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  • richardmitnick 12:50 pm on May 28, 2019 Permalink | Reply
    Tags: "The Message of Really, “Kimberley” formation of Gale Crater on Mars taken by NASA’s Curiosity rover, Ethiopia’s Danakil Depression, Extremophiles, , Really Extreme Life"   

    From Many Worlds: “The Message of Really, Really Extreme Life” 

    NASA NExSS bloc

    NASA NExSS

    Many Words icon

    From Many Worlds

    May 28, 2019
    Marc Kaufman

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    Hydrothermal system at Ethiopia’s Danakil Depression, where uniquely extreme life has been found in salt chimneys and surrounding water. The yellow deposits are a variety of sulphates and the red areas are deposits of iron oxides. Copper salts color the water green. (Felipe Gomez/Europlanet 2020 RI)

    Ethiopia’s Dallol volcano and hot springs have created an environment about as hostile to life as can be imagined.

    Temperatures in the supersaturated water reach more than 200 degrees F (94 C) and are reported to approach pure acidity, with an extraordinarily low pH of 0.25. The environment is also highly salty, with salt chimneys common.

    Yet researchers have just reported finding ultra-small bacteria living in one of the acidic, super-hot salt chimneys. The bacteria are tiny — up to 20 times smaller than the average bacteria — but they are alive and in their own way thriving.

    In the world of extremophiles, these nanohaloarchaeles order bacteria are certainly on the very edge of comprehension. But much the same can be said of organisms that can withstand massive doses of radiation, that survive deep below the Earth’s surface with no hint of life support from the sun and its creations, that keep alive deep in glacier ice and even floating high in the atmosphere. And as we know, spacecraft have to be well sterilized because bacteria (in hibernation) aboard can survive the trip to the moon or Mars.

    Not life it is generally understood. But the myriad extremophiles found around the globe in recent decades have brought home the reality that we really don’t know where and how life can survive; indeed, these extremophiles often need their conditions to be super-severe to succeed.

    And that’s what makes them so important for the search for life beyond Earth. They are proof of concept that some life may well need planetary and atmospheric conditions that would have been considered utterly uninhabitable not long ago.

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    Montage from the Dallol site: (A) the sampling site, (B) the small chimneys (temperature of water 90 ºC. (C) D9 sample from a small chimney in (A). (D-L) Scanning Electron Microscope and (M-O) Scanning Transmission Electron Microscope images of sample D9 showing the morphologies of ultra-small microorganisms entombed in the mineral layers. (Gomez et al/Europlanet 2020 Research Infrastructure)

    The unusual and extreme life and geochemistry of Dallol has been studied by a team led by Felipe Gómez from Astrobiology Center in Spain.

    The samples were collected during a field trip to the Dallol volcano and the Danakil Depression in northern Ethiopia in January 2017, which was funded by the Europlanet 2020 Research Infrastructure (RI). The results were published this week in the journal, Scientific Reports.

    The area is consistently one of the hottest in the world, both because of its near-equatorial location, the Dallol volcano and hot springs, and that much of it is below sea level.

    Its psychedelic appearance comes from the condensation of superheated water saturated with various salts, including silver chloride, zinc iron sulphide, manganese dioxide and normal rock-salt.

    The team collected samples of the thin layers of salt deposits from the wall of a yellow chimney stack and a bluish pool of water surrounding the outcrop (above.)

    The samples were brought in sterile, sealed vials to state-of-the-art facilities in Spain, where they were analyzed using a range of techniques, including electron microscopy, chemical analysis and DNA sequencing.

    The team identified tiny, spherical structures within the salt samples that had a high carbon content, demonstrating an unambiguously biological origin.

    “This is an exotic, multi-extreme environment, with organisms that need to love high temperature, high salt content and very low pH in order to survive,” Gómez said. And love it they do, raising the most interesting question of whether they adapted to the conditions or emerged from them.

