[meteorite-list] Planetary and lunar craters not what they seem

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Thu Oct 20 17:29:36 2005
Message-ID: <r93gl193r1kbkbe1jn7714d414u6a1vc4u_at_4ax.com>

Planetary and lunar craters not what they seem
Direct impact from asteroids and comets less common than thought

Updated: 10:39 p.m. ET Oct. 19, 2005
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 Space.com.

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.
Received on Thu 20 Oct 2005 05:34:47 PM PDT