planetary science

Neptune’s atmospheric temperature is on an unexpected roller-coaster ride, and it could take decades for scientists to piece together what’s happening at the distant planet.

The ice giant’s global temperature dropped about 8 degrees Celsius between 2003 and 2012 at the start of Neptune’s summer, researchers report April 11 in Planetary Sciences Journal. Then from 2018 to 2020, thermal images show that the planet’s south pole brightened dramatically, indicating a spike of 11 degrees C (SN: 10/2/07).

Naomi Rowe-Gurney, a planetary scientist at NASA Goddard Space Flight Center in Greenbelt, Md., and colleagues looked at 17 years of mid-infrared data from ground-based telescopes and the no-longer-functioning Spitzer Space Telescope (SN: 7/18/18; SN: 1/28/20). The researchers used infrared light to pierce Neptune’s top cloud layer and peer at its stratosphere, where the planet’s atmospheric chemistry comes into view.

Each Neptune year lasts 165 Earth years, so the time period analyzed — from 2003 to 2020 — is essentially equivalent to five weeks on Earth. The wildest temperature shift occurred from 2018 to 2020, when the atmospheric temperature at Neptune’s south pole rose from –121° C to –110° C.

“We weren’t expecting any seasonal changes to happen in this short time period, because we’re not even seeing a full season,” says Rowe-Gurney. “It’s all very strange and interesting.”

The researchers don’t yet know what’s causing the temperature changes. The sun’s ultraviolet rays break up methane molecules in the stratosphere, so that chemistry or even the sun’s activity cycle could be a trigger. Nailing down specifics requires more observations. “We need to keep observing over the next 20 years to see a full season and see if something else changes,” says Rowe-Gurney.

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On Mars, the speed of sound depends on its pitch.

All sound travels slower through Mars’ air compared with Earth’s. But the higher-pitched clacks of a laser zapping rocks travels slightly faster in the thin Martian atmosphere than the lower-pitched hum of the Ingenuity helicopter, researchers report April 1 in Nature.

These sound speed measurements from NASA’s Perseverance rover are part of a broader effort to monitor minute-by-minute changes in atmospheric pressure and temperature, like during wind gusts, on the Red Planet.

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“The wind is the sound of science for us,” says astrophysicist Baptiste Chide of Los Alamos National Laboratory in New Mexico.

To listen to the wind, Perseverance carries two microphones. One was meant to record audio during the mission’s complex entry, descent and landing, and while it didn’t work as hoped, it is now turned on occasionally to listen to the rover’s vitals (SN: 2/22/21; SN: 2/17/21). The other microphone is part of the rover’s SuperCam instrument, a mast-mounted mishmash of cameras and other sensors used to understand the properties of materials on the planet’s surface.

But these microphones also pick up other sounds, such as those made by the rover itself as its wheels crunch the surface, and by Perseverance’s flying companion, the robotic helicopter Ingenuity. The SuperCam instrument, for example, has a laser, which Perseverance fires at interesting rocks for further analysis (SN: 7/28/20). The microphone on SuperCam captures sounds from those laser shots, which helps researchers learn about the hardness of the target material, says planetary scientist Naomi Murdoch of the Institut Supérieur de l’Aéronautique et de l’Espace in Toulouse, France.

Murdoch, Chide and their colleagues listened to the laser’s clack-clack when zapping rocks. (“It doesn’t do, really, ‘pew pew,’” Murdoch says). When the laser hits a target, that blast creates a sound wave. Because scientists know when the laser fires and how far away a target is, they can measure the speed at which that sound wave travels through the air toward the SuperCam microphone.

The speed of this sound is about 250 meters per second, the team reports. That’s slower than on Earth, where sound travels through the air at about 340 m/s.

The slower speed isn’t surprising. What we hear as sound is actually pressure waves traveling through a medium like air, and the speed of those waves depends on the medium’s density and composition (SN: 10/9/20). Our planet’s atmosphere is 160 times as dense as the Martian atmosphere, and Earth’s air is mostly nitrogen and oxygen, whereas the Martian air is predominately carbon dioxide. So sound on Mars travels slower in that different air.

