Light and shadow on the surface of Mars (Forwarded)

ESA News
http://www.esa.int

17 February 2006

Light and shadow on the surface of Mars

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.

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.

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.

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.

For more information on Mars Express HRSC images, you might like to read
our updated 'Frequently Asked Questions'.

More information

* Mars Express image browser
http://www.esa.int/SPECIALS/Mars_Exp...ZF77ESD_0.html
* High Resolution Stereo Camera
http://berlinadmin.dlr.de/Missions/e...indexeng.shtml
* Behind the lens ...
http://www.esa.int/SPECIALS/Mars_Exp...XE1PGQD_0.html

Recent images

* Pits and tectonic grabens in Phlegethon Catena
http://www.esa.int/SPECIALS/Mars_Exp...4SLVGJE_0.html
* Claritas Fossae region of Mars
http://www.esa.int/SPECIALS/Mars_Exp...WONZCIE_0.html
* Sulphate deposits in Juventae Chasma
http://www.esa.int/SPECIALS/Mars_Exp...LKMZCIE_0.html
* 'Butterfly' impact crater in Hesperia Planum
http://www.esa.int/SPECIALS/Mars_Exp...LM8A9HE_0.html
* Fly over the Chasma Boreale at Martian north pole
http://www.esa.int/SPECIALS/Mars_Exp...T86Y3EE_0.html
* The Biblis Patera volcano
http://www.esa.int/SPECIALS/Mars_Exp...W9A5QCE_0.html
* Water ice in crater at Martian north pole
http://www.esa.int/SPECIALS/Mars_Exp...KA808BE_0.html
* Nicholson Crater on Mars
http://www.esa.int/SPECIALS/Mars_Exp...WL6DIAE_0.html
* Coprates Chasma and Coprates Catena
http://www.esa.int/SPECIALS/Mars_Exp...RE1DU8E_0.html
* Ancient floods on Mars: Iani Chaos and Ares Vallis
http://www.esa.int/SPECIALS/Mars_Exp...KO0DU8E_0.html
* Crater Holden and Uzboi Vallis
http://www.esa.int/SPECIALS/Mars_Exp...YX2IU7E_0.html
* Tithonium Chasma up close
http://www.esa.int/SPECIALS/Mars_Exp...FP2IU7E_0.html
* The mesas of Aureum Chaos
http://www.esa.int/SPECIALS/Mars_Exp...19NQS7E_0.html

IMAGE CAPTIONS:

[Image 1:
http://www.esa.int/SPECIALS/Mars_Exp...5MVGJE_1.html]
The HRSC on ESA's Mars Express spacecraft took this image of Phobos's
shadow on the surface of Mars during orbit 2345 on 10 November 2005.
This is a nadir view (i.e., vertical view).

Credits: ESA/DLR/FU Berlin (G. Neukum)

[Image 2:
http://www.esa.int/SPECIALS/Mars_Exp...html#subhead1]
This artist impression shows ESA’s Mars Express spacecraft scanning the
fast-moving shadow of the moon Phobos as it moved across the Martian
surface.

Credits: ESA/DLR/FU Berlin

[Image 3:
http://www.esa.int/SPECIALS/Mars_Exp...html#subhead3]
This image, taken by the High Resolution Stereo Camera (HRSC) on board
ESA’s Mars Express spacecraft, is one of the highest-resolution pictures
so far of the Martian moon Phobos.

The image shows the Mars-facing side of the moon, taken from a distance
of less than 200 kilometres with a resolution of about seven metres per
pixel during orbit 756, on 22 August 2004.

This colour image was calculated from the three colour channels and the
nadir channel on the HRSC. Due to geometric reasons the scale bar is
only valid for the centre of the image.

Credits: ESA/DLR/FU Berlin (G. Neukum)

[Movie:
http://www.esa.int/SPECIALS/Mars_Exp...html#subhead2]
An animation of HRSC images taken 10 November 2005, as the shadow of
Phobos moves on the surface of Mars. The HRSC has nine line sensors,
which cover the surface one after the other due to the movement of the
orbiter over the surface.

While the first and forward-looking channel images the ground, the
shadow of Phobos is still outside the imaged area (first image in the
sequence). Then the HRSC covers the field of view subsequently with the
green channel, the panchromatic nadir channel, and the blue channel
(second to fourth images in sequence).

The complete imaging time for all channels is only a few minutes. By the
time the last channel, another backward-looking panchromatic channel,
observes the 41 km-wide field on the ground, the shadow has left the
imaged area again.

Credits: ESA/DLR/FU Berlin (G. Neukum)