New Research Suggests Mars’ Moon Phobos Could Be a Captured Comet

By Lydia Amazouz Published on May 25, 2024 12:00
New Research Suggests Mars' Moon Phobos Could Be a Captured Comet

New research suggests that Mars' moon Phobos may not be what we previously thought. Historically believed to be a captured asteroid, recent studies propose that Phobos could be a trapped comet or at least a fragment of one.

This theory emerges from the analysis of previously unpublished photos and detailed studies of the moon's surface properties. Understanding the true nature of Phobos could provide significant insights into the history and dynamics of our solar system.

Details About Mars' Moon Discovery

Phobos, one of Mars' two moons, has long puzzled scientists due to its unusual orbit and surface characteristics. High-resolution images taken by the European Space Agency’s Mars Express spacecraft provided new insights into Phobos' surface.

Mars' Moon Phobos

These photos, analyzed by a team led by Sonia Fornasier at Paris Cité University, show that Phobos' surface does not reflect light uniformly, a feature more characteristic of comets than asteroids. The surface appears porous and highly reflective in certain areas, similar to known comets.

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The detailed analysis of these images revealed that Phobos exhibits properties typically associated with cometary bodies. These findings challenge the long-standing view that Phobos and its twin moon Deimos are captured asteroids. Instead, they suggest a more complex origin involving the capture and fragmentation of a comet.

The Comet Hypothesis

The hypothesis that Phobos is a captured comet rather than an asteroid is based on several key observations. Firstly, the photometric properties of Phobos match closely with those of Jupiter-family comets, particularly Comet 67P, which was studied by ESA’s Rosetta mission.

The surface of Phobos exhibits an opposition surge, where it appears brighter when the Sun is directly overhead, and the reflectivity decreases when viewed from other angles. This behavior is typical of cometary surfaces covered with fine, dusty material.

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Additionally, the surface composition of Phobos shows a high level of porosity and an irregular albedo, or reflectivity, which are characteristics more commonly associated with cometary nuclei than with typical asteroids. These observations suggest that Phobos may have a composition similar to that of comets, which are known for their icy and dusty surfaces.

Computer Simulations and Orbital Dynamics

Previous computer models simulating the capture process of asteroids by Mars' gravity could not replicate Phobos' near-circular orbit. However, the comet capture theory provides a more plausible explanation.

The dynamic simulations suggest that Phobos and its twin moon, Deimos, could have once been part of a single bilobed comet. Mars' gravity could have captured this comet, eventually breaking it apart into the two moons we see today. This scenario aligns with the moons' current orbital characteristics and their surface composition.

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The simulations also indicate that the process of capturing a comet and breaking it into smaller fragments could create moons with the specific orbital and physical characteristics observed in Phobos and Deimos. These findings support the idea that Mars' moons are remnants of a once larger comet that was gravitationally captured and disintegrated by Mars.

Implications for Deimos

If Phobos is indeed a captured comet, it has significant implications for Deimos, Mars' other moon. The study suggests that both moons may have originated from the same cometary body. This finding could reshape our understanding of how small celestial bodies are captured and transformed by planetary gravities. Further analysis and comparison of the surface properties of Deimos will be essential in testing this hypothesis.

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Understanding the true nature of Deimos in the context of the comet capture hypothesis could provide valuable insights into the processes of moon formation and the dynamic interactions between celestial bodies in the solar system. It may also shed light on the broader mechanisms of planetary capture and fragmentation.

Future Research and Missions

The Japan Aerospace Exploration Agency's (JAXA) upcoming Martian Moons eXploration (MMX) mission, scheduled to launch in 2026, aims to provide definitive answers about the origins of Phobos and Deimos. This mission will collect samples from Phobos and return them to Earth for detailed analysis. The data obtained from MMX will help determine the precise nature of Phobos' surface materials and potentially confirm its cometary origins.

The MMX mission is expected to provide unprecedented insights into the composition and structure of Phobos, offering a definitive test of the comet hypothesis. By analyzing samples directly, scientists will be able to determine the presence of specific minerals and isotopes that are indicative of cometary origin.

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Broader Implications for Planetary Science

The potential reclassification of Phobos as a captured comet would have broader implications for planetary science. It would challenge existing models of moon formation and capture, prompting a reevaluation of the processes that lead to the formation of natural satellites around planets.

This could lead to a deeper understanding of the early solar system and the interactions between planets and smaller celestial bodies.

Moreover, the study of Phobos and Deimos as potential comet fragments could provide new insights into the history and evolution of Mars itself. Understanding the origins and characteristics of its moons could reveal important information about the planet's past and its interactions with other objects in the solar system.


An editor specializing in astronomy and space industry, passionate about uncovering the mysteries of the universe and the technological advances that propel space exploration.

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