The team also used the SuperCam microphone to listen to the lower-pitch whirl of Ingenuity’s helicopter blades (SN: 12/10/21). From this lower-pitched sound, the researchers learned that there is a second speed of sound at the Martian surface at frequencies below 240 hertz, or slightly deeper than middle C on a piano: 240 m/s.

In contrast, at Earth’s surface, sound moves through the air at only one speed, no matter the pitch. The two speeds on Mars, the researchers say, are because of its carbon dioxide–rich atmosphere. Carbon dioxide molecules behave differently with one another when sound waves with frequencies above 240 hertz move through the air compared with those below 240 hertz, affecting the waves’ speed.

“We’ve proved that we can do science with a microphone on Mars,” Chide says. “We can do good science.”

The SuperCam microphone captures thousands of sound snippets per second. Those sounds are affected by air pressures, so the researchers can use that acoustic data to track detailed changes in air pressures over short timescales, and, in doing so, learn more about the Martian climate. While other Mars rovers have had wind, temperature and pressure sensors, those could sense changes only over longer periods.

“Listening to sounds on another planet is another way that helps all of us place ourselves as if we were there,” says Melissa Trainer, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md., who was not part of this work.

The team is focusing on next collecting acoustic data at different times of day and different seasons on Mars.

“The pressure changes a lot on Mars throughout the year with the seasons,” Trainer says. “I’m really excited to see how the data might change as it gets collected through proceeding seasons.” More

A treasure trove of diamonds may be sown into Mercury’s cratered crust.

Billions of years of meteorite impacts may have flash-baked much of Mercury’s surface into the glittery gemstones, planetary scientist Kevin Cannon reported March 10 at the Lunar and Planetary Science Conference in The Woodlands, Texas. His computer simulations predict that such impacts may have transformed about one-third of the little planet’s crust into a diamond stockpile many times that of Earth’s.

Diamonds are forged under immense pressures and temperatures. On Earth, the gemstones crystallize deep underground — at least 150 kilometers down — then ride to the surface during volcanic eruptions (SN: 9/14/20). But studies of meteorites suggest diamonds can also form during impact.

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“When those [impacts] happen, they create very high pressures and temperatures that can transform carbon into diamond,” says Cannon, of the Colorado School of Mines in Golden.

With impact-born diamonds on his mind, Cannon turned to the closest planet to the sun. Surveys of the planet’s surface and experiments with molten rock suggest that the planet’s crust may retain fragments of an old shell of graphite — a mineral made from carbon (SN: 3/7/16). “What we think happened is that when [Mercury] first formed, it had a magma ocean, and that graphite crystallized out of that magma,” Cannon says.

Then, the bombardment. Mercury’s surface today is heavily cratered, evidence of an impact-rich history. Much of the purported graphite crust would have been battered and transformed into diamond, Cannon hypothesized.

Curious how pervasive this diamond forging could have been, Cannon used computers to simulate 4.5 billion years of impacts on a graphite crust. The findings show that if Mercury had possessed a skin of graphite 300 meters thick, the battering would have generated 16 quadrillion tons of diamonds — about 16 times Earth’s estimated reserves.

“There’s no reason to doubt that diamonds could be produced in this way,” says Simone Marchi, a planetary scientist at the Southwest Research Institute in Boulder, Colo., who was not involved with the research. But how many might have survived, that’s another story, he says. Some of the gemstones would probably have been destroyed by later impacts.

Cannon agrees that subsequent impacts would probably obliterate some diamonds. But the losses would have been “very limited,” he says, as the ultimate melting point of diamond exceeds 4000° Celsius. Future simulations will incorporate remelting from impacts, he says, to refine the potential size of Mercury’s present day diamond reserves.

An opportunity to scout for diamonds on Mercury may come in 2025, when the BepiColombo mission reaches the planet. Diamonds reflect a distinct signature of infrared light, Cannon says. “And potentially, this could be detected.” More

A bevy of craters formed by material blasted from the carving of another, larger crater — a process dubbed secondary cratering — have finally been spotted on Earth. Several groupings of craters in southeastern Wyoming, including dozens of pockmarks in all, have the hallmarks of secondary cratering, researchers report February 11 in GSA Bulletin.

