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 fourth
largest 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.
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