NASA's image of the day revisits the giant crater on Mars's tiny moon
The October 9 feature uses a 2008 HiRISE observation to explain Stickney crater and Phobos's puzzling surface.
NASA's Astronomy Picture of the Day for October 9 turns to Stickney, the enormous crater on the Martian moon Phobos. More than nine kilometres across, the feature spans nearly half the tiny moon's diameter.
The enhanced-colour view comes from the HiRISE camera aboard Mars Reconnaissance Orbiter in March 2008. Today's publication revisits that observation; it is not an announcement of a newly captured image or a new impact.
NASA explains that streaks within the crater suggest loose material moved down its walls despite gravity below one-thousandth of Earth's. Bluish areas near the rim may indicate more recently exposed material.
The grooves across Phobos remain less straightforward. Possible explanations include tidal stresses and processes associated with the crater-forming impact. The image shows why a small moon can preserve a complicated history: its surface carries clues, but some of the mechanisms behind them are still being investigated.