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    Earth sweeps up 5,200 tons of extraterrestrial dust each year

    As our planet orbits the sun, it swoops through clouds of extraterrestrial dust — and several thousand metric tons of that material actually reaches Earth’s surface every year, new research suggests.

    During three summers in Antarctica over the past two decades, researchers collected more than 2,000 micrometeorites from three snow pits that they’d dug. Extrapolating from this meager sample to the rest of the world, tiny pebbles from space account for a whopping 5,200 metric tons of weight gain each year, researchers report in the April 15 Earth and Planetary Science Letters.

    Much of Antarctica is the perfect repository for micrometeorites because there’s no liquid water to dissolve or otherwise destroy them, says Jean Duprat, a cosmochemist at Sorbonne University in Paris (SN: 5/29/20). Nevertheless, collecting the samples was no easy chore.

    First, Duprat and colleagues had to dig down two meters or more to reach layers of snow deposited before 1995, the year when researchers set up a field station at an inland site dubbed Dome C. Then they used ultraclean tools to collect hundreds of kilograms of snow, melt it and sieve the tiny treasures from the frigid water.

    To hunt for micrometeorites that have fallen to Antarctica in recent decades, researchers dig trenches (pictured) to collect snow that is later melted and then sieved for the space dust.J. Duprat, C. Engrand, CNRS Photothèque

    In all, the team found 808 spherules that had partially melted as they blazed through Earth’s atmosphere and another 1,280 micrometeorites that showed no such damage. The particles ranged in size from 30 to 350 micrometers across and all together weigh mere fractions of a gram. But the micrometeorites were all found within three areas totaling just a few square meters, the merest fraction of Earth’s surface. Assuming that particles of space dust are just as likely to fall in Antarctica as anywhere else let the team estimate how much dust fell over the entire planet.

    The team’s findings “are a wonderful complement to previous studies,” says Susan Taylor, a geologist at the Cold Regions Research and Engineering Laboratory in Hanover, N.H., who was not involved in the new study. That’s because Duprat and colleagues found a lot of the small stuff that would have dissolved elsewhere, she notes.

    About 80 percent of the micrometeorites originate from comets that spend much of their orbits closer to the sun than Jupiter, the researchers estimate. Much of the rest probably derive from collisions of objects in the asteroid belt. All together, these tiny particles deliver somewhere between 20 and 100 metric tons of carbon to Earth each year, Duprat and colleagues suggest, and could have been an important source of carbon-rich compounds such as amino acids early in Earth’s history (SN: 12/4/20). More

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    A meteor may have exploded over Antarctica 430,000 years ago

    Seventeen tiny particles recovered from a flat-topped mountain in eastern Antarctica suggest that a space rock shattered low in the atmosphere over the ice-smothered continent about 430,000 years ago.

    The nickel- and magnesium-rich bits were sifted from more than 6 kilograms of loose sediments collected atop the 2,500-meter-tall summit of Walnumfjellet, says Matthias van Ginneken, a cosmochemist at the University of Kent in England. Their exotic chemistry doesn’t match Earth rocks, but it does match the proportions of elements seen in a type of meteorite called a carbonaceous chondrite, van Ginneken and his colleagues report March 31 in Science Advances.

    Most of the particles range in size from 0.1 to 0.3 millimeters across, and more than half consist of spherules that are fused together into odd-shaped globs. The elemental mix in the spherules closely matches that of particles found at two other far-flung sites in Antarctica— one more than 2,750 kilometers away — which suggests that all of the materials originated in the same event. Because the other particles were found in ice cores and dated to about 430,000 years ago, the team presumes that the newly found particles from Walnumfjellet fell then too.  

