Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2023 July 31
A dark irregularly-shaped moon is seen in front of the red
planet Mars. Craters are visible in the foreground and the edge
of the planet is just visible at the top of the image. 
Please see the explanation for more detailed information.

Phobos over Mars
Image Credit: ESA, DLR, FU Berlin, Mars Express; Processing & CC BY 2.0 License: Andrea Luck

Explanation: Why is Phobos so dark? Phobos, the largest and innermost of the two Martian moons, is the darkest moon in the entire Solar System. Its unusual orbit and colour indicate that it may be a captured asteroid composed of a mixture of ice and dark rock. The featured assigned-colour picture of Phobos near the edge of Mars was captured in late 2021 by ESA's robot spacecraft Mars Express, currently orbiting Mars. Phobos is a heavily cratered and barren moon, with its largest crater located on the far side. From images like this, Phobos has been determined to be covered by perhaps a metre of loose dust. Phobos orbits so close to Mars that from some places it would appear to rise and set twice a day, while from other places it would not be visible at all. Phobos' orbit around Mars is continually decaying -- it will likely break up with pieces crashing to the Martian surface in about 50 million years.

Your Sky Surprise: What picture did APOD feature on your birthday? (post 1995)
Tomorrow's picture: monster at the Sun's edge


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