Key facts
- Asaph Hall discovered both Martian moons in the year 1877.
- Stickney Crater measures over 9 kilometers across (about 5.6 miles).
- Phobos surface gravity is less than one‑thousandth of Earth’s gravity.
- Streaks inside Stickney Crater suggest loose material has slid down the crater walls over time.
How did the enormous Stickney Crater on Phobos form, and what does it reveal about the moon’s origin and future exploration? This question guides current research into one of the most striking features in the Martian system.
According to NASA Science, Stickney Crater is the largest crater on the martian moon Phobos. The feature was named for Chloe Angeline Stickney Hall, the mathematician and wife of astronomer Asaph Hall, who discovered both Phobos and its companion Deimos in 1877.
NASA Science also notes that the crater spans more than nine kilometers across, which is about 5.6 miles. This width is nearly half the mean diameter of Phobos itself.
ScienceDaily reports that Phobos has a mean diameter of about 22.2 kilometers, or 13.8 miles. The moon completes one orbit around Mars every seven hours and thirty‑nine minutes.
The impact that created Stickney likely came close to breaking Phobos apart, according to NASA Science. Such a massive blow would have released enormous energy, potentially melting and compressing stone beneath the crater and leaving a denser region that could produce a detectable gravitational signature.
Researchers are investigating whether this compressed layer exists beneath the crater’s floor, ScienceDaily adds. If present, the denser material would leave a gravitational signature that a spacecraft in close orbit could measure.
NASA Science states that the HiRISE camera on the Mars Reconnaissance Orbiter captured an enhanced‑color image of Stickney Crater and its surroundings while passing within about six thousand kilometers of Phobos in March 2008. The image shows streaks that suggest loose material has slid down the crater walls over time.
Light bluish regions near the crater’s rim may show a freshly exposed surface, NASA Science notes. These color variations point to recent mass‑movement processes on a body whose surface gravity is less than one‑thousandth of Earth’s.
NASA Science also confirms that Phobos’ surface gravity is less than 1/1000th of Earth’s gravity. This weak gravity allows regolith to migrate easily, which may contribute to the formation and evolution of surface features.
Inside the crater, streaks observed by HiRISE suggest that material has slid down the walls over time, NASA Science reports. Such sliding is consistent with the low‑gravity environment that makes loose material prone to gradual movement.
The origin of the curious grooves that cover Phobos’ surface remains mysterious, according to NASA Science. The grooves could result from tidal stresses from Phobos’ close orbit around Mars or from the crater‑forming impact itself.
ScienceDaily explains that if Phobos formed from debris ejected by a giant impact on Mars, the Stickney impact may have occurred roughly 4.2 billion years ago. Conversely, if Phobos arrived as a captured asteroid, the impact could have taken place around 2.6 billion years ago.
Current estimates suggest Phobos has a porous interior that may contain water ice, ScienceDaily adds. The moon also resembles a rubble‑pile asteroid—an assemblage of rock fragments held together loosely by gravity.
Due to tidal interactions, Phobos’ orbit is decaying; the moon is spiraling inward and will eventually break apart or strike Mars, ScienceDaily reports. This inward drift makes Phobos a dynamic object despite its small size.
Japan’s Martian Moons Exploration (MMX) mission will carry a core sampler capable of collecting material from depths of up to two centimeters, ScienceDaily states. A pneumatic sampler, provided by NASA, will loft surface material into a collection container.
MMX will attempt to operate in a quasi‑stable orbit around Phobos despite the moon’s weak gravity being strongly overshadowed by Mars’ pull, ScienceDaily explains. A quasi‑stable orbit is a trajectory that balances the gravitational influences of Mars and Phobos enough to allow close operations.
Scientists consider Stickney Crater a prime study site for MMX to probe Phobos’ internal structure and origin, ScienceDaily reports. By examining the crater’s floor and the possible dense layer beneath it, the mission could determine whether Phobos bears the signature of a massive impact or of a captured asteroid.
The samples returned from Stickney could help distinguish between the two formation scenarios, ScienceDaily notes. Data from the mission’s gravity measurements, spectrometer readings and returned samples will clarify the crater’s subsurface composition and the moon’s broader evolution.
What to watch next is the MMX spacecraft’s arrival in the Martian system after its launch, its approach to Phobos, and the execution of sampling operations at Stickney Crater. These results will feed into models of moon formation around terrestrial planets and inform future exploration of small planetary satellites.
The APOD feature published by NASA Science on October 9, 2026, highlighted Stickney Crater as the day’s picture, bringing renewed public attention to this giant impact scar. ScienceDaily’s article of October 11, 2026, discussed how the crater may reveal the origin of Mars’ doomed moon, linking the geological story to upcoming mission plans.
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