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      Oort cloud comets may spin themselves to death

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      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

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      Asteroid impacts might have created some of Mars’ sand

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      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

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      Over time, Betelgeuse changed color. Now it’s also lost its rhythm

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      The star Betelgeuse has always been a diva.

      Astronomers from antiquity through the present day have watched the red supergiant pulsing at the shoulder of the constellation Orion, and the star has continually put on a show, two new studies suggest. Betelgeuse may still be recovering from a deep dimming episode a few years ago, one team reports. And the star appears to have put on its reddish stage makeup just 2,000 years ago, before which it wore yellow, another team says.

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      Together, these studies could tell researchers about how stars spew their guts into space and hint at how long it will be before Betelgeuse explodes in a supernova.

      “This star always fools you,” says astronomer Edward Guinan of Villanova University in Pennsylvania, who has studied Betelgeuse for decades and was not involved in the new works. “You think you have it, and all of a sudden, it changes.”

      The “Great Dimming”

      In late 2019, Betelgeuse captured astronomers’ attention when it suddenly grew dark for several months — an event astronomers now call the Great Dimming. Months of subsequent observations led researchers to an explanation: The star had coughed out a big bubble of plasma. That material cooled, condensed into dust and blocked the star’s face from the perspective of Earth months later (SN: 11/29/20). The surface of the star also cooled down, contributing to the dimming (SN: 6/16/21).

      But what happened next was equally surprising, astrophysicist Andrea Dupree and colleagues report in a paper submitted August 2 to arXiv.org. The star’s regular pulsating brightness, it seems, went completely out of whack.

      In its non–Great Dimming life, Betelgeuse’s brightness was on a quasi-periodic dimmer switch. As the star breathed in and out — ballooning out before shrinking back down — its brightness went up and down. “For 200 years, it had a nice, 400-day oscillation in brightness,” says Dupree, of the Harvard & Smithsonian Center for Astrophysics in Cambridge, Mass. “But that’s gone now.”

      That regular drumbeat has since grown erratic. Instead of a regular thrum, the oscillations are “like an unbalanced washing machine, going ‘wonka wonka wonka,’” Dupree says.

      The wonkiness is a sign of the star struggling to recover from the loss of material in 2019, Dupree says. She calculates that Betelgeuse ejected several times the mass of the moon from its surface, leaving a large cool spot behind. The star’s surface plasma is sloshing around as it returns to equilibrium.

      If this picture is correct, it means red supergiants like Betelgeuse can spray material into interstellar space in discrete bursts, rather than a continuous stream. That’s important to know because many of the elements that make up planets and people were formed in stars undergoing what Betelgeuse is going through right now. Studying Betelgeuse’s growing pains and death throes can tell us about our own origins.

      But while this picture of Betelgeuse holds together, it is still speculative, Guinan cautions.

      One confounding factor is a new set of observations of Betelgeuse during the four-month period when it’s usually out of view. From May through August every year, Betelgeuse is too close to the sun from Earth’s perspective to be seen at night. Usually that leaves a hole in the datasets of astronomers who track its periodic behavior.

      But amateur observer Otmar Nickel of Mainz, Germany, developed a technique to measure Betelgeuse’s brightness using multiple images taken during the day. Dupree’s paper is the first to include those daytime data.

      “That’s cool,” Guinan says. “You can follow the star all year round.”

      Those extra observations might reveal recurring changes that have always been there, rather than picking up on something truly new. “Those little variations you’re seeing…could easily be present right before the Great Dimming,” Guinan says.

      Dupree’s team predicts that the dust Betelgeuse lost could become visible to some telescopes on Earth in 2023. “That would be proof” that the brightness changes were due to a single outburst, Guinan says.

      Seeing yellow

      The Great Dimming isn’t the first time humans have recorded a major change in Betelgeuse’s personality. Two millennia ago, the star was a completely different color, astrophysicist Ralph Neuhäuser and colleagues report in a paper in press in Monthly Notices of the Royal Astronomical Society.

      The team analyzed ancient descriptions of more than 200 stars whose colors should have been visible to the naked eye in the past few thousand years. Most stars observed over human history had the same color recorded in the past as they display today, the team found. But not Betelgeuse.

      The ancient Roman astronomer Gaius Julius Hyginus, who lived from about 64 B.C. to A.D. 17, and is thought to have written the Latin work De Astronomia, described the star in the right shoulder of Orion has having a similar color to Saturn ­— which is yellow. Astrologer and archivist Sima Qian, working during the Chinese Han dynasty around 100 B.C., independently described the star as yellow. Observers from other ancient cultures conspicuously left Betelgeuse out of their lists of red stars.

