The Moons of Mars
Sen—The existence of Mars’s moons were described by the 18th century novelist Jonathan Swift 151 years before their actual discovery. In Swift’s Gulliver’s Travels the astronomers of Laputa, “discovered two lesser stars, or satellites, which revolve about Mars; whereof the innermost is distant from the primary planet exactly three of his diameters, and the outermost five; the former revolved in a space of ten hours, the latter in twenty-one and a half.”Although part of a fictional work, Swift’s predictions were uncanny.
Using a 26-inch refracting telescope, American astronomer Asaph Hall spotted two objects orbiting Mars in August 1877 at the U.S. Naval Observatory In Washington DC. The closest was 1.38-times Mars's diameter away from the planet; the farthest, 3.46. They orbited the 3,389 kilometer-wide Mars in 7.65 and 30.31 hours respectively. Not quite the 'ten and twenty-one' predicted by Swift and also closer than he said, but still a reasonable figure.
Mars lies between Earth and Jupiter. In Swift’s time astronomers reasoned that as the first and second planets, Mercury and Venus, had no moons, Earth having one and Jupiter, four (that were known of then), then Mars must have two—i.e. the number doubling with each planet. Moreover, they reasoned that as they hadn’t spotted any Martian moons yet, they had to be small. Although the doubling theory's dubious, Hall proved that there were two small moons. They were named Phobos and Deimos, after the Greek for ‘fear’ and ‘panic’.
Two faces of Deimos, the smaller of Mars's moons. Image credit: NASA/JPL-Caltech/University of Arizona.
In more recent times it was believed that 27 kilometer-wide Phobos and 15 kilometer-wide Deimos were captured from the Asteroid Belt that lies between the orbits of Mars and Jupiter. It’s easy to see why. Both look a lot like asteroids and have densities similar to them. But observations of Phobos, at least, in infrared wavelengths show that it is more like Mars than an asteroid. So now it's thought that they originated like our own Moon did.
The Moon's is believed to have coalesced from a stream of molten debris when a large body collided with Earth early in its history. Although contested, it's the best, current theory as to the Moon's origins. Phobos and Deimos are thought to have formed in the same way. Phobos also has three prominent craters. The largest, Stickney, is 10 kilometres in size—37% the size of the Phobos itself. Deimos looks smoother because its craters have filled up with fine dust. The surface of Phobos shows deep, 20 kilometre-long scars. Something clearly scraped it in the past.