ESA Closes In On The
Origin Of Mars' Larger Moon
(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.
This image was obtained by the High Resolution Stereo Camera (HRSC) on board ESA’s Mars Express on 28 July 2008 (orbit 5870), at a distance of 351 km from the moon’s centre. The image was taken using the camera’s nadir channel, at a resolution of 14 m/pixel. The origin of Phobos is debated. While its density, lower than the density of the Martian surface rocks, make it appear to belong to D-class asteroids, the moon appears to share many surface characteristics with the class of carbonaceous C-type asteroids, which suggests it might have been captured from this population. However, it is difficult to explain either the capture mechanism or the following evolution of its orbit into the equatorial plane of Mars. An alternative hypothesis is that it formed in its present position, and is therefore a remnant from the planetary formation period. (courtesy: ESA/ DLR/ FU Berlin (G. Neukum))
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.
This striking close-up image of Phobos was obtained by the Super Resolution Channel (or SRC, a part of the High Resolution Stereo Camera experiment) on board ESA’s Mars Express on 26 July 2008 (orbit 5861). The distance from the moon’s centre was 2295 km, and the image resolution is 20 m/pixel. The Super Resolution Channel (SRC), is an additional camera sharing HRSC’s processing electronics. Unlike HRSC, SRC is a framing camera, taking a complete image during a single exposure like a conventional pocket camera. Its 975 mm Maksutov-Cassegrain telescope gives it a pixel-resolution about four times higher than that of HRSC. Early in the mission, the thermal conditions of the instrument in space caused a distortion in its precision optics. This resulted in a performance lower than expected, with some blur and ghosting visible in the raw images. However, a significant part of the degradation has now been compensated for by analysing the effect of the distortion on test images (with point-like stars as target) and applying a corrective processing. (courtesy: ESA/ DLR/ FU Berlin (G. Neukum))
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.
This mosaic image is composed by 53 pictures obtained by the Super Resolution Channel (or SRC, a part of the High Resolution Stereo Camera experiment) on board ESA’s Mars Express. The SRC images covered 70% of the moon's surface. The remaining area is filled with 16 images previously obtained by NASA’s Viking mission. The mean resolution is 12 m/pixel. (courtesy: ESA/ DLR (S. Semm, M. Wählisch, K.Willner)/ FU Berlin (G. Neukum))
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.
This image, obtained by ESA’s Mars Express on 30 August 2008 (orbit 5984), is the first HRSC colour image of Phobos. It was taken at a distance of 2366 km from the moon’s centre, and it is the composite result of the nadir, green and blue channels. The resolution is 95 m/pixel. The colour of the moon is very uniform, however it is possible to notice some subtle chromatic variations. (courtesy: ESA/ DLR/ FU Berlin (G. Neukum))
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 image was obtained by the High Resolution Stereo Camera (HRSC) on board ESA’s Mars Express on 23 July 2008 (orbit 5851), at a distance of 93 km from the moon’s centre. The image was taken using the camera’s stereo-1 channel, at a resolution of 3.7 m/pixel. The ellipses show the previously (red) and currently (blue) considered landing sites for the Phobos-Grunt Russian mission: based on Mars Express’ image series, the Russian Federal Space Agency is now considering to move the landing location slightly to the north of the previous one, to a safer area located between 7°-21°S and 214°-233°W. Phobos-Grunt (meaning Phobos Soil) will land on Phobos, collect a soil sample and return it to Earth for analysis. The landing site has been selected to be on the side of the moon facing away from Mars - a place now only imaged by Mars Express after the Viking orbiters in the 1970s. (courtesy: ESA/ DLR/ FU Berlin (G. Neukum))
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.
Geometry of the Phobos fly-by. Phobos and Mars Express are not to scale. (courtesy: ESA/ DLR/ FU Berlin (G. Neukum))
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.
This series of images was obtained by the Super Resolution Channel (SRC) (left and centre of the panel) and by the High Resolution Stereo Camera (HRSC, right of the panel) on board ESA’s Mars Express. The images were gathered on 28 July 2008 (orbit 5870), at a distance of 351 km from the moon’s centre. The two SRC images show a resolution of 3.2 m/pixel, while the HRSC one has a resolution of 14 m/pixel. The left SRC image is raw (shown as it has been collected), while the centre SRC one has been corrected for mirror distortion. The right image is a portion of the HRSC image taken using the nadir channel and showing the same portion of the surface imaged by SRC. The latter shows significantly more detail than the HRSC nadir image. (courtesy: ESA/ DLR/ FU Berlin (G. Neukum))
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.
(source:
ESA)