Mars and its Moons: An Orbital Architecture Under Gravitational Tension

Phobos and Deimos: An Orbital Dynamics Under High Gravitational Stress

Visual comparison of Phobos and Deimos with Mars

The Martian system: Mars accompanied by its moons Phobos and Deimos, two small irregular bodies likely originating from the asteroid belt.
Image source: astronoo.com — AI-generated image, public domain..

Scientific Summary

The article examines the moons Phobos and Deimos through the lens of their orbital dynamics and controversial origin. Their respective orbits present a fascinating temporal dichotomy: Phobos, in a spiral, is doomed to be disrupted by tidal forces in 30 to 50 million years, while Deimos is gradually moving away from Mars. Although their spectra and low albedo liken them to D/T-type asteroids, favoring the capture hypothesis, their in-situ formation following a giant impact is not excluded. The MMX mission (JAXA) is key to settling this debate, by sampling Phobos which could contain materials from the Martian crust, thus offering privileged access to the red planet's geological history.

Why is the study of Mars' moons crucial for science?

The study of Phobos and Deimos is crucial because these small satellites are veritable time capsules. Their composition, similar to that of asteroids, could reveal the secrets of the formation of the inner solar system and the migration of giant planets. Moreover, Phobos, inexorably approaching Mars, offers a unique chance to observe a destruction process that will lead to the creation of a ring, an event never seen in real-time. Finally, these moons, due to their low gravity, serve as testbeds and potential bases for future crewed missions, preparing for human exploration of Mars.

A duo of moons with controversial origins

Unlike the multiple moon systems of the gas giants, Mars possesses 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 since posed an enigma to planetologists. With respective dimensions of 27×22×18 km and 15×12×10 km, they resemble asteroids more than "classical" moons formed by accretion around their planet.

Why do these moons fascinate scientists?

Detailed comparison of the two Martian moons

Comparison of the two moons of Mars (2026 data)
Parameter	Phobos	Deimos	Comparison
Dimensions (km)	27.0 × 21.6 × 18.8	15.0 × 12.2 × 10.4	Phobos is 2.7 times larger 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 a different composition (rock + regolith vs. hydrated materials)
Distance from Mars (km)	9,376 (6,000 km above the surface)	23,460	Deimos is 2.5 times farther away
Orbital period	7h 39m 12s	30h 18m 43s	Phobos makes 3 orbits while one Martian day (sol = 24h 39m) elapses
Rotation period	7h 39m 12s (synchronous)	30h 18m 43s (synchronous)	Both moons always show the same face to Mars
Albedo	0.071 ± 0.012	0.068	Very dark surfaces, similar to D/T-type asteroids
Surface temperature	-4 °C to -112 °C	-40 °C to -130 °C	Phobos is slightly warmer due to its proximity to Mars
Surface gravity (m/s²)	0.0057	0.0039	A 70 kg human would weigh ~400 g on Phobos and ~280 g on Deimos
Escape velocity (m/s)	11.39 (41 km/h)	6.00 (22 km/h)	A vigorous jump could eject an astronaut from Deimos
Main crater	Stickney (9 km, i.e., 40% of the diameter)	No dominant crater (max. 3 km: Swift and Voltaire)	The impact that created Stickney almost destroyed Phobos
Assumed composition	Mixture of carbonaceous rocks, regolith (100-200 m thick), phyllosilicates (linked to Mars)	Similar but with more hydrated materials and ice	Spectra close to D/T-type asteroids, but with traces of Martian materials
Future fate	Disintegration by tidal forces within 30-50 million years (Ma)	Slow escape from Martian orbit (moving away by ~20 cm per century)	Phobos will form a ring around Mars; Deimos will eventually leave Martian orbit

Comparison of the two moons of Mars (2026 data)

Parameter

Phobos

Deimos

Comparison

Dimensions (km)

27.0 × 21.6 × 18.8

15.0 × 12.2 × 10.4

Phobos is 2.7 times larger 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 a different composition (rock + regolith vs. hydrated materials)

Distance from Mars (km)

