planetary science

Comets from the solar system’s deep freezer often don’t survive their first encounter with the sun. Now one scientist thinks he knows why: Solar warmth makes some of the cosmic snowballs spin so fast, they fall apart.

This suggestion could help solve a decades-old mystery about what destroys many “long-period” comets, astronomer David Jewitt reports in a study submitted August 8 to arXiv.org. Long-period comets originate in the Oort cloud, a sphere of icy objects at the solar system’s fringe (SN: 8/18/08). Those that survive their first trip around the sun tend to swing by our star only once every 200 years.

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“These things are stable out there in the Oort cloud where nothing ever happens. When they come toward the sun, they heat up, all hell breaks loose, and they fall apart,” Jewitt says.

The Dutch astronomer Jan Oort first proposed the Oort cloud as a cometary reservoir in 1950. He realized that many of its comets that came near Earth were first-time visitors, not return travelers. Something was taking the comets out, but no one knew what.

One possibility was that the comets die by sublimating all of their water away as they near the heat of the sun until there’s nothing left. But that didn’t fit with observations of comets that seemed to physically break up into smaller pieces. The trouble was, those breakups are hard to watch in real time.

“The disintegrations are really hard to observe because they’re unpredictable, and they happen quickly,” Jewitt says.

He ran into that difficulty when he tried to observe Comet Leonard, a bright comet that put on a spectacular show in winter 2021–2022. Jewitt had applied for time to observe the comet with the Hubble Space Telescope in April and June 2022. But by February, the comet had already disintegrated. “That was a wake-up call,” Jewitt says.

So Jewitt turned to historical observations of long-period comets that came close to the sun since the year 2000. He selected those whose water vapor production had been indirectly measured via an instrument called SWAN on NASA’s SOHO spacecraft, to see how quickly the comets were losing mass. He also picked out comets whose movements deviating from their orbits around the sun had been measured. Those motions are a result of water vapor jets pushing the comet around, like a spraying hose flopping around a garden.

That left him with 27 comets, seven of which did not survive their closest approach to the sun.

Jewitt expected that the most active comets would disintegrate the fastest, by puffing away all their water. But he found the opposite: It turns out that the least active comets with the smallest dirty snowball cores were the most at risk of falling apart.

“Basically, being a small nucleus near the sun causes you to die,” Jewitt says. “The question is, why?”

It wasn’t that the comets were torn apart by the sun’s gravity — they didn’t get close enough for that. And simply sublimating until they went poof would have been too slow a death to match the observations. The comets are also unlikely to collide with anything else in the vastness of space and break apart that way. And a previous suggestion that pressure builds up inside the comets until they explode like a hand grenade doesn’t make sense to Jewitt. Comets’ upper few centimeters of material would absorb most of the sun’s heat, he says, so it would be difficult to heat the center of the comet enough for that to work.

The best remaining explanation, Jewitt says, is rotational breakup. As the comet nears the sun and its water heats up enough to sublimate, jets of water vapor form and make the core start to spin like a catherine wheel firework. Smaller cores are easier to push around than a larger one, so they spin more easily.

“It just spins faster and faster, until it doesn’t have enough tensile strength to hold together,” Jewitt says. “I’m pretty sure that’s what’s happening.”

That deadly spin speed is actually quite slow. Spinning at about half a meter per second could spell curtains for a kilometer-sized comet, he calculates. “You can walk faster.”

But comets are fragile. If you held a fist-sized comet in front of your face, a sneeze would destroy it, says planetary astronomer Nalin Samarasinha of the Planetary Science Institute in Tucson, who was not involved in the study.

Samarasinha thinks Jewitt’s proposal is convincing. “Even though the sample size is small, I think it is something really happening.” But other things might be destroying these comets too, he says, and Jewitt agrees.

Samarasinha is holding out for more comet observations, which could come when the Vera Rubin Observatory begins surveying the sky in 2023. Jewitt’s idea “is something which can be observationally tested in a decade or two.” More

Sand on Earth is continuously being created by the slow erosion of rocks. But on Mars, violent asteroid impacts may play an important role in making new sand.

As much as a quarter of Martian sand is composed of spherical bits of glass forged in the intense heat of impacts, a new study shows. Since windblown sand sculpts the Martian landscape, this discovery reveals how asteroid impacts contribute to shaping Mars, even long after the collisions occur, Purdue University planetary scientist Briony Horgan and colleagues suggest. The team will present their results August 18 at the 85th Annual Meeting of the Meteoritical Society in Glasgow, Scotland.