When an asteroid or another type of space rock smacks into a planet or moon, it blasts material from the surface and creates a crater (SN: 12/18/18). Large blocks of that material can be thrown far from the initial crater and blast out their own holes when they land, explains Thomas Kenkmann, a planetary scientist at the Albert Ludwig University of Freiburg in Germany. Astronomers have long observed secondary cratering on our moon, Mars and other orbs in the solar system, but never on Earth.

When Kenkmann and his colleagues first investigated a series of craters near Douglas, Wyo., in 2018, they thought the pockmarks were formed by fragments of a large meteorite that had broken up in the atmosphere. But Kenkmann and his team later discovered similar groups of craters of the same age, somewhere around 280 million years old, throughout the region.

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Altogether, the team found more than 30 impact craters that range between 10 and 70 meters in diameter at six different locales. Based on subtle but distinct differences in the alignment of elliptical craters in the groups, the researchers suggest that the impactors that blasted each set of craters struck the ground from slightly different directions.

The impactors that created these secondary craters probably ranged between 4 and 8 meters in diameter and struck the ground at speeds between 2,520 and 3,600 kilometers per hour, Kenkmann says. Extrapolating the paths of these impactors back to their presumed sources suggests the original crater from which they flew straddles the Wyoming–Nebraska border northeast of Cheyenne.

The team’s evidence “comes together very well to make a compelling story,” says Gareth Collins, a planetary scientist at Imperial College London who was not involved in the new study.

The original crater was probably between 50 and 65 kilometers across and was created by an impactor 4 to 5.4 kilometers wide, Kenkmann and the team estimate. The crater is also probably buried under more than 2 kilometers of sediment that accumulated in the 280 million years since it formed. An equivalent amount of sediment eroded away to expose the secondary craters when the Rocky Mountains rose in the meantime.

“What a fortuitous discovery that these folks have made,” says Beau Bierhaus, a planetary scientist at Lockheed Martin Space Systems in Littleton, Colo. He likens the short geological interval during which these craters could be discovered to the brief period between the time a fossil is first exposed to the elements and when it is eroded to dust.

Scouring measurements of gravitational and magnetic fields in the region for anomalies could help reveal the buried crater, the researchers note. The team may also look for heavily fractured rock and other evidence of the ancient crater in sediment cores that have been drilled during oil and gas exploration in the region, Kenkmann says. More

A recently found space rock is schlepping along with Earth around the sun. This “Trojan asteroid” is only the second one discovered that belongs to our planet. And it’s probably a visitor.

Trojan asteroids, which are also found accompanying Mars, Jupiter and Neptune, hang out in two locations near a planet where the gravitational pulls of that planet and the sun balance each other (SN: 10/15/21). Because of this balancing act, these locations are stable spots in space. In 2010, astronomers discovered the first known Earth Trojan — called 2010 TK7 — orbiting within one of these two regions, known as L4, tens of millions of kilometers from Earth and leading our planet around the sun (SN: 8/2/11).

Now, researchers have found another one. Dubbed 2020 XL5, this roughly 1-kilometer-wide asteroid is also at L4, astronomer Toni Santana-Ros of the University of Barcelona and colleagues report February 1 in Nature Communications.

The space rock was first spotted in December 2020, and follow-up observations suggested that it might be at L4. To confirm this, Santana-Ros and colleagues observed the asteroid using ground-based telescopes in 2021. Measurements of its brightness let the researchers estimate the asteroid’s size — about three to four times as wide as 2010 TK7. They also scoured archival data and found the object in images dating to 2012.

“There is no doubt this is an Earth Trojan,” Santana-Ros says. That decade-worth of observations let the team calculate the rock’s orbit thousands of years into the future, confirming the asteroid’s nature. It will hang around at L4 for at least 4,000 years, the team predicts. 2010 TK7, for comparison, will stick around for some 10,000 years.

Now that scientists know of two just-visiting Earth Trojans, they can envision more. The fact that the team discovered a second object means that 2010 TK7 isn’t a rarity or loner, Santana-Ros says. “It might be part of a family or population.” More

The hunt for meteorites may have just gotten some new leads. A powerful new machine learning algorithm has identified over 600 hot spots in Antarctica where scientists are likely to find a bounty of the fallen alien rocks, researchers report January 26 in Science Advances.  

