What Are Mars Moons Called? | Celestial Companions

When we consider planetary companions, Earth’s singular, prominent Moon often comes to mind. Mars, our neighboring red planet, also hosts moons, though they are quite distinct from our own. Understanding these two small celestial bodies offers valuable insights into the solar system’s formation and the ongoing processes shaping planetary systems.

The Names: Phobos and Deimos

The two moons orbiting Mars are named Phobos and Deimos. These names originate from Greek mythology, specifically from the sons of Ares, the Greek god of war, who is the counterpart to the Roman god Mars. Phobos embodies fear, while Deimos represents panic or dread. This naming convention reflects Mars’s association with war and conflict, a tradition common in astronomical nomenclature.

Discovery and Early Observations

The existence of Mars’s moons was first theorized by Johannes Kepler in the 17th century, based on an incorrect interpretation of Galileo’s observations of other planets. However, it was American astronomer Asaph Hall who definitively discovered both Phobos and Deimos in 1877. Hall made these discoveries while working at the United States Naval Observatory in Washington, D.C., using a 26-inch refractor telescope.

He first observed Deimos on August 12, 1877, and then Phobos on August 18, 1877, after a period of poor weather had initially hampered his search.

• Initial Challenges: Hall’s initial attempts to find Martian moons were difficult due to their small size and close proximity to the bright planet Mars. He nearly abandoned his search before his wife, Angeline Stickney Hall, encouraged him to persevere for one more night.
• Confirmation: The observations were confirmed by multiple astronomers shortly after, solidifying their place in our understanding of the solar system. The discovery provided concrete evidence that not all planetary moons are large, spherical bodies like Earth’s Moon.

Phobos: The Inner, Doomed Moon

Phobos is the larger and closer of Mars’s two moons, exhibiting an irregular, potato-like shape rather than a spherical one. Its dimensions are approximately 27 km × 22 km × 18 km. Phobos orbits Mars incredibly close to the planet, at an average distance of about 6,000 kilometers from the Martian surface. This proximity places it within the Roche limit for Mars, meaning tidal forces from Mars are slowly but surely tearing Phobos apart.

Orbital Characteristics

Phobos completes an orbit around Mars in a remarkably short period, approximately 7 hours and 39 minutes. This orbital period is faster than Mars’s own rotation, which means that from the Martian surface, Phobos appears to rise in the west and set in the east, traversing the sky multiple times a day. This unique orbital behavior makes Phobos a significant subject for study regarding tidal interactions.

• Orbital Period: ~7 hours, 39 minutes
• Average Distance from Mars: ~6,000 km
• Orbital Inclination: ~1.09 degrees relative to Mars’s equator
• Synchronous Rotation: Phobos is tidally locked with Mars, meaning the same side always faces the planet.

Surface Features and Composition

The surface of Phobos is heavily cratered, indicating a long history of impacts from space debris. The most prominent feature is Stickney crater, a large impact basin about 9 kilometers in diameter, named after Asaph Hall’s wife. This crater is so large relative to Phobos itself that the impact that created it nearly shattered the moon. Phobos’s surface is dark and appears to be composed of carbonaceous chondrite-like material, suggesting an asteroid-like origin. Data from various missions, including Viking and Mars Global Surveyor, have provided detailed images and topographical maps of its surface.

Deimos: The Outer, Smaller Moon

Deimos is the smaller and more distant of Mars’s two moons, also possessing an irregular shape, measuring approximately 15 km × 12 km × 10 km. Its orbit is further from Mars compared to Phobos, averaging about 23,460 kilometers from the Martian surface. Deimos’s smoother appearance, compared to Phobos, is due to the partial filling of its craters with regolith, the loose surface material.

Orbital Characteristics

Deimos has a much longer orbital period than Phobos, taking approximately 30 hours and 18 minutes to complete one revolution around Mars. This period is slightly longer than a Martian day, meaning Deimos appears to rise in the east and set in the west, similar to Earth’s Moon. Its orbit is nearly circular and lies very close to Mars’s equatorial plane.

• Orbital Period: ~30 hours, 18 minutes
• Average Distance from Mars: ~23,460 km
• Orbital Inclination: ~0.93 degrees relative to Mars’s equator
• Synchronous Rotation: Like Phobos, Deimos is tidally locked, always presenting the same face to Mars.