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Using data collected by spacecraft orbiting Mars, Horgan and collaborators looked at different wavelengths of visible and infrared light reflected from the planet’s surface to determine the minerals present in Martian sand. The team found signatures of glass all over the planet, particularly at higher latitudes.

One explanation for all that glass is volcanic eruptions, which are known to produce glass when magma mixes with water. But the most glass-rich swath of Mars — the planet’s northern plains — is conspicuously bereft of volcanoes, the researchers note. That rules out volcanic eruptions as the culprit in that location and instead suggests that far more cataclysmic events — asteroid impacts — might be involved.

That’s a plausible argument, says Steven Goderis, a geochemist at the Vrije Universiteit Brussel in Belgium who was not involved in the research. “Often Mars is seen as a volcanic planet. But there’s also a very strong impact component, and this is often overlooked.”

When an asteroid moving at several kilometers per second slams into a rocky planet like Mars, the energy of the event melts nearby rocks and launches them skywards. That molten shrapnel fragments and produces sand grain–sized pieces that are roughly spherical. Those bits of glass — called impact spherules — eventually rain back onto the planet (SN: 3/31/21).

Martian sand, imaged by NASA’s Phoenix Mars Lander, contains dark, spherical grains that were most likely created by asteroid impacts.Briony Horgan/ICL/UA/JPL/NASA

Over the last 3 billion years, asteroid impacts could have plausibly blanketed the surface of Mars in a layer of impact spherules roughly half a meter thick, Horgan and her colleagues calculate. All that material added to the sand on Mars that formed through normal erosion. “Impacts helped supply sand to the surface continuously over time,” Horgan says.

Scientists might have the opportunity to analyze Martian impact spherules in the future. NASA’s Perseverance rover is currently storing samples of Martian sand and rocks for eventual return to Earth (SN: 9/10/21). That’s exciting, Horgan says. “The record of all this is in the sand.” More

Astronauts might one day dine on salad grown in asteroid soil.

Romaine lettuce, chili pepper and pink radish plants all grew in mixtures of peat moss and faux asteroid soil, researchers report in the July Planetary Science Journal.  

Scientists have previously grown crops in lunar dirt (SN: 5/23/22). But the new study focuses on “carbonaceous chondrite meteorites, known to be rich in volatile sources — water especially,” says astroecologist Sherry Fieber-Beyer of the University of North Dakota in Grand Forks. These meteorites, and their parent asteroids, are also rich in nitrogen, potassium and phosphorus — key agricultural nutrients. Pulverizing these types of asteroids, perhaps as part of space mining efforts, could potentially provide a ready supply of farming material in space.

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Fieber-Beyer purchased a material that mimics the space rocks’ composition and gave it to her graduate student Steven Russell. “I said, ‘All right, grow me some plants.’”

Russell, now an astrobiologist at the University of Wisconsin–Madison, chose a type of radish, lettuce and chili pepper — all of which have grown aboard the International Space Station. He, Fieber-Beyer and their colleague Kathryn Yurkonis, also of the University of North Dakota, compared how the plants grew in only faux asteroid soil, only peat moss and various mixes of the two.

Peat moss keeps soil loose and improves water retention. In all mixtures with peat moss, the plants grew. Faux asteroid soil on its own, however, compacted and couldn’t retain water, and so plants couldn’t grow.

Next, Fieber-Beyer will try growing hairy vetch seeds in that faux asteroid dirt, let the plants decay and then mix the dead plant matter throughout the soil. That, she says, could ensure that the soil doesn’t compact. Plus, seeds weigh a lot less than peat moss, making them easier to carry to space to help with any future farming attempts. More

If every mineral tells a story, then geologists now have their equivalent of The Arabian Nights.

For the first time, scientists have cataloged every different way that every known mineral can form and put all of that information in one place. This collection of mineral origin stories hints that Earth could have harbored life earlier than previously thought, quantifies the importance of water as the most transformative ingredient in geology, and may change how researchers look for signs of life and water on other planets. 

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“This is just going to be an explosion,” says Robert Hazen, a mineralogist and astrobiologist at the Carnegie Institution for Science in Washington, D.C. “You can ask a thousand questions now that we couldn’t have answered before.”

