Phobos (moon)

• 1. Introduction

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  Phobos is a tiny, lumpy rock orbiting Mars so close to the planet's surface that it travels faster than Mars itself spins. On the 18th of August 1877, American astronomer Asaph Hall spotted it through the world's largest refracting telescope, the 26-inch Great Equatorial at the United States Naval Observatory in Washington, D.C. He had already found Mars's other moon, Deimos, just days before. Two worlds discovered in a single week, circling a planet named for the Roman god of war.

  Hall named his find after the Greek god of fear and panic. Phobos was, in mythology, the twin brother of Deimos and the son of Ares. It is a fitting name. This small, scarred body is caught in a slow spiral toward destruction, and the grooves etched across its surface have puzzled scientists for decades. What carved them? Where did Phobos come from in the first place? And what will become of it? Those questions have driven generations of astronomers, spacecraft designers, and theorists to keep returning to one of the Solar System's most peculiar objects.

• 2. A World Unlike Any Other

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  Phobos has a mean radius of just 11 km, far too small for gravity to pull it into a sphere. Its density, measured directly by spacecraft at 1.887 grams per cubic centimetre, is lower than solid rock. Calculations suggest that somewhere between a quarter and a third of its volume is simply empty space, giving it a porosity of 30 percent, plus or minus 5 percent.

  Its albedo sits at 0.071, meaning it reflects only about 7 percent of the sunlight that strikes it, placing it among the least reflective bodies in the entire Solar System. The surface is dark and chemically rich in carbon, with infrared spectra resembling carbonaceous chondrite meteorites. The similarity to C- and D-type asteroids is so strong that one longstanding hypothesis holds that Phobos and Deimos are captured main-belt asteroids.

  Images from Mars Global Surveyor revealed that Phobos is blanketed by a layer of fine-grained regolith at least 100 metres thick. How that material stays put on a body with almost no gravity remains an open question. Spectral observations show that the regolith's surface layer lacks hydration, though ice below it has not been ruled out. Some areas of the surface appear reddish, while others show a bluish hue, a contrast scientists attribute to Martian gravity dragging weathered reddish regolith across the surface and exposing fresher, unweathered material beneath.

• 3. An Orbit Like No Other

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  Phobos orbits Mars at an altitude of just 5,989 km, closer to its parent planet than any other known natural satellite in the Solar System. A complete trip around Mars takes only 7 hours and 39 minutes, far faster than the Martian day of roughly 24.6 hours. Because of this speed, Phobos rises in the west and sets in the east, the reverse of what any moon-watcher on Earth would expect.

  From the Martian surface, Phobos crosses the sky in 4 hours and 15 minutes or less, and it completes this journey roughly twice per Martian day, which lasts about 11 hours and 6 minutes. The moon cannot be seen at all from latitudes above 70.4 degrees, because its orbit hugs the equator so tightly. At the horizon it appears about 0.14 degrees wide; directly overhead it grows to 0.20 degrees, roughly a third the apparent diameter of the full Moon as seen from Earth. The Sun, by comparison, spans about 0.35 degrees in Mars's sky, making Phobos far too small to produce a total solar eclipse. The Mars Rover Opportunity photographed several of Phobos's transits across the Sun, capturing it as a small dark disk sliding across the solar face.

  Because Phobos orbits below Mars's synchronous orbit radius, tidal forces are gradually pulling it inward. The orbit shrinks by approximately 2 metres every 100 years, or about 2 cm per year, making the orbital dynamics of Phobos what researchers have called the best-studied in the Solar System.

• 4. Stickney and the Grooves

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  The crater Stickney dominates Phobos. Nine kilometres in diameter, it occupies a substantial fraction of the moon's entire surface area. It is named after Angeline Stickney, born in 1830, the wife of Asaph Hall, the man who discovered Phobos. The impact that carved Stickney almost certainly came close to shattering the moon entirely, in the same way that the crater Herschel tested the Saturnian moon Mimas.

  For decades after Phobos was first imaged, the grooves scoring its surface were assumed to be fractures radiating outward from the Stickney impact. Analysis from the Mars Express spacecraft complicated that picture. The grooves are not radial to Stickney at all; instead, they converge on the leading apex of Phobos as it moves through its orbit. By 2015, researchers suspected the grooves were more like stretch marks produced by tidal deformation from the moon's unusually close orbit. Later modelling found the stresses were probably too weak to fracture a moon of that size unless Phobos is a rubble pile wrapped in roughly 100 metres of powdery regolith.

