planetary science

An uncanny resemblance to the Death Star might not be the only intriguing thing about Saturn’s moon Mimas. It could also harbor a vast ocean of liquid water beneath its pockmarked exterior.

A new look at data from NASA’s Cassini probe reveals that the point in Mimas’ orbit where it comes closest to Saturn changed slightly over 13 years, researchers report February 7 in Nature. Because Mimas’ internal composition affects the gravitational dance between the moon and its planet, these orbital dynamics, along with some previously seen moon wobbles, point to a liquid interior, astronomer Valéry Lainey of the Paris Observatory and colleagues say. More

Around 2.5 million years ago, an asteroid may have exploded over Antarctica. 

The evidence comes from a chemical analysis of more than 100 tiny pieces of rock entrained within the White Continent’s ice, researchers report in the Feb. 1 Earth and Planetary Science Letters. The timing makes the midair detonation the oldest known airburst, the team says. Only two other ancient airburst events are known in the geologic record, dating to 480,000 and 430,000 years ago (SN: 3/31/21).   More

Future interplanetary explorers beware: Hitchhiking bacteria brought to Mars on human bodies might not only survive the harsh conditions on the Red Planet’s surface but also potentially thrive.

Recent experiments exposed four common disease-causing microbes to a simulated Mars-like environment, with its lack of water, scant atmospheric pressure, deadly ultraviolet radiation and toxic salts. The bacteria remained alive for various periods of time and, in some cases, even grew in the imitation Martian sands, researchers report in the January Astrobiology. More

After nearly three years, NASA’s Ingenuity helicopter, the first spacecraft to undertake a powered flight on another world, has ended its mission. Officials at the agency confirmed on January 25 that the history-making quad-copter has sustained damage to one of its rotor blades and is no longer capable of flying.

“While we knew this day was inevitable, it doesn’t make it any easier,” said Lori Glaze, NASA’s planetary science division director, during a news conference on the status of the quad-copter. More

Mercury’s surface might not be quite so terra firma, at least on geologic timescales.

The closest planet to the sun is a world sculpted by volatiles — ephemeral compounds that can freeze, flow or float into space over time, analogous to water on Earth. Salt, the primary volatile on Mercury, appears to have reshuffled the planet’s landscape over billions of years and might even flow — very slowly — in glacierlike landforms, researchers report in the November Planetary Science Journal. The volatile could possibly even form habitable niches deep underground, the authors speculate. More

SAN FRANCISCO — For those delighted by the possibility of alien life, Enceladus, the wintry moon of Saturn, is a gift that just keeps giving.

Key compounds that could support alien microbes or help life emerge have been detected in the enormous plume of water that erupts from vents in Enceladus’ icy shell, biophysicist Jonah Peter of Harvard University reported December 15 at the American Geophysical Union annual meeting. These include hydrogen cyanide, which can be toxic to humans, though Peter noted it’s also “a key building block for synthesizing more complex compounds including amino acids, sugars and nucleobases, which in turn are precursors for proteins, RNA and DNA.”

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The findings are good tidings for the NASA team developing the Exobiology Extant Life Surveyor, or EELS, a snakelike robot that could one day crawl into Enceladus’ frozen crust to seek out signs of life in the ocean below.

Though that ocean has never been directly observed, it is thought to be the source of the huge water plume spouting from Enceladus’ south pole (SN: 6/9/23). Researchers had previously reported that the plume contains phosphorous in the form of phosphate, establishing Enceladus as the first alien ocean world known to possess all the elements essential for life (SN: 12/16/23). The detection of phosphorus bolstered the moon as a prime candidate for hosting extraterrestrial life, and the new findings from Peter’s team offer even more support.

With hydrogen cyanide and phosphate, “Enceladus might be seen as a favorable prebiotic system,” or a chemical setting capable of giving rise to life, said Christopher Glein, a geochemist at the Southwest Research Institute in San Antonio who was not involved in the study.

Peter and colleagues conducted a detailed analysis of data collected by NASA’s Cassini probe as it flew through Enceladus’ plume in 2011 and 2012. In addition to hydrogen cyanide, the spray contains acetylene, ethane and various alcohols, the team also reported December 14 in Nature Astronomy. These compounds fuel some microbes on Earth, Peter said, “and could supply large amounts of metabolic energy for any potential life” on Enceladus.

Though NASA has no missions scheduled for Enceladus, its habitability has already spurred the development of EELS. The 100-kilogram, 4-meter-long robot consists of ringed, cylindrical segments. These pieces can be angled and rotated in different ways to make EELS slither, sidewind or move vertically inside crevasses.