For over 100 years, scientists have defined minerals in terms of “what,” focusing on their structure and chemical makeup. But that can make for an incomplete picture. For example, though all diamonds are a kind of crystalline carbon, three different diamonds might tell three different stories, Hazen says. One could have formed 5 billion years ago in a distant star, another may have been born in a meteorite impact, and a third could have been baked deep below the Earth’s crust.

Diamonds have the same carbon structure, but they can form in different ways. This particular gem originated deep within the Earth.Rob Lavinsky/ARKENSTONE

So Hazen and his colleagues set out to define a different approach to mineral classification. This new angle focuses on the “how” by thinking about minerals as things that evolve out of the history of life, Earth and the solar system, he and his team report July 1 in a pair of studies in American Mineralogist. The researchers defined 57 main ways that the “mineral kingdom” forms, with options ranging from condensation out of the space between stars to formation in the excrement of bats. 

The information in the catalog isn’t new, but it was previously scattered throughout thousands of scientific papers. The “audacity” of their work, Hazen says, was to go through and compile it all together for the more than 5,600 known types of minerals. That makes the catalog a one-stop shop for those who want to use minerals to understand the past.

The compilation also allowed the team to take a step back and think about mineral evolution from a broader perspective. Patterns immediately popped out. One of the new studies shows that over half of all known mineral kinds form in ways that ought to have been possible on the newborn Earth. The implication: Of all the geologic environments that scientists have considered as potential crucibles for the beginning of life on Earth, most could have existed as early as 4.3 billion years ago (SN: 9/24/20). Life, therefore, may have formed almost as soon as Earth did, or at the very least, had more time to arise than scientists have thought. Rocks with traces of life date to only 3.4 billion years ago (SN: 7/26/21). 

“That would be a very, very profound implication — that the potential for life is baked in at the very beginning of a planet,” says Zachary Adam, a paleobiologist at the University of Wisconsin–Madison who was not involved in the new studies.

The exact timing of when conditions ripe for life arose is based on “iffy” models, though, says Frances Westall, a geobiologist at the Center for Molecular Biophysics in Orléans, France, who was also not part of Hazen’s team. She thinks that scientists need more data before they can be sure. But, she says, “the principle is fantastic.”

The new results also show how essential water has been to making most of the minerals on Earth. Roughly 80 percent of known mineral types need H2O to form, the team reports.

“Water is just incredibly important,” Hazen says, adding that the estimate is conservative. “It may be closer to 90 percent.”

Some minerals would not form in certain ways without the influence of life. Photosynthesizing bacteria helped bring about the oxygen-rich conditions needed for this azurite (left), while the opalized ammonite (right) was created by the mineral opal filling the space where an ammonite shell used to be.Rob Lavinsky/ARKENSTONE

Taken one way, this means that if researchers see water on a planet like Mars, they can guess that it has a rich mineral ecosystem (SN: 3/16/21). But flipping this idea may be more useful: Scientists could identify what minerals are on the Red Planet and then use the new catalog to work backward and figure out what its environment was like in the past. A group of minerals, for example, might be explainable only if there had been water, or even life.

Right now, scientists do this sort of detective work on just a few minerals at a time (SN: 5/11/20). But if researchers want to make the most of the samples collected on other planets, something more comprehensive is needed, Adam says, like the new study’s framework.

And that’s just the beginning. “The value of this [catalog] is that it’s ongoing and potentially multigenerational,” Adam says. “We can go back to it again and again and again for different kinds of questions.” 

“I think we have a lot more we can do,” agrees Shaunna Morrison, a mineralogist at the Carnegie Institution and coauthor of the new studies. “We’re just scratching the surface.” More

Samples of the asteroid Ryugu are the most pristine pieces of the solar system that scientists have in their possession.

A new analysis of Ryugu material confirms the porous rubble-pile asteroid is rich in carbon and finds it is extraordinarily primitive (SN: 3/16/20). It is also a member of a rare class of space rocks known as CI-type, researchers report online June 9 in Science. 

Their analysis looked at material from the Japanese mission Hayabusa2, which collected 5.4 grams of dust and small rocks from multiple locations on the surface of Ryugu and brought that material to Earth in December 2020 (SN: 7/11/19; SN: 12/7/20). Using 95 milligrams of the asteroid’s debris, the researchers measured dozens of chemical elements in the sample and then compared abundances of several of those elements to those measured in rare meteorites classified as CI-type chondrites. Fewer than 10 meteorites found on Earth are CI chondrites.