    Just last month, the same international team published a review in the journal Astrobiology describing the close parallels between the Dallol area and the hydrothermal environments found on Mars — including the Gusev Crater, where NASA’s Spirit Mars Exploration Rover landed in .

    As is the norm in the effort to understand life in extreme conditions and astrobiology generally, they focused on the geology and geochemistry of the site that gave rise to the extreme life.

    “The physical and compositional features of the Dallol deposits, their mineralogies, sedimentary and alteration features, and their location in a region of basaltic volcanism of planetary-scale importance, are testament to the novelty of this extreme environment and its ability to host life-forms and to preserve biosignatures,” they wrote.

    “It is therefore also a reliable analog to ancient martian environments and habitats. Deep investigation of the characteristics of this unique geological site will improve our understanding of the limits of life on Earth and inform the search for life on Mars.”

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    A view from the “Kimberley” formation of Gale Crater on Mars taken by NASA’s Curiosity rover. The mission has confirmed the long-ago presence of large amounts of water on the planet, as well as organic compounds needed for life. Curiosity was not equipped to be a life detection mission, but the follow-up Mars 2020 rover mission will be. The colors are adjusted so that rocks look approximately as they would if they were on Earth, to help geologists interpret the rocks. day, or sol, of the mission. (NASA/JPL-Caltech/MSSS)

    While this summation is surely accurate, it is also true that findings like these tell a larger story that goes well beyond Mars. Because the discovery of such a vast number and variety of extremophiles on Earth is one of the key factors that has led many space scientists and astrobiologists to conclude that life beyond Earth is likely.

    If life can survive such unusual and extreme conditions on Earth, logic says that this flexibility would no doubt be present on other potentially habitable planets and moons.

    Other major factors pointing to the plausibility of life beyond Earth are now broadly accepted:

    We now know there are billions upon billions of stars in our own Milky Way galaxy, and that most of them have planets orbiting them. The Kepler Space Telescope was crucial to reaching that consensus through its survey of one small bit of the distant sky.
    The most common planets are small and rocky ones, and some of them are within the habitable zones of their host star. This means the planet can at least sometimes support liquid water; in other words that it is neither too hot (close to its star) or too cold (far from its star.) Liquid water is considered to be essential to assemble and support life.
    The physics and chemistry of the cosmos appear to be consistent with what exists on Earth.

    None of this means any particular planet will support life since there are many other factors at play, such as how circular or elliptical the planet’s orbit might be, as well as the presence and composition of an atmosphere and a protective magnetic field. But our increasingly better understanding of exoplanets, solar systems and extreme life has brought legions of scientists into that hunt for extraterrestrial life — and they have found many ways to move forward as well as to avoid errors.

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    An overview of the past, present, and future of research on remotely detectable biosignatures from an Astrobiology journal paper by NASA NExSS participants. Research historically has focused on cataloguing lists of substances or physical features that yield spectral signatures as indicators of potential life on exoplanets. Recent progress has led to an understanding of how environmental context is critical to interpret signatures of nonliving planets that may mimic some effects of biota. Exoplanet observing telescopes in the near future hold promise to provide direct spectral imaging that can chemically characterize rocky planets in the habitable zone of their parent star. Anticipating these capabilities, the field should seek to develop frameworks to utilize widespread but sparse data to deliver quantitative assessments of whether or not a given planet has life. (Aaron Gronstal)

    While the Dallol discoveries (and others like them) are encouraging, they are sobering as well. Finding these creatures here on Earth has been very difficult, so imagine how challenging it will be to detect the presence of comparable microbial life on now desiccated Mars or a distant planet.

    Indeed, it would be impossible because their small numbers and limited metabolism don’t provide enough of a chemical biosignature to be detected even by telescopes and spectrographs a million times more powerful than what we have now. In terms of exoplanets, what is needed is a planet where plentiful life is providing a strong global biosignature of some kind.

