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PLANETARY SCIENCE
Where Did Mars’s Moons Come From?
New results from a U.A.E. orbiter suggest Mars’s moons may be pieces of
the planet. A Japanese mission will tell us for sure

By Jonathan O'Callaghan on May 2, 2023
Where Did Mars's Moons Come From?
Two views of Deimos, the smaller of Mars's two moons, from the HiRISE
camera aboard NASA's Mars Reconnaissance Orbiter. Fresh studies of
Deimos by the United Arab Emirates' Hope spacecraft suggest the moon was
formed from pieces of Mars ejected by giant impacts. Credit:
NASA/JPL-Caltech/University of Arizona
Where did the moons of Mars come from? That’s a question scientists
still can’t answer. We know that Earth’s moon was likely formed from a
giant impact on our planet about 4.5 billion years ago. Some moons in
the solar system, such as several of Jupiter’s smaller satellites,
appear to be captured asteroids. It remains unclear which of these two
formation routes holds true for Mars’s moons, Phobos and Deimos—but we
may soon have an answer. A Japanese spacecraft launching next year will
attempt to bring samples back from Phobos. The mission will build on
exciting new results from a United Arab Emirates (U.A.E.) orbiter at
Mars that suggest a planetary origin for the two moons. “There’s room to
be surprised, but I think we’re going to figure it out,” says Jemma
Davidson of Arizona State University.

On April 24 the U.A.E. announced that its orbiter, Hope, had studied the
smaller of Mars’s two moons, Deimos. The spacecraft returned some of the
best data and images of Deimos yet from as low as 100 kilometers above
the moon’s surface. Those results suggest the Deimos’s composition more
closely matches Mars than that of a class of asteroids that was
previously flagged as the likely raw material for Deimos and Phobos
alike: D-type asteroids in the outer asteroid belt between Mars and
Jupiter. “We don’t believe that [Deimos] is an asteroid,” says Hessa Al
Matroushi, science lead of the mission at the Mohammed Bin Rashid Space
Center in Dubai.

To find out for sure, scientists want to return samples of Phobos to
Earth. An attempt by Russia to do so ended in failure in 2012, when its
Phobos-Grunt spacecraft crashed into the Pacific Ocean shortly after
launch. “It never got out of Earth orbit,” says John Logsdon, a space
historian and professor emeritus at George Washington University’s Space
Policy Institute. The Japan Aerospace Exploration Agency (JAXA) is
hoping to avoid the same fate with its Martian Moons eXploration (MMX)
mission. The solar-powered spacecraft, expected to launch in September
2024, weighs in at more than three metric tons and is roughly the size
of an SUV. It will aim to enter Martian orbit in August 2025 before
sidling up to Phobos in 2026 to scoop samples and return them to Earth
by 2029. The mission is “super complex” but should be highly rewarding,
says Patrick Michel of the Côte d’Azur Observatory in France, a European
collaborator on MMX and a member of the mission’s science board.

On April 17 NASA and JAXA announced they would be partnering on the
mission. As part of the partnership, NASA selected 10 U.S. scientists to
work on MMX and will also supply two instruments for the spacecraft.
“We’ve got great partners at JAXA, and they are leading this ambitious
mission to bring back the first samples of the Martian moon Phobos,”
said Bill Nelson, NASA’s administrator, in a video message posted to
Twitter. “Together, we’re going to deepen our knowledge of the solar
system.”

Of Mars’s two moons, Phobos is slightly larger. Both are irregularly
shaped, like potatoes. Phobos is about 27 km across on its longest side,
and Deimos is 15 km across. Phobos is also the closer of the two to
Mars. It orbits just 6,000 km above the surface and completes an orbit
every seven hours and 39 minutes. Deimos, at more than 23,000 km in
altitude, takes slightly more than 30 hours to orbit. Both moons have
been imaged by several spacecraft before, most notably by NASA’s Viking
2 orbiter in 1977 and by the Mars Reconnaissance Orbiter in the 2000s
and even by the Curiosity rover from the surface of Mars in 2013. But no
spacecraft has ever landed on either moon.