    The chemistry of nickel- and magnesium-rich spherules (pictured) found on a mountaintop in Antarctica match that of a certain type of stony meteorites.Scott Peterson/micro-meteorites.com

    The chemistry of nickel- and magnesium-rich spherules (pictured) found on a mountaintop in Antarctica match that of a certain type of stony meteorites.Scott Peterson/micro-meteorites.com

    The meteor that broke up over Antarctica was between 100 to 150 meters across, the team’s simulations suggest, and probably burst at low altitude. Blast waves may have pummeled a 100,000-square-kilometer area of the ice sheet, the team estimates. The explosion left no crater, but peak temperatures where the plume of hot gases reached Earth’s surface would have hit 5,000° Celsius and may have melted up to a few centimeters of ice. A similar airburst over a densely populated area today would result in millions of casualties and severely damage an area hundreds of kilometers across (SN: 5/2/17). More

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    50 years ago, experiments hinted at the possibility of life on Mars

    Organics on Mars — Science News, March 27, 1971

    [Researchers] have exposed a mixture of gases simulating conditions believed to exist on the surface of Mars to ultraviolet radiation. The reaction produced organic compounds. They conclude that the ultraviolet radiation bombarding the surface of Mars could be producing organic matter on that planet.… The fact that such organic compounds may be produced on the Martian surface increases the possibility of life on Mars.

    Update

    In 1976, a few years after those experiments, NASA took its search for organic molecules to the Red Planet’s surface. That year, the Viking landers became the first U.S. mission to land on Mars. Though the landers failed to turn up evidence in the soil, NASA has continued the hunt. In 2018, the Curiosity rover found hints of life: organic molecules in rocks and seasonal shifts in atmospheric methane. A new phase of the hunt began in February when the Perseverance rover landed on Mars (SN Online: 2/17/21). It will find and store rocks that might preserve signs of past life for eventual return to Earth. More

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    ‘Oumuamua may be a chip knocked off an icy, Pluto-like exoplanet

    Since its discovery, the interstellar object known as ‘Oumuamua has defied explanation. First spotted in 2017, it has been called an asteroid, a comet and an alien spaceship (SN: 10/27/17). But researchers think they finally have the mystery object pegged: It could be a shard of nitrogen ice broken off a Pluto-like planet orbiting another star.

    “The idea is pretty compelling,” says Garrett Levine, an astronomer at Yale University not involved in the work. “It does a really good job of matching the observations.”

    ‘Oumuamua’s origin has been a mystery because it looks sort of like a comet, but not quite (SN: 12/18/17). After whipping by the sun, ‘Oumuamua zoomed away slightly faster than gravity alone would allow. That happens when ices on the sunlit sides of comets vaporize, giving them a little rocketlike boost in speed. But unlike comets, ‘Oumuamua didn’t appear to have a tail from typical cometary ices, such as carbon monoxide or carbon dioxide, streaming off it.

    Alan Jackson and Steven Desch, planetary scientists at Arizona State University in Tempe, set out to discover what other kind of evaporating ice could give ‘Oumuamua a big enough nudge to explain its movement. The pair reported their results March 17 at the virtual Lunar and Planetary Science Conference and in two studies published online March 16 in the Journal of Geophysical Research: Planets.

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    The amount of force that a vaporizing ice exerts on a comet depends on factors such as how much the ice heats up when it absorbs energy, the mass of its molecules and even the ice’s crystal structure. By calculating the rocketlike push on ‘Oumuamua if it were made of ices such as nitrogen, hydrogen and water, “we learned that nitrogen ice would work perfectly well,” Desch says.

    Because nitrogen ice covers outer solar system bodies such as Pluto and Neptune’s moon Triton, but not smaller objects like comets, ‘Oumuamua is probably a chip off a Pluto-like exoplanet, the researchers report.

    To determine how realistic that scenario is, Jackson and Desch calculated how many chunks of nitrogen ice could have been knocked off Pluto-like bodies in the early solar system. Back then, the Kuiper Belt of objects beyond Neptune was much more crowded than it is today, including thousands of Pluto-like bodies iced with nitrogen. But some 4 billion years ago, Neptune’s orbit expanded. That disruption caused many Kuiper Belt objects to collide with each other, and most sailed out of the solar system altogether.