    That ups the ante quite a bit in the search for life beyond Earth. But there are a vast multitude of planets out there, and a logic to the possibility that some have enough life on them for us to some day detect it.

    See the full article here .


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    About Many Worlds
    There are many worlds out there waiting to fire your imagination.

    Marc Kaufman is an experienced journalist, having spent three decades at The Washington Post and The Philadelphia Inquirer, and is the author of two books on searching for life and planetary habitability. While the “Many Worlds” column is supported by the Lunar Planetary Institute/USRA and informed by NASA’s NExSS initiative, any opinions expressed are the author’s alone.

    This site is for everyone interested in the burgeoning field of exoplanet detection and research, from the general public to scientists in the field. It will present columns, news stories and in-depth features, as well as the work of guest writers.

    About NExSS

    The Nexus for Exoplanet System Science (NExSS) is a NASA research coordination network dedicated to the study of planetary habitability. The goals of NExSS are to investigate the diversity of exoplanets and to learn how their history, geology, and climate interact to create the conditions for life. NExSS investigators also strive to put planets into an architectural context — as solar systems built over the eons through dynamical processes and sculpted by stars. Based on our understanding of our own solar system and habitable planet Earth, researchers in the network aim to identify where habitable niches are most likely to occur, which planets are most likely to be habitable. Leveraging current NASA investments in research and missions, NExSS will accelerate the discovery and characterization of other potentially life-bearing worlds in the galaxy, using a systems science approach.
    The National Aeronautics and Space Administration (NASA) is the agency of the United States government that is responsible for the nation’s civilian space program and for aeronautics and aerospace research.

    President Dwight D. Eisenhower established the National Aeronautics and Space Administration (NASA) in 1958 with a distinctly civilian (rather than military) orientation encouraging peaceful applications in space science. The National Aeronautics and Space Act was passed on July 29, 1958, disestablishing NASA’s predecessor, the National Advisory Committee for Aeronautics (NACA). The new agency became operational on October 1, 1958.

    Since that time, most U.S. space exploration efforts have been led by NASA, including the Apollo moon-landing missions, the Skylab space station, and later the Space Shuttle. Currently, NASA is supporting the International Space Station and is overseeing the development of the Orion Multi-Purpose Crew Vehicle and Commercial Crew vehicles. The agency is also responsible for the Launch Services Program (LSP) which provides oversight of launch operations and countdown management for unmanned NASA launches. Most recently, NASA announced a new Space Launch System that it said would take the agency’s astronauts farther into space than ever before and lay the cornerstone for future human space exploration efforts by the U.S.

    NASA science is focused on better understanding Earth through the Earth Observing System, advancing heliophysics through the efforts of the Science Mission Directorate’s Heliophysics Research Program, exploring bodies throughout the Solar System with advanced robotic missions such as New Horizons, and researching astrophysics topics, such as the Big Bang, through the Great Observatories [Hubble, Chandra, Spitzer, and associated programs. NASA shares data with various national and international organizations such as from the [JAXA]Greenhouse Gases Observing Satellite.

     
  • richardmitnick 10:42 am on November 13, 2017 Permalink | Reply
    Tags: A Johns Hopkins biologist has deputized an army of "citizen scientists" to collect samples out in the field, , Extremophiles, , The cosmos is too vast and too crowded with the hundreds of billions or perhaps trillions of galaxies filled with stars and planets for there not to be life out there somewhere, We found some colonization and are working now to upload it all to the Rockiology website   

    From Hopkins: “Johns Hopkins researcher enlists citizen scientists to track down rocks harboring earthly ‘extraterrestrials'” 

    Johns Hopkins
    Johns Hopkins University

    Nov 9, 2017
    Arthur Hirsch

    1
    To collect and examine rocks that could house microbes, a Johns Hopkins biologist has deputized an army of “citizen scientists” to collect samples out in the field. Image credit: Darci J. Harland

    In a small New Mexico town called Truth or Consequences, a pair of homeschooled brothers are on the hunt for extraterrestrials.

