No planet holds as much intrigue for humans as Mars. The Red Planet has played an outsized role in the human imagination ever since H.G Wells imagined it as home base to an alien race determined to conquer Earth. Now, people on Earth see Mars as another potential tourist location, or even home. But for all its fame, the planet’s moons never get much credit. Here’s what you should know about them.
How Many Moons Does Mars Have?
Two: the larger, innermost moon, Phobos, and the smaller, outermost moon, Deimos. Perhaps one of the reasons they don’t draw headlines is that they’re among the solar system’s smallest moons. Phobos is just 4 213.2 kilometers, while Deimos is a puny 689 Kilometers in size.
What Are Phobos and Deimos Like?
They’re both made out of what NASA designates as C-type rock, similar to blackish, carbonaceous chondrite asteroids. Since they’re both so small, they’re constantly spinning around the Red Planet. Phobos spins around Mars three times a day, while Deimos completes an orbit every 30 hours.
Neither moon fits the traditional spherical model for planetary bodies. Both are irregularly shaped, often compared to potatoes. Both have been smashed by asteroids and are littered with impact craters. Both are suspected to have been asteroids in a previous life, captured by Martian gravity, possibly from the nearby asteroid belt. Neither have the gravity to sustain any satellites of their own.
Do Mars’s Moons Have Any Distinguishing Features?
Between the two of them, there’s just one. But it’s a pretty fascinating story.
Stickney is the largest crater on Phobos, and with a diameter of 5.6 miles, it takes up a significant part of the moon’s surface. There are smaller craters within Stickney, and the massive crater is large enough to be seen with the naked eye from the Martian surface. Phobos and Deimos were both discovered by American astronomer Asaph Hall in 1877. Speculation about the planet’s moons had existed as far back as Galileo, and there’s an imagined reference to them in Jonathan Swift’s classic Gulliver’s Travels. At the time, he was working off an assumption that since the inner planets had no moons, Earth had one, and Jupiter had four, it stood to reason that a planet in between them would have two.
Hall had grown frustrated in his search, during which he had come close to identifying the planets, but had been hindered by bad weather. But his wife, the mathematician Angeline Hall, strongly encouraged him to continue. Like clockwork, Hall discovered Deimos on August 12, 1877 and found Phobos less than a week later, writing that he “might have abandoned the search [for Martian satellites] had it not been for the encouragement of [his] wife.”
Like many women historically involved in science, however, Angeline never received the credit she deserved in life. When she asked her husband for equal compensation for her work assisting him, he refused to give her a man’s wage. Low blow.
But discoveries made in space have a way of outlasting their discoverers. Flash forward to 1973, when NASA’s Mariner 9 discovered a large crater on Phobos. Carl Sagan, aware of the story of inequality between the Halls, offered a small historical correction: the crater was named Stickney, after Angeline Hall’s maiden name.
What Does the Future Hold for Phobos and Deimos?
Unfortunately, nothing good. Phobos is slowly but surely getting closer to Mars, a speed NASA estimates as “a rate of six feet (1.8 meters) every hundred years.” The Agency expects that at that rate, it will either crash into Mars in 50 million years or break up into a ring. If humans are living on Mars by that point, they’ll just have to deal with it.
But until then, Phobos has a possible future as a pit stop on the way to the Red Planet. When the moon is on the far side of the planet, it would protect astronauts from potentially fatal solar radiation. So until it destroys itself, it could become a useful docking port.
This article was written by David Grossman and published to Popular Mechanics on 3/10/2019