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This comparison confirmed Ryugu is a CI-type chondrite. But it also showed that unlike Ryugu, the meteorites appear to have been altered, or contaminated, by Earth’s atmosphere or even human handling over time. “The Ryugu sample is a much more fresh sample,” says Hisayoshi Yurimoto, a geochemist at Hokkaido University in Sapporo, Japan.

The researchers also measured the abundances of manganese-53 and chromium-53 in the asteroid and determined that melted water ice reacted with most of the minerals around 5 million years after the solar system’s start, altering those minerals, says Yurimoto. That water has since evaporated, but those altered minerals are still present in the samples. By studying them, the researchers can learn more about the asteroid’s history.   More

Four billion years ago, lava spilled onto the moon’s crust, etching the man in the moon we see today. But the volcanoes may have also left a much colder legacy: ice.

Two billion years of volcanic eruptions on the moon may have led to the creation of many short-lived atmospheres, which contained water vapor, a new study suggests. That vapor could have been transported through the atmosphere before settling as ice at the poles, researchers report in the May Planetary Science Journal.

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Since the existence of lunar ice was confirmed in 2009, scientists have debated the possible origins of water on the moon, which include asteroids, comets or electrically charged atoms carried by the solar wind (SN: 11/13/09). Or, possibly, the water originated on the moon itself, as vapor belched by the rash of volcanic eruptions from 4 billion to 2 billion years ago.

“It’s a really interesting question how those volatiles [such as water] got there,” says Andrew Wilcoski, a planetary scientist at the University of Colorado Boulder. “We still don’t really have a good handle on how much are there and where exactly they are.”

Wilcoski and his colleagues decided to start by tackling volcanism’s viability as a lunar ice source. During the heyday of lunar volcanism, eruptions happened about once every 22,000 years. Assuming that H2O constituted about a third of volcano-spit gasses — based on samples of ancient lunar magma — the researchers calculate that the eruptions released upward of 20 quadrillion kilograms of water vapor in total, or the volume of approximately 25 Lake Superiors.

Some of this vapor would have been lost to space, as sunlight broke down water molecules or the solar wind blew the molecules off the moon. But at the frigid poles, some could have stuck to the surface as ice.

For that to happen, though, the rate at which the water vapor condensed into ice would have needed to surpass the rate at which the vapor escaped the moon. The team used a computer simulation to calculate and compare these rates. The simulation accounted for factors such as surface temperature, gas pressure and the loss of some vapor to mere frost.

About 40 percent of the total erupted water vapor could have accumulated as ice, with most of that ice at the poles, the team found. Over billions of years, some of that ice would have converted back to vapor and escaped to space. The team’s simulation predicts the amount and distribution of ice that remains. And it’s no small amount: Deposits could reach hundreds of meters at their thickest point, with the south pole being about twice as icy as the north pole.

The results align with a long-standing assumption that ice dominates at the poles because it gets stuck in cold traps that are so cold that ice will stay frozen for billions of years.

“There are some places at the lunar poles that are as cold as Pluto,” says planetary scientist Margaret Landis of the University of Colorado Boulder.

Volcanically sourced water vapor traveling to the poles, though, probably depends on the presence of an atmosphere, say Landis, Wilcoski and their colleague Paul Hayne, also a planetary scientist at the University of Colorado Boulder. An atmospheric transit system would have allowed water molecules to travel around the moon while also making it more difficult for them to flee into space. Each eruption triggered a new atmosphere, the new calculations indicate, which then lingered for about 2,500 years before disappearing until the next eruption some 20,000 years later.

This part of the story is most captivating to Parvathy Prem, a planetary scientist at Johns Hopkins Applied Physics Laboratory in Laurel, Md., who wasn’t involved in the research. “It’s a really interesting act of imagination.… How do you create atmospheres from scratch? And why do they sometimes go away?” she says. “The polar ices are one way to find out.”

If lunar ice was belched out of volcanoes as water vapor, the ice may retain a memory of that long-ago time. Sulfur in the polar ice, for example, would indicate that it came from a volcano as opposed to, say, an asteroid. Future moon missions plan to drill for ice cores that could confirm the ice’s origin.