    With their mom and a small group of other families, Caleb, 10, and Corban, 6, scour the scrubby desert ground at the base of nearby Turtle Back Mountain, searching for certain kinds of rocks that could be home to microorganisms so resilient and so tough that they might be able to survive outside their rock hosts and live on other planets or moons.

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    Corban Harland, 6, inspects a rock, looking for the telltale green haze that indicates the presence of microbes called extremophiles.
    Image credit: Darci J. Harland

    These citizen scientists were deputized by Johns Hopkins biologist Jocelyne DiRuggiero—who specializes in astrobiology, or the study of the origins, evolution, and distribution of life in the universe—through her crowdsourcing research project, Rockiology. DiRuggiero believes that rocks in deserts and other extreme locations around the world could be home to single-cell microbes that may shed light on whether life could exist outside of our planet.

    After all, suggests DiRuggiero, the cosmos is too vast and too crowded with the hundreds of billions or perhaps trillions of galaxies filled with stars and planets for there not to be life out there somewhere.

    But the hunt begins at home.

    To learn more about these microbes, named extremophiles for the extreme conditions in which they live, DiRuggiero needs to collect samples. To gather those samples, she needs help reaching the most dry, barren places on Earth: deserts, dry valleys in Antarctica, places that resemble other planets.

    “We can go to some places and collect rocks, but we can’t go everywhere,” said DiRuggiero, an associate research professor in the Department of Biology in the Krieger School of Arts and Sciences. “We try to be creative and conserve resources.”

    That’s where sleuths like Caleb, Corban, and their mom, Darci J. Harland, come in. While scouting for home-school projects, Harland came across the Rockiology website, which features instructions on what sort of rocks the researchers are seeking, what characteristics to look for, and how to send in photos of the rocks—and perhaps eventually the rocks themselves—along with information on where they were found.


    Video: David Schmelick and Deirdre Hammer

    “I’ve always enjoyed getting kids out into the field to collect data, not just talking about it,” said Harland, who is a former public school science and English teacher and university professor of education. “And what better way to do that than to collect data for an actual scientist who needs your help?”

    Locations that are potentially rich with extremophile-housing rocks are very dry, very salty, or both. The Atacama desert in Chile, for instance, with its expanses of desolate, reddish terrain—cracked in some spots, littered with stones in others, and broken with jagged cliffs and rock formations—could easily pass for Mars.

    DiRuggiero has conducted several rock-collecting expeditions there, discovering a number of Atacama sodium chloride rocks that have been “colonized,” as she likes to put it, by microbes. In the exposed innards of a cracked rock, the colonies give away their position in a faint green haze on the white surface. There the creatures find refuge from the more dry, sunny, and windy conditions in the desert.

    So far Harland, her sons, and other homeschool families have taken basic lessons in rocks, extremophiles, and DiRuggiero’s work, and they have embarked on sample-gathering expeditions to Turtle Back Mountain.

    “We found some colonization and are working now to upload it all to the Rockiology website,” Harland said in an email. “Being able to communicate with the scientist on this project has been very rewarding both for me and for the students. They were careful in their data collection, knowing it was ‘for real.'”

    Microbes that can live in a salt rock might help a scientist learn something about creatures living in, say, briny water. That could be significant for astrobiologists wondering about the prospect of liquid water on Mars, which could be a sign that the place could support life. If water exists there as a liquid it is likely to be very salty, perhaps toxic.

    The hunt goes on for more information about extremophiles, the search party now expanded to include anyone who signs on for citizen Rockiology.

    See the full article here .

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    Johns Hopkins Campus

    The Johns Hopkins University opened in 1876, with the inauguration of its first president, Daniel Coit Gilman. “What are we aiming at?” Gilman asked in his installation address. “The encouragement of research … and the advancement of individual scholars, who by their excellence will advance the sciences they pursue, and the society where they dwell.”

