Mars’ Moon Phobos Could Be the Remains of an Ancient Comet

Phobos’ surface reflects light like a comet’s—hinting that Mars’ moon may have cosmic origins.

Key Takeaways:

  1. Newly analyzed photos reveal that Phobos shares key reflective properties with Jupiter-family comets.
  2. High-resolution ESA photos, previously unpublished, show that Phobos’ surface behaves much like Comet 67P.
  3. Scientists suggest Mars’ moons could have once been a single bilobed comet torn apart by gravity.
  4. Japan’s upcoming 2026 MMX mission will sample Phobos’ surface to determine its true origins.
  5. If Phobos is a captured comet, other terrestrial planets may also have hidden cometary moons.

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A New Perspective on Phobos

A new study suggests that Mars’ moon Phobos might actually be a comet, or at least part of one, that was captured by the planet’s gravity. This surprising revelation comes from a fresh analysis of previously unpublished images taken by the Mars Express spacecraft, a European Space Agency (ESA) orbiter that has studied Mars since 2003.

For years, scientists have debated Phobos’ origins. Some theories propose that both Phobos and its twin moon Deimos were asteroids pulled into orbit around Mars. However, this theory struggles to explain their nearly circular orbits. Another idea suggests a massive impact on Mars created both moons, but Phobos’ composition does not match that of the Red Planet.

A Comet Connection?

While preparing for the upcoming Martian Moons eXploration (MMX) mission, set to launch in 2026, lead researcher Sonia Fornasier and her team examined high-resolution images of Phobos. They used a technique called photometry to analyze how Phobos reflects sunlight. Their findings revealed that Phobos’ surface shares striking similarities with comets.

One key observation was Phobos’ “opposition surge,” a phenomenon where an object appears brighter when the Sun is directly overhead. This is common in airless celestial bodies, but Phobos’ photometric properties matched those of Comet 67P, a Jupiter-family comet studied by ESA’s Rosetta mission in 2016. Additionally, Phobos’ porous, dust-covered surface resembles cometary material.

A close-up photo of Phobos. Stickney Crater is the large dent visible in the lower right corner. (Image credit: NASA/JPL-Caltech/University of Arizona)

These similarities led the team to propose that Phobos might have once been a comet that Mars captured long ago. If true, this also raises the possibility that Deimos could have a similar origin. The study even suggests that Phobos and Deimos may have started as a single bilobed comet, eventually torn apart by Mars’ gravitational pull.

The Search for More Evidence

Despite these findings, some aspects of Phobos’ light-scattering behavior do not fully match known comets, leaving room for doubt. To settle the debate, future studies will use dynamical simulations to test whether a cometary origin is plausible. However, the most definitive answers may come from the MMX mission, which will physically sample Phobos’ surface.

If confirmed, Phobos would be the first known “comet moon” of a terrestrial planet, raising new questions about how often comets are captured by planets like Mars. This discovery could reshape our understanding of planetary moon formation and the dynamic history of our solar system.

The new study is forthcoming in the journal Astronomy and Astrophysics, and available on the preprint server arXiv.

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