Surface Features and Composition

Deimos’s surface is less rugged than Phobos’s, with fewer large craters. The two named craters on Deimos are Swift and Voltaire, honoring the authors who speculated about Martian moons before their discovery. The moon’s dark, reddish surface is covered by a deep layer of regolith, which has softened the appearance of its craters. The composition of Deimos is also thought to be similar to carbonaceous chondrite asteroids, reinforcing the idea of an external origin.

Comparative Data: Phobos and Deimos

Characteristic	Phobos	Deimos
Shape	Irregular (potato-like)	Irregular (smoother)
Average Diameter	~22 km	~12 km
Orbital Period	~7 hours 39 minutes	~30 hours 18 minutes
Average Orbital Altitude	~6,000 km	~23,460 km
Tidal Locking	Yes	Yes

Origin Theories: Captured Asteroids or Martian Ejecta?

The origin of Phobos and Deimos remains a topic of active scientific inquiry, with two primary hypotheses dominating discussions. The first, and historically most popular, theory posits that both moons are captured asteroids. This idea is supported by their irregular shapes, dark surfaces, and spectroscopic similarities to C-type (carbonaceous) asteroids found in the outer asteroid belt. If they were captured, they would have likely been gravitationally pulled into orbit around Mars during the solar system’s early, chaotic period.

The alternative hypothesis suggests that Phobos and Deimos formed from debris ejected into orbit after a large impact on Mars itself. This “giant impact” theory mirrors the leading explanation for Earth’s Moon. In this scenario, a massive collision with Mars would have sent a ring of rocky material into orbit, which then coalesced to form the two moons. This theory better explains their current nearly circular and equatorial orbits, which are not typical for randomly captured objects. Recent research, including orbital simulations and compositional analysis, continues to refine our understanding, with some models suggesting a hybrid origin or multiple impact events. NASA provides extensive resources on these ongoing investigations into planetary origins.

A third, more recent idea combines elements of both, proposing that an initial large impact created a temporary ring of debris, which then formed a larger moon that subsequently broke apart to form Phobos and Deimos. Understanding their genesis is not just about Mars; it offers a window into the broader processes of planet formation and moon evolution across our solar system.

Future of Mars’s Moons

The gravitational interactions between Mars and its moons are not static; they are forces shaping their long-term destinies. Phobos, being so close to Mars, experiences significant tidal forces. These forces are gradually drawing Phobos closer to the planet, causing its orbit to decay at a rate of about 1.8 meters per century. Scientists predict that in approximately 30 to 50 million years, Phobos will either crash into Mars or be torn apart by tidal stresses, forming a temporary ring of debris around the planet. This process offers a natural laboratory for studying planetary tidal effects.

Deimos, in contrast, is moving slowly away from Mars. Its orbital distance is gradually increasing, though at a much slower rate than Phobos’s decay. This divergence in their fates highlights the complex interplay of gravitational forces, orbital mechanics, and the physical properties of celestial bodies. The study of these orbital changes provides insights into the long-term evolution of planetary systems. European Space Agency missions have contributed to precise orbital tracking.

Key Discovery Facts

Fact	Detail
Discoverer	Asaph Hall
Discovery Location	United States Naval Observatory, Washington, D.C.
Telescope Used	26-inch refractor
Deimos Discovery Date	August 12, 1877
Phobos Discovery Date	August 18, 1877

Observing Phobos and Deimos

Observing Phobos and Deimos from Earth is a significant challenge due to their small size and close proximity to the much brighter Mars. Even with powerful terrestrial telescopes, they appear as faint points of light. Spacecraft missions to Mars have provided the most detailed views and scientific data. Orbiters like the Viking missions, Mars Global Surveyor, Mars Express, and the Mars Reconnaissance Orbiter have captured high-resolution images and collected spectral data, allowing scientists to characterize their surfaces and compositions.

• From Earth: Amateur astronomers with large telescopes might glimpse them under ideal conditions, but they are very difficult to resolve. They are often obscured by the glare of Mars itself.
• From Mars: For human missions to Mars, Phobos and Deimos would offer unique perspectives. Phobos, in particular, with its rapid orbit, would be a frequent and prominent sight in the Martian sky, potentially serving as a valuable target for robotic or human exploration due to its accessibility.