[meteorite-list] Phobos=faux bos?

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Thu, 16 Oct 2008 12:52:53 -0400
Message-ID: <20sef41tpo8joaiusmu7nk9soh5d28dh2d_at_4ax.com>

http://www.redorbit.com/news/space/1588924/closing_in_on_the_origin_of_phobos/

amazing image:

http://www.redorbit.com/modules/imglib/download.php?Url=/modules/news/upload/dcd8170ba805c62e3e752b20335678e8.jpg

Closing in on the Origin of Phobos

Posted on: Thursday, 16 October 2008, 06:55 CDT |

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 Universit?t 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,
Universit?t 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 1.072 1016 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.
 
More Information

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.
Received on Thu 16 Oct 2008 12:52:53 PM PDT