[meteorite-list] Cassini Images Help Solve Mystery of Titan's Missing Craters

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Jun 7 22:46:03 2006
Message-ID: <200606072316.QAA09892_at_zagami.jpl.nasa.gov>

http://www.newscientistspace.com/article/dn9291-images-help-solve-mystery-of-titans-missing-craters.html

Images help solve mystery of Titan's missing craters
Stephen Battersby
New Scientist
07 June 2006

What happened to Titan's craters? NASA's Cassini mission should have
seen hundreds of impact craters on Saturn's giant moon, but so far it
has only spotted a handful.

The latest clues in the mystery of the missing craters suggest a
conspiracy between volcanoes, rain and settling soot - perhaps aided by
an eggshell-thin crust.

Cassini has aimed its radar at Titan five times, mapping five narrow
strips of terrain. In a paper published in Nature, the radar team
analyse the second strip in detail.

If Titan were like other dead moons in the outer Solar System, this
strip would be scarred by perhaps 100 craters bigger than 20 kilometres
across, created by cosmic impacts. But only two appear, meaning the
others must have been destroyed.

One is a ring 80 kilometres across called Sinlap. The other, named
Menrva, is an impact basin 450 kilometres in diameter and there is a
clue to the mystery on its rim - gaps in its ramparts and fluid drainage
patterns nearby.

Radar team member Steven Wall at the Jet Propulsion Laboratory in
Pasadena, California, US, thinks that the gaps have been cut by flowing
fluids, probably the liquid methane that is thought to rain down
occasionally on Titan. Methane streams might have completely washed away
smaller craters.

Buried or obliterated

But other processes are probably at work too. "We see so much evidence
for surface modification, it is likely that craters are being buried or
obliterated," says Wall.

On other passes, Cassini has seen bright lobes of material that are
thought to be volcanic flows. Many craters might have been filled in by
such cryovolcanism - floods of liquid water from Titan's interior.

Others might be slowly buried by the organic soot that rains out of the
atmosphere. Burial would be especially easy if Titan's ice crust is so
thin that it cannot support the weight of high crater walls.

This theory could also fit radar altimetry measurements, which show few
hills more than about 200 metres high. The walls of Sinlap reach 1300
metres, however, so the crust might vary in thickness.

The latest radar swath, taken on 30 April in a bright region called
Xanadu, shows more crater-like forms. Although some of these features
may have other origins, Wall thinks he knows why there are more impact
craters in this region. "A hypothesis we're starting to chase is that
Xanadu is higher and washed clean of organics - and so these higher
craters may be re-exposed, or never covered," he says.

Journal reference: Nature (vol 441, p 709)
Received on Wed 07 Jun 2006 07:16:26 PM PDT