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    Organic molecules in an ancient Mars meteorite formed via geology, not alien life

    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

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    Oxygen-rich exoplanets may be geologically active

    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

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    Enceladus’ plumes might not come from an underground ocean

    Saturn’s icy moon Enceladus sprays water vapor into space. Scientists have thought that the plumes come from a deep subsurface ocean — but that might not be the case, new simulations suggest.

    Instead, the water could come from pockets of watery mush in the moon’s icy shell, scientists report December 15 at the American Geophysical Union’s fall meeting.

    “Maybe we didn’t get the straw all the way through the ice shell to the ocean. Maybe we’re just getting this weird pocket,” says planetary scientist Jacob Buffo of Dartmouth College.

    The finding is “a cautionary tale,” Buffo says. The hidden ocean makes Enceladus one of the best places to search for life in the solar system (SN: 4/8/20). Concepts for future missions to Enceladus rely on the idea that taking samples of the plumes would directly test the contents of the ocean, without needing to drill or melt through the ice. “That could be true,” Buffo says. But the simulations suggest “you could be sampling this slushy region in the middle of the shell, and that might not be the same chemistry as is down in the ocean.”

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    Enceladus has beguiled planetary scientists since NASA’s Cassini spacecraft revealed the moon’s dramatic plumes in 2005 (SN: 8/23/05). At the time, researchers wondered if the spray originated on Enceladus’ icy surface, where friction from quakes could melt ice and let it escape as pure water vapor into space. But later evidence collected by Cassini convinced most scientists that the geysers are from fractures in the shell that reach all the way to a salty, subsurface sea (SN: 8/4/14).

    One of the most convincing pieces of evidence was the fact that the plumes contain salts, said physicist Colin Meyer of Dartmouth in a talk at the meeting, which was held virtually and in New Orleans. Early versions of the quake idea couldn’t account for those salts, and instead suggested that any salts in the melted ice would be left on the surface as the water escaped into space, like the sheen of salt left on your skin after you sweat, he says.

    But Meyer, who has studied the physics of sea ice on Earth, realized that pockets of meltwater in the ice shell could concentrate salts and other compounds. He, Buffo and colleagues applied computer simulations developed for sea ice on Earth to the observed icy conditions on Enceladus. The team found that Enceladus could easily generate pockets of mush within its shell and vent the contents of that mush out into space, salts and all.

    That does not mean Enceladus doesn’t have an ocean, Meyer says — it almost certainly does. And it does not mean the ocean isn’t habitable, Buffo adds.

    The implications of the results “are huge,” especially for proposed life-finding missions to Enceladus, says planetary scientist Emily Martin of the Smithsonian National Air and Space Museum in Washington, D.C., who was not involved in the work.

    “If those plumes aren’t tapping into the ocean, it will really shift our perspective on what that plume is telling us about the interior of Enceladus,” Martin says. “And that’s a big deal.” More

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    A space rock called Kamoʻoalewa may be a piece of the moon

    The moon’s violent history is written across its face. Over billions of years, space rocks have punched craters into its surface, flinging out debris. Now, for the first time, astronomers may have spotted rubble from one of those ancient smashups out in space. The mysterious object known as Kamoʻoalewa appears to be a stray fragment of the moon, researchers report online November 11 in Communications Earth & Environment.

    Discovered in 2016, Kamoʻoalewa — also known as 2016 HO3 — is one of Earth’s five known quasisatellites (SN: 6/24/16). These are rocks that stick fairly close to the planet as they orbit the sun. Little is known about Earth’s space rock entourage because these objects are so small and faint. Kamoʻoalewa, for instance, is about the size of a Ferris wheel and strays between 40 and 100 times as far from Earth as the moon, as its orbit around the sun weaves in and out of Earth’s. That has left astronomers to wonder about the nature of such tagalong rocks.

    “An object in a quasisatellite orbit is interesting because it’s very difficult to get into this kind of orbit — it’s not the kind of orbit that an object from the asteroid belt could easily find itself caught in,” says Richard Binzel, a planetary scientist at MIT not involved in the new work. Having an orbit nearly identical to Earth’s immediately raises suspicions that an object like Kamoʻoalewa originated in the Earth-moon system, he says.

