Benjamin Haser was standing in a Vienna conference hall, gesturing toward a jagged crater on a screen—the kind of scar that could rewrite the origin story of Mars’ mysterious moon Phobos. Stickney Crater, a 9-kilometer gash across the 22.2-kilometer-wide moon, isn’t just a dent in space rock; it’s a time capsule, and Haser, a doctoral student at Germany’s Universität der Bundeswehr München, believes its gravitational whisper may finally answer a decades-old question: Did Phobos form from debris after a colossal impact on Mars, or was it a stray asteroid captured by the Red Planet’s gravity?
The answer hinges on what lies beneath Stickney. If Phobos were a solid boulder, the impact that created the crater should have shattered it. But it didn’t. Instead, scientists like Haser and co-author Thomas Andert suspect the moon is more like a planetary sponge—porous, compressible, possibly containing water ice, with a denser mass concentrated near its equator. Their 2026 paper in the Monthly Notices of the Royal Astronomical Society models how the Stickney impact may have compressed material beneath it, altering Phobos’ gravitational signal, wobble, and inertia in detectable ways.
The age of the impact is a crucial clue. Under the giant-impact theory, Stickney could be 4.2 billion years old—almost as ancient as the solar system itself. But if Phobos was captured later, the crater might be a relative youngster at 2.6 billion years. These competing timelines reflect deeper uncertainties about Phobos’ spectral properties, density, and orbital dance with Mars—a dance that’s slowly pulling the moon inward, dooming it to eventual destruction or collision.
Phobos’ proximity to Mars makes studying its gravity a formidable challenge. Mars’ gravitational pull drowns out the moon’s faint signal, and its irregular shape complicates modeling. Yet, as Haser explains, “In the paper, we investigate how a compressed mass beneath Stickney Crater affects the tiny moon's gravitational signal, moments of inertia and libration amplitude.” These subtle measurements could tip the scales toward one origin theory or the other.
Hope lies ahead with the Japanese Martian Moons Exploration (MMX) mission, set to launch in late 2026. The spacecraft will attempt the near-impossible: maintaining a quasi-stable orbit around Phobos to collect surface samples using two tools—a core sampler reaching 2 cm deep and a NASA-contributed pneumatic sampler that uses gas to lift material into a container. When those samples return to Earth, they could reveal not just the composition of Phobos, but the story of how it came to be.
For now, the moon remains a riddle wrapped in a porous, wobbling enigma. But with each gravitational clue and every planned mission, we’re one step closer to understanding not just Phobos, but the chaotic early history of our solar system.