      “I thought, ‘Oh, how can this be?’” says Neuhäuser, of AIU Jena in Germany. “I was not expecting such a result … to find a star to change color in historical time.”

      A star’s color is a sign of its evolutionary stage (SN: 7/23/21). When stars burn through the hydrogen fuel in their cores, they puff up and expel gases into space. That expansion makes their surface temperatures drop, and they change color from blue to red in fairly short order — about 10,000 years for a giant star like Betelgeuse, which is around 14 times as massive as the sun.

      [embedded content]
      Measuring a star’s age isn’t as easy as you’d think. Here’s how scientists get their ballpark estimates.

      That relatively recent color change suggests Betelgeuse has just reached the end of its hydrogen-burning life and became the red supergiant we know it as today while human observers were watching.

      “It’s fully consistent with astrophysical knowledge,” Neuhäuser says. “It could have been expected, but no one really checked.”

      That result means anyone waiting for Betelgeuse to go supernova will have a very long wait. If the star just became a supergiant in the last few millennia, it has more than 1 million years to go before the boom. More

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      How balloons could one day detect quakes on Venus

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      The balloon was floating over the Pacific Ocean when the first sound waves hit. For 11 seconds, a tiny device dangling beneath the large, transparent balloon recorded sudden, jerky fluctuations in air pressure: echoes of an earthquake more than 2,800 kilometers away.

      That scientific instrument was one of four hovering high above the Malay Archipelago on December 14, 2021. That day the quartet became the first network of devices to monitor an earthquake from the air, researchers report in the Aug. 16 Geophysical Research Letters.

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      The finding could help scientists track earthquakes in remote areas on Earth, and also opens the door to one day sending specially equipped balloons to study the geology of other worlds, including our closest planetary neighbor.

      “Venus is the sister planet of Earth, but it’s the evil twin sister,” says David Mimoun, a planetary scientist at the University of Toulouse in France. “We don’t know why the two planets are so different. That’s why we need measurements.”

      The idea of using balloons to study far-off rumblings on Earth has its roots in the Cold War. In the 1940s, the U.S. military launched a top secret project to spy on Soviet nuclear weapons testing using microphones attached to balloons floating high in the atmosphere. When the ground shakes, it releases low-frequency sound waves that can travel long distances in the atmosphere. The military planned on using the microphones to pick up on the sound of the ground shaking from a nuclear explosion. But the project was eventually deemed too expensive and dropped — though not before one of the balloons crashed in New Mexico, launching the Roswell conspiracy.

      For decades after, balloon science stayed mostly in the realm of meteorology. Then in the early 2000s, Mimoun and his colleagues started experimenting with using balloons for space exploration, specifically for studying extraterrestrial quakes.

      Analyzing temblors is one of the main ways that scientists can learn about a planet’s interior. On worlds with thin atmospheres, such as Mars or Earth’s moon, this generally means sending a lander to the surface and measuring quakes directly on the ground (SN: 5/13/22).

      But doing that on Venus isn’t really an option. The dense atmosphere means that the planet’s surface has about the same pressure as Earth’s deep ocean, with temperatures averaging around 450° Celsius — hot enough to melt lead. “Basically, it’s hell,” Mimoun says.

      Landers have made it to the surface of Venus before (SN: 6/19/76). But these probes lasted only a few hours before succumbing to the extreme heat and pressure. The chances of measuring a quake in that short time frame are slim, says Siddharth Krishnamoorthy, a research technologist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., who wasn’t involved in the study. So while radar images of Venus have revealed a world dotted with volcanoes, scientists still don’t know for sure if Venus is geologically active, he says.

      Scientists have previously experimented with the idea of detecting quakes on Venus using orbiters (SN: 9/02/05). But quake-detecting balloons have better resolution, says Mimoun, meaning they could provide the key to revealing the planet’s interior life. But first Mimoun and his colleagues had to show that they could design devices small enough to be carried by balloons but sensitive enough to pick up earthquakes far below.

      In 2021, the team attached micro-barometers to 16 balloons launched from the Seychelles Islands, off the coast of East Africa. In December, four balloons — having drifted thousands of kilometers apart — recorded similar, low-frequency sound waves. These changes in air pressure resembled ground readings of a 7.3 magnitude earthquake near the Indonesian island of Flores, indicating that the sound waves were produced by the earthquake. The researchers were able to use the changes in air pressure to pinpoint the epicenter of the quake and calculate its magnitude.

      “This is a huge step forward in demonstrating the utility of this technology,” says Paul Byrne, a planetary scientist at Washington University in St. Louis, who was not involved with the study.

