Lightning strikes seen from space. pic.twitter.com/tygrsZPrmw
— Wonder of Science (@wonderofscience) February 2, 2021
LIGHTNING STRIKES AGAIN
Lightning Sparked the Origin of Life
by Tim McMillan / March 16, 2021
“Could lightning be responsible for the origin of life on Earth? According to a new study published by researchers at Yale and the University of Leeds, yes. In their study, published today in the journal Nature Communications, researchers contend that over billions of years, up to a quintillion lightning strikes is what unlocked usable phosphorus and ultimately sparked the creation of life on planet Earth. “This work helps us understand how life may have formed on Earth and how it could still be forming on other, Earth-like planets,” said lead author Benjamin Hess, a graduate student in Yale’s Department of Earth & Planetary Sciences, in a press release issued today by Yale. The origin of life has been one of humanity’s most enduring mysteries. Throughout history, theologians, philosophers, and scientists have all applied their respective tradecrafts to explain how life could have suddenly emerged billions of years ago on such an inhospitable early Earth.
Since the 1980s, the popular scientific view is that life erupted during a hypothetical “RNA world” stage in evolutionary history. Under this theory, scientists propose self-replicating RNA molecules set the stage for the evolution of DNA and proteins, which produced the necessary chemical reactions for the development of life. Recently, The Debrief published a study conducted by a team of American and Japanese scientists who announced they had discovered an organic molecule critical to the development of life within meteorite samples. These results give credence to the panspermia theory or the hypothesis that the building blocks for life on Earth have extraterrestrial origins. The theory of panspermia, however, only answers a piece of the puzzle. The other lingering question is how did conditions on Earth 3.8 to 4.1 billion years ago change from incredibly inhospitable to even unicellular life to suddenly becoming a homeworld ripe for the evolution of complex multicellular organisms.
“(c) Mass range of reduced phosphorus produced per year from fulgurite-forming lightning strikes and meteorites as calculated in (a, b). Inset shows model inputs for atmospheric pCO2 in bars, lightning strikes per km2 per year, and annual total P in kg per year from meteorites11 as a function of time.”
One of the keys to answering how Earth went from being hostile to hospital rests on the sudden emergence of usable phosphorus gas. Phosphorus was tightly locked billions of years ago within insoluble minerals on Earth’s surface. Yet, usable phosphorus – an essential component for all life and a necessary ingredient for creating DNA, RNA, and other critical biomolecules – was scarce. Under the premise of panspermia, scientists have examined the possibility that meteorites containing the phosphorus mineral schreibersite may account for the emergence of usable phosphorus on Earth. The biggest problem with this extraterrestrial origin hypothesis for Earthly phosphorus is that when life is believed to have emerged, roughly 3.5 to 4.5 billion years ago, meteorite impacts are thought to have been relatively low. “The early bombardment is a once in a solar system event. As planets reach their mass, the delivery of more phosphorus from meteors becomes negligible,” said Dr Jason Harvey, Associate Professor of Geochemistry in Leeds’ School of Earth and Environment, in a press release issued by Leeds. With meteorite strikes offering a low probability solution, the team of researchers at Yale and the University of Leeds began looking for more likely candidates to create usable phosphorus gas.
“Fulgurites or natural tubes, clumps, or masses of fused soil, sand, rock, and organic debris can form when lightning discharges into the ground.”
When a lightning strike discharges into the ground, the powerful bolt of electrical energy can cause nearby soil, sand, rock, or other organic debris to fuse into bizarre-looking branches of mineraloid glass called: Fulgurite. A fulgurite’s chemical composition is determined by the physical properties of whatever material has been struck by lightning. However, the twisting glass figures which resemble electrical discharges contain a soluble form of phosphorus from the surface rock they are made from. The inspiration for examining the role fulgurite may have played on the early evolution life came during while the study’s primary author, Benjamin Hess, was undergoing undergraduate research at the University of Leeds, School of Earth and Environment. In a release issued today by Leeds, Hess said he and his mentors were studying a large sample of fulgurite formed from a lightning strike in Glen Ellyn, Illinois. Initially, interested in how fulgurite is formed, researchers say they were fascinated to discover the Glen Ellyn sample contained large amounts of the highly unusual phosphorus mineral schreibersite.
