Moons of Mars

• 1. Introduction

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  The moons of Mars, Phobos and Deimos, were discovered by American astronomer Asaph Hall in August 1877 and named for the twin sons of Ares who accompanied their father into battle. Fear and Panic. Terror and Dread. These are the companions of the Red Planet, two tiny, irregular rocks so small that they look nothing like Earth's Moon. What are they, really? Where did they come from? And what happens when a moon is doomed to spiral inward toward its own destruction? These are the questions Phobos and Deimos force us to ask.

• 2. A Long Anticipation

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  Johannes Kepler misread an anagram in 1610. Galileo had encoded a message about Saturn's rings as smaismrmilmepoetaleumibunenugttauiras, and Kepler, attempting to decode it, arrived instead at a Latin phrase meaning "Hello, furious twins, sons of Mars." He had predicted something that did not yet exist. Perhaps that error planted a seed. Jonathan Swift, in his 1726 satire Gulliver's Travels, described Laputan astronomers who had discovered two Martian satellites. He gave them orbital distances of 3 and 5 Martian diameters and periods of 10 and 21.5 hours. The actual distances turned out to be 1.4 and 3.5 Martian diameters, and the periods 7.66 and 30.35 hours. Swift's estimates were rough but not absurd. Most astronomers believe he was following a logical analogy of the time: Venus and Mercury had no moons, Earth had one, and Jupiter had four known satellites, so Mars by the same reasoning should have two. His friend the mathematician John Arbuthnot may have helped him work out the numbers. Voltaire followed in 1752, writing two Martian moons into his short story "Micromégas," apparently drawing on Swift. In recognition, two craters on Deimos were named Swift and Voltaire, and features on Phobos were named after fictional places in Gulliver's Travels, including Laputa Regio and Lagado Planitia.

• 3. Hall's Sleepless August

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  Asaph Hall discovered Deimos on the 12th of August 1877 at about 07:48 UTC and Phobos six days later, on the 18th of August 1877 at about 09:14 GMT, at the US Naval Observatory in Washington, D.C., in the neighbourhood called Foggy Bottom. He had been deliberately searching for Martian satellites. On the 10th of August he thought he glimpsed something, but bad weather stopped him from confirming it. His own notebook records what followed: fog from the river cut his observation short on the night of the 11th. Cloud intervened for several days. A thunderstorm on the 15th cleared the sky, but Mars was "blazing and unsteady" and nothing could be confirmed. On the 16th, he found the object again and realised it was moving with the planet. He told his assistant George Anderson to keep quiet, but as Hall wrote, "the thing was too good to keep and I let it out myself." On the 17th, while reducing his observations, he showed his measurements to Professor Newcomb. That same night, while watching for the outer moon, he found the inner one. The discovery was publicly announced by Admiral Rodgers. The instrument Hall used was the 26-inch refractor at Foggy Bottom, and in 1893 its lens was remounted into a new dome where it still sits. The names Phobos and Deimos were suggested by Henry Madan, Science Master of Eton, drawn from Book XV of the Iliad. A curious genealogical thread connects that decision to Pluto: Madan's brother Falconer Madan had a granddaughter named Venetia Burney, who first suggested the name for the ninth planet.

• 4. Two Small Worlds

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  Phobos has a diameter of 22.2 km and a mass of 1.08 kg, orbiting Mars at a semi-major axis of 9,377 km with a period of just 7.66 hours. Deimos measures 12.6 km across, with a mass of 1.5 kg and a semi-major axis of 23,460 km, completing one orbit in about 30.35 hours. Seen from the Martian equator, a full Phobos would appear about one-third the size of Earth's full Moon, with an angular diameter between 8 and 12 arcminutes depending on whether it is rising or overhead. Deimos, by contrast, would look more like a bright star, with an angular diameter of only about 2 arcminutes. The Sun as seen from Mars has an angular diameter of about 21 arcminutes, which means neither moon can produce a total solar eclipse. Phobos's motion is stranger still. Because it orbits faster than Mars rotates, it rises in the west, sets in the east, and completes that full arc in just eleven hours. Deimos moves so slowly in the opposite direction that despite having a 30-hour orbit, it takes 2.7 days to set in the west. Both moons are tidally locked, always showing the same face to Mars. Phobos is not visible from the Martian polar ice caps at all, because its close orbit carries it below the horizon for observers at high latitudes. Total lunar eclipses of Phobos, however, happen almost every night.

