[meteorite-list] Martian Craters With Rays May Be Sources for Mars Meteorites Found on Earth

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Mar 21 13:24:57 2005
Message-ID: <200503162338.j2GNcVq28753_at_zagami.jpl.nasa.gov>

http://www.astronomy.com/default.aspx?c=a&id=2980

Chipping pieces off Mars

Martian craters with rays may be the long-sought sources for the Mars
meteorites found on Earth.

Robert Burnham
Astronomy Magazine
March 16, 2005

Researchers using the Mars Odyssey spacecraft have discovered a handful
of small craters on Mars surrounded by far-flung patterns of rays. Ray
craters are common on the Moon, but they are quite rare on Mars, where
the atmosphere stops the flying jets of shattered rock that make rays
after an impact.

Prior to the discovery, announced this week at the 36th Lunar and
Planetary Science Conference in Texas, only one ray crater had been
identified on Mars. Named Zunil and spanning 6 miles (10 kilometers),
the crater lies in Cerberus Planum in southern Elysium. Now a team of
researchers led by Livio Tornabene of the University of Tennessee,
Knoxville, reports finding an additional four martian ray craters plus
three probables.

"It's fascinating we're seeing ray craters on Mars," Tornabene says,
noting the discovery used the THEMIS infrared imager aboard Mars Odyssey.

"We didn't find ray craters before because we were looking in visible
light," he says. "But martian ray craters don't have the bright rays
that you see on the Moon or Mercury. On Mars, rays show up in the
thermal infrared."

The ray craters cluster in two groups. A pair of them lies south of
Tharsis, while the others, plus the probables, join Zunil in Elysium.
The ray craters range in diameter from 1 to 6 miles (1.5 to 6 km), and
most lie on volcanic plains dating from the most recent period in
martian geological history, the Amazonian.

That suggested a link to the martian meteorites - 34 pieces (latest
count) of the Red Planet that were ejected from Mars between 700,000 and
20 million years ago through impacts and which eventually fell to Earth.
Their extraterrestrial origin is considered proven because the rocks
contain gases matching the atmosphere of Mars as sampled by the Viking
landers in the 1970s. But the meteorites' source locations on Mars have
remained stubbornly unknown.

"We've done incredibly exhaustive studies of these meteorites," says
Jeff Moersch, a team member also at the University of Tennessee. Adds
Tornabene, "We've inferred an entire geological history of Mars from
these meteorites, yet we don't know where they're from."

"Most martian meteorite ages are young," says Tornabene. "So most of
them should come from Amazonian-age terrains - and most of our ray
craters occur in Amazonian-age terrains."

Getting rocks launched from Mars without being destroyed in the process
may be a unique feature of ray craters, says Tornabene. The key is a
process called spallation.

"In spallation," Tornabene explains, "material on top of the surface
close to the impact is ejected at high speed without being greatly
shocked. Spallation can throw debris to great distances - or potentially
off Mars." Escape velocity from Mars is 3 miles (5 km) per second.

Impacts that hit the ground at an angle of 30 degrees to 45 degrees are
most effective in generating spallation and ejecting lots of ray
material. The craters from these moderately oblique impacts typically
have asymmetric shapes and ray patterns, just as the researchers find.

Says Tornabene, "If we could send a rover to one of these sites, we
could confirm the idea - or disprove it.
Received on Wed 16 Mar 2005 06:38:31 PM PST