      Even without being able to pick up quakes, the balloons, if designed to survive in the Venusian atmosphere, might be able to detect changes in air pressure that reveal clues about the planet’s volcanic eruptions and mysterious highlands, Byrne says.

      Venus is entering a renaissance of interest from space agencies. At least two NASA missions to visit the planet are planned for the end of this decade (SN: 6/2/21). Mimoun is hoping that earthquake-detecting balloons will feature in the next major mission, emphasizing that their data could help researchers understand why Earth and Venus — alike in size and distance from the sun, relative to the other planets — have gone down such different paths.

       “We have no clue,” Mimoun says. “So we need to go back.” More

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      Scientists mapped dark matter around galaxies in the early universe

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      Scientists have mapped out the dark matter around some of the earliest, most distant galaxies yet.

      The 1.5 million galaxies appear as they were 12 billion years ago, or less than 2 billion years after the Big Bang. Those galaxies distort the cosmic microwave background — light emitted during an even earlier era of the universe — as seen from Earth. That distortion, called gravitational lensing, reveals the distribution of dark matter around those galaxies, scientists report in the Aug. 5 Physical Review Letters.

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      Understanding how dark matter collects around galaxies early in the universe’s history could tell scientists more about the mysterious substance. And in the future, this lensing technique could also help scientists unravel a mystery about how matter clumps together in the universe.

      Dark matter is an unknown, massive substance that surrounds galaxies. Scientists have never directly detected dark matter, but they can observe its gravitational effects on the cosmos (SN: 7/22/22). One of those effects is gravitational lensing: When light passes by a galaxy, its mass bends the light like a lens. How much the light bends reveals the mass of the galaxy, including its dark matter.

      It’s difficult to map dark matter around such distant galaxies, says cosmologist Hironao Miyatake of Nagoya University in Japan. That’s because scientists need a source of light that is farther away than the galaxy acting as the lens. Typically, scientists use even more distant galaxies as the source of that light. But when peering this deep into space, those galaxies are difficult to come by.

      So instead, Miyatake and colleagues turned to the cosmic microwave background, the oldest light in the universe. The team used measurements of lensing of the cosmic microwave background from the Planck satellite, combined with a multitude of distant galaxies observed by the Subaru Telescope in Hawaii (SN: 7/24/18). “The gravitational lensing effect is very small, so we need a lot of lens galaxies,” Miyatake says. The distribution of dark matter around the galaxies matched expectations, the researchers report.

      The researchers also estimated a quantity called sigma-8, a measure of how “clumpy” matter is in the cosmos. For years, scientists have found hints that different measurements of sigma-8 disagree with one another (SN: 8/10/20). That could be a hint that something is wrong with scientists’ theories of the universe. But the evidence isn’t conclusive.

      “One of the most interesting things in cosmology right now is whether that tension is real or not,” says cosmologist Risa Wechsler of Stanford University, who was not involved with the study. “This is a really nice example of one of the techniques that will help shed light on that.”

      Measuring sigma-8 using early, distant galaxies could help reveal what’s going on. “You want to measure this quantity, this sigma-8, from as many perspectives as possible,” says cosmologist Hendrik Hildebrandt of Ruhr University Bochum in Germany, who was not involved with the study.

      If estimates from different eras of the universe disagree with one another, that might help physicists craft a new theory that could better explain the cosmos. While the new measurement of sigma-8 isn’t precise enough to settle the debate, future projects, such as the Rubin Observatory in Chile, could improve the estimate (SN: 1/10/20). More

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      Mini-Neptunes may become super-Earths as the exoplanets lose their atmospheres

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      Mini-Neptunes and super-Earths may have a lot more in common than just being superlatives.

      Four gaseous exoplanets, each a bit smaller than Neptune, seem to be evolving into super-Earths, rocky worlds up to 1.5 times the width of our home planet. That’s because the intense radiation of their stars appears to be pushing away the planets’ thick atmospheres, researchers report in a paper submitted July 26 at arXiv.org. If the current rate of atmospheric loss keeps up, the team predicts, those puffy atmospheres will eventually vanish, leaving behind smaller planets of bare rock.

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      Studying how these worlds evolve and lose their atmospheres can help scientists understand how other exoplanets lose their atmospheres. And that, says astronomer Heather Knutson of Caltech, can provide intel on what types of planets might have habitable environments. “Because if you can’t keep an atmosphere,” she says, “you can’t be habitable.”

      Knutson and her colleagues’ new study bolsters a previous suspicion. Earlier this year, the same researchers reported that helium seemed to be escaping the atmosphere of one these mini-Neptunes. But the team wasn’t sure if their discovery was a one-off. “Maybe we just got very lucky for this one planet, but every other planet is different,” says exoplanet researcher Michael Zhang, also of Caltech.