“Glen Ellyn fulgurite sample during excavation. The ruler shown is 15cm long”
Now a PhD student at Yale, Hess, and co-authors Sandra Piazolo and Jason Harvey from the University of Leeds decided to examine if enough usable phosphorus could have been produced to spurn life by estimating how many lightning strikes might have occurred on Earth billions of years ago. Using computer modeling, researchers found that early Earth saw as many as 1 to 5 billion lightning strikes every year. By comparison, Earth today encounters around 560 million lightning flashes annually. Researchers estimate that anywhere from 100 million to 1 billion lightning strikes would reach the ground per year Earth’s early days. Ultimately, scientists concluded after around a billion years, the Earth would have been hit by up to 0.1 to 1 quintillion bolts of lightning. Researchers suggest that many strikes would have produced enormous numbers of fulgurites, resulting in a wealth of usable phosphorus to give rise to life’s origin.
“Glen Ellyn fulgurite sample after excavation. It weighed around 30kg in total”
In their paper, researchers also note several other advantages to their lightning strike theory. Unlike meteorite impacts, the annual number of lightning strikes would have remained constant for billions of years. Making lightning a more likely progenitor for phosphorus and ultimately the origins of life. The scientists also point out that lightning strikes were likely most prevalent in tropical regions of early Earth, which would have provided for more concentrated areas of usable phosphorus following Darwin’s famous “warm little pond” concept. Ultimately, with this newly published study, researchers say the “lightning in a bottle” event that led to life on Earth, could be just that – lighting in a bottle. Or, in this particular case, fulgurite glass. “It makes lightning strikes a significant pathway toward the origin of life,” said Hess in today’s release. “Perhaps more importantly, this also means that the formation of life on other Earth-like planets remains possible long after meteorite impacts have become rare.”
Comets May Have Kick-Started Life on Earth
by Brad Bergan / Mar 05, 2021
“Way back in 2016, a tiny icy wanderer from the fringes of our solar system shot past Earth at incredible speeds. During its flyby, it was momentarily visible to stargazers — called Comet Catalina — before it slingshotted around the sun, and disappeared from us forever. However, using NASA’s plane-based telescope called Stratospheric Observatory for Infrared Astronomy (SOFIA), scientists detected a crucial fingerprint of life, according to a recent study published in the Planetary Science Journal. Within the dusty majesty of the comet’s tail, SOFIA detected carbon.
This single-serving visitor to the inner solar system is helping scientists come to understand the origins of life on Earth — as it’s become likely that comets like Catalina may have played an essential role as a primary source of carbon on planets like Earth and Mars during the very young era of our solar system. “Carbon is key to learning about the origins of life,” said Charles “Chick” Woodward, lead author on the paper, in a NASA blog post. Woodward is also an astrophysicist and professor at the University of Minnesota’s Minnesota Institute of Astrophysics — based in Minneapolis. “We’re still not sure if Earth could have trapped enough carbon on its own during its formation, so carbon-rich comets could have been an important source delivery this essential element that led to life as we know it,” added Woodward.
Comet Catalina came from the Oort could on the farthest fringes of the solar system — where similar comets have long, elliptical orbits that cause them to enter our proverbial doorstep with little-to-no interference in their cosmic trajectory. This makes them a celestial time capsule in space, giving researchers a rare chance to study the conditions of the early solar system during which the comets formed. The infrared observations from SOFIA provided data on the composition of dust and gas as it evaporated off of the comet’s surface, creating the tail. These observations revealed that Comet Catalina is rich in carbon, which means it formed in the outer regions of the primordial solar system, which contained a reservoir of carbon that may have been critical to seeding the origin of life on Earth. Carbon is an essential ingredient of life, but the young Earth and other terrestrial planets of this era of the inner solar system were so indescribably hot from the perils of formation that elements like carbon were simply lost, or depleted.
Researchers think a minor shift in Jupiter’s orbit enabled small, early precursors of comets to mix carbon from outer regions into inner ones, where it was then pulled into planets like Mars and Earth. Comet Catalina’s carbon-heavy composition helps show how planets forming in hot, carbon-lacking regions of the young solar system were able to evolve into rich, life-supporting environments. “All terrestrial worlds are subject to impacts by comets and other small bodies, which carry carbon and other elements,” said Woodward, in the blog post. “We are getting closer to understanding exactly how these impacts on early planets may have catalyzed life.” Without carbon, life as we know it would never have evolved on Earth. We’ve yet to confirm the existence of life on Mars, but since we know it has salty, subsurface lakes and suspect it once had oceans, life may be in the cards for the Red Planet. And if life found a way, it might have comets like Catalina to thank for seeding it with carbon, just like us.”