Antarctica isn’t necessarily the No. 1 landing spot for meteorites, bits of extraterrestrial rock that offer a window into the birth and evolution of the solar system. Previous estimates suggest more meteorites probably land closer to the equator (SN: 5/29/20). But the southern continent is still the best place to find them, says Veronica Tollenaar, a glaciologist at the Université libre de Bruxelles in Belgium. Not only are the dark specks at the surface starkly visible against the white background, but quirks of the ice sheet’s flow can also concentrate meteorites in “stranding zones.”

The trouble is that so far, meteorite stranding zones have been found by luck. Satellites help, but poring through the images is time-consuming, and field reconnaissance is costly. So Tollenaar and her colleagues trained computers to find these zones more quickly.

Such stranding zones form when the slow creep of the ice sheet over the land encounters a mountain or hidden rise in the ground. That barrier shifts the flow upward, carrying any embedded space rocks toward the surface.

Combining a machine learning algorithm with data on the ice’s velocity and thickness, surface temperatures, the shape of the bedrock and known stranding zones, Tollenaar and colleagues created a map of 613 probable meteorite hot spots, including some near existing Antarctic research stations.

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To date, about 45,000 meteorites have been plucked from the ice. But that’s a fraction of the 300,000 bits of space rock estimated to lie somewhere on the continent’s surface.

The team has yet to test the map on the ground; a COVID-19 outbreak at the Belgian station in December halted plans to try it during the 2021–2022 field season. It will try again next year. Meanwhile, the team is making these data freely accessible to other researchers, hoping they’ll take up the hunt as well. More

When researchers in 1996 reported they had found organic molecules nestled in an ancient Martian meteorite discovered in Antarctica, it caused quite a buzz. Some insisted the compounds were big-if-true evidence of life having existed on Mars (SN: 3/8/01). Others, though, pointed to contamination by earthly life-forms or some nonbiological origins (SN: 1/10/18).

Now, a geochemical analysis of the meteorite provides the latest buzzkill to the idea that alien life inhabited the 4.09-billion-year-old fragment of the Red Planet. It suggests instead that the organic matter within probably formed from the chemical interplay of water and minerals mingling under Mars’ surface, researchers report in the Jan. 14 Science. Even so, the finding could aid in the search for life, the team says.

Organic molecules are often produced by living organisms, but they can also arise from nonbiological, abiotic processes. Though myriad hypotheses claim to explain what sparked life, many researchers consider abiotic organic molecules to be necessary starting material. Martian geologic processes could have been generating these compounds for billions of years, the new study suggests.

“These organic chemicals could have become the primordial soup that might have helped form life on [Mars],” says Andrew Steele, a biochemist from the Carnegie Institution for Science in Washington, D.C. Whether life ever existed there, however, remains unknown.

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Steele and his colleagues initially sought to study how ancient Martian water may have morphed minerals in the meteorite, known as ALH84001. The team used microscopic and spectroscopic imaging methods to analyze tiny slivers from parts of the meteorite that appeared to have reacted with water.

In their samples, the researchers discovered by-products of two chemical reactions — serpentinization and carbonation — which occur when underground fluids interact with minerals and transform them. Amid these by-products, the researchers detected complex organic molecules. Based on the identification of these two processes, the team concluded the organics probably formed during the reactions, just as they do on Earth.

Analysis of the relative amounts of different types of hydrogen in the organic matter supported the notion that the organic compounds developed while on Mars; they didn’t emerge later on from Earth’s microbes or materials used in the team’s experiments.

Altogether the findings suggest that at least two geologic processes probably produced organic matter on the Red Planet, says Mukul Sharma, a geochemist at Dartmouth College who was not involved in the study.

The study is not the only to propose that organic material in Martian rocks could form without life. Researchers attributed the formation of complex organics in the 600-million-year-old Tissint meteorite, also from Mars, to chemical interactions of water and rock (SN: 10/11/12).