      So the team looked at three more mini-Neptunes orbiting other stars and compared those worlds to the first planet they had observed. Each of these planets occasionally blocks some of the light from its star (SN: 7/21/21).  Zhang, Knutson and colleagues tracked how long each planet blocked its stars’ light and how much of that starlight was absorbed by helium enveloping the planets. Together, these observations let the team measure the sizes and shapes of the planets’ atmospheres.

      “When a planet is losing its atmosphere, you get this big, sort of cometlike tail of gas coming out from the planet,” Knutson says. If the gas instead is still bound to the planet — as is the case for Neptune in our solar system — the astronomers would have seen a circle. “We don’t fully understand all the shapes that we see in the outflows,” she says, “but we see they’re not spherical.”

      In other words, each planet is steadily losing its helium. “I never would have guessed that every single planet we looked at, that we would see such a clear detection,” Knutson says.

      The astronomers also calculated how much mass those exoplanets were losing (SN: 6/19/17). “This mass loss rate is high enough to strip the atmospheres of at least most of these planets, so that some of them, at least, will become super-Earths,” Zhang says.

      These rates, though, are just snapshots in time, says Ian Crossfield, an exoplanet researcher at the University of Kansas in Lawrence who was not involved with this work. For each planet, “you don’t know exactly how it’s been losing atmosphere throughout its entire history and into the future,” he says. “All we know is what we see today.” Even with such open questions, he adds, the idea that mini-Neptunes turn into super-Earths “seems plausible.”

      Theories and computer simulations of how planets form and lose their atmospheres can help fill in some of the blanks on individual planets, Crossfield says.

      Measurements of more mini-Neptunes will also help. Zhang plans to observe another handful. In addition, “we’ve already looked at one more target, and that target also has a pretty strong escaping helium [signal],” he says. “Now we have five for five.” More

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      How Mars rovers have evolved in 25 years of exploring the Red Planet

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      Few things are harder than hurling a robot into space — and sticking the landing. On the morning of July 4, 1997, mission controllers at the Jet Propulsion Laboratory in Pasadena, Calif., were hoping to beat the odds and land a spacecraft successfully on the Red Planet.

      Twenty-five years ago that little robot, a six-wheeled rover named Sojourner, made it — becoming the first in a string of rovers built and operated by NASA to explore Mars. Four more NASA rovers, each more capable and complex than the last, have surveyed the Red Planet. The one named Curiosity marked its 10th year of cruising around on August 5. Another, named Perseverance, is busy collecting rocks that future robots are supposed to retrieve and bring back to Earth. China recently got into the Mars exploring game, landing its own rover, Zhurong, last year.

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      Other Mars spacecraft have done amazing science from a standstill, such as the twin Viking landers in the 1970s that were the first to photograph the Martian surface up close and the InSight probe that has been listening for Marsquakes shaking the planet’s innards (SN Online: 2/24/20). But the ability to rove turns a robot into an interplanetary field geologist, able to explore the landscape and piece together clues to its history. Mobility, says Kirsten Siebach, a planetary scientist at Rice University in Houston, “makes it a journey of discovery.”

      Each of the Mars rovers has gone to a different place on the planet, enabling scientists to build a broad understanding of how Mars evolved over time. The rovers revealed that Mars contained water, and other life-friendly conditions, for much of its history. That work set the stage for Perseverance’s ongoing hunt for signs of ancient life on Mars.

      Each rover is also a reflection of the humans who designed and built and drove it. Perseverance carries on one of its wheels a symbol of Mars rover tracks twisted into the double helix shape of DNA. That’s “to remind us, whatever this rover is, it’s of human origin,” says Jennifer Trosper, an engineer at the Jet Propulsion Lab, or JPL, who has worked on all five NASA rovers. “It is us on Mars, and kind of our creation.”

      The little microwave that could

      Sojourner, that first rover, was born in an era when engineers weren’t sure if they even could get a robot to work on Mars. In the early 1990s, then-NASA Administrator Daniel Goldin was pushing the agency to do things “faster, better and cheaper” — a catchphrase that engineers would mock by saying only two of those three things were possible at the same time. NASA had no experience with inter­planetary rovers. Only the Soviet Union had operated rovers — on the moon in 1970 and 1973.

      JPL began developing a Mars rover anyway. Named after the abolitionist Sojourner Truth, the basic machine was the size of a microwave oven. Engineers were limited in where they could send it; they needed a large flat region on Mars because handling a precision landing near mountains or canyons was beyond their abilities. NASA chose Ares Vallis, a broad outflow channel from an ancient flood, and the mission landed there successfully.