    The mission laid out by Gilman remains the university’s mission today, summed up in a simple but powerful restatement of Gilman’s own words: “Knowledge for the world.”

    What Gilman created was a research university, dedicated to advancing both students’ knowledge and the state of human knowledge through research and scholarship. Gilman believed that teaching and research are interdependent, that success in one depends on success in the other. A modern university, he believed, must do both well. The realization of Gilman’s philosophy at Johns Hopkins, and at other institutions that later attracted Johns Hopkins-trained scholars, revolutionized higher education in America, leading to the research university system as it exists today.

     
  • richardmitnick 6:27 am on November 2, 2017 Permalink | Reply
    Tags: , , , , , , Earth-sized alien worlds are out there. Now astronomers are figuring out how to detect life on them, Exobiology, Extremophiles, , NASA Deep Space Climate Observatory, NASA HabEx, , , ,   

    From Science: “Earth-sized alien worlds are out there. Now, astronomers are figuring out how to detect life on them” 

    ScienceMag
    Science Magazine

    Nov. 1, 2017
    Daniel Clery

    Stephen Kane spends a lot of time staring at bad pictures of a planet. The images are just a few pixels across and nearly featureless. Yet Kane, an astronomer at the University of California, Riverside, has tracked subtle changes in the pixels over time. They are enough for him and his colleagues to conclude that the planet has oceans, continents, and clouds. That it has seasons. And that it rotates once every 24 hours.

    He knows his findings are correct because the planet in question is Earth.

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    An image from the Deep Space Climate Observatory satellite (left), degraded to a handful of pixels (right), is a stand-in for how an Earth-like planet around another star might look through a future space telescope.
    (LEFT TO RIGHT) NASA EPIC TEAM; STEPHEN KANE

    Kane took images from the Deep Space Climate Observatory satellite, which has a camera pointing constantly at Earth from a vantage partway to the sun, and intentionally degraded them from 4 million pixels to just a handful.

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    NASA Deep Space Climate Observatory

    The images are a glimpse into a future when telescopes will be able to just make out rocky, Earth-sized planets around other stars. Kane says he and his colleagues are trying to figure out “what we can expect to see when we can finally directly image an exoplanet.” Their exercise shows that even a precious few pixels can help scientists make the ultimate diagnosis: Does a planet harbor life?

    Finding conclusive evidence of life, or biosignatures, on a planet light-years away might seem impossible, given that space agencies have spent billions of dollars sending robot probes to much closer bodies that might be habitable, such as Mars and the moons of Saturn, without detecting even a whiff of life. But astronomers hope that a true Earth twin, bursting with flora and fauna, would reveal its secrets to even a distant observer.

    Detecting them won’t be easy, considering the meager harvest of photons astronomers are likely to get from such a tiny, distant world, its signal almost swamped by its much brighter nearby star. The new generation of space telescopes heading toward the launch pad, including NASA’s mammoth James Webb Space Telescope (JWST), have only an outside chance of probing an Earth twin in sufficient detail.

    NASA/ESA/CSA Webb Telescope annotated

    But they will be able to sample light from a range of other planets, and astronomers are already dreaming of a space telescope that might produce an image of an Earth-like planet as good as Kane’s pixelated views of Earth. To prepare for the coming flood of exoplanet data, and help telescope designers know what to look for, researchers are now compiling lists of possible biosignatures, from spectral hints of gases that might emanate from living things to pigments that could reside in alien plants or microbes.

    There is unlikely to be a single smoking gun. Instead, context and multiple lines of evidence will be key to a detection of alien life. Finding a specific gas—oxygen, say—in an alien atmosphere isn’t enough without figuring out how the gas could have gotten there. Knowing that the planet’s average temperature supports liquid water is a start, but the length of the planet’s day and seasons and its temperature extremes count, too. Even an understanding of the planet’s star is imperative, to know whether it provides steady, nourishing light or unpredictable blasts of harmful radiation.

