[meteorite-list] Astronomers Puzzled by Titan's Missing Craters

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 26 Mar 2007 14:36:45 -0700 (PDT)
Message-ID: <200703262136.l2QLajH22203_at_zagami.jpl.nasa.gov>

http://space.com/scienceastronomy/070326_titan_craters.html

Astronomers Puzzled by Titan's Missing Craters
By David Powell
space.com
26 March 2007

The Cassini spacecraft's radar sweep of Saturn's largest moon Titan in
January revealed a portion of what appears to be a 110 mile (180
kilometer) diameter impact crater.

If its impact origin is confirmed it would only be the fourth such
crater discovered on Titan, a surprisingly small number.

Impact cratering is pervasive in our solar system, and the number of
craters on the surface of a moon or planet can reveal its age in just
the same way the accumulation of potholes on a highway reveals how long
ago the asphalt was laid.

Earth's Moon remains heavily pockmarked because it has no significant
weather or geological processes to wipe its face clean. Earth, similarly
bombarded over the eons, shows many scars from relatively recent impacts
that have not had time to weather away. Craters are common on several
other satellites of Saturn.

"If Titan's surface had the same density of craters that other Saturnian
moons have, there should be thousands of craters," said Ralph Lorenz of
the Johns Hopkins University Applied Physics Laboratory. "With radar
having examined about 10 percent of Titan's surface, we have only three
definite craters, and perhaps a half-dozen probables."

Far too few

The trio of impact craters confirmed to date are named Menrva, Sinlap
and Ksa and have diameters of 273 miles (440 kilometers), 50 miles (80
kilometers) and 17 miles (28 kilometers), respectively.

Titan's thick nitrogen atmosphere hinders the formation of impact
craters less than about 12 miles (20 kilometers) in diameter, because
smaller space rocks burn up before they reach the surface. This is one
reason the shrouded moon's crater count is so modest.

As Cassini continues to map up to 30 percent of Titan's surface at radar
wavelengths the crater tally is set to grow.

"We've seen three craters on 10 percent of the surface, so we will
probably find another 10 to 30. Maybe there are 30-100 in total,
although we will need a follow-on mission to Titan to find
and document them all," Lorenz told SPACE.com.

Still, researchers consider the surface nearly pothole free when it
should be on the road to ruin. Something yet to be determined must be
keeping the crater count down.

Buried

Clues to Titan's smooth finish can be seen in the presence of vast
tracts of sand dunes, river channels and evidence for cryovolcanism
visible in Cassini images.

It is likely that a combination of burial in sand, erosion by methane or
obliteration by the cold hand of cryovolcanism is responsible for paving
over the craters. Cassini has already spotted vague circular features
among Titan's sand dunes that may be evidence of craters undergoing burial.

Pinning down the rate of crater removal will be an important factor in
dating the age of Titan's surface features, including those craters that
have survived.

"We have no way of knowing how recent the known craters are, although
Menrva is old enough that it has a fairly eroded rim, cut by river
channels," Lorenz said.

The road toward life

Though craters are the most obvious legacy of impacts, these dramatic
events can leave much more than a hole in the ground. Indeed, the shock
of an impact can affect the atmosphere. In Titan's case, a big blow can
add ethylene to the mix and aid the conversion of ammonia into molecular
nitrogen.

Chemical reactions on Titan's surface might also be instigated by
impacts and though the craters may be few and far between, fresher
examples such as Ksa could be biologically interesting sites for future
missions to veer toward.

It's possible that for a brief period, the heat of an impact could
create liquid water on a moon that is otherwise icy cold.

"One of the most exciting aspects about impact craters on Titan, is that
by melting a chunk of Titan's icy crust, it forms a local area of liquid
water on the surface, which can then chemically react with all the
photochemical organics drizzling down from the atmosphere," Lorenz
explained.

Such organics when exposed to water in the laboratory have been seen to
form the prebiotic molecules essential to the formation of proteins and DNA.

"So in the thousands of years it might take for this water to freeze,
some very interesting prebiotic synthesis can occur. No-one knows how
close to self-replicating chemical systems such environments might get
before they freeze solid. Perhaps by visiting Titan in the future with a
lander or balloon able to sample such material we can find out," Lorenz
said.
Received on Mon 26 Mar 2007 05:36:45 PM PDT