      Sojourner spent nearly three months poking around the landscape. It was slow going. Mission controllers had to communicate with Sojourner constantly, telling it where to roll and then assessing whether it had gotten there safely. They made mistakes: One time they uploaded a sequence of computer commands that mistakenly told the rover to shut itself down. They recovered from that stumble and many others, learning to quickly fix problems and move forward.

      In 1997, NASA’s first rover, Sojourner, rolled down a landing ramp and became the first mobile Mars robot. Solar panels provided power throughout its 12-week mission.JPL-CALTECH/NASA

      Although Sojourner was a test mission to show that a rover could work, it managed to do some science with its one X-ray spectrometer. The little machine analyzed the chemical makeup of 15 Martian rocks and tested the friction of the Martian soil.

      After surviving 11 weeks beyond its planned one-week lifetime, Sojourner ultimately grew too cold to operate. Trosper was in mission control when the rover died on September 27, 1997. “You build these things, and even if they’re well beyond their lifetime, you just can’t let go very easily, because they’re part of you,” she says.

      Jennifer Trosper, an engineer at the Jet Propulsion Laboratory, is part of a small group of people who have worked on all five NASA Mars rovers. Here she is in 2021 with a model of Perseverance.CHRISTOPHER MICHEL/WIKIMEDIA COMMONS (CC BY-SA 4.0)

      Twin explorers

      In 1998 and 1999, NASA hurled a pair of spacecraft at Mars; one was supposed to orbit the planet and another was supposed to land near one of the poles. Both failed. Stung from the disappointment, NASA decided to build a rover plus a backup for its next attempt.

      Thus were born the twins Spirit and Opportunity. Each the size of a golf cart, they were a major step up from Sojourner. Each had a robotic arm, a crucial development in rover evolution that enabled the machines to do increasingly sophisticated science. The two had beefed-up cameras, three spectrometers and a tool that could grind into rocks to reveal the texture beneath the surface.

      But there were a lot of bugs to work out. Spirit and Opportunity launched several weeks apart in 2003. Spirit got to Mars first, and on its 18th Martian day on the surface it froze up and started sending error messages. It took mission controllers days to sort out the problem — an overloaded flash-memory system — all while Opportunity was barreling toward Mars. Ultimately, engineers fixed the problem, and Opportunity landed safely on the opposite side of the planet from Spirit.

      Both rovers lasted years beyond their expected three-month lifetimes. And both did far more Martian science than anticipated.

      Spirit broke one of its wheels early on and had to drive backward, dragging the broken wheel behind it. But the rover found plenty to do near its landing site of Gusev crater, home to a classic Mars landscape of dust, rock and hills. Spirit found rocks that appeared to have been altered by water long ago and later spotted a pair of iron-rich meteorites. The rover ultimately perished in 2010, stuck in a sand-filled pit. Mission controllers tried to extract it in an effort dubbed “Free Spirit,” but salts had precipitated around the sand grains, making them particularly slippery.

      Opportunity, in contrast, became the Energizer Bunny of rovers, exploring constantly and refusing to die. Immediately after landing in Meridiani Planum, Opportunity had scientists abuzz.

      The pale rock at center, seen beneath the Opportunity rover’s robotic arm in 2013, was one of many at the rover’s landing site that held long-awaited evidence that liquid water once flowed on Mars.
      JPL-CALTECH/NASA, CORNELL UNIV., ARIZONA STATE UNIV.

      “The images that the rover first sent back were just so different from any other images we’d seen of the Martian surface,” says Abigail Fraeman, a planetary scientist at JPL. “Instead of these really dusty volcanic plains, there was just this dark sand and this really bright bedrock. And that was just so captivating and inspiring.”

      Right at its landing site, Opportunity spotted the first definitive evidence of past liquid water on Mars, a much-anticipated and huge discovery (SN: 3/27/04, p. 195). The rover went on to find evidence of liquid water at different times in the Martian past. After years of driving, the rover reached a crater called Endeavour and “stepped into a totally new world,” Fraeman says. The rocks at Endeavour were hundreds of millions of years older than others studied on Mars. They contained evidence of different types of ancient water chemistry.

      Opportunity ultimately drove farther than any rover on any extraterrestrial world, breaking a Soviet rover’s lunar record. In 2015, Opportunity passed 26.2 miles (42.2 km) on its odometer; mission controllers celebrated by putting a marathon medal onto a mock-up of the rover and driving it through a finish line ribbon at JPL. Opportunity finally died in 2019 after an intense dust storm obscured the sun, cutting off solar power, a must-have for the rover to recharge its batteries (SN: 3/16/19, p. 7).