    “Each [observation] will provide crucial evidence to piece together to say if there is life,” says Mary Voytek, head of NASA’s astrobiology program in Washington, D.C.

    In the heady early days following the discovery of the first exoplanet around a normal star in 1995, space agencies drew up plans for extremely ambitious—and expensive—missions to study Earth twins that could harbor life. Some concepts for NASA’s Terrestrial Planet Finder and the European Space Agency’s Darwin mission envisaged multiple giant telescopes flying in precise formation and combining their light to increase resolution. But neither mission got off the drawing board. “It was too soon,” Voytek says. “We didn’t have the data to plan it or build it.”

    Instead, efforts focused on exploring the diversity of exoplanets, using both ground-based telescopes and missions such as NASA’s Kepler spacecraft.

    NASA/Kepler Telescope

    Altogether they have identified more than 3500 confirmed exoplanets, including about 30 roughly Earth-sized worlds capable of retaining liquid water. But such surveys give researchers only the most basic physical information about the planets: their orbits, size, and mass. In order to find out what the planets are like, researchers need spectra: light that has passed through the planet’s atmosphere or been reflected from its surface, broken into its component wavelengths.

    Most telescopes don’t have the resolution to separate a tiny, dim planet from its star, which is at least a billion times brighter. But even if astronomers can’t see a planet directly, they can still get a spectrum if the planet transits, or passes in front of the star, in the course of its orbit. As the planet transits, starlight shines through its atmosphere; gases there absorb particular wavelengths and leave characteristic dips in the star’s spectrum.

    Astronomers can also study a transiting planet by observing the star’s light as the planet’s orbit carries it behind the star.

    Planet transit. NASA/Ames

    Before the planet is eclipsed, the spectrum will include both starlight and light reflected from the planet; afterward, the planet’s contribution will disappear. Subtracting the two spectra should reveal traces of the planet.

    Teasing a recognizable signal from the data is far from easy. Because only a tiny fraction of the star’s light probes the atmosphere, the spectral signal is minuscule, and hard to distinguish from irregularities in the starlight itself and from absorption by Earth’s own atmosphere. Most scientists would be “surprised at how horrible the data is,” says exoplanet researcher Sara Seager of the Massachusetts Institute of Technology in Cambridge.

    In spite of those hurdles, the Hubble and Spitzer space telescopes, plus a few others, have used these methods to detect atmospheric gases, including sodium, water, carbon monoxide and dioxide, and methane, from a handful of the easiest targets.

    NASA/ESA Hubble Telescope

    NASA/Spitzer Infrared Telescope

    Most are “hot Jupiters”—big planets in close-in orbits, their atmospheres puffed up by the heat of their star.

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    In an artist’s concept, a petaled starshade flying at a distance of tens of thousands of kilometers from a space telescope blocks a star’s light, opening a clear view of its planets. NASA/JPL.

    The approach will pay much greater dividends after the launch of the JWST in 2019. Its 6.5-meter mirror will collect far more light from candidate stars than existing telescopes can, allowing it to tease out fainter exoplanet signatures, and its spectrographs will produce much better data.

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    https://jwst.nasa.gov/mirrors.html

    And it will be sensitive to the infrared wavelengths where the absorption lines of molecules such as water, methane, and carbon monoxide and dioxide are most prominent.

    Once astronomers have such spectra, one of the main gases that they hope to find is oxygen. Not only does it have strong and distinctive absorption lines, but many believe its presence is the strongest sign that life exists on a planet.

    Oxygen-producing photosynthesis made Earth what it is today. First cyanobacteria in the oceans and then other microbes and plants have pumped out oxygen for billions of years, so that it now makes up 21% of the atmosphere—an abundance that would be easily detectable from afar. Photosynthesis is evolution’s “killer app,” says Victoria Meadows, head of the NASA-sponsored Virtual Planet Laboratory (VPL) at the University of Washington in Seattle. It uses a prolific source of energy, sunlight, to transform two molecules thought to be common on most terrestrial planets—water and carbon dioxide—into sugary fuel for multicellular life. Meadows reckons it is a safe bet that something similar has evolved elsewhere. “Oxygen is still the first thing to go after,” she says.