Japan’s MMX mission will attempt to change that. It builds on the
success of the nation’s asteroid sampling missions, Hayabusa and
Hayabusa2, which returned samples of asteroids in 2010 and 2020,
respectively. Both of those, however, spent mere seconds brushing across
the surfaces of their targets. MMX will land on Phobos in two locations
and spend two hours on the surface collecting about 10 grams of material
in total. “That’s a big difference with Hayabusa,” Michel says. Surface
operations on Phobos pose many challenges because the moon has just a
thousandth of Earth’s gravity—and an uneven gravity field at that, given
its unusual shape. MMX will gather samples using two methods: a coring
sampler on an extendable arm to collect specimens from deeper than two
centimeters and a pneumatic sampler to kick up material from the surface.

Before MMX collects its samples, however, it will seek to ensure a
smaller landing takes place. In 2026 or 2027 the spacecraft will deploy
a small rover on the surface, developed by scientists in France and
Germany. The rover, the size of a microwave, will be dropped from a
height of 45 meters when the spacecraft performs a practice landing
attempt. After tumbling on the surface, the rover will then be righted
by its four extendable wheels to begin a 100-day mission. The moon’s
weak, irregular gravitational pull means that the rover, despite
weighing just 25 kilograms, will not be able to travel faster than a
snail’s pace because it would otherwise risk launching itself into space.

“If we’re going quicker than 80 millimeters per second, we might flip
over the rover or even leave the Phobos system,” says Markus Grebenstein
of the German Aerospace Center, the project manager for the rover.
Accounting for the rover’s limited lifetime, that speed limit “basically
restricts our range to about 100 meters.” Even so, the rover should
prove invaluable. It will study the surface of Phobos and give the main
MMX spacecraft vital information on the moon’s surface properties that
will be incorporated into the two landing attempts. The rover will also
test robotic operations on a small body such as Phobos. A stretch goal
might be to push the rover to its limits by spinning up its back wheels
at the end of the mission in an attempt to flip it. “The rover would
easily be able to do a backflip,” Grebenstein says. “We might be allowed
to do experiments like that at end of its life.”

The target for MMX will be sampling “the most pristine material on
Phobos,” Michel says, which may include hints to its origin. The samples
may have a hidden bonus, too. The surface of Phobos is thought to be
covered in some material that was ejected from Mars via impacts and then
settled on the moon. So when Japan brings its samples to Earth in 2029,
they may well contain the first pristine ones collected from the planet
itself, beating NASA’s multi-billion-dollar Mars Sample Return effort,
which is not expected to send samples back to our planet until 2033 at
the earliest, by a considerable margin. MMX’s samples are unlikely to
contain any evidence of past life or habitability on Mars, but they may
provide useful information about its past geology. “We hope we can
capture them in the sampling mechanism,” Michel says. “We could have the
first retrieved samples from Mars with this mission.”

After its two landings, MMX will leave the surface and send its
collected samples back to Earth in a capsule. While the main spacecraft
itself will stay in Mars orbit, subsequently performing flybys of Deimos
to study that moon from afar, the sample capsule will touch down in an
Australian desert in July 2029. Davidson is one of the scientists
selected by NASA who will then investigate its samples back on Earth.
“By looking at the minerals, we’ll be able to tell if it’s a mineral
from Mars or a captured asteroid,” she says.

If the samples prove to be captured asteroids, this finding will pose
interesting implications for how they migrated from the outer asteroid
belt to Mars. But if they are pieces of Mars, formed by an impact early
in its history, that poses its own problems—not least by raising the
question of how smaller objects such as these formed around a planet,
compared with the size of our own moon around Earth, which is
unfathomably larger at some 3,500 km across. “It doesn’t fit the models
we have for what material from a giant impact would look like,” Davidson
says. “Whatever we figure out, we have to rethink what we’ve assumed we
know about these processes.”

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MMX and Hope represent a renewed interest in the moons of Mars, which
were suggested by the Planetary Society in 2015 as prime locations to
begin human exploration of the Red Planet. “If we couldn’t send humans
to the surface of Mars, maybe we could send them to rendezvous with
Phobos and Deimos,” says Logsdon, a co-author on the Planetary Society
report. Now we are closer than ever to working out where they came from,
which could help us understand more about how our solar system and its
myriad of planets, moons and asteroids came to be. “Understanding how
the moons formed is really fundamental to us understanding the dynamics
of our solar system,” Davidson says.

On Tuesday, May 2, 2023 at 11:20:43 AM UTC-5, a425couple wrote:
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