The EELS robot (seen here) operates on the snow at the Big Bear Mountain Resort in Southern California, during a field test in February 2023.JPL-Caltech/NASA

The robot’s most recent field test occurred in September, on the Athabasca Glacier in Alberta, Canada, reported technologist Masahiro Ono on December 13 at the meeting. On the glacier, Ono, of the Jet Propulsion Laboratory in Pasadena, Calif., and his colleagues tested the vertical mobility of EELS inside three ice shafts, or moulins. The robot first succeeded at holding itself steady in the moulins, unfurling its body to lodge itself between the frozen walls. Then, by carefully rotating treaded rings on its body, EELS safely completed multiple 1.5-meter descents.

Following further tests on the glacier, the next step for EELS might be Earth’s moon. For instance, a small version of the space snake could be deployed to assist stationary lunar landers, Ono said. This robot “is a game changer,” he said, “and not just for Enceladus.” More

Enormous polygon patterns in rock lie dozens of meters below Mars’ surface, ground-penetrating radar data suggest.

Similar patterns develop on the surface in Earth’s polar regions when icy sediments cool and contract. A comparable process long ago may have created the shapes on Mars, found near the planet’s dry equator, researchers report November 23 in Nature Astronomy.

If so, the finding hints that the Red Planet’s equator was much wetter and icier, more like a polar region, when the polygons formed 2 billion to 3 billion years ago.

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“Buried possible polygons at that depth have yet to be reported” on Mars, says planetary scientist Richard Soare of Dawson College in Montreal, who was not involved in the study. Searching for ancient polygonal terrain on Mars using ground-penetrating radar is a new idea that “could be powerful,” he adds, and could help scientists understand how Mars’ climate has changed in the past.

On Earth, polygonal terrain forms in chilly climes when sharp temperature drops cause icy ground to contract and crack open. These thermal fractures are small at first. But the little cracks can fill with ice, sand, or a bit of both, forming “wedges” that prevent the cracks from healing and gradually pry open the earth as they grow. Because this wedging process requires multiple cycles of freezing and thawing, polygonal ground is a good hint that the terrain was icy when the patterns formed.

Polygonal surface patterns on Mars — like these in western Utopia Planitia, glimpsed from orbit by the Mars Global Surveyor satellite —might have been carved by “ice wedges” that form over many cycles of freezing and thawing.JPL/NASA, Malin Space Science Systems

But the Chinese Zhurong rover’s landing site, on a part of Mars called Utopia Planitia, is not the kind of place one would expect to find the terrain on Earth — at least not today (SN: 5/19/21). Polygons have been spotted at higher latitudes on the Martian surface from orbit, but the landing site sits near the Martian equator in a dry, sandy dune field (SN: 8/24/04).

The polygons appear to be roughly 70 meters across and are bordered by wedges nearly 30 meters wide and tens of meters deep — about 10 times as large as typical polygons and wedges on Earth. So it’s possible, Soare says, the structures here formed a bit differently than ice-wedge polygons on Earth.Forming polygons near the Martian equator wouldn’t be possible today, says study coauthor Ross Mitchell, a geoscientist also at the Institute of Geology and Geophysics. To form polygons, the region must have been colder and wetter in the past, he says — much like a polar region.

Changes in the tilt of Mars’ axis could explain such a shift in climate. Simulations of Mars’ orbit have suggested that the planet’s spin axis has at times been so extremely tilted that the planet essentially lay halfway on its side. This would cause the poles to receive more direct sunlight while equatorial regions froze. Finding potential polygons buried near the Martian equator, Mitchell says, is “smoking gun evidence” supporting the idea that the tilt of Mars’ axis has varied so substantially in the past.

“We think of every planet other than Earth as dead,” Mitchell says. But if Mars’s axis does swing around often, he says, our neighboring planet’s climate would be far more dynamic than currently believed. More

A long-ago reshuffling of the giant planets in our solar system may have been instrumental in giving Earth its moon.

For decades, planetary scientists have hypothesized that Jupiter, Saturn, Uranus and Neptune were born much closer to the sun and that gravitational interactions among those planets jolted them into their contemporary trajectories (SN: 5/10/22). But the timing of that “giant planet orbital instability” has been tricky to nail down.

Now, an analysis of meteorite data suggests that the instability took place between 60 million and 100 million years after the solar system started forming, planetary scientist Alessandro Morbidelli reported October 5 in San Antonio at a meeting of the American Astronomical Society’s Division for Planetary Sciences. That timing also roughly coincides with when Earth’s moon is thought to have formed in the wake of a Mars-sized planet running into our own.