      The twin rovers were a huge advance over Sojourner. But the next rover was an entirely different beast.

      Mission project scientist Ashwin Vasavada stands with several rovers, which learn to traverse various surfaces in the Mars Yard at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.JPL-Caltech/NASA

      The SUV of rovers

      By the mid-2000s, NASA had decided it needed to go big on Mars, with a megarover the size of a sports utility vehicle. The one-ton Curiosity was so heavy that its engineers had to come up with an entirely new way to land on Mars. The “sky crane” system used retro-rockets to hover above the Martian surface and slowly lower the rover to the ground.

      Against all odds, in August 2012, Curiosity landed safely near Mount Sharp, a 5-kilometer-high pile of sediment within the 154-kilometer-wide Gale crater (SN: 8/25/12, p. 5). Unlike the first three Mars rovers, which were solar-powered, Curiosity runs on energy produced by the radioactive decay of plutonium. That allows the rover to travel farther and faster, and to power a suite of sophisticated science instruments, including two chemical laboratories.

      Curiosity introduced a new way of exploring Mars. When the rover arrives in a new area, it looks around with its cameras, then zaps interesting rocks with its laser to identify which ones are worth a closer look. Once up close, the rover stretches out its robotic arm and does science, including drilling into rocks to see what they are made of.

      When Curiosity arrived near the base of Mount Sharp, it immediately spotted rounded pebbles shaped by a once-flowing river, the first close­up look at an ancient river on Mars. Then mission controllers sent the rover rolling away from the mountain, toward an area in the crater known as Yellowknife Bay. There Curiosity discovered evidence of an ancient lake that created life-friendly conditions for potentially many thousands of years.

      Curiosity then headed back toward the foothills of Mount Sharp. Along the way, the rover discovered a range of organic molecules in many different rocks, hinting at environments that had been habitable for millions to tens of millions of years. It sniffed methane gas sporadically wafting within Gale crater, a still-unexplained mystery that could result from geologic reactions, though methane on Earth can be formed by living organisms (SN: 7/7/18, p. 8). The rover measured radiation levels across the surface — helpful for future astronauts who’ll need to gauge their exposure — and observed dust devils, clouds and eclipses in the Martian atmosphere and night sky.

      Shimmering clouds of ice crystals appear in the sky above Gale crater on Mars, as seen by the Curiosity rover in March 2021. The ability to drive across Mars gives rovers a humanlike ability to interact with the landscape.
      MSSS, JPL-Caltech/NASA

      “We’ve encountered so many unexpectedly rich things,” says Ashwin Vasavada of JPL, the mission’s project scientist. “I’m just glad a place like this existed.”

      Ten years into its mission, Curiosity still trundles on, making new discoveries as it climbs the foothills of Mount Sharp. It recently departed a clay-rich environment and is now entering one that is heavier in sulfates, a transition that may reflect a major shift in the Martian climate billions of years ago.

      In the course of driving more than 28 kilometers, Curiosity has weathered major glitches, including one that shuttered its drilling system for over a year. And its wheels have been banged up more than earthbound tests had predicted. The rover will continue to roll until some unknown failure kills it or its plutonium power wanes, perhaps five years from now.

      Over nearly 10 years of driving on Mars’ rocky surface, Curiosity’s wheels have taken more of a beating than its designers expected.
      MSSS, JPL-Caltech/NASA

      A rover and its sidekick

      NASA’s first four rovers set the stage for the most capable and agile rover ever to visit Mars: Perseverance. Trosper likens the evolution of the machines to the growth of children. “We have a preschooler in Sojourner, and then … your happy-go-lucky teenagers in Spirit and Opportunity,” she says. “Curiosity is certainly a young adult that’s able to do a lot of things on her own, and Perseverance is kind of that high-powered mid­career [person] able to do pretty much anything you ask with really no questions.”

      Perseverance is basically a copy of Curiosity built from its spare parts, but with one major modification: a system for drilling, collecting and storing slender cores of rock. Perseverance’s job is to collect samples of Martian rock for future missions to bring to Earth, in what would be the first robotic sample return from Mars. That would allow scientists to do sophisticated analyses of Martian rocks in their earthbound labs. “It feels, even more than previous missions, that we are doing this for the next generation,” Siebach says.

      The rover is working fast. Compared with Curiosity’s leisurely exploration of Gale crater, Perseverance has been zooming around its landing site, the 45-kilometer-wide Jezero crater, since its February 2021 arrival. It has collected 10 rock cores and is already eyeing where to put them down on the surface for future missions to pick up. “We’re going to bring samples back from a diversity of locations,” says mission project scientist Kenneth Farley of Caltech. “And so we keep to a schedule.”