    Under such chaotic conditions, collisions could have broken trillions of nitrogen ice fragments off Pluto-like bodies, Jackson and Desch estimate. If other planetary systems throw out as many shards of ice, those objects could make up about 4 percent of the bodies in interstellar space. That would make seeing an object like ‘Oumuamua mildly unusual but not exceptional, the researchers say.

    “When I first started reading it, I was skeptical … but it does tick a lot of the necessary boxes,” says Scott Sheppard, an astronomer at the Carnegie Institution for Science in Washington, D.C. not involved in the work. “It’s definitely plausible that this could be a fragment of an icy dwarf planet.” But plausible, he notes, does not necessarily mean correct.

    ‘Oumuamua is now too far away to confirm this idea with more observations. But the upcoming Vera Rubin Observatory and European Space Agency’s Comet Interceptor mission could detect more interstellar objects, says Yun Zhang, a planetary scientist at Côte d’Azur Observatory in Nice, France not involved in the research. The Vera Rubin Observatory is expected to spot, on average, one interstellar visitor per year, and the Comet Interceptor spacecraft may actually visit one.

    Getting a closer look at more of these objects could narrow down which possible explanations for ‘Oumuamua are most reasonable, she says (SN: 2/27/19). More

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    Most of Mars’ missing water may lurk in its crust

    An ocean’s worth of water may be lurking in minerals below Mars’ surface, which could help explain why the Red Planet dried up.

    Once home to lakes and rivers, Mars is now a frigid desert (SN: 12/8/14). Scientists have typically blamed that on Mars’ water wafting out of the planet’s atmosphere into space (SN: 11/12/20). But measurements of atmospheric water loss made by spacecraft like NASA’s MAVEN orbiter are not enough to account for all of Mars’ missing water — which was once so abundant it could have covered the whole planet in a sea up to 1,500 meters deep. That’s more than half the volume of the Atlantic Ocean.

    Computer simulations of water moving through Mars’ interior, surface and atmosphere now suggest that most of the Red Planet’s water molecules may have gotten lodged inside the crystal structures of minerals in the planet’s crust, researchers report online March 16 in Science. 

    The finding “helps bring focus to a really important mechanism for water loss on Mars,” says Kirsten Siebach, a planetary geologist at Rice University in Houston who was not involved in the work. “Water getting locked up in crustal minerals may be equally important as water loss to space and could potentially be more important.”

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    Planetary scientist Eva Scheller of Caltech and colleagues simulated possible scenarios for water loss on Mars, based on observations of the Red Planet made by rovers and orbiting spacecraft, and lab analyses of Martian meteorites. These simulations accounted for possible water loss to space and into the planet’s crust through bodies of water or groundwater interacting with rock.

    In order for the simulations to match how much water was on Mars 4 billion years ago, how much is left in polar ice caps today and the observed abundance of hydrogen in Mars’ atmosphere, 30 to 99 percent of Mars’ ancient water must be stashed away inside its crust. The rest was lost to space.

    Judging by modern Martian landscapes, like this image taken by the Curiosity rover at the base of Mount Sharp, the Red Planet appears bone dry. But an entire ocean’s worth of water may be lurking underground, in the minerals of the planet’s crust.MSSS/JPL-Caltech/NASA

    Water gets locked inside minerals on Earth, too, says Scheller, who presented the results March 16 in a news conference at the virtual Lunar and Planetary Science Conference. But unlike on Mars, that underground water is eventually belched back out into the atmosphere by volcanoes. That difference is important for understanding why one rocky planet may be lush and wet and habitable, while another is an arid wasteland. 

