By Jonny Lupsha, Wondrium Staff Writer
Mars has two moons, both of which are very small—the largest is just 13 miles in diameter. For some time, scientists have debated what their origins are. Are Phobos and Deimos really asteroids?
Mars’s two moons are almost impossibly small. Phobos, the largest, is just 13 miles or 22 kilometers in diameter. Deimos, the smallest, is about half that—theoretically, if Deimos’s gravity were close to Earth’s, an astronaut could walk around it, in its entirety, in about four and a half hours. Due to their size, it’s long been a popular theory that Phobos and Deimos are asteroids captured by Mars’s gravitational pull.
However, a satellite from the United Arab Emirates that has been orbiting Mars since 2021 recently caught the most highly detailed images of Deimos in history, and it turns out the minuscule moon may once have been part of Mars. In her video series A Field Guide to the Planets, Dr. Sabine Stanley, Bloomberg Distinguished Professor in the Morton K. Blaustein Department of Earth and Planetary Sciences at Johns Hopkins University, reveals Deimos and its fascinating qualities.
What Is Deimos?
Deimos is not only smaller than Phobos, but it also differs in many other aspects. Phobos experiences major tidal forces from Mars that are slowly decreasing its orbit. In 50 million years, Phobos may be ripped apart and Mars would have a ring created from its remains.
“Deimos, on the other hand, acts much more like a normal moon,” Dr. Stanley said. “Its orbit is almost four times as far away, over 23,000 kilometers from Mars’s surface. Deimos has an orbital period around 30 hours, so, longer than Mars’s rotation period. That means Deimos rises in the east and sets in the west, just like Earth’s Moon does.”
However, it takes a very long time to do so, since Deimos’s orbital period is so close to Mars’s rotational period. According to Dr. Stanley, if you stood on the Martian equator, it would take two and a half Martian days to watch Deimos rise in the east and set in the west. Due to its diminutive size, Deimos doesn’t cause solar eclipses when it passes the Sun. Additionally, it won’t share Phobos’s fate: Deimos is slowly moving away from Mars.
Where Did the Moons of Mars Come From?
“There is some debate on the origins of Mars’s moons,” Dr. Stanley said. “Because they are so small and similar in composition to objects in the asteroid belt, they might be asteroids that were perturbed out of the asteroid belt and then captured by Mars’s gravity. The problem with this theory is that both moons’ orbits are very circular.”
The circular orbits matter because Jupiter has many “captured moons,” but all of them wind up with very eccentric orbiting patterns due to their high speeds when Jupiter captured them. It doesn’t totally rule out the captured moons theory for Mars, but it does require an explanation as to their change in orbit from eccentric to circular.
“Atmospheric drag or tidal forces might provide an explanation, but it’s still hard to explain the two moons’ current orbital properties,” Dr. Stanley said. “Or perhaps the moons accreted from a disk surrounding Mars, just like Earth’s Moon accreted from debris around Earth. Or possibly Mars once had many asteroid-sized objects in orbit from a large collision, and Phobos and Deimos are simply the last remaining objects.”
Thanks to the new images of Deimos from the United Arab Emirates, we may be one massive step closer to solving the mystery.
A Field Guide to the Planets is now available to stream on Wondrium.