LAST UNIVERSAL COMMON ANCESTOR
Why some scientists believe life may have started on Mars
by Nicole Karlis / February 8, 2021
“On February 18, NASA’s Perseverance rover will parachute through thin Martian air, marking a new era in red planet exploration. Landing on the Jezero Crater, which is located north of the Martian equator, will be no easy feat. Only about 40 percent of the missions ever sent to Mars succeed, according to NASA. If it does, Perseverance could drastically change the way we think about extraterrestrial life. That’s because scientists believe Jezero, a 28 mile-wide impact crater that used to be a lake, is an ideal place to look for evidence of ancient microbial life on Mars.
Once it lands, Perseverance will collect and store Martian rock and soil samples, which will eventually be returned to Earth. This is known as a “sample-return mission,” an extremely rare type of space exploration mission due to its expense. (Indeed, there has never been a sample return mission from another planet.) And once Martian soil is returned to Earth in a decade, scientists will set about studying the material to figure out if there was ever ancient life on Mars. Yet some scientists believe that these samples could answer an even bigger question: Did life on Earth originate on Mars?
Though the idea that life started on Mars before migrating on Earth sounds like some far-fetched sci-fi premise, many renowned scientists take the theory seriously. The general idea of life starting elsewhere in space before migrating here has a name, too: Panspermia. It’s the hypothesis that life exists elsewhere in the universe, and is distributed by asteroids and other space debris. To be clear, the notion of life on Earth originating on Mars isn’t a dominant theory in the scientific community, but it does appear to be catching on. And scientists like Gary Ruvkun, a professor of genetics at Harvard Medical School, say that it does sound “obvious, in a way.” The evidence starts with how space debris moved around in the young solar system.
Indeed, we have evidence of an exchange of rocks from Mars to Earth. Martian meteorites have been found in Antarctica and across the world — an estimated 159, according to the International Meteorite Collectors Association. “You can assign them to Mars based on the gaseous inclusions that they have, that are sort of the equivalent of the gases that were shown by the Viking spacecraft” to exist in Mars’ atmosphere, Ruvkun said. In other words, small bubbles of air in these rocks reveal that they were forged in the Martian air. “So, there is exchange between Mars and Earth — probably more often going from Mars to Earth because it goes ‘downhill,’ going to Mars is ‘uphill,’ gravitationally-speaking.”
But for Ruvkun, whose area of expertise is genomics, it’s the timing of cellular life that he believes makes a strong case that life on Earth came from somewhere else — perhaps Mars, or perhaps Mars vis-a-vis another planet. Ruvkun noted that our genomes reveal the history of life, and provide clues about the ancestors that preceded us by millions or even billions of years. “In our genomes, you can kind of see the history, right?” he said. “There’s the RNA world that predated the DNA world and it’s very well supported by all kinds of current biology; so, we know the steps that evolution took in order to get to where we are now.” Thanks to the advancement of genomics, the understanding of LUCA (the Last Universal Common Ancestor) — meaning the organism from which all life on Earth evolved from — has greatly advanced. By studying the genetics of all organisms on Earth, scientists have a very good sense of what the single-celled ancestor of every living thing (on Earth) looked like. They also know the timeline: all modern life forms descend from a single-celled organism that lived about 3.9 billion years ago, only 200 million years after the first appearance of liquid water. In the grand scheme of the universe, that’s not that long.
And the last universal common ancestor was fairly complicated as far as organisms go. That leaves two possibilities, Ruvkun says. “Either evolution to full-on modern genomes is really easy, or the reason you see it so fast was that we just ‘caught’ life, it didn’t actually start here.” He adds, “I like the idea that we just caught it and that’s why it’s so fast, but I’m an outlier.” If that’s the case, then Erik Asphaug, is a professor of planetary science at the University of Arizona, is also an outlier. Asphaug said that what we know about the oldest rocks on Earth — which have chemical evidence of carbon isotopes, tracing back to nearly 4 billion years ago — tell us that life started “started forming on Earth almost as soon as it was possible for it to happen.”
If that’s the case, it makes for an interesting precedent. “Let’s say you expect life to be flourishing whenever a planet cools down to the point where it can start to have liquid water,” Asphaug said. “But just looking at our own solar system, what planet was likely to be habitable first? Almost certainly Mars.” This is because, Asphaug said, Mars formed before Earth did. Early in Martian history when Mars was cooling down, Mars would have had a “hospitable” environment before Earth. “If life was going to start anywhere it might start first on Mars,” Asphaug said. “We don’t know what the requirement is — you know, if it required something super special like the existence of a moon or some factors that are unique to the Earth — but just in terms of what place had liquid water first, that would have been Mars.”
