[meteorite-list] Mars Express To Investigate Phobos

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:11 2004
Message-ID: <200106071609.JAA23767_at_zagami.jpl.nasa.gov>

ESA Science News
http://sci.esa.int

07 Jun 2001

Mars Express to investigate Phobos

Phobos, the tiny innermost moon of Mars, is to come under unprecedented
scrutiny after Europe's mission to Mars goes into orbit around the Red
Planet late in 2003. Mars Express is due to pass within 3000 km of the
22 km diameter moon a few hundred times during its two-year nominal
mission lifetime.

"There will be many more opportunities for close fly-bys than during NASA's
Viking mission," says Tom Duxbury from NASA's Jet Propulsion Laboratory, a
veteran of previous Phobos observations who is helping to plan the Mars
Express observation campaign.

At a distance of 3000 km, the High Resolution Stereo Camera (HRSC) on board
Mars Express will be capable of taking images with a resolution at least as
good as any taken by Viking. At 1000 km, the Infrared and Visible Mapping
Spectrometer (OMEGA) will also be switched on to map the mineral composition
of the tiny moon's surface; and when the distance is only a few hundred
kilometres, the Planetary Fourier Spectrometer (PFS) and the Sub-surface
Sounding Radar/Altimeter (MARSIS) will record measurements. The Energetic
Neutral Atoms Analyser (ASPERA) will monitor the plasma environment around
the orbit of Phobos during all fly-bys.

"Mars Express will provide the first global map of Phobos. This is important
because we know one side of this moon well, but the other side is less
well-known," says Agustin Chicarro, ESA's Mars Express project scientist.
"We should get a complete view of the satellite in terms of its topography,
sub-surface and composition. This should help us determine once and for all
whether it is a captured asteroid or not," he adds.

Mars Express could generate the most comprehensive suite of observations
ever recorded for Phobos. They will help to resolve some of the outstanding
puzzles about the Martian moon, many of which came to light during previous
observation missions. In addition to Viking, these included the ill-fated
Russian mission, Phobos-2, and NASA's Mars Global Surveyor, which is still
on mission around the Red Planet. Phobos-2 was due to send a lander to the
tiny moon in 1989, but was lost just 100 km above the surface.

Phobos is a world of mysterious origin and destiny. It is light, with a
density less than twice that of water, and orbits just 5980 km above the
Martian surface. One idea is that Phobos and Deimos, Mars's other moon,
are captured asteroids. Data returned by the infrared mapping spectrometer
experiment (ISM) on board the Phobos-2 mission supported this view.

"In general the composition matched that of the inner asteroids. But
we don't know whether Phobos is primordial or whether it has undergone
transformation," says Jean-Pierre Bibring, principal investigator for ISM
and also for OMEGA on Mars Express. "We found some tantalising evidence
for transformation in the form of interesting minerals at the bottom of a
crater, which were absent elsewhere on Phobos, suggesting that the bulk
composition is inhomogeneous. I'm very excited about observing Phobos
again because OMEGA will have much higher resolution and will be able to
determine the surface composition unambiguously and pick out minerals at
the bottom of craters, thus determining the bulk composition."

However, there are other ideas about the origin of these two moons. One,
favoured by Duxbury, is that they are lightly accumulated ejecta from
asteroid impacts on the Martian surface, with Phobos composed of ejecta
orbiting Mars faster than the planet rotates and Deimos, whose orbit is
further out and orbital motion slower, composed of ejecta orbiting more
slowly than the planet rotates.

Another mystery about Phobos concerns the origin of deep grooves on its
surface. Some of these radiate from Stickney, the large impact crater
that distorts the shape of the moon, and are thought to indicate that
the force of the impact nearly shattered the small body. However, others
on the central portion of Phobos run almost parallel and have prompted
another thought.

John Murray, from the Open University in the United Kingdom, and a
co-investigator on the HRSC, thinks that the grooves are strings of
impact craters made by ejecta thrown up by major impacts on Mars. He has
calculated the velocity of such ejecta and the position and direction of
the 'grooves' they might be expected to leave on Phobos. "The direction
of the grooves corresponds to what the hypothesis predicts," he says.
"There are no grooves on the trailing edge of Phobos, which is also what
you'd expect."

However, the leading edge of Phobos has never been imaged in close up
before and the appearance of grooves here will be critical to the theory.
The HRSC on Mars Express should provide the required images for the first
time. "If I'm right, we should see wider chains of grooves crossing at
all angles here," says Murray.

For further information please contact:

ESA Science Programme Communication Service
Tel: +31 71 5653183

USEFUL LINKS FOR THIS STORY

* Mars Express home page
  http://sci.esa.int/marsexpress
* Mars Express instruments
  http://sci.esa.int/structure/content/index.cfm?aid=9&cid=1995
* Phobos and Deimos -- some facts and historical anecdotes
  http://spdext.estec.esa.nl/content/doc/ca/27338_.htm

IMAGE CAPTIONS:

[Image 1:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=9&cid=12&oid=27328&ooid=27332]
The giant Stickney crater on Phobos is clearly visible on this Viking image.
One of the most striking features on the 27 km diameter, irregularly shaped
Phobos is the presence of grooves over most of its surface. The grooves seem
to radiate in all directions from the giant Stickney crater (left) and
converge on the opposite side of the satellite at a region close to the
Stickney antipode. The grooves are best developed near Stickney, where some
measure 700 m across and 90 m deep. However, most of the grooves have widths
and depths in the 100 - 200 m and 10 - 20 m ranges, respectively.

[Image 2:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=9&cid=12&oid=27328&ooid=27333]
Deimos, the companion moon of Phobos, was also imaged by the Viking Orbiter.
This computer mosaic of Deimos was made with images acquired from the Viking
Orbiter during one of its close approaches to the moon. The 15 km diameter
Deimos circles Mars every 30 hours. Scientists speculate that Deimos and its
companion moon Phobos were once passing asteroids that were pulled in by the
gravity of Mars.

[Image 3:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=9&cid=12&oid=27328&ooid=27329]
One of the highest resolution images to date of Phobos obtained with the Mars
Orbiter Camera. This image of Phobos was taken with the Mars Orbiter camera
on the Mars Global Surveyor (MGS) on August 19, 1998, 10 AM PDT. The MGS
spacecraft was approximately 1080 km from Phobos at closest approach. This
image, about 8.2 km wide by 12 km tall, shows the full field-of-view of the
Mars Orbiter Camera (MOC) as spacecraft motion swept across the satellite.
The image as shown here has a scale of 12 m per picture element. Credit:
Malin Space Science Systems/NASA

[Image 4:
http://sci.esa.int/content/searchimage/searchresult.cfm?aid=9&cid=12&oid=27328&ooid=27132]
ESA's Mars Express mission will also obtain valuable measurements on Phobos,
the largest of the Martian moons.
Received on Thu 07 Jun 2001 12:09:29 PM PDT