This image of Jupiter's moon Europa was taken by the Galileo spacecraft in 1996. Scientists used Galileo images to measure Europa's craters for the study.
updated 10/19/2005 10:39:45 PM ET 2005-10-20T02:39:45

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

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

It was previously thought that most of the craters seen on moons and planets were the work of direct, or "primary" impacts from asteroids and comets. The new finding suggests that most of those craters might instead be "secondaries," impacts that formed by the material ejected from primary impacts.

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

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

Earth's moon, on the other hand, is pockmarked with millions of craters because it lacks both atmosphere and geologic activity. Similarly, Mars has thin atmosphere and relatively little geologic activity.

On both the moon and Mars, teasing out the primary impacts from the secondaries is difficult because the craters are just too numerous, said Edward B. Bierhaus, a researcher at Lockheed Martin's Space Exploration Systems in Denver, Colorado and an author on the study.

The researchers instead turned to Europa, Jupiter's smallest moon and a world covered in a thick crust of ice. More importantly, Europa is geologically active like Earth. Its surface is constantly being repaved 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 on Europa. They then ran a computer simulation of meteors randomly striking Europa but with the condition that the number and size of the craters had to match the real number and size observed in the images. After running the simulations hundreds of times and comparing the results to the images, they found that the crater distributions were not similar as would be expected if most of the craters were caused by primary impacts.

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

"[If] it turns out that most of these small caters are secondary and not primary, then that means the calibrated age from [Earth's] moon is not right," Bierhaus told

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

Most of the objects that strike Jupiter and its moons come from a region of the solar system known as the Kuiper Belt. Therefore, another implication of the finding may be that there are fewer small asteroids in the Kuiper Belt than previously thought, the researchers wrote.  It may be that small asteroids are rarely made or perhaps some process depletes them before they can reach Jupiter and its moons.

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

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