An intriguing and convincing piece of evidence relates to how material moved between the two neighboring planets. Indeed, the further you go back in time, the bigger the collisions of rocks between Mars and Earth, Asphaug said. These impact events could have been huge “mountain-sized blocks of Mars” that were launched into space. Such massive asteroids could serve as a home for a hardy microorganism. “When you collide back into a planet, some fraction of that mountain-sized mass is going to survive as debris on the surface,” he said. “It’s taken a while for modeling to show that you can have a relatively intact survival of what we call ‘ballistic panspermia’ — firing a bullet into one planet, knocking bits off, and having it end up on another planet. But it’s feasible, we think it happens, and the trajectory would tend to go from Mars to the Earth, much more likely than from Earth to Mars.”
Asphaug added that surviving the trip, given the mass of the vehicle for the microorganisms, wouldn’t be a problem — and neither would surviving on a new, hospitable planet. “Any early life form would be resistant to what’s going on at the tail end of planet formation,” he said. “Any organism that’s going to be existing has to be used to the horrific bombardment of impacts, even apart from this, swapping from planet to planet.” In other words, early microbial life would have been fine with harsh environments and long periods of dormancy. Harvard professor Avi Loeb told Salon via email that one of the Martian rocks found on Earth, ALH 84001, “was not heated along its journey to more than 40 degrees Celsius and could have carried life.” All three scientists believe that Perseverance might be able to add credibility to the theory of panspermia. “If you were to go and find remnants of life on Mars, which we hope to do with Perseverance rover and these other Martian adventures, I would be personally surprised if they were not connected at the hip to terrestrial life,” Asphaug said.
Ruvkun said he hopes to be one of the scientists to look for DNA when the sample from Mars hopefully, eventually, returns. “Launching something from Mars is a seriously difficult thing,” he said. But what would this mean for human beings, and our existential understanding of who we are and where we came from? “In that case, we might all be Martians,” Loeb said. He joked that the self-help book “Men are from Mars, Women are from Venus” may have been more right than we know. Or perhaps, as Ruvkun believes, we’re from a different solar system, and life is just scattering across the universe. “To me the idea that it all started on Earth, and every single solar system has their own little evolution of life happening, and they’re all independent — it just seems kind of dumb,” Ruvkun said. “It’s so much more explanatory to say ‘no, it’s spreading, it’s spreading all across the universe, and we caught it too, it didn’t start here,” he added. “And in this moment during the pandemic — what a great moment to pitch the idea. Maybe people will finally believe it.”
POST LATE HEAVY BOMBARDMENT
Fossil Discoveries Challenge Ideas About Earth’s Start
by Rebecca Boyle / January 22, 2018
“In the arid, sun-soaked northwest corner of Australia, along the Tropic of Capricorn, the oldest face of Earth is exposed to the sky. Drive through the northern outback for a while, south of Port Hedlund on the coast, and you will come upon hills softened by time. They are part of a region called the Pilbara Craton, which formed about 3.5 billion years ago, when Earth was in its youth. Look closer. From a seam in one of these hills, a jumble of ancient, orange-Creamsicle rock spills forth: a deposit called the Apex Chert. Within this rock, viewable only through a microscope, there are tiny tubes. Some look like petroglyphs depicting a tornado; others resemble flattened worms. They are among the most controversial rock samples ever collected on this planet, and they might represent some of the oldest forms of life ever found. Last month, researchers lobbed another salvo in the decades-long debate about the nature of these forms. They are indeed fossil life, and they date to 3.465 billion years ago, according to John Valley, a geochemist at the University of Wisconsin. If Valley and his team are right, the fossils imply that life diversified remarkably early in the planet’s tumultuous youth.
The fossils add to a wave of discoveries that point to a new story of ancient Earth. In the past year, separate teams of researchers have dug up, pulverized and laser-blasted pieces of rock that may contain life dating to 3.7, 3.95 and maybe even 4.28 billion years ago. All of these microfossils — or the chemical evidence associated with them — are hotly debated. But they all cast doubt on the traditional tale. As that story goes, in the half-billion years after it formed, Earth was hellish and hot. The infant world would have been rent by volcanism and bombarded by other planetary crumbs, making for an environment so horrible, and so inhospitable to life, that the geologic era is named the Hadean, for the Greek underworld. Not until a particularly violent asteroid barrage ended some 3.8 billion years ago could life have evolved.
