In fall 2017, the Thermal Emission Imaging System (THEMIS) camera on NASA's Mars Odyssey orbiter took a series of images of the Martian moon Phobos. Because of the viewing angle from the spacecraft, the images mainly revealed the moon's surface texture and features.
A new series of THEMIS images taken April 24, 2019, caught Phobos at a full moon phase, and this time the results will help scientists tell more about what the moon is made of. The new observations may resolve the longstanding question of whether Phobos is a captured asteroid or a broken-off piece of Mars itself.
"We've been at Mars since 2001," Arizona State University's Philip Christensen said. "And to me it's highly gratifying that THEMIS continues to help us advance our understanding of Mars."
A Regents' Professor in ASU's School of Earth and Space Exploration, Christensen is the designer and principal investigator for THEMIS, a multiband visible and infrared heat-sensing camera.
"This new image is a kind of a temperature bullseye — warmest in the middle and gradually cooler moving out," said Jeff Plaut, Odyssey project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, which leads the mission. "Each Phobos observation is done from a slightly different angle or time of day, providing a new kind of data."
ASU's Jonathon Hill, THEMIS mission planner, explains, "Our goal is to observe Phobos at many different phases, which are like what Earth's moon goes through. These images will be able to tell us whether the surface is rocky, dusty or somewhere in between."
On April 24, 2019, THEMIS looked at Phobos dead-on, with the sun behind the spacecraft. This full moon view is better for studying material composition, whereas half-moon views are better for looking at surface textures.
"With the half-moon views, we could see how rough or smooth the surface is and how it's layered," said Joshua Bandfield, a THEMIS co-investigator and senior research scientist at the Space Sciences Institute in Boulder, Colorado. "Now we're gathering data on what minerals are in it, including metals."
Iron and nickel are two such metals. Depending on how abundant they are and how they're mixed with other minerals, their presence could suggest whether Phobos is an asteroid or churned up bits of the Martian interior.
This sequence of 19 images was taken in visible-wavelength light as the Thermal Emission Imaging System (THEMIS) camera on NASA's Mars Odyssey orbiter scanned across the Martian moon Phobos. Image credit: NASA/JPL-Caltech/ASU
These recent observations won't definitively explain Phobos' origin, Bandfield said. But THEMIS is collecting vital data on a moon scientists still know little about — and which future missions might want to visit.
Human exploration of Phobos has been discussed in the space community as a distant, future possibility, and a Japanese sample return mission to the moon is scheduled for launch in the 2020s.
"By studying the surface features, we're learning where the rockiest spots on Phobos are and where the fine, fluffy dust is," Bandfield said. "Identifying landing hazards and understanding the space environment could help design safer missions to the surface."
Odyssey has been orbiting Mars since 2001 and is NASA's longest-lived Mars mission. It takes thousands of images of the Martian surface each month, many of which help scientists select landing sites for future missions. The spacecraft also serves an important role relaying data for Mars' newest visitor, NASA's InSight lander. But studying Phobos is a new chapter for the orbiter.
"I think it's a great example of taking a spacecraft that's been around a very long time and finding new things you can do with it," Bandfield said. "It's great that you can still use this tool to collect interesting science."
Says Hill, "These images are just the start of our plans to learn more about Phobos using THEMIS. We are planning to observe Phobos at many additional phases over the next year, including during an eclipse when Phobos will pass into the shadow of Mars. All together, these data will help us understand Phobos' origin and prepare for future robotic and human missions."