Planetary scientists studying Phobos, the largest moon of Mars, are using computer models of the 9-kilometer-wide Stickney Crater to resolve a long-standing debate over whether the doomed satellite is a captured asteroid or the product of an ancient planetary collision. According to research presented at the European Geosciences Union General Assembly in Vienna and published in a 2026 study in Monthly Notices of the Royal Astronomical Society (MNRAS), unlocking the origin of Phobos depends on analyzing its hidden internal structure and gravitational field.
The Mystery of Phobos and the Stickney Crater
Phobos orbits just 22.2 kilometers in mean diameter and completes a circuit around Mars every 7 hours and 39 minutes. Despite decades of observation, researchers still debate its birth. One leading theory suggests Mars captured the irregular body as a passing asteroid from space. Another proposes that a massive impact on the Martian surface blasted debris into orbit, which eventually coalesced to form both Phobos and its smaller companion, Deimos, according to findings discussed by Benjamin Haser and co-author Thomas Andert.
At the center of this investigation lies Stickney Crater. According to Haser, a doctoral student in planetary science at Germany’s Universität der Bundeswehr München, the impact that carved out the crater serves as one of the most critical events in the moon’s history. By modeling how a massive impact alters the geophysical measurements of a small body, researchers aim to match the physical scars on the surface with distinct timelines for the moon’s formation.
Crater Timelines and Internal Structure
The age of the Stickney impact varies significantly depending on the origin theory. If Phobos formed from debris ejected by a giant impact on Mars, the collision creating Stickney likely occurred roughly 4.2 billion years ago. If the moon is instead a captured asteroid, the crater-forming impact took place much later, around 2.6 billion years ago, as outlined in the MNRAS research.
Current estimates indicate that Phobos features a porous interior that may contain water ice, alongside a denser mass concentration near its equatorial region. Haser notes that an impact capable of forming a 9-kilometer crater would normally shatter a small body unless the moon possesses an extremely low, uniform density. This sponge-like structure allowed the interior to absorb the kinetic energy of the collision while extreme heat melted and compressed the rock beneath the crater floor.
Mapping the Gravitational Field
Connecting the present-day gravity field, shape, density, and orbital evolution of Phobos into a single consistent geophysical model remains challenging due to the moon’s proximity to Mars and its irregular geometry. To resolve these contradictions, researchers are focusing on how a compressed mass beneath Stickney Crater affects the moon’s gravitational signal, moments of inertia, and libration amplitude.
Detailed mapping of the gravitational field is essential for testing whether the moon operates as a rubble-pile asteroid or a collisional fragment of Mars. The upcoming Martian Moons Exploration (MMX) mission by Japan’s space agency JAXA aims to provide the precise data needed to test these geophysical models and determine the true nature of Mars’ doomed inner moon.
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