[meteorite-list] Mars Express Images: Light and Shadow on the Surface of Mars (Phobos)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Feb 17 13:07:31 2006
Message-ID: <200602171731.k1HHVOJ07303_at_zagami.jpl.nasa.gov>


Light and shadow on the surface of Mars
Mars Express
European Space Agency
February 17, 2006

[Black and white HRSC view of Phobos's shadow]
These images, taken by the High Resolution Stereo Camera (HRSC) on board
ESA's Mars Express spacecraft, show the fast-moving shadow of the moon
Phobos as it moved across the Martian surface.
The HRSC obtained these unique images during orbit 2345 on 10 November
2005. These observations would not have been possible without the close
co-operation between the camera team at the Institute of Planetary
Research at DLR and the ESA teams, in particular the mission engineers
at ESA's European Space Operations Centre (ESOC) in Darmstadt, Germany.
[Artist impression of Mars Express and Phobos]

They confirm the model of the moon's orbit around Mars, as it was
determined earlier in 2004 also on the basis of HRSC images. They also
show that with accurate planning even moving objects can be captured
exactly at their predicted position.

The basis for such observations is the accurate knowledge of the
spacecraft position in its orbit as well as of the targeted location on
Mars to within a few hundred metres.

Phobos is the larger of the two Martian moons, 27 kilometres by 22
kilometres in size, and travels around Mars in an almost circular orbit
at an altitude of about 6000 kilometres. Phobos takes slightly more than
7.5 hours to complete a full revolution around the planet.

[An animation of the shadow of Phobos on the surface of Mars]

When it is between the Sun and Mars, Phobos casts a small and diffuse
shadow onto the Martian surface. To an observer on Mars, this would
appear as a very quick eclipse of the Sun. This is similar to an eclipse
on Earth, when the Moon covers the solar disk but much slower.

The shadow of Phobos has an elliptical shape on the Martian surface,
because the shadow's cone hits the surface at an oblique angle. This
shadow appears to be distorted even more because of the imaging
technique of the HRSC.

The shadow moves across the surface with a speed of roughly 7200
kilometres per hour from west to east. The spacecraft travels with a
higher speed of about 12 600 kilometres per hour on its almost polar
orbit from south to north.

[Phobos in colour, close-up]

Since HRSC scans the surface synchronised with the flight velocity of
Mars Express, it takes some time to cover the shadow in its full
dimension. Within this short time, however, the moon moves on, and
therefore the shape of its shadow is "smeared" in the HRSC image.

Another phenomenon, that the shadow is darker at its centre than the
edges, can be explained by again imagining the observer on Mars. With
its small size, Phobos would only cover some 20% of the solar disk.

Even if the observer stood in the centre of the shadow, they would still
be illuminated by the uncovered part of the Sun's disk, in a partial
solar eclipse instead of a total eclipse.

Members of the HRSC Science Team recalculated the orbit of Phobos on the
basis of images taken in 2004. With the help of the improved orbit
determination - the moon has advanced about 12 kilometres with respect
to its previously predicted position along its orbit - it was possible
to calculate those precise times when the shadow observations could be
made. In turn, it was possible to verify the accuracy of the improved
orbit determination by the shadow's position in the new images.
Received on Fri 17 Feb 2006 12:31:23 PM PST

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