    Researchers used the Large Binocular Telescope and the Lowell Discovery Telescope, in Safford and Happy Jack, Ariz., respectively, to peer at Kamoʻoalewa in visible and near-infrared wavelengths. “The real money is in the infrared,” says Vishnu Reddy, a planetary scientist at the University of Arizona in Tucson. Light at those wavelengths contains important clues about the minerals in rocky bodies, helping distinguish objects such as the moon, asteroids and terrestrial planets.

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    Kamoʻoalewa reflected more sunlight at longer, or redder, wavelengths. This pattern of light, or spectrum, looked unlike any known near-Earth asteroid, Reddy and colleagues found. But it did look like grains of silicate rock from the moon brought back to Earth by Apollo 14 astronauts (SN: 2/20/71).

    “To me,” Binzel says, “the leading hypothesis is that it’s an ejected fragment from the moon, from a cratering event.”

    Martin Connors, who was involved in the discovery of Earth’s first known quasisatellites but did not participate in the new research, also suspects that Kamoʻoalewa is a chip off the old moon. “This is well-founded evidence,” says Connors, a planetary scientist at Athabasca University in Canada. But, he cautions, “that doesn’t mean it’s right.”

    More detailed observations could help confirm Kamoʻoalewa is made of moon stuff. “If you really wanted to put that nail in the coffin, you’d want to go and visit, or rendezvous with this little quasisatellite and take a lot of up-close observations,” says Daniel Scheeres, a planetary scientist at the University of Colorado Boulder not involved in the work. “The best would be to get a sample.”

    China’s space agency has announced plans to send a probe to Kamoʻoalewa to scoop up a bit of rock and bring it back to Earth later this decade. More

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    Pluto’s dark side reveals clues to its atmosphere and frost cycles

    Pluto’s dark side has come into dim view, thanks to the light of the dwarf planet’s moon.

    When NASA’s New Horizons spacecraft flew past Pluto in 2015, almost all the images of the dwarf planet’s unexpectedly complex surface were of the side illuminated by the sun (SN: 7/15/15). Darkness shrouded the dwarf planet’s other hemisphere. Some of it, like the area near the south pole, hadn’t seen the sun for decades.

    Now, mission scientists have finally released a grainy view of the dwarf planet’s dark side. The researchers describe the process to take the photo and what it tells them about how Pluto’s nitrogen cycle affects its atmosphere October 20 in the Planetary Science Journal.

    Before New Horizons passed by Pluto, the team suspected the dwarf planet’s largest moon, Charon, might reflect enough light to illuminate the distant world’s surface. So the researchers had the spacecraft turn back toward the sun to take a parting peek at Pluto.

    New Horizons captured this view of the backlit dark side of Pluto as the spacecraft receded from the dwarf planet in 2015. Some light and dark splotches were illuminated by the dim light of Pluto’s moon Charon.NOIRLab, SwRI, JHUAPL, NASA

    At first, the images just showed a ring of sunlight filtering through Pluto’s hazy atmosphere (SN: 7/24/15). “It’s very hard to see anything in that glare,” says planetary scientist John Spencer of the Southwest Research Institute in Boulder, Colo. “It’s like trying to read a street sign when you’re driving toward the setting sun and you have a dirty windshield.”

    Spencer and colleagues took a few steps to make it possible to pull details of Pluto’s dark side out of the glare. First, the team had the spacecraft take 360 short snapshots of the backlit dwarf planet. Each was about 0.4 seconds long, to avoid overexposing the images. The team also took snapshots of the sun without Pluto in the frame so that the sun could be subtracted out after the fact.

    Tod Lauer of the National Optical Astronomy Observatory in Tucson, Ariz., tried to process the images when he got the data in 2016. At the time, the rest of the data from New Horizons was still fresh and took up most of his attention, so he didn’t have the time to tackle such a tricky project.

    But “it was something that just sat there and ate away at me,” Lauer says. He tried again in 2019. Because the spacecraft was moving as it took the images, each image was a little bit smeared or blurred. Lauer wrote a computer code to remove that blur from each individual frame. Then he added the reflected Charon light in each of those hundreds of images together to produce a single image.

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    “When Tod did that painstaking analysis, we finally saw something emerging in the dark there … giving us a little bit of a glimpse of what the dark pole of Pluto looks like,” Spencer says.