“A sliver of a nearly 3.5-billion-year-old rock from the Apex Chert deposit in Western Australia (top). An example of one of the microfossils discovered in a sample of rock from the Apex Chert (bottom).”
But this story is increasingly under fire. Many geologists now think Earth may have been tepid and watery from the outset. The oldest rocks in the record suggest parts of the planet’s crust had cooled and solidified by 4.4 billion years ago. Oxygen in those ancient rocks suggest the planet had water as far back as 4.3 billion years ago. And instead of an epochal, final bombardment, meteorite strikes might have slowly tapered off as the solar system settled into its current configuration. “Things were actually looking a lot more like the modern world, in some respects, early on. There was water, potentially some stable crust. It’s not completely out of the question that there would have been a habitable world and life of some kind,” said Elizabeth Bell, a geochemist at the University of California, Los Angeles. Taken together, the latest evidence from the ancient Earth and from the moon is painting a picture of a very different Hadean Earth: a stoutly solid, temperate, meteorite-clear and watery world, an Eden from the very beginning.
About 4.54 billion years ago, Earth was forming out of dust and rocks left over from the sun’s birth. Smaller solar leftovers continually pelted baby Earth, heating it up and endowing it with radioactive materials, which further warmed it from within. Oceans of magma covered Earth’s surface. Back then, Earth was not so much a rocky planet as an incandescent ball of lava. Not long after Earth coalesced, a wayward planet whacked into it with incredible force, possibly vaporizing Earth anew and forming the moon. The meteorite strikes continued, some excavating craters 1,000 kilometers across. In the standard paradigm of the Hadean eon, these strikes culminated in an assault dubbed the Late Heavy Bombardment, also known as the lunar cataclysm, in which asteroids emigrated to the inner solar system and pounded the rocky planets. Throughout this early era, ending about 3.8 billion years ago, Earth was molten and couldn’t support a crust of solid rock, let alone life.
“Graphic of Early Cataclysm Under Assault”
But starting around a decade ago, this story started to change, thanks largely to tiny crystals called zircons. The gems, which are often about the size of the period at the end of this sentence, told of a cooler, wetter and maybe livable world as far back as 4.3 billion years ago. In recent years, fossils in ancient rock bolstered the zircons’ story of calmer climes. The tornadic microfossils of the Pilbara Craton are the latest example. Today, the oldest evidence for possible life — which many scientists doubt or outright reject — is at least 3.77 billion years old and may be a stunningly ancient 4.28 billion years old. In March 2017, Dominic Papineau, a geochemist at University College London, and his student Matthew Dodd described tubelike fossils in an outcrop in Quebec that dates to the basement of Earth’s history. The formation, called the Nuvvuagittuq (noo-voo-wog-it-tuck) Greenstone Belt, is a fragment of Earth’s primitive ocean floor. The fossils, about half the width of a human hair and just half a millimeter long, were buried within. They are made from an iron oxide called hematite and may be fossilized cities built by microbial communities up to 4.28 billion years ago, Dodd said. “They would have formed these gelatinous, rusty-red-colored mats on the rocks around the vents,” he said. Similar structures exist in today’s oceans, where communities of microbes and bloody-looking tube worms blossom around sunless, black-smoking chimneys.
“Bright red rock in Nuvvuagittuq Greenstone Belt appears to contain tube-shaped microfossils dating to at least 3.77 billion years ago”
Dodd found the tubes near graphite and with carbonate “rosettes,” tiny carbon rings that contain organic materials. The rosettes can form through varying nonbiological processes, but Dodd also found a mineral called apatite, which he said is diagnostic of biological activity. The researchers also analyzed the variants, or isotopes, of carbon within the graphite. Generally, living things like to use the more lightweight isotopes, so an abundance of carbon 12 over carbon 13 can be used to infer past biological activity. The graphite near the rosettes also suggested the presence of life. Taken together, the tubes and their surrounding chemistry suggest they are remnants of a microbial community that lived near a deep-ocean hydrothermal vent, Dodd said. Geologists debate the exact age of the rock belt where they were found, but they agree it includes one of the oldest, if not the oldest, iron formations on Earth. This suggests the fossils are that old, too — much older than anything found previously and much older than many scientists had thought possible.
“microfossils resemble sea life that grows near deep-sea hydrothermal vents.”
