Phobos: The Fascinating Descent to Mars

An orbit too low to last

A miniature world of modest dimensions

Phobos, the closest of Mars' two moons, has an average radius of only 11.1 km, with dimensions of about 26 × 23 × 18 km. Its orbit, located about 5,989 km above the Martian surface, is so low that its orbital period is only 7 hours and 39 minutes. This duration is shorter than Mars' rotation on its axis, which classifies Phobos as a sub-synchronous orbit.

The inexorable spiral of tidal forces

Under the influence of tidal forces (differential gravitational forces exerted by Mars on Phobos, stronger on the side closest to the planet), Phobos gradually loses its orbital energy. Result: its trajectory tightens in a spiral, inexorably bringing it closer to Mars. Eventually, this moon is doomed to disintegrate into a debris ring, or possibly crash onto the planet.

N.B.:
A sub-synchronous orbit refers to an orbital trajectory whose period is shorter than the planet's rotation period. In this case, the satellite moves faster than the planet's rotation beneath it. The tidal forces generated by this speed difference cause a transfer of angular momentum from the orbit to the planet, resulting in a slow spiraling (decrease of the orbital radius over time) of the satellite towards the surface. Conversely, a super-synchronous orbit (orbital period >planetary rotation period) leads to a gradual distancing of the satellite.

A slow descent towards disintegration

A precisely measured approach

Altimetric measurements from the Mars Global Surveyor mission (NASA) indicate that Phobos approaches Mars by about 1.8 cm per year. This movement leads to gravitational disruption, or possibly a collision, within a horizon generally estimated at a few tens of millions of years.

The critical threshold: the Roche limit

The critical threshold is defined by the Roche limit (minimum distance before a satellite is torn apart by tides), whose numerical factor 2.44 for a rigid body goes back to the work of French mathematician Édouard Roche (1848): \( d_R = R_p \times 2.44 \left( \frac{\rho_p}{\rho_s} \right)^{1/3} \) where \(R_p\) is Mars' radius, \(\rho_p\) its average density and \(\rho_s\) that of the satellite. The application of this limit to the specific case of Phobos was notably studied by astronomer James A. Burns (1972, 1978), who laid the foundations for the orbital dynamics of the two Martian moons.

An inevitable fragmentation

When Phobos reaches this critical distance, on the order of 2 to 2.4 Martian radii depending on the models (the exact figure strongly depending on the actual internal cohesion of the satellite, which is poorly known), its internal stresses will no longer be sufficient to resist tidal forces, likely causing its fragmentation into a temporary ring around Mars.

Origin: capture or accretion after a giant impact?

A persistent mystery

The origin of Phobos remains debated. Its low density (\(\rho_s \approx 1.88\ \text{g.cm}^{-3}\)) and very low albedo (about 0.07) suggest a composition rich in silicates and carbon, akin to C- or D-type asteroids.

The gravitational capture hypothesis: an imperfect explanation

Long favored, the hypothesis of a gravitational capture of a primitive asteroid from the main belt, however, encounters a major difficulty: it poorly explains the almost circular and nearly equatorial orbits of the two moons, which a random capture would have rather left highly eccentric and inclined.

The accretion hypothesis: a promising scenario

This is why the hypothesis of accretion within a debris disk, generated by a giant impact on Mars several billion years ago, is gaining increasing acceptance in the scientific literature, particularly since the work of Pascal Rosenblatt and his colleagues (2016). In this scenario, a major impact would have ejected a disk of mixed material (Martian and from the impactor), from which Phobos and Deimos would have formed by accretion, naturally explaining their current orbits. Variants of this model, combining several cycles of destruction and tidal reaccretion of more massive ancient moons, are also being studied. The debate remains open, and this is precisely one of the central questions that the MMX space mission aims to answer.

Internal structure and progressive fracturing: an ongoing debate

Mysterious grooves on the surface

High-resolution images from the Mars Reconnaissance Orbiter and Mars Express probes reveal parallel grooves several kilometers long on Phobos' surface, largely concentrated on its leading hemisphere. The origin of these characteristic structures has been the subject of a long-standing scientific debate, and several competing hypotheses still coexist today.

Tidal origin or impact ejecta?

The idea of a tidal origin, notably defended by the modeling of Terry Hurford and his colleagues (2016), suggests that these grooves are stretch marks caused by tidal stresses exerted by Mars on a Phobos with low internal cohesion. However, subsequent work has shown that these tidal stresses alone would be too weak to fracture a body of this size, unless assuming a very particular internal structure (a rubble pile coated with a layer of loose regolith).

A body with well-kept secrets

The most supported hypothesis today, carried by the numerical simulations of Kenneth Ramsley and James Head (2018), instead attributes the grooves to rocky blocks ejected during the impact that formed the large Stickney crater, which would have rolled, bounced, and slid across Phobos' surface at very low gravity, some traveling more than a full revolution around the satellite. This diversity of hypotheses illustrates how much Phobos' internal structure — probably a poorly cohesive rubble pile — remains poorly constrained in the absence of in-situ observations.

Towards in-situ exploration: the Japanese MMX mission

A historic rendezvous with Phobos

After several decades of flybys and remote observations, Phobos is about to receive its first dedicated visit. The MMX (Martian Moons eXploration) mission by the Japanese space agency JAXA, scheduled for launch in November or December 2026 from the Tanegashima Space Center, has the primary objective of settling the debate on the origin of Mars' moons by returning a sample of Phobos' soil to Earth.

A journey with multiple stages

After a cruise of about one year, the probe is to enter orbit around Mars in 2027 and begin a phase of close observation of Phobos and Deimos. It will be accompanied by the small Franco-German rover IDEFIX (jointly developed by CNES and DLR), which will land as a scout to prepare for the landing of the main vehicle under Phobos' extremely low gravity conditions.

Samples to solve the mystery

Landing and sampling of at least 10 grams of material are planned for 2029, before a return of the samples to Earth expected in 2031. By analyzing the precise composition of this material in the laboratory, scientists hope to finally determine whether Phobos is a captured asteroid or a fragment born from the accretion of Martian debris.

Phobos and Deimos: a striking orbital contrast

Compared to its sister Deimos, Phobos illustrates two opposing fates: one is inexorably approaching Mars, the other is very slowly moving away. These opposite trajectories result from their relative position to the Martian synchronous orbit (synchronous orbit where the satellite's orbital period equals the planet's rotation) (~6 Martian radii): Phobos orbits below it, Deimos just above.