[meteorite-list] ESA Closes in on the Origin of Phobos

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 23 Oct 2008 15:06:59 -0700 (PDT)
Message-ID: <200810232206.PAA19809_at_zagami.jpl.nasa.gov>

http://www.esa.int/SPECIALS/Mars_Express/SEM8MUSG7MF_0.html

ESA closes in on the origin of Mars' larger moon
European Space Agency
16 October 2008

European space scientists are getting closer to unravelling the origin
of Mars'larger moon, Phobos. Thanks to a series of close encounters by
ESA's Mars Express spacecraft, the moon looks almost certain to be a
"rubble pile", rather than a single solid object. However, mysteries
remain about where the rubble came from.
 
Unlike Earth, with its single large moon, Mars plays host to two small
moons. The larger one is Phobos, an irregularly sized lump of space rock
measuring just 27 km x 22 km x 19 km.

During the Summer, Mars Express made a series of close passes to Phobos.
It captured images at almost all fly-bys with the High Resolution Stereo
Camera (HRSC). A team led by Gerhard Neukum, Freie Universitat Berlin,
also involving scientists from the German Aerospace Centre (DLR), is now
using these and previously collected data to construct a more accurate
3D model of Phobos, so that its volume can be determined with more
precision.
 
In addition, during one of the nearest fly-bys, the Mars Express Radio
Science (MaRS) Experiment team led by Martin P??tzold, Rheinisches
Institut fuer Umweltforschung at the University of Cologne, carefully
monitored the spacecraft's radio signals. They recorded the changes in
frequency brought about by Phobos' gravity pulling Mars Express. This
data is being used by Tom Andert, Universitat der Bundeswehr Muenchen
and Pascal Rosenblatt, Royal Observatory of Belgium, both members of the
MaRS team, to calculate the precise mass of the martian moon.

Putting the mass and volume data together, the teams will be able to
calculate the density. Eventually, this will be a new important clue to
how the moon formed.

Previously, radio tracking from the Soviet Phobos 88 mission and from
the spacecraft orbiting Mars in the past decades had provided the most
accurate mass. "We can be ten times more precise in our frequency shift
measurements today," says Rosenblatt.

The team's current mass estimate for Phobos is 1.072x1016 kg, or about
one billionth the mass of the Earth.

Preliminary density calculations suggest that it is just 1.85 grams per
cubic centimetre. This is lower than the density of the martian surface
rocks, which are 2.7-3.3 grams per cubic centimetre, but very similar to
that of some asteroids.
 
The particular class of asteroids that share Phobos' density are known
as D-class. They are believed to be highly fractured bodies containing
giant caverns because they are not solid. Instead, they are a collection
of pieces, held together by gravity. Scientists call them rubble piles.

Also, spectroscopic data from Mars Express and previous spacecraft show
that Phobos has a similar composition to these asteroids. This suggests
that Phobos, and probably its smaller sibling Deimos, are captured
asteroids. However, one observation remains difficult to explain in this
scenario.

Usually captured asteroids are injected into random orbits around the
planet that gravitationally tie them, but Phobos orbits above Mars'
equator - a very specific case. Scientists do not yet understand how it
could do this.

In another scenario, Phobos could have been made of martian rocks that
were blasted into space during a large meteorite impact. These pieces
have not fallen completely together, thus creating the rubble pile.

So the question remains, where did the original material come from -
Mars' surface or the asteroid belt? The MARSIS radar on board Mars
Express has also collected historic data about Phobos' subsurface. This
data, together with that from the moon's surface and surroundings
gathered by the other Mars Express instruments, will also help put
constraints on the origin. It's clear though that the whole truth will
only be known when samples of the moon are brought back to Earth for
analysis in laboratories.
 
This exciting possibility might soon become reality because the Russians
will attempt to do this with the Phobos-Grunt mission, to be launched
next year. To land on Phobos, they will require the precise knowledge of
the mass as measured by the MaRS Experiment in order to navigate
correctly, and are also making use of the HRSC images to select the
landing site.
 

Note for editors
 
Many more Phobos pictures are accessible following this link
<http://www.esa.int/SPECIALS/Mars_Express/SEMAKVSG7MF_mg_1.html>.

Between 23 July and 15 September 2008 Mars Express performed a series of
eight fly-bys of the martian moon Phobos, at distances ranging between
4500 and 93 km from the centre of the moon, conducting some of the most
detailed investigations of the Moon to date. In observing Phobos, Mars
Express benefits from its highly elliptical orbit which takes it from a
closest Mars approach of 270 km above the surface up to a maximum of 10
000 km from the planet's centre, crossing the 9 400 km orbit of the
moon. Like our Moon, Phobos always shows the same side to the planet, so
it is only by flying outside the orbit that it becomes possible to
observe the far side. The other spacecraft presently orbiting Mars do so
at much lower altitudes, and therefore only see the planet-facing side
of the moon.
 
The High-Resolution Stereo Camera (HRSC) collected pictures of the
moon???s surface with the highest resolution possible, in colour and in
3-D, and provided images of areas never glimpsed before. By September,
also the Super Resolution (SRC) Camera, part of the HRSC experiment,
collected plenty of images. During the second fly-by, all efforts were
concentrated on accurately determining the mass of the moon using the
MaRS experiment.

The Visible and Infrared Mineralogical Mapping Spectrometer, OMEGA, the
Planetary Fourier Spectrometer, PFS, and the Ultraviolet and Infrared
Atmospheric Spectrometer, SPICAM, gathered details on the surface
composition, geochemistry and temperature of Phobos.
 
The MARSIS radar collected information on the topography of the moon's
surface and on the structure of its interior. The Energetic neutral
atoms analyser, ASPERA studied the environment around Phobos, in
particular the plasma that surrounds the moon and also the interaction
of the moon with the solar wind.

For more information

Gerhard Neukum, HRSC Principal Investigator, Freie Universitat Berlin,
Germany
Email: gneukum _at_ zedat.fu-berlin.de

Martin P??tzold, MaRS Principal Investigator, Rheinisches Institut fuer
Umweltforschung, University of Cologne
Email: Paetzold _at_ geo.uni-koeln.de

Pascal Rosenblatt, MaRS science team, Royal Observatory of Belgium
Email: Pascal.Rosenblatt _at_ oma.be

Tom Andert, Universitat der Bundeswehr Muenchen, Germany
Email: Tom.Andert _at_ unibw-muenchen.de

Agustin Chicarro, ESA Mars Express Project Scientist
Email: Agustin.Chicarro _at_ esa.int
Received on Thu 23 Oct 2008 06:06:59 PM PDT