Craters in Planets and Moons Not What They Seemed

A hole in a moon or planet does not always mean whatastronomers thought.

Most of the craters on Jupiter's moon Europa are formed bychunks of rock and ice splashing back down onto the moon's surface after ameteor strike, a new study suggests.

It was previously thought that most of the craters seen onmoons and planets were the work of direct, or "primary" impacts fromasteroids and comets. The new finding suggests that most of those craters mightinstead be "secondaries," impacts that formed by the material ejectedfrom primary impacts.

For Europa, secondaries account for as much as 95 percent ofall the small craters--those less than a mile in diameter--observed on the moon,the researchers concluded. The finding has implications for how astronomersdate the ages of planetary surfaces.

Asteroids, comets and chunks of cosmic debris routinely bombardthe surface of planets and moons. Earth's atmosphere protects us from most ofthese impacts, incinerating most objects before they hit the ground. Even so,Earth has experienced countless meteor impacts throughout its long history. Theevidence for most of those impacts have been erased by erosion from wind andrain and by constant turnover of the Earth's crust.

Earth's Moon, on the other hand, is pockmarked with millions ofcraters because it lacks both atmosphere and geologic activity. Similarly, Marshas thin atmosphere and relatively little geologic activity.

On both the Moon and Mars, teasing out the primary impacts fromthe secondaries is difficult because the craters are just too numerous, saidEdward B. Bierhaus, a researcher at Lockheed Martin's Space Exploration Systemsin Denver, Colorado and an author on the study.

The researchers instead turned to Europa, Jupiter's fourthlargest moon and a world covered in a thick crust of ice. More importantly,Europa is geologically active like Earth. Its surface is constantly beingrepaved with new ice and as a result, Europa has very few craters.

Using high-resolution images from NASA's Galileo spacecraft,the researchers measured the number, size and distribution of craters onEuropa. They then ran a computer simulation of meteors randomly striking Europabut with the condition that the number and size of the craters had to match thereal number and size observed in the images. After running the simulationshundreds of times and comparing the results to the images, they found that thecrater distributions were not similar as would be expected if most of thecraters were caused by primary impacts.

The finding is important because scientists typically usecrater counts to date the ages of planet and moon surfaces. When comparing twosimilar regions on a moon, for example, scientists generally assume that theregion with more impact craters is older. Scientists can also use a region'scrater density to calculate it's absolute age. They usually use our own Moon asa reference because scientists have reliably dated the age of some its cratersbased on rocks brought back by astronauts.

"[If] it turns out that most of these small caters aresecondary and not primary, then that means the calibrated age from[Earth's]Moon is not right," Bierhaus told SPACE.com.

Bierhaus stresses, however, that large primary craters can stillbe reliably used to date a region. It's only in regions where large, primaryimpacts are scarce or nonexistent that dating becomes difficult.

Most of the objects that strike Jupiter and its moons come froma region of the Solar System known as the Kuiper Belt. Therefore, anotherimplication of the finding may be that there are fewer small asteroids in theKuiper Belt than previously thought, the researchers wrote.  It may bethat small asteroids are rarely made or perhaps some process depletes them beforethey can reach Jupiter and its moons.

The finding was reported in the Oct. 20 issue of the journal Nature.

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