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The instability of the giant planets “is related to a complete reshaping of the solar system, the formation of the cometary reservoirs, the sculpting of the asteroid belt,” says Morbidelli, of the Observatoire de la Côte d’Azur in Nice, France. “Understanding when it occurred means fixing a milestone in the history of the solar system.”

The giant planet migration, first proposed by Morbidelli and colleagues in 2005, is a widely accepted hypothesis for explaining much about the solar system. In particular, the giant planets travel along slightly elongated orbits that are askew to one another. But observations of other planetary systems and computer simulations of planet formation suggest that giant planets, in general, form on closer-in orbits that are circular and coplanar. Using simulations, Morbidelli and others showed that if the giant planets in our solar system formed like that, they wouldn’t stay that way. Gravitational interactions would eventually knock the planets into the orbits they have today.

At first, the team thought this instability occurred about 600 million years after the birth of the solar system. That timing meant the migration of the giant planets could also explain an apparent asteroid bombardment of the terrestrial planets — Mercury, Venus, Earth and Mars — as evidenced by craters on the moon and lunar rocks brought back by Apollo astronauts. More recent work, however, has cast doubt on the reality of this “lunar cataclysm.”

Now, Morbidelli suspects the orbital instability came a lot earlier. His reasoning starts with a rare type of meteorite called EL enstatite chondrites.

The mix of elements in these meteorites suggests they must be the remnants of a large rocky body, a few hundred kilometers across, born near the terrestrial planets in the dusty disk that once swirled around our sun.

Then, in 2022, Morbidelli and colleagues showed that any of these enstatites that land on Earth today must come from a collection of bits and pieces in the asteroid belt between Mars and Jupiter. Those bits all broke off Athor, one of the many asteroids there, after it collided long ago with some other object in the belt. Altogether, that family of material adds up to an asteroid roughly 60 kilometers across — much smaller than the one thought to have birthed the enstatites in the first place. That means Athor must be only one piece of that larger rocky body, left over from a collision that destroyed it much closer to the sun.

“The question is, which dynamical mechanism can allow the implantation of Athor in the asteroid belt?” Morbidelli said at the meeting.

He tried computer simulations of several possibilities, but so far, only the giant planet instability seems capable of sending Athor to a stable orbit in the asteroid belt.

This couldn’t have happened earlier than 60 million years after the birth of the solar system, Morbidelli said. Radioactive elements in the enstatite chondrites indicate their parent body was slowly cooling until then, which means it was still large. It hadn’t yet collided with one of the many other planetesimals stirred around by the forming terrestrial planets.

On the other hand, simulations suggest the instability can’t have occurred later than about 100 million years after the solar system began. A study in 2018 concluded that if the giant planets migrated later, a pair of asteroids dubbed Patroclus-Menoetius, trailing Jupiter around the sun while orbiting each other, would have been pulled apart.

That 60-million- to 100-million-year window makes the instability a prime suspect in the diversion of a hypothesized planet that hit Earth, creating the moon (SN: 3/15/23).  The timing “seems right,” says Matthew Clement, an astrophysicist at the Johns Hopkins Applied Physics Laboratory in Laurel, Md.  “Lots of things were happening in the solar system’s early history. However, dynamically speaking, we don’t have a whole lot of reason to believe that things changed much after the moon-forming impact.”

But he cautions that Morbidelli’s estimate is based on “one data point, of the breakup of one asteroid, pieces of which serendipitously happened to get to Earth.”

Still, “it’s nice that [the new result] is actually based on some real data, even if it’s indirectly, rather than just computer models,” says planetary scientist John Chambers. He has questions, though. “They suggest this happened when the formation of the terrestrial planets was more or less complete, apart maybe from the giant impact that formed the moon,” says Chambers, of the Carnegie Institution for Science in Washington, D.C. “But then there’s a good chance it would have messed up the orbits of the terrestrial planets and possibly led to some of them colliding,” which the current lineup of planets suggests did not happen.

Both Chambers and Matthews have worked on scenarios in which the orbital instability occurred even earlier, just a few million years after the solar system began. That earlier time for the instability would help explain one of the outstanding riddles of the solar system: the relatively small size of Mars compared with Earth and Venus. That’s because the instability would have removed many objects from near the orbit of Mars before it could grow to the size of Earth or larger. The new result from Morbidelli’s team seems to exclude that solution.

“I’m prepared to rely on the evidence,” Chambers says. But he isn’t convinced yet because many facets of the solar system’s present structure must be reconciled with any date for the giant planet instability.

Clement agrees. “There’s problems if the instability happened at 500 million years. There’s still problems that we have to resolve if it happened when they say it happened,” he says. “And there’s still problems if it happened immediately after the planets formed, in the first few million years. This story is not done being told yet.” More