[FPSPACE] Martian Moon Phobos from Mars Express

[David R. Woods]

Folks,

For a bit of a change of subject, the web site*Astronomy Picture of the 
Day Archive* at http://antwrp.gsfc.nasa.gov/apod/archivepix.html has 
some really neat pictures every day going back to June 1995.  Check out 
today's showing Phobos against the disc of Mars.

The comment about Phobos eventually crashing on Mars has to do with 
tidal dynamics.  Phobos revolves faster than Mars rotates.  Presumably 
it generates a tidal bulge on Mars that provides a gentle tug on 
Phobos.  The bulge lags behind Phobos, providing a retarding force that 
is slowing Phobos down in orbit and causing it to slowly spiral in.  
Deimos, the outer moon, revolves slower than Mars rotates, so its tidal 
bulge leads Deimos, providing an acceleration force that slowly raising 
its orbit to a point where it will eventually escape the Mars 
gravitational field.  Because the moon masses are so low, the tidal 
bulges, if they even exist on Mars with little or no molten core, will 
take a really long time to have any effect, if all all.  Phobos has an 
extremely low density which may mean that a substantial portion of it is 
ice which would be great for human missions: oxygen and water for people 
to consume, and oxygen and hydrogen for propellants.  Bases on Phobos 
would probably precede missions to the surface.

Dave

http://antwrp.gsfc.nasa.gov/apod/ap101201.html

*Martian Moon Phobos from Mars Express *
*Credit: * G. Neukum 
<http://www.esa.int/esaSC/SEMGQOXLDMD_people_0_iv.html> (FU Berlin 
<http://www.fu-berlin.de/>) et al., Mars Express 
<http://www.esa.int/export/SPECIALS/Mars_Express/SEMVQ95V9ED_0.html>, 
DLR <http://www.dlr.de/pf>, ESA <http://www.esa.int/>; /Acknowledgement: 
/ Peter Masek

*Explanation: * Why is Phobos so dark? Phobos 
<http://en.wikipedia.org/wiki/Phobos_%28moon%29>, the largest and 
innermost of two Martian moons, is the darkest moon in the entire Solar 
System <http://solarsystem.nasa.gov/planets/>. Its unusual orbit and 
color indicate that it may be a captured asteroid 
<http://bb.nightskylive.net/asterisk/viewtopic.php?f=24&t=18014> 
composed of a mixture of ice and dark rock. The above picture 
<http://www.unmannedspaceflight.com/index.php?showtopic=480&st=195&p=167059&#entry167059> 
of Phobos near the limb of Mars was captured 
<http://www.planetary.org/blog/article/00002791/> last month by the 
robot spacecraft Mars Express 
<http://www.esa.int/SPECIALS/Mars_Express/SEMFU55V9ED_0.html> currently 
orbiting Mars. Phobos <http://antwrp.gsfc.nasa.gov/apod/ap080414.html> 
is a heavily cratered and barren 
<http://antwrp.gsfc.nasa.gov/apod/ap031109.html> moon, with its largest 
crater <http://antwrp.gsfc.nasa.gov/apod/ap080410.html> located on the 
far side. From images like this, Phobos 
<http://antwrp.gsfc.nasa.gov/apod/ap061203.html> has been determined to 
be covered by perhaps a meter of loose dust 
<http://antwrp.gsfc.nasa.gov/apod/ap980914.html>. Phobos orbits 
<http://www.youtube.com/watch?v=7rxqZcO-0uI> so close to Mars that from 
some places it would appear to rise and set twice a day, but from other 
places it would not be visible at all. Phobos 
<http://antwrp.gsfc.nasa.gov/apod/ap990313.html>' orbit around Mars is 
continually decaying -- it will likely break up 
<http://adsabs.harvard.edu/abs/2008AGUFM.P51C1423H> with pieces crashing 
to the Martian surface in about 50 million years.