What exactly are Mars’ two moons? Japan’s new mission will land on one to find out
By Ashley Strickland, CNN
(CNN) — Mars’ largely unexplored moons are getting their moment in the spotlight as a first-of-its-kind mission prepares to take flight.
The red planet has many unsolved riddles, but the Japanese-led Martian Moons eXploration project will focus solely on two of the biggest curiosities orbiting nearby. Phobos and Deimos have long baffled scientists, who have been eyeing the small, lumpy moons for clues about the early days of Mars.
“This is kind of like one of the major outstanding mysteries of solar system formation, the origin of Phobos and Deimos, so we really do need a mission like MMX to figure this out,” said Dr. Daniel Glavin, senior scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and participating scientist on the MMX mission.
Previous Mars missions have observed the peculiar pair whenever convenient. Orbiters circling the red planet have snapped images of the rocky bodies over the years, while rovers including Curiosity and Perseverance have captured some fantastic “googly eye” eclipses when Phobos moved between Mars and the sun. But even after decades of observations, scientists still don’t know how Phobos and Deimos came to be.
“What are these little space potatoes? We can’t explain Phobos or Deimos right now. They shouldn’t exist, but they do,” said Dr. Abigail Fraeman, research scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California. “They’re super weird, they’re super small and they’re super puzzling.”
MMX is set to lift off aboard an H3 rocket from the Yoshinobu Launch Complex at the Japan Aerospace Exploration Agency’s Tanegashima Space Center in southwestern Japan’s Kagoshima Prefecture at 3:41 p.m. ET Monday (4:41 a.m. JST Tuesday).
The risky robotic mission is an international collaboration — including scientific instruments from NASA, the European Space Agency, France’s National Center for Space Studies and the German Aerospace Center — that aims to collect more than 0.4 ounces (10 grams) of material from Phobos and return to Earth in 2031.
The detailed observations and samples captured by MMX might reveal the missing pieces to Phobos and Deimos’ origin story — and could even unveil secrets about the red planet itself. If the samples contain pieces of Mars, MMX could potentially be the first mission to successfully bring back Martian material to scientists on Earth in a race alongside China.
Landing on an enigmatic moon
American astronomer Asaph Hall discovered the red planet’s unusual moons in 1877. He named them after the two mythological sons of the Roman god Mars: Phobos, meaning fear, and Deimos, signifying dread.
Phobos is a doomed moon, drawing 6 feet (1.8 meters) closer to Mars every 100 years. Scientists expect the lumpy rock to crash into the planet in roughly 50 million years or break up into a rocky ring that stays in orbit. The object’s defining feature is Stickney crater — a gaping cavity, about 6 miles (9.7 kilometers) wide, that takes up approximately half of the tiny moon’s surface. Thousands of meteorite impacts have also scarred Phobos over time.
Meanwhile, more distant and diminutive Deimos is slowly spiraling away from Mars. The moon has a smoother surface than Phobos but is still heavily pockmarked and covered in a perplexing amount of dust.
Scientists are weighing two leading hypotheses about how these oddballs came to orbit around Mars. One concept suggests that a large object slammed into Mars, launching rocky material that coalesced into Phobos and Deimos, Glavin said. The other hypothesis purports that the two moons are asteroids that came too close to Mars and were drawn in by its gravity.
“None of our data fit either hypothesis correctly,” said Fraeman, who is also a participating scientist on MMX. Japan’s expedition to the enigmatic moons could find new data that supports one of the existing hypotheses or illuminate an entirely different formation backstory, Fraeman added.
The mission will primarily focus on scouting and visiting Phobos, the larger and closer of the two moons to Mars, but along the way it will also take images of Deimos to help scientists better understand similarities and differences between the fraternal moons.
Carrying a small rover named Idefix, the MMX spacecraft — a stack of three modules for propulsion, exploration and returning to Earth — will spend about a year traveling to Mars and then orbit Phobos for about three years.
A suite of instruments aboard the spacecraft’s lander, called the exploration module, will enable scientists to gather a variety of data, including measurements on terrain, minerals, chemical elements and space radiation. Two Super Hi-Vision cameras will take the highest-definition images of Mars and its moons in 4K and 8K.
The spacecraft will first deploy the Idefix rover, developed by the German Aerospace Center and France’s National Center for Space Studies, in early 2029. The robotic explorer will make initial observations that will help mission scientists and engineers determine sites for MMX’s four-legged lander to collect samples directly on Phobos.
The rover will drop from about 131 to 328 feet (40 to 100 meters) down to the surface of Phobos; the tiny moon’s weak gravity should cushion its fall and enable the rover to bounce before sticking its landing.
