near-perfect ellipse, but ended up looking kidney-shaped because lava has since obscured part of it (scroll down for image).
Catastrophic effects
At roughly 8500 by 10,600 kilometres across, it is nearly 15 times the area of the Moon’s South Pole-Aitken basin which, at 2500 kilometres in diameter, is the largest undisputed impact scar in the solar system.
The Mars crater was probably created by an object as large as 2700 kilometres across – over half of the diameter of Mercury. The effects of such an impact would have been catastrophic, says Andrews-Hanna.
“Within the basin you’d have had a magma ocean – it would have been easily several tens of kilometres deep,” he says. “Outside the basin you would have had a tremendous amount of ejecta raining back down on the surface.”
Herbert Frey of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, had been sceptical of the giant impact theory because of the structure’s irregular shape, but now calls it the “best working hypothesis” to explain the differences between the Red Planet’s hemispheres. “They make a compelling case,” he says.
Journal reference: (vol 453, p 1216)
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