    That the team got anything at all is impressive, says planetary scientist Carly Howett, also of the Southwest Research Institute and who is on the New Horizons team but was not involved in this work. “This dataset is really, really hard to work with,” she says. “Kudos to this team. I wouldn’t have wanted to do this.”

    The image, Howett says, can help scientists understand how Pluto’s frigid nitrogen atmosphere varies with its decades-long seasons. Pluto’s atmosphere is controlled by how much nitrogen is in a gas phase in the air and how much is frozen on the surface. The more nitrogen ice that evaporates, the thicker the atmosphere becomes. If too much nitrogen freezes to the ground, the atmosphere could collapse altogether.

    In this image of Pluto’s sunlit side from NASA’s New Horizons spacecraft, different colors represent different kinds of ices. A faint glimpse of Pluto’s dark hemisphere in newly released mission images reveals some new details about how those ices behave.SwRI, JHUAPL, NASA

    When New Horizons was there, Pluto’s south pole looked darker than the north pole. That suggests there was not a lot of fresh nitrogen frost freezing out of the atmosphere there, even though it was nearing winter. “The previous summer ended decades ago, but Pluto cools off pretty slowly,” Spencer says. “Maybe it’s still so warm [that] the frost can’t condense there, and that keeps the atmosphere from collapsing.”

    There was a bright spot in the middle of the image, which could be a fresh ice deposit. That’s also not surprising, Howett says. The ices may still be moving from the north pole to the south pole as Pluto moves deeper into its wintertime.

    “We’ve thought this for a long time. It makes sense,” she says. “But it’s nice to see it happening.” More

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    What the Perseverance rover’s quiet landing reveals about meteor strikes on Mars

    The lander was listening. On February 18, NASA’s InSight lander on Mars turned its attention to the landing site for another mission, Perseverance, hoping to detect its arrival on the planet.

    But InSight heard nothing.

    Tungsten blocks ejected by Perseverance during entry landed hard enough to create craters on the Martian surface. Collisions like these — whether from space missions or meteor strikes — send shock waves through the ground. Yet in the first experiment of its kind on another world, InSight failed to pick up any seismic waves from the blocks’ impacts, researchers report October 28 in Nature Communications.

    As a result, researchers think that less than 3 percent of the energy from the impacts made its way into the Martian surface. The intensity of impact-generated rumblings varies from planet to planet and is “really important for understanding how the ground will change from a big impact event,” says Ben Fernando, a geophysicist at the University of Oxford.

    Perseverance left behind several craters (one indicated with the arrow) after pieces of the mission disengaged as planned during entry, creating a rare opportunity to see how Mars absorbs energy from impacts. Univ. of Arizona, JPL-Caltech/NASA

    But getting these measurements is tricky. Scientists need sensitive instruments placed relatively near an impact site. Knowing when and where a meteor will strike is nearly impossible, especially on another world.

    Enter Perseverance: a hurtling space object set to hit Mars at an exact time and place (SN: 2/17/21). To help with its entry, Perseverance dropped about 78 kilograms of tungsten as the rover landed about 3,450 kilometers from InSight. The timing and weight of the drop provided a “once-in-a-mission opportunity” to study the immediate seismic effects of an impact from space, Fernando says.

    The team had no idea whether InSight would be able to detect the blocks’ impacts or not, but the quiet arrival speaks volumes. “It lets us put an upper limit on how much energy from the tungsten blocks turned into seismic energy,” Fernando says. “We’ve never been able to get that number for Mars before.”

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    5 cool things to know about NASA’s Lucy mission to the Trojan asteroids

    For the first time, a spacecraft is headed to Jupiter’s odd Trojan asteroids. What Lucy finds there could provide a fresh peek into the history of the solar system.

    “Lucy will profoundly change our understanding of planetary evolution in our solar system,” Adriana Ocampo, a planetary scientist at NASA Headquarters in Washington, D.C., said at a news briefing October 14.

    The mission is set to launch from the Kennedy Space Center at Cape Canaveral, Fla., as early as October 16. Live coverage will air on NASA TV beginning at 5 a.m. EDT, in anticipation of a 5:34 a.m. blast off.

    The Trojan asteroids are two groups of space rocks that are gravitationally trapped in the same orbit as Jupiter around the sun. One group of Trojans orbits ahead of Jupiter; the other follows the gas giant around the sun. Planetary scientists think the Trojans could have formed at different distances from the sun before getting mixed together in their current homes. The asteroids could also be some of the oldest and most pristine objects in the solar system.

