“A boulder as big as a house”

A large boulder sits on the surface of Mars' moon

A high-res image of Mars’ moon Phobos, acquired on January 9 by ESA’s Mars Express, shows a large boulder on the irregularly-shaped moon’s southern hemisphere. It rests near the edge of a large crater…when I rotated the image it looked as if it should just start rolling backwards and slide right over the edge! (The crop above was rotated 90º counter-clockwise.) This is the largest boulder visible on the southern end of Phobos in the High Resolution Stereo Camera (HRSC) images. Craters and grooves cover the rest of the 13-mile-wide moon.

Illustration of Mars Express flyby

During its 8,974th orbit of the red planet, Mars Express passed by Phobos at a distance of 62 miles (100 km) on January 9, 2011, at a speed of about 1.43 miles (2.3 km) a second. At that speed the orbiter had only about nine seconds to image Phobos using five of its nine imaging sensors. Images were downloaded and received at the German Aerospace Center (DLR) on January 18 and were processed and released to the public two days later. See more images and information from the flyby here.

Image ESA / DLR / FU Berlin (G. Neukum)

The High Resolution Stereo Camera (HRSC) experiment on the European Space Agency’s Mars Express mission is led by the Principal Investigator (PI) Prof. Dr Gerhard Neukum, who was also responsible for the technical design of the camera.

Mars Express, so called because of the rapid and streamlined development time, represents ESA’s first visit to another planet in the Solar System. Launched June 2, 2003, it established orbit around Mars on December 25, 2003.

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  1. Corinna says:

    Is that the lump at 12 o’clock high and slightly down from the top Jason? There are a lot of lumps, which I presume are dust [?] covered chunks?


    1. J. Major says:

      Most of the “lumps” are actually craters, the boulder is the bright object casting a small shadow towards the right near the center of the image.

      Sometimes your eye wants to see craters as raised bumps. Once you know which direction the light is coming from (in this case, from the left side in this rotated view) then it’s easier to see what’s raised and what’s a depression.


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