Everything we thought we knew about Mars' moon Deimos could be wrong

New data from the probe Hope reveals that Deimos may not be a captured asteroid at all

By Matthew Rozsa

Staff Writer

Published April 28, 2023 9:00AM (EDT)

Mars and Deimos (Getty Images/ANDRZEJ WOJCICKI)
Mars and Deimos (Getty Images/ANDRZEJ WOJCICKI)

The red planet Mars, fourth from Earth's sun, has two little moons: Phobos and Deimos. Neither is anything like Earth's moon: small and irregularly shaped, astronomers have long believed that they are more likely captured asteroids, pulled into the Martian orbit by the red planet's gravity and then kept there indefinitely as makeshift moons. That makes sense, given Mars' proximity to the Asteroid Belt.

Yet a new report by Hope, an orbiter sent around the red planet by the United Arab Emirates, offered the first highly detailed image of Deimos — and the data from Hope has caused scientists to reevaluate that assumption.

Deimos was probably not a captured asteroid but a chunk of Mars that broke off from the planet at some point in its history. 

During a fly-by mission of Deimos on March 10, the Hope mission team scanned the planet's surface using instruments that detected light waves ranging from the infrared to the extreme ultraviolet, according to Nature. Using spectrometry, the scientists could analyze the readings and learn about the type of elements on the planet's surface. The Hope onboard instruments all showed a flat spectrum in their readings, meaning that Deimos is composed of the same minerals seen on Mars — as opposed to the carbon-rich rocks detected in asteroids. That suggests an entirely different origin than theorized.

These types of spectrographic analyses are a common means of figuring out the composition, and therefore origin, of different bodies int he solar system. The geologic history of every body in the solar system is unique — so much so that scientists have been able to figure out with little doubt that some small meteorites that struck Earth millions or billions of years ago originated on Mars. 175 such Martian meteorites have been discovered on Earth, including one that may have held evidence of past life on Mars.

The scientists involved in the study said that Deimos' composition did not resemble the carbon-rich nature of the asteroids that dominate the Asteroid Belt. "If there were carbon or organics, we would see spikes in specific wavelengths," Hessa Al Matroushi, the lead scientist at the Emirates Mars Mission (EMM), told Nature. Al Matroushi first reported these findings on April 24th to the European Geosciences Union meeting in Vienna.

Hence, through spectrometry, it became readily apparent to scientists Deimos was probably not a captured asteroid but a chunk of Mars that broke off from the planet at some point in its history. If indeed Deimos is composed of the same minerals found on Mars, this opens the door to a new hypothesis: That Deimos, and possibly its sister moon Phobos, were formed after a large celestial object collided with Mars, sloughing off some of Mars' surface material in the process. This isn't an unprecedented theory, as a similarly catastrophic strike from a much larger celestial body was what created the Earth-moon system over 4 billion years ago. 

Of course, as The New York Times reported, it is not yet clear with any certainty how Deimos was formed from Mars. In other words, this theory holds that before Deimos was Deimos – as opposed to Deimos being an asteroid — that Deimos was originally part of Mars.

The Martian probe to Deimos is historic for one additional reason. Deimos is tidally locked with Mars, meaning that the same side of Deimos is always facing the same side of Mars. This means that previous probes that visited Mars only studied one side of Deimos' surface — until Hope. Hope was launched in mid-2020 and reached Mars in early 2021. Initially, Hope was designed to study the Martian atmosphere; it completed that mission and still had extra propellant, and so the mission engineers decided to move the space probe into the region around Deimos so they could learn more about the moon.

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As mentioned, this is not the first probe to explore Deimos. In 1976, NASA's Viking 2 orbiter got as close as 19 miles above the surface of Deimos, although its cameras and other equipment were much more primitive that Hope's. Viking 2 was also — like every spacecraft before Hope — unable to capture information about the side of Deimos that does not face the Martian surface. Although Hope did not set a record for being the spacecraft that got closest to Deimos, it is easily the one that captured the most information.

The new data on Deimos comes at a heady moment for Mars studies. Earlier this month, data from the InSight probe's seismograph provided novel data about what Mars' core looks like. Last year, a paper in the journal Nature Astronomy suggested that life may have once flourished in Martian regoliths (that is, the loose dust and rock on top of Mars' main layer of bedrock). That paper suggested that if said life emitted methane gas, this may have altered the planet's climate so much that it could not longer support it. (The life forms in question though would have resembled Earth microbes.) And last year, scientists found water on nakhlites, or Martian meteors that struck Earth roughly 11 million years ago, although there is debate over whether the water originated on Mars or not.

By Matthew Rozsa

Matthew Rozsa is a staff writer at Salon. He received a Master's Degree in History from Rutgers-Newark in 2012 and was awarded a science journalism fellowship from the Metcalf Institute in 2022.

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Astronomy Brief Deimos Mars Phobos Science Solar System