      Perseverance went to Jezero to study an ancient river delta, which contains layers of sediment that may harbor evidence of ancient Martian life. But the rover slightly missed its target, landing on the other side of a set of impassable sand dunes. So it spent most of its first year exploring the crater floor, which turned out to be made of igneous rocks (SN: 9/11/21, p. 32). The rocks had cooled from molten magma and were not the sedimentary rocks that many had expected.

      Scientists back on Earth will be able to precisely date the age of the igneous rocks, based on the radioactive decay of chemical elements within them, providing the first direct evidence for the age of rocks from a particular place on Mars.

      Perseverance collected its 9th rock core, barely the size of a pinky finger, on July 7. Future missions will return the stored samples to Earth for study.
      JPL-CALTECH/NASA, ARIZONA STATE UNIV.

      Once it finished exploring the crater floor in March, the rover drove quickly toward the delta. Each successive NASA rover has had greater skills in autonomous driving, able to identify hazards, steer around them and keep going without needing constant instructions from mission control.

      Perseverance has a separate computer processor to run calculations for autonomous navigation, allowing it to move faster than Curiosity. (It took Curiosity two and a half years to travel 10 kilometers; Perseverance traveled that far in a little over a year.) “The rover drives pretty much every minute that we can give it,” Farley says.

      In April, Perseverance set a Martian driving record, traveling nearly five kilometers in just 30 Martian days. If all goes well, it will make some trips up and down the delta, then travel to Jezero crater’s rim and out onto the ancient plains beyond.

      Perseverance has a sidekick, Ingenuity, the first helicopter to visit another world. The nimble flier, only half a meter tall, succeeded beyond its designers’ wildest dreams. The helicopter made 29 flights in its first 16 months when it was only supposed to make five in one month. It has scouted paths ahead and scientific targets for the rover (SN Online: 4/19/22). Future rovers are almost certain to carry a little buddy like this.

      An engineer at NASA’s Jet Propulsion Laboratory measures light on the Perseverance rover during a 2019 test. The rover landed on Mars last year and has been exploring it ever since.JPL-CALTECH/NASA

      China’s debut

      While the United States has led in Mars rover exploration, it is not the only player on the scene. In May 2021, China became the second nation to successfully place a rover on Mars. Its Zhurong rover, named after a mythological fire god, has been exploring part of a large basin in the planet’s northern hemisphere known as Utopia Planitia.

      The landing site lies near a geologic boundary that may be an ancient Martian shoreline. Compared with the other Mars rover locations, Zhurong’s landing site is billions of years younger, “so we are investigating a different world on Mars,” says Lu Pan, a planetary scientist at the University of Copenhagen who has collaborated with Zhurong scientists.

      In many ways, Zhurong resembles Spirit and Opportunity, in size as well as mobility. It carries cameras, a laser spectrometer for studying rocks and ground-penetrating radar to probe underground soil structures (SN Online: 5/19/21).

      After landing, Zhurong snapped pictures of its rock-strewn surroundings and headed south to explore a variety of geologic terrains, including mysterious cones that could be mud volcanoes and ridges that look like windblown dunes. The rover’s initial findings include that the Martian soil at Utopia Planitia is similar to some desert sands on Earth and that water had been present there perhaps as recently as 700 million years ago.

      In May, mission controllers switched Zhurong into dormant mode for the Martian winter and hope it wakes up at the end of the season, in December. It has already traveled nearly two kilometers across the surface, farther than the meager 100 meters that Sojourner managed. (To be fair, Sojourner had to keep circling its lander because it relied on that lander to communicate with Earth.)

      The China National Space Administration released this image on June 11, 2021 of Zhurong with its landing platform on Mars.CNSA/Handout via Xinhua

      From Sojourner to Zhurong, the Mars rovers show what humankind can accomplish on another planet. Future rovers might include the European Space Agency’s ExoMars, although its 2022 launch was postponed after Russia attacked Ukraine (SN: 3/26/22, p. 6). Europe terminated all research collaborations with Russia after the invasion, including launching ExoMars on a Russian rocket.

      Vasavada remembers his sense of awe at the Curiosity launch in 2011: “Standing there in Florida, watching this rocket blasting off and feeling it in your chest and knowing that there’s this incredibly fragile complex machine hurtling on the end of this rocket.… It just gave me this full impression that here we are, humans, blasting these things off into space,” he says. “We’re little tiny human beings sending these things to another planet.” More

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      in Space & Astronomy

      The Windchime experiment could use gravity to hunt for dark matter ‘wind’

      by

      The secret to directly detecting dark matter might be blowin’ in the wind.