Looking for sulfur will be important when thinking about lunar resources. These water reserves could someday be harvested by astronauts for water or rocket fuel, the researchers say. But if all the lunar water is contaminated with sulfur, Landis says, “that’s a pretty critical thing to know if you plan on bringing a straw with you to the moon.” More

Any Martians out there should learn to duck and cover.

On May 4, the Red Planet was rocked by a roughly magnitude 5 temblor, the largest Marsquake detected to date, NASA’s Jet Propulsion Laboratory in Pasadena, Calif., reports. The shaking lasted for more than six hours and released more than 10 times the energy of the previous record-holding quake.

The U.S. space agency’s InSight lander, which has been studying Mars’ deep interior since touching down on the planet in 2018 (SN: 11/26/18), recorded the event. The quake probably originated near the Cerberus Fossae region, which is more than 1,000 kilometers from the lander.

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Cerberus Fossae is known for its fractured surface and frequent rockfalls. It makes sense that the ground would be shifting there, says geophysicist Philippe Lognonné, principal investigator of the Seismic Experiment for Interior Structure, InSight’s seismometer. “It’s an ancient volcanic bulge.”

Just like earthquakes reveal information about our planet’s interior structure, Marsquakes can be used to probe what lies beneath Mars’ surface (SN: 7/22/21). And a lot can be learned from studying this whopper of a quake, says Lognonné, of the Institut de Physique du Globe de Paris. “The signal is so good, we’ll be able to work on the details.” More

On Jupiter’s moon Io, lava creeping beneath frost may give rise to fields of towering dunes.

That finding, described April 19 in Nature Communications, suggests that dunes may be more common on other worlds than previously thought, though the lumps may form in odd ways.  

“In some sense, these [other worlds] are looking more familiar,” says George McDonald, a planetary scientist at Rutgers University in Piscataway, N.J. “But the more you think about it, they feel more and more exotic.”

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Io is a world crowded with erupting volcanoes, created when the gravitational forces of Jupiter and some of its other moons pull on Io and generate heat (SN: 8/6/14). Around 20 years ago, scientists reported another type of feature on Io’s dynamic surface — hummocky ridges. The features resemble dunes, but that couldn’t be the case, scientists reasoned, because Io’s atmosphere is too thin for winds to whip up a dunescape.

But in recent years, dunelike features have been discovered on comet 67P (SN: 9/21/20) and Pluto (SN: 8/24/21), planetary bodies that also lack thick atmospheres. Inspired by those alien dunescapes, McDonald and his colleagues revisited the matter of Io’s mysterious lumps. All they needed was some type of airborne force to sculpt the moon’s dunes.

On Earth, powerful explosions of steam occur when flows of molten rock encounter bodies of water. While water isn’t found on Io, sulfur dioxide frost is pervasive. So the scientists hypothesize that when lava slowly flows into and just under a frost layer, jets of sulfur dioxide gas could burst from the frost. Those jets could send grains of rock and other material flying and forming dunes.

The researchers calculate that an advancing lava flow, buried under at least 10 centimeters of frost, could sublimate some of the frost into pockets of hot vapor. When enough vapor accumulates and the pressure becomes high enough to match or overcome the weight of the overlying frost, the vapor could burst out at velocities over 70 kilometers per hour. These bursts could propel grains with diameters from 20 micrometers to 1 centimeter in size, the team estimates.  

Analyzing images of Io’s surface, collected by NASA’s now-defunct Galileo probe, revealed highly reflective streaks of material radiating outward over dunes in front of lava flows — possibly material newly deposited at the time by vapor jets.

This image, taken by NASA’s now-defunct Galileo spacecraft, shows dunelike lumps on Jupiter’s third-largest moon Io. The dark area (lower left) is a lava flow, and the bright streaks that radiate outward may be evidence of material strewn by jets of vapor that burst from frost heated by the lava.JPL-Caltech/NASA, Rutgers Univ.

What’s more, using the images to measure the hummocky features showed that their dimensions align with those of dunes on other planetary bodies. Some of the Ionian dunes are over 30 meters high, the team estimates.

“I think a lot of [scientists] looked at those and thought, hey, these really could be dunes,” says Jani Radebaugh, a planetary scientist at Brigham Young University in Provo, Utah, who was not involved in the study. “But what’s exciting about it is that they’ve come up with a good physical mechanism to explain how it’s possible.”

Io is typically thought of as a world of volcanoes. The possibility of dunes suggests that there might be more going on there than scientists thought, McDonald says. “It certainly adds a layer of complexity.” More