    Mars’ underground water could be mined by future explorers, says Jack Mustard, a planetary geologist at Brown University in Providence, R.I., not involved in the work. The most easily accessible water on Mars may be at its polar ice caps (SN: 9/28/20). But “to get the ice, you’ve got to go up to [high latitudes] — kind of cold, harder to live there,” Mustard says. If water can be extracted from minerals, it could support human colonies at warmer climes closer to the equator.  More

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    Watch real video of Perseverance’s Mars landing

    This is what it looks like to land on Mars.
    NASA’s Perseverance rover took this video on February 18 as a jetpack lowered it onto the Red Planet’s surface.
    “It gives me goosebumps every time I see it,” said engineer David Gruel of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., at a news briefing on February 22.
    The movie begins with the rover’s parachute opening above it as the rover and its landing gear enter the Martian atmosphere. Seconds later, a camera on the rover’s underside shows the heat shield falling toward the ground. If you look carefully, you can see one of the springs that pushed the heat shield off the rover came loose, said NASA engineer Allen Chen, the rover’s entry, descent and landing lead.
    [embedded content]
    NASA’s Perseverance rover captured video of its own landing using a set of cameras on the back of the entry vehicle, the sky crane and the rover itself.
    “There’s no danger to the spacecraft here, but it’s something we didn’t expect, and wouldn’t have seen” without the videos, he said.
    The rover filmed the ground coming closer and closer, getting glimpses of a river delta, craters, ripples and fractured terrain. Cameras on the top and bottom of the rover captured clouds of dust billowing as the rover’s jetpack, the sky crane, lowered it down to the ground on three cables. A camera on the sky crane showed the rover swinging slightly as it descended. Finally, the sky crane disconnected the cables and flew away, leaving Perseverance to begin its mission.
    “It’s hard to express how emotional it was and how exciting it was to everybody” to see the movie for the first time, said deputy project manager Matt Wallace. “Every time we got something, people were overjoyed, giddy. They were like kids in a candy store.”
    The movie looks so much like animations of the sky crane landing technique that NASA had released in the past that it almost doesn’t look real, says imaging scientist Justin Maki. “I can attest to, it’s real,” he says. “It’s stunning and it’s real.”

    The rover also captured audio from the surface of the Red Planet for the first time, including a gust of Martian wind.
    Perseverance landed in an ancient lakebed called Jezero crater, about two kilometers from what looks like an ancient river delta feeding into the crater (SN: 2/18/21). The rover’s primary mission is to search for signs of past life and to cache rock samples for a future mission to return to Earth.
    The first images Perseverance sent back from Mars showed its wheels on a flat expanse. The ground is strewn with rocks that are shot through with holes, said deputy project scientist Katie Stack Morgan in a news briefing on February 19.
    “Depending on the origins of the rocks, these holes could mean different things,” she said. The science team thinks the holes could be from gases escaping volcanic rock as lava cooled, or from fluid moving through the rock and dissolving it away. “Both would be equally exciting for the team.”

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    Here’s how to watch NASA’s Perseverance lander touch down on Mars

    All eyes are on Mars — and all ears, too. When NASA’s Perseverance rover touches down on the Red Planet on February 18, the landing will be recorded with sight, sound and maybe even touch.
    The rover will cap off a month of Mars arrivals from space agencies around the world (SN: 7/30/20). Perseverance joins Hope, the first interplanetary mission from the United Arab Emirates, which successfully entered Mars orbit on February 9; and Tianwen-1, China’s first Mars mission, which arrived on February 10 and will deploy a rover to the Martian surface in May.
    NASA will broadcast Perseverance’s landing on YouTube starting at 2:15 p.m. EST. The actual moment of touchdown is expected at approximately 3:55 p.m. EST. Perseverance is designed to explore an ancient river delta called Jezero crater, searching for signs of ancient life and collecting rocks for a future mission to return to Earth (SN: 7/28/20).
    The rover will use the landing system pioneered by its predecessor, Curiosity, which has been exploring Mars since 2012 (SN: 8/6/12). But in a first for Mars touchdowns, this rover will record its own landing with dedicated cameras and a microphone.
    As the craft carrying Perseverance zooms through the thin Martian atmosphere, three cameras will look up at the parachute slowing it down from supersonic speeds. When a rocket-powered “sky crane” platform lowers the rover to the ground, a fourth camera on the platform will record the rover’s descent. Another camera on the rover will look back up at the platform, and a sixth camera will look at the ground.
    Perseverance will use the “sky crane” landing system pioneered by its predecessor, Curiosity. The landing involves dangling the rover from a floating platform on cables and touching down directly on its wheels.JPL-Caltech/NASA
    Perseverance will use the “sky crane” landing system pioneered by its predecessor, Curiosity. The landing involves dangling the rover from a floating platform on cables and touching down directly on its wheels.JPL-Caltech/NASA
    “The goal is to see the video and the action of getting from high up in the atmosphere down to the surface,” says engineer David Gruel of NASA’s Jet Propulsion Laboratory in Pasadena, who was the engineering lead for that six-camera system, called EDL-Cam. He hopes every engineer on the team has an image of the rover hanging below the descent stage as their computer desktop background six months from now.
    Because it will take more than 11 minutes for signals to travel between Earth and Mars, the cameras won’t stream the landing movie in real time. And after Perseverance lands, engineers will be focused on making sure the rover is healthy and able to collect science data, so the landing videos won’t be among the first data sent back. Gruel expects to be able to share what the rover saw four days after landing, on February 22.