    The mission will mark several other firsts, from the types of objects it will visit to the way it powers its instruments. Here are five cool things to know about our first visit to the Trojans.

    1. The Trojan asteroids are a solar system time capsule.

    The Trojans occupy spots known as Lagrangian points, where the gravity from the sun and from Jupiter effectively cancel each other out. That means their orbits are stable for billions of years.

    “They were probably placed in their orbits by the final gasp of the planet formation process,” the mission’s principal investigator Hal Levison, a planetary scientist at Southwest Research Institute in Boulder, Colo.,  said September 28 in a news briefing.

    But that doesn’t mean the asteroids are all alike. Scientists can tell from Earth that some Trojans are gray and some are red, indicating that they might have formed in different places before settling in their current orbits. Maybe the gray ones formed closer to the sun, and the red ones formed farther from the sun, Levison speculated.

    Studying the Trojans’ similarities and differences can help planetary scientists tease out whether and when the giant planets moved around before settling into their present positions (SN: 4/20/12). “This is telling us something really fundamental about the formation of the solar system,” Levison said.

    2. The spacecraft will visit more individual objects than any other single spacecraft. 

    Lucy will visit eight asteroids, including their moons. Over its 12-year mission, it will visit one asteroid in the main asteroid belt between Mars and Jupiter, and seven Trojans, two of which are binary systems where a pair of asteroids orbit each other.

    “We are going to be visiting the most asteroids ever with one mission,” planetary scientist Cathy Olkin, Lucy’s deputy principal investigator, said in the Oct. 14 briefing.

    The spacecraft will observe the asteroids’ composition, shape, gravity and geology for clues to where they formed and how they got to the Lagrangian points.

    The spacecraft’s first destination, in April 2025, will be an asteroid in the main belt. Next, it will visit five asteroids in the group of Trojans that orbit the sun ahead of Jupiter: Eurybates and its satellite Queta in August 2027; Polymele in September 2027; Leucus in April 2028; and Orus in November 2028. Finally, the spacecraft will shift to Jupiter’s other side and visit the twin asteroids Patroclus and Menoetius in the trailing group of space rocks in March 2033.

    The spacecraft won’t land on any of its targets, but it will swoop within 965 kilometers of their surfaces at speeds of 3 to 5 meters per second relative to the asteroids’ speed through space.

    There’s no need to worry about collisions while zipping through these asteroid clusters, Levison said. Although there are about 7,000 known Trojans, they’re very far apart. “If you were standing on any one of our targets, you wouldn’t be able to tell you were part of the swarm,” he said.

    The Trojan asteroids trail and follow Jupiter in its orbit around the sun, but they’re actually quite far from the giant planet. In fact, Earth is closer to Jupiter than either swarm of Trojans is.NASA, adapted by T. TibbittsThe Trojan asteroids trail and follow Jupiter in its orbit around the sun, but they’re actually quite far from the giant planet. In fact, Earth is closer to Jupiter than either swarm of Trojans is.NASA, adapted by T. Tibbitts

    3. Lucy will have a weird flight path.

    In order to make so many stops, Lucy will need to take a complex path. First, the spacecraft will swoop past Earth twice to get a gravitational boost from our planet that will help propel it onward to its first asteroid.

    The closest Earth flyby, in October 2022, will take it within 300 kilometers of the planet’s surface, closer than the International Space Station, the Hubble Space Telescope and many satellites, Olkin said. Observers on Earth might even be able to see it. “I’m hoping to go near where it flies past and look up and see Lucy flying by a year from now,” she said.

    Then in December 2030, after more than a year exploring the “leading” swarm of Trojans, Lucy will come back to the vicinity of Earth for one more boost. That final gravitational slingshot will send the spacecraft to the other side of the sun to visit the “trailing” swarm. This will make Lucy the first spacecraft ever to venture to the outer solar system and come back near Earth again.

    4. Lucy will travel farther from the sun than any other solar-powered craft.

    Another record Lucy will break has to do with its power source: the sun. Lucy will run on solar power out to 850 million kilometers away from the sun, making it the farthest-flung solar powered spacecraft ever.