However, ALH84001 is one of the oldest Martian meteorites ever found. The new findings, when considered alongside other discoveries of Martian organic matter, suggest that abiotic processes may have been generating organic material across the Red Planet for much of its history, Sharma says. “Nature has had a huge amount of time on its hands to produce this stuff.”

Though the work doesn’t bring us any closer to proving or disproving the existence of life on Mars, identifying abiotic sources of organic compounds there is crucial for the search, Steele explains. Once you’ve figured out how Martian organic chemistry acts without meddlesome life, he says, “you can then look to see if it’s been tweaked.” More

Humble oxygen is more than just a building block of life. The element could also help scientists sneak a peek into the innards of planets orbiting faraway stars, a new study suggests.

Laboratory experiments show that rocks exposed to higher concentrations of oxygen melt at lower temperatures than rocks exposed to lower amounts. The finding suggests that oxygen-rich rocky exoplanets could have a thick layer of soupy mantle, possibly giving rise to a geologically active world, researchers report in the Nov. 9 Proceedings of the National Academy of Sciences.

A gooey interior is thought to have profound effects on a rocky planet. Molten rock deep within a planet is the magma that powers geologic activity on the surface, like what happens on Earth (SN: 7/31/13). During volcanic eruptions, volatiles such as water vapor and carbon dioxide can fizzle out of the magmatic ooze, setting up atmospheres that are potentially friendly to life (SN: 9/3/19). But the factors that drive mantle melting on Earth aren’t well-understood, and scientists have tended to focus on the role of metals, such as iron.

The impact of oxygen on rock melting has been overlooked, says Yanhao Lin, a planetary scientist at the Center for High Pressure Science and Technology Advanced Research in Beijing. Oxygen is one of the most abundant elements on Earth and probably on rocky exoplanets too, he says. As such, other scientists may have previously thought that it is just too common of an element to play such a literally earthshaking role, adds Lin.

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In the new study, Lin and colleagues measured the melting temperatures of synthetic, iron-free basalt rock under rock in two environments: under oxygen-starved conditions and exposed to oxygen-rich air. The team used the faux rock to isolate oxygen’s effect on melting and rule out the effects of iron, which can also influence rock melting.

As the molten rocks cooled to less than 1000° Celsius, the minerals in the oxygenated basalt stayed melted longer than the oxygen-depleted samples, the team observed. The oxygenated rocks consistently solidified at temperatures 100° Celsius lower than their counterparts.

Just as salt lowers the melting temperature of ice, oxygen similarly makes it easier for rocks to melt, the researchers conclude. Lin hypothesizes that oxygen can break up long chains of silicon and oxygen atoms in solid rock, coaxing them to form smaller bits. These fragments are more mobile and can flow more easily compared to the longer, tangly groups.

The degree of oxidation could determine how a young exoplanet’s syrupy insides eventually settle into subterranean layers. A more oxidized and more melt-prone gut at lower temperatures may lead to a smaller solid core, a thicker sludgy mantle and a more metal-deprived crusty shell, the researchers say.

A caveat to the work is that the researchers tested the impact of only oxygen on the melting temperature of rocks. The team has yet to consider other factors such as iron concentration and high pressure, which are also probably part of many real-world exoplanet interiors. These additional factors will further induce melting, Lin predicts.

The findings are “a very good effort,” says planetary scientist Tim Lichtenberg of the University of Oxford who was not involved in the study. Other caveats to mantle melting may surpass oxygen’s contribution, but the new results are still useful, he says. Understanding oxygen’s potential impact, for example, could be valuable for explaining the inner workings and history of any exoplanet that scientists come across in their astronomical observations. That understanding could be even more valuable — and opportune — as scientists prepare to use the newly launched James Webb Space Telescope to probe the atmospheres of other worlds (SN: 10/6/21).

Lab experiments, of course, can’t capture all the nuances of real-life planetary interiors. But the work is necessary to guide — and confirm — the formulation of theories about how certain types of exoplanets came to be, Lichtenberg says. Simulations can then extend the reach of experimental results when combined with other techniques, such as modeling.

“Observations, the modeling and the experiments,” Lichtenberg says, “there’s a trifecta.” These three prongs feed off each other to advance exoplanet science as a whole, long before humankind ever sets foot on such distant worlds. More