• 5. A Doomed Orbit

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  Phobos is slowly falling. Because it orbits faster than Mars rotates, tidal forces are steadily reducing its orbital radius. At some point in the future, when it crosses the Roche limit, those same tidal forces will tear it apart. The debris will either crash into Mars or spread into a ring. Several strings of craters on the Martian surface, arranged at angles that increase with age, suggest that earlier small moons may already have met this fate, and that the Martian crust shifted between those events. Deimos faces the opposite trajectory. Far enough from Mars that tidal forces boost its orbit rather than drain it, Deimos is slowly drifting away, much as Earth's Moon is doing. The two moons of Mars illustrate, in one planetary system, both directions a moon can travel toward its end.

• 6. The Question of Origins

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  Phobos and Deimos share spectral and density properties with carbonaceous C- and D-type asteroids, which led early researchers to propose that both are captured main-belt asteroids. The problem is that captured asteroids would arrive in highly eccentric, tilted orbits, and both moons orbit almost exactly in Mars's equatorial plane on nearly circular paths. Circularizing those orbits through atmospheric drag and tidal forces is difficult to reconcile with available time, especially for Deimos. Geoffrey Landis raised the possibility that the original body was a binary asteroid whose components separated under tidal forces, which might ease the capture problem. Against the asteroid hypothesis, the high porosity of Phobos, with voids estimated at 25 to 35 percent of its volume, is inconsistent with an asteroidal origin. Thermal infrared observations indicate that Phobos contains mainly phyllosilicates, minerals well known from the surface of Mars itself, and its spectrum does not match any class of chondrite meteorites. These findings point toward an origin from material ejected by a Martian impact that then reaccreted in orbit, paralleling the leading theory for Earth's Moon. One version of this scenario envisions a collision between Mars and a protoplanet roughly one-third of Mars's mass. The collision formed a ring. The inner ring coalesced into a large moon. Gravitational interactions with that moon shaped Phobos and Deimos from the outer ring. The large moon later crashed into Mars, leaving the two small moons behind. Simulations constrain the impactor's size to somewhere between Ceres and Vesta; a larger body would have produced moons too massive to leave Phobos and Deimos intact. Researchers from ETH Zurich and the US Naval Observatory, led by Amirhossein Bagheri, analyzed seismic and orbital data from the Mars InSight Mission and proposed that the two moons are fragments of a common parent body disrupted 1 to 2.7 billion years ago. A separate paper, using N-body simulations, found that a single ancestral moon split directly into Phobos and Deimos is unlikely, because the two fragments would collide and produce a debris ring within 10,000 years.

• 7. Failed Missions, Future Samples

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  Two probes in the Soviet Phobos program were launched in 1988, but neither completed its intended landing on the moons' surfaces, though Phobos 2 did photograph Phobos from orbit. The post-Soviet Fobos-Grunt probe was designed as the first sample return mission from Phobos, but a rocket failure in 2011 left it stranded in Earth orbit. Efforts to recover it failed, and it re-entered the atmosphere in an uncontrolled descent on the 15th of January 2012, falling over the Pacific Ocean west of Chile. In 1997 and 1998, NASA's Aladdin mission concept, which would have visited both moons and collected ejected material during slow flybys, reached finalist status in the Discovery Program before NASA chose MESSENGER, a mission to Mercury, instead. JAXA is planning to launch the Martian Moons eXploration mission in 2026. The spacecraft will orbit Mars, descend to Phobos, and land once or twice to collect at least 10 grams of sand-like regolith using a pneumatic system. After departing Phobos, the craft will make several flybys of Deimos before the return module heads back to Earth, with arrival targeted for July 2029. Those samples may finally resolve which origin hypothesis is correct.