    Fifteen years ago, when exoplanets were new and researchers started thinking about how to scan them for life, “Champagne would have flowed” if oxygen had been detected, Meadows recalls. But since then, researchers have realized that things are not that simple: Lifeless planets can have atmospheres full of oxygen, and life can proliferate without ever producing the gas. That was the case on Earth, where, for 2 billion years, microbes practiced a form of photosynthesis that did not produce oxygen or many other gases. “We’ve had to make ourselves more aware of how we could be fooled,” Meadows says.

    To learn what a genuine biosignature might look like, and what might be a false alarm, Meadows and her colleagues at the VPL explore computer models of exoplanet atmospheres, based on data from exoplanets as well as observations of more familiar planets, including Earth. They also do physical experiments in vacuum chambers. They recreate the gaseous cocktails that may surround exoplanets, illuminate them with simulated starlight of various kinds, and see what can be measured.

    Over the past few years, VPL researchers have used such models to identify nonbiological processes that could make oxygen and produce a “false positive” signal. For example, a planet with abundant surface water might form around a star that, in its early years, surges in brightness, perhaps heating the young planet enough to boil off its oceans. Intense ultraviolet light from the star would bombard the resulting water vapor, perhaps splitting it into hydrogen and oxygen. The lighter hydrogen could escape into space, leaving an atmosphere rich in oxygen around a planet devoid of life. “Know thy star, know thy planet,” recites Siddharth Hegde of Cornell University’s Carl Sagan Institute.

    Discovering methane in the same place as oxygen, however, would strengthen the case for life. Although geological processes can produce methane, without any need for life, most methane on Earth comes from microbes that live in landfill sites and in the guts of ruminants. Methane and oxygen together make a redox pair: two molecules that will readily react by exchanging electrons. If they both existed in the same atmosphere, they would quickly combine to produce carbon dioxide and water. But if they persist at levels high enough to be detectable, something must be replenishing them. “It’s largely accepted that if you have redox molecules in large abundance they must be produced by life,” Hegde says.

    Some argue that by focusing on oxygen and methane—typical of life on Earth—researchers are ignoring other possibilities. If there is one thing astronomers have learned about exoplanets so far, it is that familiar planets are a poor guide to exoplanets’ huge diversity of size and nature. And studies of extremophiles, microbes that thrive in inhospitable environments on Earth, suggest life can spring up in unlikely places. Exobiology may be entirely unlike its counterpart on Earth, and so its gaseous byproducts might be radically different, too.

    But what gases to look for? Seager and her colleagues compiled a list of 14,000 compounds that might exist as a gas at “habitable” temperatures, between the freezing and boiling points of water; to keep the list manageable they restricted it to small molecules, with no more than six nonhydrogen atoms. About 2500 are made of the biogenic atoms carbon, nitrogen, oxygen, phosphorus, sulfur, and hydrogen, and about 600 are actually produced by life on Earth. Detecting high levels of any of these gases, if they can’t be explained by nonbiological processes, could be a sign of alien biology, Seager and her colleagues argue.


    A. CUADRA/SCIENCE

    Light shining through the atmospheres of transiting exoplanets is likely to be the mainstay of biosignature searches for years to come. But the technique tends to sample the thin upper reaches of a planet’s atmosphere; far less starlight may penetrate the thick gases that hug the surface, where most biological activity is likely to occur. The transit technique also works best for hot Jupiters, which by nature are less likely to host life than small rocky planets with thinner atmospheres. The JWST may be able to tease out atmospheric spectra from small planets if they orbit small, dim stars like red dwarfs, which won’t swamp the planet’s spectrum. But these red dwarfs have a habit of spewing out flares that would make it hard for life to establish itself on a nearby planet.