    To accomplish that, Lucy has a pair of enormous solar arrays. Each 10-sided array is more than 7.3 meters across and includes about 4,000 solar cells per panel, Lucy project manager Donya Douglas-Bradshaw said in a news briefing on October 13. Standing on one end, Lucy and its solar panels would be as tall as a five-story building.

    “It’s a very intricate, sophisticated design,” she said. The advantage of using solar power is that the team can adjust how much power the spacecraft needs based on how far from the sun it is.

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    5. The inspiration for Lucy’s name is decidedly earthbound.

    NASA missions are often named for famous scientists, or with acronyms that describe what the mission will do. Lucy, on the other hand, is named after a fossil.

    The idea that the Trojans hold secrets to the history of the solar system is part of how the mission got its unusual name. To understand, go back to 1974, when paleoanthropologist Donald Johanson and a graduate student discovered a fossil of a human ancestor who had lived 3.2 million years ago. After listening to the Beatles song “Lucy in the Sky with Diamonds” at camp that night, Johanson’s team named the fossil hominid “Lucy.” (In a poetic echo, the first asteroid the Lucy spacecraft will visit is named Donaldjohanson.)

    Planetary scientists hope the study of the Trojans will revolutionize our understanding of the solar system’s history in the same way that studying Lucy’s fossil revolutionized our understanding of human history.

    “We think these asteroids are fossils of solar system formation,” Levison said. So his team named the spacecraft after the fossil. 

    The spacecraft even carries a diamond in one of its instruments, to help split beams of light. Said planetary scientist Phil Christensen of Arizona State University in Tempe at the Oct. 14 briefing: “We truly are sending a diamond into the sky with Lucy.” More

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    Earth is reflecting less light. It’s not clear if that’s a trend

    The amount of sunlight that Earth reflects back into space — measured by the dim glow seen on the dark portions of a crescent moon’s face — has decreased measurably in recent years. Whether the decline in earthshine is a short-term blip or yet another ominous sign for Earth’s climate is up in the air, scientists suggest.

    Our planet, on average, typically reflects about 30 percent of the sunlight that shines on it. But a new analysis bolsters previous studies suggesting that Earth’s reflectance has been declining in recent years, says Philip Goode, an astrophysicist at Big Bear Solar Observatory in California. From 1998 to 2017, Earth’s reflectance declined about 0.5 percent, the team reported in the Sept. 8 Geophysical Research Letters.

    Using ground-based instruments at Big Bear, Goode and his colleagues measured earthshine — the light that reflects off our planet, to the moon and then back to Earth — from 1998 to 2017. Because earthshine is most easily gauged when the moon is a slim crescent and the weather is clear, the team collected a mere 801 data points during those 20 years, Goode and his colleagues report.

    Much of the decrease in reflectance occurred during the last three years of the two-decade period the team studied, Goode says. Previous analyses of satellite data, he and his colleagues note, hint that the drop in reflectance stems from warmer temperatures along the Pacific coasts of North and South America, which in turn reduced low-altitude cloud cover and exposed the underlying, much darker and less reflective seas.

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    “Whether or not this is a long-term trend [in Earth’s reflectance] is yet to be seen,” says Edward Schwieterman, a planetary scientist at University of California, Riverside, who was not involved in the new analysis. “This strengthens the argument for collecting more data,” he says.

    Decreased cloudiness over the eastern Pacific isn’t the only thing trimming Earth’s reflectance, or albedo, says Shiv Priyam Raghuraman, an atmospheric scientist at Princeton University. Many studies point to a long-term decline in sea ice (especially in the Arctic), ice on land, and tiny pollutants called aerosols — all of which scatter sunlight back into space to cool Earth.

    With ice cover declining, Earth is absorbing more radiation. The extra radiation absorbed by Earth in recent decades goes toward warming the oceans and melting more ice, which can contribute to even more warming via a vicious feedback loop, says Schwieterman.

    Altogether, Goode and his colleagues estimate, the decline in Earth’s reflectance from 1998 to 2017 means that each square meter of our planet’s surface is absorbing, on average, an extra 0.5 watts of energy. For comparison, the researchers note in their study, planet-warming greenhouse gases and other human activity over the same period boosted energy input to Earth’s surface by an estimated 0.6 watts of energy per square meter. That means the decline in Earth’s reflectance has, over that 20-year period, almost doubled the warming effect our planet experienced. More