Phobos and Deimos: Two Moons, From One Source?

Running the clock back on the enigmatic pair of Martian moons Phobos and Deimos gives researchers insight to their possible origin.

A recent study provides crucial clues on the possible ‘origin story’ for the two tiny moons of Mars, Deimos and Phobos.

Modern astronomy provides us with a snapshot, a look at the present state of affairs across the solar system… but what were things like in the distant past? The existence of the two tiny moons seen orbiting Mars presents a particular dilemma for astronomers. Close up, Phobos and Deimos resemble tiny misshapen captured asteroids… but how did they get into the neat, tidy orbits that we see today?

A new study conducted by the Institute of Geophysics at ETH Zürich, the Physics Institute at the University of Zürich, and the United States Naval Observatory published in the February 2021 issue of Nature: Astronomy recently tackled this problem, with intriguing results.

The Curious Tale of Two Martian Moons

The two moons of Mars were discovered by American astronomer Asaph Hall using the newly-installed 26-inch refractor at the U.S. Naval Observatory during the favorable opposition of August 1877.

The very presence of the two moons has even sparked some outlandish theories over the years. Mathematically inclined theorists claimed Mars should have two moons even before Hall’s discovery, fitting the exponential cadence of Venus (with zero), Earth (with one moon) and Jupiter (with four)… this assertion famously turns up in Jonathan Swift’s 1726 satire Gulliver’s Travels, though of course, the discovery of Jupiter’s 5^th moon Amalthea in 1892 broke this nifty pattern, tossing this coincidence into the footnote section of astronomical history.

But Phobos and Deimos continued to capture controversy, right on through the 20^th century. Russian astronomer Iosif Shklovsky even made the bizarre claim in 1958 that Phobos was an artificial space station (!), echoing an earlier deliberate April Fool’s hoax perpetrated by astronomy popularizer Walter Scott Houston claiming the same. The very first looks at Phobos by Mariner 7 in 1969 and Deimos by Mariner 9 in 1971 revealed more a prosaic-looking pair of natural space rocks.

22 kilometer (13.7 mile)-wide Phobos is certainly unique as moons go: it orbits Mars once every 7 hours and 39 minutes, just 6,000 kilometers (3,700 miles) above the Red Planet’s surface, far closer than any other moon in the solar system versus its primary host. This also means that unlike Deimos (and our own Moon), Phobos orbits Mars faster than the planet rotates, meaning that viewed from the surface, it actually rises in the west and sets in the east. 12 kilometer (7.5 mile)-wide Deimos is just above Mars synchronous orbit, and orbits Mars once every 30 hours and 20 minutes.

An Origin Story for the Moons of Mars

Over the years, Two competing scenarios have emerged to explain the existence of the pair of curious Martian moons:

Option 1: They’re captured asteroids. This is not likely, however, from an orbital mechanics perspective, as such events are high energy, and would not produce the neat and tidy orbits with a low ellipticity and a low orbital inclination that we see today.

Option 2: The moons formed along with Mars early on in the solar system’s history, possibly from a single ‘proto-moon’ or a giant impact on Mars, similar to the impactor proposed in the ‘Theia Hypothesis’ that formed Earth’s large moon.

“Most moons are big and round objects, not small and irregularly shaped as Phobos and Deimos,” Amir Khan (Zürich Institute of Geophysics) told Universe Today. “This is why the Martian moons were originally considered to be captured objects from the nearby asteroid belt. Yet, because of dynamical problems associated with captured objects, this scenario fell into disfavor, as a result of which, in situ formation models were put forward.”

Subtle aspects of the overall masses of the moons predict how their orbits will evolve. Tidal forces act on the moons over time, but the total energy in the system always remains constant overall, in accordance with the Law of Conservation of Angular Momentum.

“There exists a critical altitude which is called the synchronous radius,” Michael Efroimsky (U.S. Naval Observatory) told Universe Today. “If the moon is below the synchronous radius, the tides of the planet will drag the moon down. This is the case with Phobos”

For Mars, the synchronous radius is at an altitude of 17,032 kilometers (10,583 miles) above the planet’s surface. Phobos is well below this limit, with the tidal force of Mars lagging behind it and dragging it downward. Deimos is above this limit, and the tidal force leads the tiny moon, pushing it gently outward from Mars. Deimos outward motion is a tiny 2 millimeters (0.08 inches) per year, slower than our Moon’s own 3.8 centimeters (1.5 inches) per year. Deimos probably won’t escape from Mars in the lifespan of our solar system.

The research team posed the question of what the Martian moon system might have looked like in the past, based on the snapshot that we see today. They developed a mathematical model of the propagation of the moons’ orbits over time. Lead researcher in the study Amirhossein Bagheri (Institute of Geophysics, ETH Zurich) carried out hundreds of numerical runs of the evolution of the orbits of Phobos and Deimos back into the far past.

