the ringer

Martian moon holds clues to an ancient ring around the planet

For most of its history, Mars may have sported a prominent ring.


Out of all the moons of the Solar System, Deimos might be the least impressive. From the over 200 moons that populate our Solar System, the Martian satellite is one of the smallest, measuring at about only seven miles in diameter, and is severely misshapen with an uneven surface. Named after the Roman god of dread, Deimos is not anyone's idea of a vacation spot.

However, this moon may hold secrets about Mars' past and what the Red Planet may have looked like billions of years ago.

A new study uses Deimos' unusual orbital tilt to suggest that Mars may have had a prominent ring for most of its history.

The research was presented at the 236th Meeting of the American Astronomical Society, held virtually on June 1-3, and was accepted for publication in the Astrophysical Journal Letters.

Mars' moon Deimos has an irregular shape, and is the smallest of two satellites orbiting the Red Planet.


The moons of Mars

Mars has two moons orbiting around it, Deimos and Phobos. Deimos is the smaller and outermost of the two, and orbits around the planet every 30 hours.

Deimos and Phobos appear to be made up of the same material that makes up asteroids found in the outer asteroid belt. In fact, following the pair's discovery in 1977, scientists believed them to be asteroids that had been captured by Mars' gravitational force.

"I wanted to see what kind of history they might have, and what was there before during Mars' past," Matija Ćuk, a research scientist at the SETI Institute and lead author of the new study, tells Inverse.

Ćuk was drawn to a subtle difference in Deimos, the moon's orbit is slightly tilted by two degrees.

“The fact that Deimos’s orbit is not exactly in plane with Mars’s equator was considered unimportant, and nobody cared to try to explain it,” Ćuk says. “But once we had a big new idea and we looked at it with new eyes, Deimos’s orbital tilt revealed its big secret.”

Cyclic-moon theory

The idea was first proposed in 2017 by co-author of the study, David Minton, a professor of planetary science at Purdue University. He had suggested that over the billions of years of Mars' existence, a number of moons had been pulled apart by Mars' gravity and created a ring around the Red Planet, from which a newborn moon would be created.

The researchers found that Phobos' orbit was getting closer and closer to Mars, and that it would soon meet its end by Mars' gravitational pull. On the other hand, Deimos is the newborn moon on the Martian block.

After it was created, Deimos moved into its outward orbit. The only way to explain its odd orbital tilt is that it was being pushed away by the ring.

The team behind the new study believe that there was a "grandparent" moon about 20 times larger than Phobos that existed about 3 billion years ago. This ancient moon was eventually torn apart by Mars' gravity, and two other ring-moon cycles followed. Soon, Phobos may meet a similar fate and form a ring in its aftermath.

"What happens when there’s both a ring and a moon?" Ćuk says. "Angle momentum, the amount of orbital energy goes from the ring to the moon. The orbit of the moon gets larger, while the orbit of the ring gets smaller."

Eventually the ring gets close enough to Mars, and its particles fall into the planet.

Ćuk is looking forward to test out this theoretical model involving Mars' moons with the upcoming Japanese space agency's (JAXA) mission to Phobos, slotted for the year 2024, which will collect a sample from the moon and bring it back to Earth.

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