Last update: October 5, 2025

Phobos and Deimos: Mars’ Mysterious Companions

Visual comparison of Phobos and Deimos with Mars in the background

A duo of moons with controversial origins

Unlike the multiple-moon systems of gas giants, Mars has only two small natural satellites: Phobos and Deimos. Discovered in 1877 by Asaph Hall (1829–1907) at the Washington Naval Observatory, these irregular bodies have puzzled planetary scientists ever since. Measuring 27×22×18 km and 15×12×10 km respectively, they resemble asteroids more than "classic" moons formed by accretion around their planet.

Why do these moons fascinate scientists?

Uncertain origin: Two hypotheses compete—capture of main-belt asteroids (favored by their composition) or in situ formation from debris of a giant impact on Mars (similar to Earth’s Moon).
Opposing fates: Phobos is inexorably moving closer to Mars (1.8 m per century) and will be destroyed in 30–50 million years, while Deimos is slowly drifting away.
Exploration interest: Their low gravity (0.0057 g for Phobos) makes them ideal bases for future crewed missions, as planned by MMX (JAXA).

Detailed comparison of Mars’ two moons

Comparison of Mars’ two moons
Parameter	Phobos	Deimos	Comparison
Dimensions (km)	27.0 × 21.6 × 18.8	15.0 × 12.2 × 10.4	Phobos is 2.7 times more voluminous than Deimos
Mass (×10¹⁵ kg)	10.659	1.476	Phobos is 7.2 times more massive
Density (g/cm³)	1.876 ± 0.020	1.471 ± 0.030	Phobos is 27% denser—suggests different composition
Distance from Mars (km)	9,376 (6,000 km above surface)	23,460	Deimos is 2.5 times farther
Orbital period	7h 39m 14s	30h 18m 43s	Phobos completes 3 orbits in one Martian day
Rotation period	7h 39m 14s (synchronous)	30h 18m 43s (synchronous)	Both moons always show the same face to Mars
Albedo	0.071	0.068	Very dark surfaces, similar to D-type asteroids
Surface temperature	-4°C to -112°C	-40°C to -130°C	Phobos is slightly warmer due to proximity to Mars
Surface gravity (m/s²)	0.0057	0.003	A 70 kg human would weigh ~400 g on Phobos
Escape velocity (m/s)	11.39	5.56	A vigorous jump could eject an astronaut from Deimos
Main crater	Stickney (9 km, 40% of diameter)	No dominant crater (max. 2.3 km)	The Stickney impact nearly destroyed Phobos
Presumed composition	Mix of carbonaceous rocks and regolith (100–200 m thick)	Similar but with more hydrated materials	Spectra resemble D/T-type asteroids
Future fate	Disintegration by tidal forces in 30–50 Ma	Slow escape from Martian orbit	Phobos will form a ring around Mars

Past and future exploration missions

The study of Mars’ moons has seen several key milestones:

1970s: First close-up images by Mariner 9 (1971) and Viking (1977) revealed their irregular shapes.
2003–2025: Mars Express (ESA) and MRO (NASA) provided high-resolution data (up to 4 m/pixel for Phobos).
2024–2029: The MMX mission (JAXA) plans:
- Landing on Phobos with sample collection (10 g minimum)
- Returning samples to Earth by 2029
- Studying Deimos during close flybys
2030s: The moons could serve as relays for crewed Mars missions (Lockheed Martin’s MBC project).

Phobos and Deimos as keys to understanding Mars

These moons play a far greater scientific role than their size suggests:

Their study could resolve several mysteries:

Early Martian history: Phobos samples may contain material ejected from Mars by giant impacts, offering access to deep crustal layers without drilling.
Inner solar system dynamics: If captured asteroids, their composition would reveal clues about giant planet migration.
Preparation for crewed missions: Their low gravity makes them ideal for testing technologies before landing on Mars.

As Tomohiro Usui (1975–), MMX project scientist, explains: “Phobos is a time capsule preserving primitive solar system materials while collecting Martian dust ejected by meteorite impacts over billions of years.”

The future of Mars’ moons

Phobos’ fate is particularly dramatic. Calculations by Benjamin Black (1983–) and Tushar Mittal (1989–) (UC Berkeley, 2023) show:

In 30–50 million years, Phobos will cross the Roche limit and be torn apart by Martian tidal forces.
Debris will form a ring lasting ~100 million years, akin to Saturn’s rings but far thinner.
Deimos will continue drifting away, eventually escaping Mars’ gravity in a few billion years.

This catastrophic scenario offers a unique chance to study planetary ring formation in real time—a process never before observed in our solar system.