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    Perseverance will also carry microphones to record first-ever audio of a Mars landing. Unlike the landing cameras, the microphones will continue to work after touchdown, hopefully helping the engineering team keep track of the rover’s health. Motors sound different when they get clogged with dust, for instance, Gruel says. The team will hear the sound of the rover’s wheels crunching across the Martian surface, and maybe the sound of the wind blowing.
    “Are we going to hear a dust devil? What might a dust devil sound like? Could we hear rocks rolling down a hill?” Gruel asks. “You never know what we might stumble onto.”
    Sound will add a way to share Mars with people who have trouble seeing, Gruel notes. “It might appeal to a whole other element of the population who might not have been able to experience past missions the same way,” he says.
    [embedded content]
    Watch NASA’s live coverage of the Perseverance landing here starting at 2:15 p.m. EST.
    Elsewhere on Mars, the InSight lander will be listening to the landing too (SN: 2/24/20). The lander’s seismometer may be able to feel vibrations when two tungsten weights that Perseverance carried to Mars for stability smack into the ground before the rover lands, geophysicist Benjamin Fernando of the University of Oxford and colleagues report in a paper posted December 3 to eartharxiv.org and submitted to JGR Planets.
    “No one’s ever tried to do this before,” Fernando says.
    The ground will move by at most 0.1 nanometers per second, Fernando and colleagues calculated. “It’s incredibly small,” he says. “But the seismometer is also incredibly sensitive.”
    The team may be able to catch that tiny signal because they know exactly when and where the impact will happen. If the lander does pick up the signal, it will tell scientists something about how fast seismic waves travel through the ground, a clue to the details of Mars’ interior structure. And even if they don’t feel anything, that will put limits on the waves’ speed. “It still teaches us something,” Fernando says.
    The InSight team hopes to also feel vibrations from Tianwen-1 when its rover touches down in May. “Detecting one would be great,” Fernando says. “Detecting two would be like, amazing.” More

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    Fossil mimics may be more common in ancient rocks than actual fossils

    When it comes to finding fossils of very ancient microbial life — whether on Earth or on other worlds, such as Mars — the odds are just not in our favor.
    Actual microbial life-forms are much less likely to become safely fossilized in rocks compared with nonbiological structures that happen to mimic their shapes, new research finds. The finding suggests that Earth’s earliest rocks may contain abundant tiny fakers — minuscule objects masquerading as fossilized evidence of early life — researchers report online January 28 in Geology.
    The finding is “at the very least a cautionary tale,” says study author Julie Cosmidis, a geomicrobiologist at the University of Oxford.
    Tiny, often enigmatic structures found in some of Earth’s oldest rocks, dating back to more than 2.5 billion years, can offer tantalizing hints of the planet’s earliest life. And the hunt for ever-more-ancient signs of life on Earth has sparked intense debate — in part because the farther back in time you go, the harder it is to interpret tiny squiggles, filaments and spheres in the rock (SN: 1/3/20). One reason is that the movements of Earth’s tectonic plates over time can squeeze and cook the rocks, deforming and chemically altering tiny fossils, perhaps beyond recognition.