    To look for signs of life on a terrestrial planet around a sunlike star, astronomers will probably have to capture its light directly, to form a spectrum or even an actual image. That requires blocking the overwhelming glare of the star. Ground-based telescopes equipped with “coronagraphs,” which precisely mask a star so nearby objects can be seen, can now capture only the biggest exoplanets in the widest orbits. To see terrestrial planets will require a similarly equipped telescope in space, above the distorting effect of the atmosphere. NASA’s Wide Field Infrared Survey Telescope (WFIRST), expected to launch in the mid-2020s, is meant to fill that need.

    NASA/WFIRST

    Even better, WFIRST could be used in concert with a “starshade”—a separate spacecraft stationed 50,000 kilometers from the telescope that unfurls a circular mask tens of meters across to block out starlight. A starshade is more effective than a coronagraph at limiting the amount of light going into the telescope. It not only blocks the star directly, but also suppresses diffraction with an elaborate petaled edge. That reduces the stray scattered light that can make it hard to spot faint planets. A starshade is a much more expensive prospect than a coronagraph, however, and aligning telescope and starshade over huge distances will be a challenge.

    Direct imaging will provide much better spectra than transit observations because light will pass through the full depth of the planet’s atmosphere twice, rather than skimming through its outer edges. But it also opens up the possibility of detecting life directly, instead of through its waste gases in the atmosphere. If organisms, whether they are plants, algae, or other microbes, cover a large proportion of a planet’s surface, their pigments may leave a spectral imprint in the reflected light. Earthlight contains an obvious imprint of this sort. Known as the “red edge,” it is the dramatic change in the reflectance of green plants at a wavelength of about 720 nanometers. Below that wavelength, plants absorb as much light as possible for photosynthesis, reflecting only a few percent. At longer wavelengths, the reflectance jumps to almost 50%, and the brightness of the spectrum rises abruptly, like a cliff. “An alien observer could easily tell if there is life on Earth,” Hegde says.

    There’s no reason to assume that alien life will take the form of green plants. So Hegde and his colleagues are compiling a database of reflectance spectra for different types of microbes. Among the hundreds the team has logged are many extremophiles, which fill marginal niches on Earth but may be a dominant life form on an exoplanet. Many of the microbes on the list have not had their reflectance spectra measured, so the Cornell team is filling in those gaps. Detecting pigments on an exoplanet surface would be extremely challenging. But a tell-tale color in the faint light of a distant world could join other clues—spectral absorption lines from atmospheric gases, for example—to form “a jigsaw puzzle which overall gives us a picture of the planet,” Hegde says.

    None of the telescopes available now or in the next decade is designed specifically to directly image exoplanets, so biosignature searches must compete with other branches of astronomy for scarce observing time. What researchers really hanker after is a large space telescope purpose-built to image Earth-like alien worlds—a new incarnation of the idea behind NASA’s ill-fated Terrestrial Planet Finder.

    The Habitable Exoplanet Imaging Mission, or HabEx, a mission concept now being studied by NASA, could be the answer. Its telescope would have a mirror up to 6.5 meters across—as big as the JWST’s—but would be armed with instruments sensitive to a broader wavelength range, from the ultraviolet to the near-infrared, to capture the widest range of spectral biosignatures. The telescope would be designed to reduce scattered light and have a coronagraph and starshade to allow direct imaging of Earth-sized exoplanets.

    Such a mission would reveal Earth-like planets at a level of detail researchers can now only dream about—probing atmospheres, revealing any surface pigments, and even delivering the sort of blocky surface images that Kane has been simulating. But will that be enough to conclude we are not alone in the universe? “There’s a lot of uncertainty about what would be required to put the last nail in the coffin,” Kane says. “But if HabEx is built according to its current design, it should provide a pretty convincing case.”

    4
    NASA HabEx: The Planet Hunter

    See the full article here .

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