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Phobos (Nasa/JPL-Caltech/University of Arizona) Phobos is the larger and closer
of Mars' two moons
Scientists say they have uncovered firm evidence that Mars's biggest moon,
Phobos, is made from rocks blasted off the Martian surface in a catastrophic
event.
The origin of Mars's satellites Phobos and Deimos is a long-standing puzzle.
It has been suggested that both moons could be asteroids that formed in the
main asteroid belt and were then "captured" by Mars's gravity.
The latest evidence has been presented at a major conference in Rome.
The new work supports other scenarios. Material blasted off Mars's surface by a
colliding space rock could have clumped together to form the Phobos moon.
Alternatively, Phobos could have been formed from the remnants of an earlier
moon destroyed by Mars's gravitational forces. However, this moon might itself
have originated from material thrown into orbit from the Martian surface.
Previous observations of Phobos at visible and near-infrared wavelengths have
been interpreted to suggest the possible presence of carbonaceous chondrites,
found in meteorites that have crashed to Earth.
This carbon-rich, rocky material, left over from the formation of the Solar
System, is thought to originate in asteroids from the so-called "main belt"
between Mars and Jupiter.
But, now, data from the European Space Agency's Mars Express spacecraft appear
to make the asteroid capture scenario look less likely.
'Poor agreement'
Recent observations as thermal infrared wavelengths using the Planetary Fourier
Spectrometer (PFS) instrument on Mars Express show a poor match between the
rocks on Phobos and any class of chondritic meteorite known from Earth.
These would seem to support the "re-accretion" models for the formation of
Phobos, in which rocks from the surface of the Red Planet are blasted into
Martian orbit to later clump and form Phobos.
"We detected for the first time a type of mineral called phyllosilicates on the
surface of Phobos, particularly in the areas northeast of Stickney, its largest
impact crater," said co-author Dr Marco Giuranna, from the Italian National
Institute for Astrophysics in Rome.
These phyllosilicate rocks are thought to form in the presence of water, and
have been found previously on Mars.
"This is very intriguing as it implies the interaction of silicate materials
with liquid water on the parent body prior to incorporation into Phobos," said
Dr Giuranna.
"Alternatively, phyllosilicates may have formed in situ, but this would mean
that Phobos required sufficient internal heating to enable liquid water to
remain stable."
Rocky blocks
Other observations from Phobos appear to match the types of minerals identified
on the surface of Mars. Thus, the make-up of Phobos appears more closely
related to Mars than to asteroids from the main belt, say the researchers.
In addition, said Pascal Rosenblatt of the Royal Observatory of Belgium, "the
asteroid capture scenarios also have difficulties in explaining the current
near-circular and near-equatorial orbit of both Martian moons (Phobos and
Deimos)".
The researchers also used Mars Express to obtain the most precise measurement
yet of Phobos' density.
"This number is significantly lower than the density of meteoritic material
associated with asteroids. It implies a sponge-like structure with voids making
up 25%-45% in Phobos's interior," said Dr Rosenblatt.
A highly porous asteroid would have probably not survived if captured by Mars.
Alternatively, such a highly porous structure on Phobos could have resulted
from the re-accretion of rocky blocks in Mars' orbit.
Russia's robotic mission to Phobos, named Phobos-Grunt (grunt means ground , or
earth, in Russian) to be launched in 2011, will investigate the moon's
composition in more detail.
The study has been submitted for publication in the peer-reviewed journal
Planetary and Space Science. It was presented at the 2010 European Planetary
Science Congress in Rome.