Mars Express Captures Unprecedented Images With High-Res Stereo
European space scientists are getting closer to unraveling the
origin of Mars’ larger moon, Phobos, according to the
European Space Agency.
Thanks to a series of close encounters by the 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 (about 16 x 13 x 11
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 Universität 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 Pätzold,
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,
Universität 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 centimeter. This is lower than the density of the
Martian surface rocks, which are 2.7-3.3 grams per cubic
centimeter, 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
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
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
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.