9,376 (6,000 km above the surface)

23,460

Deimos is 2.5 times farther away

Orbital period

7h 39m 12s

30h 18m 43s

Phobos makes 3 orbits while one Martian day (sol = 24h 39m) elapses

Rotation period

7h 39m 12s (synchronous)

30h 18m 43s (synchronous)

Both moons always show the same face to Mars

Albedo

0.071 ± 0.012

0.068

Very dark surfaces, similar to D/T-type asteroids

Surface temperature

-4 °C to -112 °C

-40 °C to -130 °C

Phobos is slightly warmer due to its proximity to Mars

Surface gravity (m/s²)

0.0057

0.0039

A 70 kg human would weigh ~400 g on Phobos and ~280 g on Deimos

Escape velocity (m/s)

11.39 (41 km/h)

6.00 (22 km/h)

A vigorous jump could eject an astronaut from Deimos

Main crater

Stickney (9 km, i.e., 40% of the diameter)

No dominant crater (max. 3 km: Swift and Voltaire)

The impact that created Stickney almost destroyed Phobos

Assumed composition

Mixture of carbonaceous rocks, regolith (100-200 m thick), phyllosilicates (linked to Mars)

Similar but with more hydrated materials and ice

Spectra close to D/T-type asteroids, but with traces of Martian materials

Future fate

Disintegration by tidal forces within 30-50 million years (Ma)

Slow escape from Martian orbit (moving away by ~20 cm per century)

Phobos will form a ring around Mars; Deimos will eventually leave Martian orbit

Past and future exploration missions

Phobos and Deimos as keys to understanding Mars

As explained by Tomohiro Usui (1975-), scientist on the MMX project: "Phobos is a time capsule that has preserved primitive materials of the solar system, while collecting for billions of years Martian dust ejected by meteorite impacts."

The future of the Martian moons

The fate of Phobos is particularly dramatic. Calculations by Benjamin Black (1983-) and Tushar Mittal (1989-) (University of California, 2023) show that:

This catastrophic scenario offers a unique opportunity to study in real-time the formation of a planetary ring system - a process that has never been observed in our solar system.

References

FAQ: Everything you need to know about Mars' moons

Are Phobos and Deimos captured asteroids?

The main hypothesis suggests that Phobos and Deimos are captured asteroids, due to their composition and very dark albedo, similar to primitive D or T-type asteroids. However, the hypothesis of in-situ formation, from the debris of a giant impact on Mars (scenario comparable to that of the Earth's Moon), remains plausible. The JAXA MMX mission should settle this debate by analyzing samples taken from Phobos.

What is Phobos's fate?

Phobos is doomed to violent destruction. Due to Martian tidal forces, it approaches its planet by 1.8 meters per century. In 30 to 50 million years, it will cross the Roche limit and be disrupted, forming a ring of debris around Mars that could persist for 100 million years.

How are Mars' moons useful for human exploration?

Their gravity is extremely low (0.0057 g for Phobos), making them ideal sites for easy landings and takeoffs. They could serve as relay bases or observation posts to prepare for future crewed missions to Mars, as some projects envision (Lockheed Martin's MBC) or the JAXA MMX program which plans to sample them.

What is the MMX mission?

MMX (Martian Moons eXploration) is a mission by the Japanese space agency (JAXA) scheduled for launch around 2024. Its main objective is to explore Phobos and Deimos. It plans a landing on Phobos to collect at least 10 grams of samples, which will be returned to Earth in 2029.

Why do Phobos and Deimos have irregular shapes?

Their low gravity is not sufficient to reshape them into spheres. These moons are primitive bodies, probably fractured and accumulated through successive collisions, which explains their chaotic shape and cratered surface. Their very porous internal structure also reinforces this irregularity.

Is Deimos doomed to the same fate as Phobos?

No. Unlike Phobos, Deimos is slowly moving away from Mars due to tidal interactions. Its trajectory is stable and it will not cross the Roche limit. Deimos will therefore continue to orbit Mars for hundreds of millions of years without risk of disruption.