  In November 2018, a computational probability analysis offered another explanation: boulders ejected from the Stickney impact rolled and bounced in a predictable pattern across the surface, travelling more than 360 degrees around the moon and compressing the soft regolith into long grooves. The grooves have been grouped into 12 or more families of varying age, suggesting at least 12 separate Martian impact events sent debris into Phobos's path over geological time. The origin of the grooves remains an active dispute, one that dates back to the 1970s.

• 5. The Hollow Moon Controversy

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  Around 1958, Soviet astrophysicist Iosif Samuilovich Shklovsky was studying the secular acceleration of Phobos's orbital motion when he proposed something extraordinary. Using estimates of the Martian atmosphere's density, he calculated that for the observed braking effect to account for the measured acceleration, Phobos would have to be extraordinarily light. One calculation yielded a hollow iron sphere 16 km across but less than 6 cm thick. Shklovsky floated the possibility that Phobos was of artificial origin.

  The idea attracted serious attention. In a February 1960 letter to the journal Astronautics, Fred Singer, then science advisor to U.S. President Dwight D. Eisenhower, assessed the hypothesis carefully. Singer acknowledged that if the spiraling inward were real, a hollow and therefore artificial origin would be difficult to dismiss, but he flagged the astronomical observations as the weak link: they rested on independent measurement sets taken decades apart by different observers with different instruments, leaving systematic error as a real possibility.

  Singer's caution proved well placed. By 1969 accurate orbital measurements showed no discrepancy at all. Earlier studies had used an overestimated rate of altitude loss of 5 cm per year; the corrected figure came in at 1.8 cm per year. The secular acceleration was attributed to tidal effects, not to any unusual internal structure. Viking probe images taken in the 1970s confirmed Phobos as a natural object. The density of 1.887 g/cm3, measured directly by spacecraft, is consistent with a porous rubble pile, and subsequent Mars Express mapping confirmed the presence of voids. Shklovsky himself later received the honour of having a 2-km crater on Phobos named after him, approved in 2011.

• 6. Where Did Phobos Come From?

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  The origin of Phobos has not been settled. The surface resembles carbonaceous C- and D-type asteroids in spectrum, albedo, and density, which supports a capture hypothesis. But capturing a body of Phobos's size requires a mechanism to circularise what would initially have been a highly eccentric orbit and to tilt it into Mars's equatorial plane. The current Martian atmosphere is too thin to do that by atmospheric braking alone. Researcher Geoffrey A. Landis noted that capture might have worked if the original body was a binary asteroid that separated under tidal forces, allowing the pieces to dissipate energy during the encounter.

  Other evidence points away from an asteroid origin. The high interior porosity, estimated from the 1.88 g/cm3 density as comprising 25 to 35 percent of Phobos's volume, is inconsistent with known asteroidal composition. Thermal infrared observations suggest Phobos contains mainly phyllosilicates, minerals well known from the surface of Mars itself, and the spectra do not match any class of chondrite meteorite. Both findings lean toward an impact origin: a large object strikes Mars, ejects material into orbit, and that material reaccretes to form Phobos, a process analogous to the leading theory for the formation of Earth's Moon.

  In February 2021, a team led by Amirhossein Bagheri at ETH Zurich, working with colleagues at the U.S. Naval Observatory, analyzed seismic and orbital data from the Mars InSight Mission. Their analysis proposed that Phobos and Deimos are not independent captures but fragments of a single common parent body. That progenitor was struck by another object and shattered somewhere between 1 and 2.7 billion years ago, producing both moons simultaneously.

• 7. An Inevitable End

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  Phobos is on a one-way trip toward Mars. Tidal deceleration shrinks its orbital radius by approximately 2 metres every 100 years, and as the orbit tightens, the tidal forces pulling at the moon grow stronger. Researchers estimate that Phobos will reach the critical breakup point when it arrives at approximately 2.1 Mars radii from the planet's centre, with that moment expected somewhere in the range of 30 to 50 million years from now; one specific study placed the estimate at about 43 million years.

  At that threshold, assuming Phobos behaves as a Mohr-Coulomb rubble pile, tidal forces will overcome the loose cohesion binding the body together. The loosely bound material will disperse into a planetary ring around Mars. That ring, according to models, could persist for anywhere from 1 million to 100 million years. The more coherent, strongly bound fragments will not join the ring; instead, they will enter the Martian atmosphere.

  The fraction of Phobos that ends up in the ring versus the atmosphere depends entirely on the internal structure of the moon, which is not yet known. A dedicated sample return mission, the Japanese Aerospace Exploration Agency's Martian Moons eXploration, is scheduled to launch in 2026 and return samples to Earth in 2031, aiming to retrieve a minimum of 10 grams of Phobos material that could finally resolve the open questions about what, exactly, holds this doomed moon together.