After autonomously releasing its wheels and solar panels and maneuvering into an upright position, Idefix will embark on a 100-day mission. The gravity on Phobos is so weak that Idefix, which weighs roughly 55 pounds (25 kilograms), will only weigh about 0.5 ounces (15 grams) during its surface operations.
The drink crate-size rover will scout out areas of scientific interest and potential safe landing spots. The mission team will then select two separate sites for the MMX lander to touch down.
Images taken by current orbiters show that Phobos appears to have one lighter side and one darker side — almost as if it has two different types of surfaces, Glavin said. That’s why the plan is to collect a sample from each of these two regions — on the side of Phobos that faces Mars and the opposite side facing away from the planet, said Dr. Tomohiro Usui, a scientist at the Japan Aerospace Exploration Agency, or JAXA, who has worked on the mission for 10 years.
“We’re going to choose a first landing site and then do some landing operations, and after that we’re going to lift off and get back to orbit,” Usui, professor in the department of solar system sciences at Japan’s Institute of Space and Astronautical Science, told CNN. “It takes another several months for the search of the next landing site. Landing operations are very risky.”
The MMX lander has two different ways to collect rocks and dust from Phobos. A coring sampler will drill down 0.8 inches (2 centimeters) and store the subsurface material in a tube aboard the spacecraft’s sample return capsule. A separate pneumatic sampler will use pressurized gas to kick up surface material and collect it in a tube.
After the samples have been stored aboard the MMX lander, it will lift off, make observations of Deimos and begin the journey back to Earth in 2030.
Insights gained from MMX’s tour of the Martian moons could even aid with the planning of crewed missions to Mars in the future, Usui said.
Revealing Martian secrets
The MMX lander is like a Swiss army knife of many different components, providing scientists various ways to try to decode the mysterious origins of Phobos and Deimos.
Ahead of the samples returning to Earth, researchers will be able to get a sneak peek at initial remote-sensing data beamed back in real time from MMX instruments such as the Mars-moon Exploration with GAmma-rays and Neutrons spectrometer, or MEGANE.
MEGANE data could reveal signs of volatile elements, or chemical elements with low boiling and melting points, such as hydrogen, sodium and chlorine on Phobos and Deimos, for example. These elemental composition measurements could provide key clues that lend credibility to one origin hypothesis over another, said David Lawrence, planetary scientist at the Johns Hopkins University’s Applied Physics Laboratory and science lead for MEGANE.
“If you had a giant impact, what happens is the materials get very hot, and you will cook off these volatile elements,” Lawrence said. “In contrast, a lot of asteroids that we see have higher abundances of these volatile elements.”
Fraeman, who focused part of her doctoral thesis on Phobos’ composition, will do a combined analysis of MEGANE data and observations captured by the MMX InfraRed Spectrometer, or MIRS. The spectrometer will use near-infrared light to measure minerals, organic matter and water-related substances on Phobos.
Even though the Martian moons are mission priorities, scientists anticipate that MMX’s data will also reveal previously unknown aspects of Mars’ past.
“If Phobos is made from bits of Mars that were thrown up and then recoalesced, that could give us some important new clues about the composition of the planet and what it was like when it was much younger,” Fraeman said.
Even if the rocky neighbor doesn’t contain remnants of Mars, Phobos would be equally intriguing if it’s a captured asteroid or comet — which could have transported water and other volatile elements from the edge of the solar system to Mars’ vicinity, Fraeman said.
If Phobos and Deimos were enriched with water and organics, “that could have been a seed for life on Mars,” Glavin said.
“That’s what I’m particularly interested in: the origin of life. Are we alone? Did it happen on Mars?” he added. “Studying the samples from Phobos and analyzing Deimos as well is part of that whole story.”
Glavin has worked on previous sample collection missions, like OSIRIS-REx, which returned an unexpected wealth of organic material from the near-Earth asteroid Bennu. For MMX, his team will look for amino acids, the molecules that form proteins, and other chemical building blocks of life in the Phobos samples as well.
“Amino acids seem to be literally everywhere we look in the solar system,” Glavin said. “What we’re going to be looking at is differences in the number and complexity of amino acids, but also their relative abundances, which can tell us about their origin.”
Phobos and Deimos likely have existed for billions of years, which means the moons might be witnesses of Mars’ evolution. Given its proximity, Phobos has served as a little dustpan, collecting material that has swept away from the planet’s thin atmosphere.
“There could be pieces of Mars that have landed on the surface of Phobos, much like pieces of Mars have landed on Earth,” Fraeman said. “When we bring samples back from Phobos, there could be little bits of Mars mixed in there.”
MMX is not the only craft racing toward the red planet. China’s Tianwen-3 mission, which aims to launch in 2028 and return samples directly from Mars back to Earth, is also expected to return in 2031.
“The early 2030s could really be banner years of explanation for Mars,” Glavin said.
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