“Is it really that certain that a body above the synchronous radius is doomed to fly away, and one below is doomed to fall back toward the planet?” says Efroimsky. Running back the clock in simulations, researchers realized that “At some point in the past, Phobos’ and Deimos’ orbits intersected,” says Efroimsky. The study cites a period in time back around 1 to 2.7 billion years ago, when the moons’ orbits would have crossed paths.

“The intersection of the orbits implies that the moons were born from a common progenitor,” says Bagheri. “The parent body was most likely disrupted by an impactor within the time-span mentioned earlier. The highly cratered surface of Mars indicates that the planet was hit by many such objects.”

Of course, pinning this time down relies on knowing the precise composition of the moons, in order to understand the effect that tidal forces will have on them over time.

Many, if not most asteroids that we’ve seen up close are loose ‘rubble piles’ of rocks. It’s all down to porosity versus density: both of the moons of Mars are less than twice as dense as water, suggesting that they’re very loosely put together internally. Efroimsky notes, however, that Phobos has one large literal strike in favor of durability: the large Stickney crater on one end of the moon. First seen by Mariner 9 and named after the maiden name of Asaph Hall’s wife Chloe Angeline Stickney Hall, such a large hit didn’t actually manage to shatter the Moon.

Another piece of the puzzle also supports the Mars impact origin for the moons: a 2018 study, which cites the Borealis basin feature on the northern hemisphere of Mars as the possible site of an ancient ‘proto-moon’-forming impact, sometime around 4.5 billion years ago very early on in the history of the solar system.

Knowing just what the Martian moons are made of would go a long ways towards knowing their respective fates in the far future. Missions such as the European Space Agency’s Mars Express have imaged both moons up close. Flybys also give Mars Express a slight gravitational tug, allowing researchers to refine the masses of the two moons. Curiosity even caught several misshapen annular solar eclipses of the moons in 2019, allowing astronomers to refine our understanding of the moons’ orbits.

Journey to Mars on November 10, 2084, and you can witness a transit of Earth and the Moon across the face of the Sun… along with a ‘Phobos eclipse’:

NASA’s Mars InSight has even ‘seen’ solar eclipses caused by the passage of Phobos in tiny dips of its Seismic Experiment for Interior Structure (SEIS) instrument… but it turns out that these events weren’t due to a drop in solar power, but the dip in temperature caused by the passing moon partially obscuring the Sun, and slightly cooling and depressing the ground under the lander.

“In order to do backward integration of the orbits, we need to plug in models of the interior structures of Mars and its moons,” says Khan. “That’s where InSight and Mars Express come in.” InSight can probe the interior of the Red Planet by documenting ‘marsquakes.’ Researchers can also probe the interior structure of Mars and how its internal mass is deformed and distributed by carefully analyzing the orbit of the Mars Express spacecraft.

What researchers would really like to do is send landers to either or both moons, and return samples to Earth for further study. Russia attempted to do this with the launch of the Phobos-Grunt mission in 2011, which failed to leave Earth orbit due to a faulty Fregat upper stage, and reentered the Earth’s atmosphere on January 15, 2012.

While Russia has vague plans to attempt a follow up Phobos-Grunt 2 mission sometime in the coming decade, the Japanese Aerospace Exploration Agency (JAXA) may be the first, with their Mars Moons eXploration (MMX) mission to explore the Martian moon system, launching in 2024. Comparing and contrasting the composition of the two moons could cinch their origin story, once and for all.

“Analysis of this (a sample return) will then tell us the composition of Phobos,” Says Khan. “if it turns out the composition is Mars-like, then that points to in situ formation, whereas if the composition is different, that would suggest that the moons originated elsewhere.”

Into the Far Future

And yes, while tiny Deimos is drifting away from Mars, Phobos is doomed to crash into the Red Planet in about 40 million-odd years. It could impact the planet in one giant piece, or the tidal forces exerted by Mars could rip it to shreds first. Phobos is approaching within the Roche Limit for Mars 5,470 kilometers (3,400 miles) above the planet’s surface, the point inside which tidal forces from the planet exceed its own internal integrity due to gravity. Phobos already exhibits long grooves on its surface, evidence of the fracturing strain it’s currently under. If so, Mars may briefly sport a ring from the resulting destruction of Phobos for several thousand years, perhaps making the planet look noticeably different at the eyepiece.

You can even spot Deimos and Phobos for yourself, The biannual opposition for the Red Planet is the best time to try… but be forewarned: +11.8 magnitude Phobos and +12.9 magnitude Deimos are challenging objects to catch, as they never stray far from brilliant -2 magnitude Mars. Your very best bet is to know just when a given moon is at greatest elongation, and use an eyepiece equipped with an occulting bar to block brilliant Mars from view.

It’s amazing to think, we may soon know where the strange moons of Mars came from, and what’s in store for them in the far future.

Lead image credit: Phobos versus Mars, as seen by the European Space Agency’s Mars Express orbiter. Credit: ESA/DLR/Mars Express/Peter Masek