      The mysterious substance continues to elude scientists even though it outweighs visible matter in the universe by about 8 to 1. All laboratory attempts to directly detect dark matter — seen only indirectly by the effect its gravity has on the motions of stars and galaxies — have gone unfulfilled.

      Those attempts have relied on the hope that dark matter has at least some other interaction with ordinary matter in addition to gravity (SN: 10/25/16). But a proposed experiment called Windchime, though decades from being realized, will try something new: It will search for dark matter using the only force it is guaranteed to feel — gravity.

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      “The core idea is extremely simple,” says theoretical physicist Daniel Carney, who described the scheme in May at a meeting of the American Physical Society’s Division of Atomic Molecular and Optical Physics in Orlando, Fla. Like a wind chime on a porch rattling in a breeze, the Windchime detector would try to sense a dark matter “wind” blowing past Earth as the solar system whips around the galaxy.  

      If the Milky Way is mostly a cloud of dark matter, as astronomical measurements suggest, then we should be sailing through it at about 200 kilometers per second. This creates a dark matter wind, for the same reason you feel a wind when you stick your hand out the window of a moving car.

      The Windchime detector is based on the notion that a collection of pendulums will swing in a breeze. In the case of backyard wind chimes, it might be metal rods or dangling bells that jingle in moving air. For the dark matter detector, the pendulums are arrays of minute, ultrasensitive detectors that will be jostled by the gravitational forces they feel from passing bits of dark matter. Instead of air molecules bouncing off metal chimes, the gravitational attraction of the particles that make up the dark matter wind would cause distinctive ripples as it blows through a billion or so sensors in a box measuring about a meter per side.

      Within the Windchime detector (illustrated as an array of small pendulums), a passing dark matter particle (red dot) would gravitationally tug on sensors (blue squares) and cause a detectable ripple, much like wind blowing through a backyard wind chime.D. Carney et al/Physical Review D 2020

      While it may seem logical to search for dark matter using gravity, no one has tried it in the nearly 40 years that scientists have been pursuing dark matter in the lab. That’s because gravity is, comparatively, a very weak force and difficult to isolate in experiments. 

      “You’re looking for dark matter to [cause] a gravitational signal in the sensor,” says Carney, of Lawrence Berkeley National Laboratory in California. “And you just ask . . . could I possibly see this gravitational signal? When you first make the estimate, the answer is no. It’s actually going to be infeasibly difficult.”

      That didn’t stop Carney and a small group of colleagues from exploring the idea anyway in 2020. “Thirty years ago, this would have been totally nuts to propose,” he says. “It’s still kind of nuts, but it’s like borderline insanity.”

      The Windchime Project collaboration has since grown to include 20 physicists. They have a prototype Windchime built of commercial accelerometers and are using it to develop the software and analysis that will lead to the final version of the detector, but it’s a far cry from the ultimate design. Carney estimates that it could take another few decades to develop sensors good enough to measure gravity even from heavy dark matter.

      Carney bases the timeline on the development of the Laser Interferometer Gravitational-Wave Observatory, or LIGO, which was designed to look for gravitational ripples coming from black holes colliding (SN: 2/11/16). When LIGO was first conceived, he says, it was clear that the technology would need to be improved by a hundred million times. Decades of development resulted in an observatory that views the sky in gravitational waves. With Windchime, “we’re in the exact same boat,” he says.

      Even in its final form, Windchime will be sensitive only to dark matter bits that are roughly the mass of a fine speck of dust. That’s enormous on the spectrum of known particles — more than a million trillion times the mass of a proton.

      “There is a variety of very interesting dark matter candidates at [that scale] that are definitely worth looking for … including primordial black holes from the early universe,” says Katherine Freese, a physicist at the University of Michigan in Ann Arbor who is not part of the Windchime collaboration. Black holes slowly evaporate, leaking mass back into space, she notes, which could leave many relics formed shortly after the Big Bang at the mass Windchime could detect.

      But if it never detects anything at all, the experiment still stands out from other dark matter detection schemes, says Dan Hooper, a physicist at Fermilab in Batavia, Ill., also not affiliated with the project. That’s because it would be the first experiment that could entirely rule out some types of dark matter.

      Even if the experiment turns up nothing, Hooper says, “the amazing thing about [Windchime] … is that, independent of anything else you know about dark matter particles, they aren’t in this mass range.” With existing experiments, a failure to detect anything could instead be due to flawed guesses about the forces that affect dark matter (SN: 7/7/22).  

      Windchime will be the only experiment yet imagined where seeing nothing would definitively tell researchers what dark matter isn’t. With a little luck, though, it could uncover a wind of tiny black holes, or even more exotic dark matter bits, blowing past as we careen around the Milky Way. More