New images from two Mars-orbiting satellites capture times when the planet was pummeled by at least two meteorites at once.
The slug-shaped crater above, photographed on Aug. 4, 2010 by ESA's Mars Express spacecraft, was probably carved by a chain of projectiles coming in at a shallow angle. The 48-mile-long crater has no name, but it lies in Mars' heavily cratered southern highlands, just south of a large crater called the Huygens basin.
Astronomers have speculated that similarly oblong basins were formed by one oblique impact or by volcanic flows. But the shape of the material tossed out of this crater in the initial impact, called the ejecta blanket, suggests the unnamed scar came from a double punch. The blanket shows two distinct lobes like butterfly wings, hinting that each blob was excavated by a different incoming rock.
Three deeper areas inside the crater itself suggest that there could have been more than two impactors. Smaller craters that lie to the gouge's right probably formed later.
A photo taken Jan. 10 with NASA's HiRISE camera is even more striking. The image below shows two symmetrical, neatly overlapping craters that must have formed at the same time.
The assailing rocks could have been parts of a once-intact body that broke up on its way through Mars' thin atmosphere before slamming into the surface. Several known celestial bodies, including the rubbly asteroid Itokawa and the chicken-leg-shaped comet Hartley 2, would likely break in two and form simultaneous impact craters if they smacked into a planet.
Although planets suffer fewer slings and arrows now than they did in the solar system's youth, Mars is headed for another rough time. One of its moons, Phobos, will collide with the planet in a few tens of millions of years, breaking up in the process to form more wonky-shaped impact craters.
Images: 1) ESA/DLR/FU Berlin (G. Neukum). 2) NASA/JPL/University of Arizona
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