NASA’s Perseverance rover captured the silhouette of Phobos — the larger and inner of the two natural satellites of Mars — as it passed in front of the Sun on September 30, 2024, the 1,285th Martian day, or Sol, of the mission.
Perseverance captured the silhouette of Phobos as it passed in front of the Sun on September 30, 2024. Image credit: NASA / JPL-Caltech / ASU / MSSS.
Phobos was discovered along with its smaller companion, Deimos, by the American astronomer Asaph Hall in 1877.
It orbits Mars about 6,000 km (3,700 miles) from the surface and completes an orbit in just 7 hours and 39 minutes.
Phobos orbits so close to the Martian surface that the curvature of the planet would obscure its view from an observer standing in the planet’s polar regions.
Its orbital period is about 3 times faster than the rotation period of the planet, with the unusual result among natural satellites that Phobos rises in the west and sets in the east as seen from Mars.
Phobos has dimensions of 26 x 22 x 18 km (16.2 x 13.7 x 11.2 miles) and a very lumpy appearance. It also has impact craters and grooves on its surface.
“From its perch on the western wall of Mars’ Jezero crater, Perseverance recently spied a ‘googly eye’ peering down from space,” NASA scientists said in a statement.
“The pupil in this celestial gaze is the Martian moon Phobos, and the iris is our Sun.”
Captured by the rover’s Mastcam-Z on September 30, the event took place when Phobos passed directly between the Sun and a point on the Martian surface, obscuring a large part of the Sun’s disk.
At the same time that Phobos appeared as a large black disk rapidly moving across the face of the Sun, its shadow, or antumbra, moved across the planet’s surface.
“Due to its quick orbit, a transit of Phobos usually lasts only 30 seconds or so,” the researchers said.
This is not the first time that a NASA rover has witnessed Phobos blocking the Sun’s rays.
Perseverance has captured several transits of the small moon since landing at Mars’ Jezero crater in February 2021.
Curiosity captured a video in 2019; and Opportunity captured an image in 2004.
“By comparing the various images, we can refine our understanding of the moon’s orbit to learn how it’s changing,” the scientists said.
“Phobos is getting closer to Mars and is predicted to collide with it in about 50 million years.”