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This date in science: Asaph Hall discovers a moon for Mars

American astronomer Asaph Hall discovered Phobos, one of the two Martian moons, on this date in 1877. Did he imagine how well we’d see Mars’ moons today?

August 17, 1877. On this date, American astronomer Asaph Hall discovered one of two small Martian moons. He found the other moon, even smaller than the first, later that year. Both Martian moons are more like asteroids than they are like Earth’s large companion moon, and it’s thought they might have been asteroids that were captured by Mars’ gravity. Astronomers named the two moons Phobos and Deimos – Fear and Terror – for the horses that pulled the chariot of the Greek war god Ares, counterpart to the Roman war god Mars.

HiRISE, MRO, LPL (U. Arizona), NASA

This color-enhanced image shows Stickney Crater on Phobos. This crater is 5.6 miles (9 km) in diameter, meaning it takes up a substantial proportion of the Phobos’ surface. Notice the smaller crater within Stickney, about 1.2 miles (2 km) in diameter, resulting from a later impact. Image via HiRISE, MRO, LPL (U. Arizona), NASA.

Phobos is tiny, with a mean radius of 6.9 miles (11.1 km). But it’s more than 7 times as massive than the second moon, Deimos, which is just 5.5 miles (9 kilometers) across.

Scientists got the first good look at Phobos in 1971 and 1972, during Mariner 9’s mission to the planet. They discovered a large crater that later received the name Stickney Crater, after Chloe Angeline Stickney Hall, wife of Phobos’ discoverer.

bosHere's a view of Phobos and its large Stickney Crater from Viking I.  Viking 1 Orbiter global view of the Martian satellite Phobos. The 10 km diameter Stickney crater is at the left, centered at 5 S, 55 W. The Mars facing point is at the bottom center of Phobos, and the north pole is at about 1:00 from this point. Radiating grooves and crater chains can be seen around Stickney. Phobos is roughly 20 km across in this image.

Phobos and its large Stickney Crater as seen by Viking I. Image taken in June 1977, via NASA.

Asaph Hall probably couldn’t have imagined the video below, which was acquired by NASA’s Mars rover Curiosity on August 1, 2013. The rover had been taking a series of shots of the sky above, from Mars’ surface. This video shows both moons, Phobos and Deimos, as you might see them while standing on the surface of Mars. Large craters on Phobos are clearly visible in these images from the surface of Mars. No previous images from missions on the surface caught one moon eclipsing the other.

Bottom line: ON August 17, 1877, American astronomer Asaph Hall discovered Phobos, one of two Martian moons. He also discovered Deimos, the other known Martian moon, later that year. Both moons are believed to be captured asteroids.

Elizabeth Howell