[meteorite-list] Surprising Impacts on Mars and Europa

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:29:54 2004
Message-ID: <200309121551.IAA13572_at_zagami.jpl.nasa.gov>

http://www.astronomy.com/Content/Dynamic/Articles/000/000/001/483lymqh.asp

Surprising Impacts on Mars and Europa

New studies show secondary cratering may be of primary importance.

by Richard Talcott
astronomy.com
September 12, 2003

In the high-tech world of modern science, where sophisticated computers tear
through complicated calculations, the value of arithmetic might seem
negligible. Yet simply being able to count proves to be one of the most
powerful weapons in a planetary scientist's arsenal.

Counting the number of craters on the surface of a planet or moon is the
best way to estimate the age of the surface (assuming you can't bring a
sample back to the lab). Planetary scientists pore over detailed images to
see how many craters pock the surface. Lots of craters means the surface has
been exposed for a long time to bombardment by projectiles; pristine
surfaces imply a young age. A key to interpreting the data, however, is to
account for the number of secondary craters - those formed when material
blasted out by a primary impact rains back down, creating additional
craters.

Two new studies presented at last week's meeting of the American
Astronomical Society's Division for Planetary Sciences in Monterey,
California, highlight the significance of secondary craters. In the first,
Edward Bierhaus of Lockheed Martin reported on his team's study of Jupiter's
moon Europa.

Europa's surface ranks among the youngest in the solar system. Tidal forces
heat the interior, resulting in a global ocean hidden beneath an icy crust.
Over millions of years - short in the history of the solar system - the
craters melt away. Images from the Voyager and Galileo spacecraft show only
about 40 large craters on Europa's surface.

Yet Bierhaus and his team found that the number of craters rises
dramatically when smaller secondary craters are counted. Using
high-resolution Galileo images, "we've identified more than 26,000 craters
on less than one percent of Europa's surface," says Bierhaus. Nearly all of
those are secondaries, created in the aftermath of the few dozen large
impacts.

Planetary scientists estimating surface ages prefer using smaller craters
because they are more numerous and thus yield statistically more significant
results. But the team's findings suggest that lots of small craters do not
necessarily mean great age.

Comets create most of the impact craters in the jovian system, so the lack
of small primary craters also implies a possible lack of small
Jupiter-family comets. Because many of these comets originate in the Kuiper
Belt, Bierhaus thinks the crater study may indicate a scarcity of small
objects in the Kuiper Belt.

Finally, Bierhaus thinks that the apparently easy formation of secondary
craters may create difficulties with the small-crater record in the inner
solar system. As if to emphasize that point, Nadine Barlow of Northern
Arizona University reported on her study comparing secondary craters on Mars
to those on Earth's moon. Planetary scientists typically apply the
well-established link between crater numbers and surface ages derived from
our moon to other objects in the inner solar system.

Barlow compared the number of secondaries on Mars and the moon produced by
relatively recent impacts of asteroids with diameters of 1 kilometer, 5 km,
and 10 km. Using detailed images of Mars from the cameras onboard the Global
Surveyor and Odyssey spacecraft and of the moon from Clementine and Lunar
Orbiter, Barlow found that martian impacts produce fewer secondary craters
than do lunar impacts of similar energy. She suspects this happens because
larger impacts on Mars rapidly melt ice just below the surface surrounding
the site.

Some researchers have used crater counts to derive exceedingly young ages
for some martian terrains. But because impacts of similar age and energy
produce fewer secondaries on Mars than on the moon, Barlow argues that "the
quoted terrain ages on Mars are actually older than currently proposed."
Received on Fri 12 Sep 2003 11:51:12 AM PDT