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    But an even more pernicious and contentious problem is that such tiny filaments or spheres may not be biological in origin at all. Increasingly, scientists have found that nonbiological chemical processes can create similar shapes, suggesting the possibility of “false positives” in the biological record.
    One such discovery led to the new study, Cosmidis says. A few years ago, she and others were trying to grow bacteria and make them produce sulfur. “We were mixing sulfides with organic matter, and we started forming these objects,” she says. “We thought they were formed by the bacteria, because they looked so biological. But then we realized they were forming in laboratory tubes that happened to have no bacteria in them at all.”
    That led her to wonder about such processes happening in the rocks themselves. So she and others decided to examine what would happen if they tried to re-create the early formation stages of chert, a kind of compact, silica-rich rock common on the early Earth. “Microfossils are often found in chert formations,” says study coauthor Christine Nims, a geobiologist now at the University of Michigan in Ann Arbor. “Anything hosted in [chert] will be well-preserved.”
    Chert forms out of silica-rich water; the silica precipitates out of the water and accumulates, eventually hardening into rock. Cosmidis, Nims and colleagues added sulfur-containing bacteria called Thiothrix to solidifying chert to see what might happen during actual fossilization. To other chert samples, they added sulfur-containing “biomorphs,” spheres and filaments made of tiny crystals but shaped like bacteria.
    At first, nanoparticles of silica encrusted the bacteria and the biomorphs, Nims says. But after a week or so, the bacteria started to deform, their cells deflating from cylinders into flattened, unrecognizable ribbons as the sulfur inside the cells diffused out and reacted with the silica outside the cells, forming new minerals.
    The biomorphs, on the other hand, “had this impressive resiliency,” she says. Although they, too, lost sulfur to the surrounding solution, they kept their silica crust. As a result, “they kept their shape and showed very little change over time.” That endurance suggests that enigmatic structures found in the early rock record have a better chance of being pseudofossils, rather than actual fossils, the team says.
    In a new study, researchers produced twisted filament-shaped biomorphs (top) from the reactions of sulfide with prebiotic organic compounds. The biomorphs resemble possible microbial fossils (bottom, filaments indicated by red arrows) found in rocks dating to 3.5 billion years ago.From top: C. Nims; R.J. Baumgartner et al/Geology 2019
    The idea that once-living creatures are harder to preserve makes sense, says Sean McMahon, an astrobiologist at the University of Edinburgh who was not involved in the new study. “It’s not totally surprising,” he says. “We know that biomass does tend to break down quite quickly.”
    In fact, scientists have known for centuries that certain chemical reactions can act as “gardens” that “grow” strange-shaped mineral objects, twisting into tubes or sprouting branches or otherwise mimicking the weirdness of life. “There’s a complacency about it, a misconception that we kind of know all this and it’s already been dealt with,” McMahon says.
    Strategies to deal with this conundrum have included looking for particular structures — such as mound-shaped stromatolites — or chemical compounds in a potential fossil that are thought to be uniquely formed or modified by the presence of life (SN: 10/17/18). Those criteria are the product of decades of field studies, through which scientists have amassed a vast reference dataset of fossil structures, against which researchers can compare and evaluate any new discoveries.
    “Anything we find, we can look at through that lens,” McMahon says. But what’s lacking is a similarly rich dataset for how such structures might form in the absence of life. This study, he says, highlights that attempts “to define criteria for recognizing true fossils in very ancient rocks are premature, because we don’t yet know enough about how nonbiological processes mimic true fossils.”
    It’s an increasingly urgent problem with rising stakes, as NASA’s Perseverance rover is about to set down on Mars to begin a new search for traces of life in ancient rocks (SN: 7/28/20), he adds. “Paleontologists and Mars exploration scientists should take [this study] very seriously.” More