[meteorite-list] Cassini-Huygens Looks at Phoebe's Distant Past

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Jun 15 16:37:48 2004
Message-ID: <200406152037.NAA01098_at_zagami.jpl.nasa.gov>

http://www.esa.int/esaCP/SEMNL63VQUD_index_0.html

Cassini-Huygens looks at Phoebe's distant past
European Space Agency
14 June 2004
 
Images collected during the Cassini-Huygens close fly-by of Saturn's
moon Phoebe give strong evidence that the tiny moon may be rich in ice
and covered by a thin layer of darker material.
 
Its surface is heavily battered, with large and small craters. It might
be an ancient remnant of the formation of the Solar System.

On Friday 11 June, at 21:56 CET, the Cassini-Huygens spacecraft flew by
Saturn's outermost moon Phoebe, coming within approximately 2070
kilometres of the satellite's surface. All eleven on-board instruments
scheduled to be active at that time worked flawlessly and acquired data.

The first high-resolution images show a scarred surface, covered with
craters of all sizes and large variation of brightness across the surface.

Phoebe is a peculiar moon amongst the 31 known satellites orbiting
Saturn. Most of Saturn's moons are bright but Phoebe is very dark and
reflects only 6% of the Sun's light. Another difference is that Phoebe
revolves around the planet on a rather elongated orbit and in a
direction opposite to that of the other large moons (a motion known as
'retrograde' orbit).

All these hints suggested that Phoebe, rather than forming together with
Saturn, was captured at a later stage. Scientists, however, do not know
whether Phoebe was originally an asteroid or an object coming from the
'Kuiper Belt'.
 
The stunning images obtained by Cassini's high-resolution camera now
seem to indicate that it contains ice-rich material and is covered by a
thin layer of dark material, probably 300-500 metres thick.

Scientists base this hypothesis on the observation of bright streaks in
the rims of the largest craters, bright rays radiating from smaller
craters, grooves running continuously across the surface of the moon
and, most importantly, the presence of layers of dark material at the
top of crater walls.

"The imaging team is in hot debate at the moment on the interpretations
of our findings," said Dr Carolyn Porco, Cassini imaging team leader at
the Space Science Institute in Boulder, USA.

"Based on our images, some of us are leaning towards the view that has
been promoted recently, that Phoebe is probably ice-rich and may be an
object originating in the outer solar system, more related to comets and
Kuiper Belt objects than to asteroids."

The high-resolution images of Phoebe show a world of dramatic landforms,
with landslides and linear structures such as grooves, ridges and chains
of pits. Craters are ubiquitous, with many smaller than one kilometre.

"This means, besides the big ones, lots of projectiles smaller than 100
metres must have hit Phoebe," said Prof. Gerhard Neukum, Freie
Universitaet Berlin, Germany, and a member of the imaging team. Whether
these projectiles came from outside or within the Saturn system is
debatable.

There is a suspicion that Phoebe, the largest of Saturn's outer moons,
might be parent to the other, much smaller retrograde outer moons that
orbit Saturn. They could have resulted from the impact ejecta that
formed the many craters on Phoebe.

Besides these stunning images, the instruments on board Cassini
collected a wealth of other data, which will allow scientists to study
the surface structures, determine the mass and composition of Phoebe and
create a global map of it.

"If these additional data confirm that Phoebe is mostly ice, covered by
layers of dust, this could mean that we are looking at a 'leftover' from
the formation of the Solar System about 4600 million years ago," said Dr
Jean-Pierre Lebreton, ESA Huygens Project Scientist.

Phoebe might indeed be an icy wanderer from the distant outer reaches of
the Solar System, which, like a comet, was dislodged from the Kuiper
Belt and captured by Saturn when the planet was forming.

Whilst studying the nature of Phoebe may give scientists clues on the
origin of the building blocks of the Solar System, more data are needed
to reconstruct the history of our own neighbourhood in space.

With that aim, ESA's Rosetta mission is on its way to study one of these
primitive objects, Comet 67P/Churyumov-Gerasimenko, from close quarters
for over a year and land a probe on it.

The fly-by of Phoebe on 11 June was the only one that Cassini-Huygens
will perform with this mysterious moon. The mission will now take the
spacecraft to its closest approach to Saturn on 1 July, when it will
enter into orbit around the planet.

>From there, it will conduct 76 orbits of Saturn over four years and
execute 52 close encounters with seven other Saturnian moons. Of these,
45 will be with the largest and most interesting one, Titan. On 25
December, Cassini will release the Huygens probe, which will descend
through Titan's thick atmosphere to investigate its composition and
complex organic chemistry.
 
Received on Tue 15 Jun 2004 04:37:10 PM PDT