Then in September 2017, researchers in Japan published an examination of graphite flakes from a 3.95-billion-year-old sedimentary rock called the Saglek Block in Labrador, Canada. Yuji Sano and Tsuyoshi Komiya of the University of Tokyo argued their graphite’s carbon-isotope ratio indicates it, too, was made by life. But the graphite flakes were not accompanied by any feature that looked like a fossil; what’s more, the history of the surrounding rock is murky, suggesting the carbon may be younger than it appears. Farther to the east, in southwestern Greenland, another team had also found evidence of ancient life. In August 2016, Allen Nutman of the University of Wollongong in Australia and colleagues reported finding stromatolites, fossil remains of microbes, from 3.7 billion years ago.
“Nutman prospecting for ancient microfossils in southern Greenland”
Many geologists have been skeptical of each claim. Nutman’s fossils, for example, come from the Isua belt in southern Greenland, home to the oldest known sedimentary rocks on Earth. But the Isua belt is tough to interpret. Just as nonbiological processes can form Dodd’s carbon rosettes, basic chemistry can form plenty of layered structures without any help from life, suggesting they may not be stromatolites but lifeless pretenders. In addition, both the Nuvvuagittuq Greenstone Belt and the Isua belt have been heated and squished over billions of years, a process that melts and recrystallizes the rocks, morphing them from their original sedimentary state. “I don’t think any of those other studies are wrong, but I don’t think any of them are proof,” said Valley, the Wisconsin researcher. “All we can say is [Nutman’s rocks] look like stromatolites, and that’s very enticing.” Regarding his work with the Pilbara Craton fossils, however, Valley is much less circumspect.
“The stromatolites form small wavelike mounds in sedimentary rock.
The vertical lines are cuts made by the researchers.”
The tornadic microfossils lay in the Pilbara Craton for 3.465 billion years before being separated from their natal rock, packed up in a box and shipped to California. Paleobiologist William Schopf of UCLA published his discovery of the strange squiggles in 1993 and identified 11 distinct microbial taxa in the samples. Critics said the forms could have been made in nonbiological processes, and geologists have argued back and forth in the years since. Last year, Schopf sent a sample to Valley, who is an expert with a super-sensitive instrument for measuring isotope ratios called a secondary ion mass spectrometer. Valley’s team found that some of the apparent fossils had the same carbon-isotope ratio as modern photosynthetic bacteria. Three other types of fossils had the same ratios as methane-eating or methane-producing microbes. Moreover, the isotope ratios correlate to specific species that had already been identified by Schopf. The locations where these isotope ratios were measured corresponded to the shapes of the microfossils themselves, Valley said, adding they are the oldest samples that look like fossils both physically and chemically.
“John Valley in his mass spectrometer laboratory”
While they are not the oldest samples in the record — supposing you accept the provenance of the rocks described by Dodd, Komiya and Nutman — Schopf’s and Valley’s cyclonic miniatures do have an important distinction: They are diverse. The presence of so many different carbon isotope ratios suggests the rock represents a complex community of primitive organisms. The life-forms must have had time to evolve into endless iterations. This means they must have originated even earlier than 3.465 billion years ago. And that means our oldest ancestors are very, very old indeed. Fossils were not the first sign that early Earth might have been Edenic rather than hellish. The rocks themselves started providing that evidence as far back as 2001. That year, Valley found zircons that suggested the planet had a crust as far back as 4.4 billion years ago. Zircons are crystalline minerals containing silicon, oxygen, zirconium and sometimes other elements.
They form inside magma, and like some better-known carbon crystals, zircons are forever — they can outlast the rocks they form in and withstand eons of unspeakable pressure, erosion and deformation. As a result, they are the only rocks left over from the Hadean, making them invaluable time capsules. Valley chipped some out of Western Australia’s Jack Hills and found oxygen isotopes that suggested the crystal formed from material that was altered by liquid water. This suggested part of Earth’s crust had cooled, solidified and harbored water at least 400 million years earlier than the earliest known sedimentary rocks. If there was liquid water, there were likely entire oceans, Valley said. Other zircons showed the same thing. “The Hadean was not hell-like. That’s what we learned from the zircons. Sure, there were volcanoes, but they were probably surrounded by oceans. There would have been at least some dry land,” he said. Zircons suggest there may even have been life. In research published in 2015, Bell and her coauthors presented evidence for graphite embedded within a tiny, 4.1-billion-year-old zircon crystal from the same Jack Hills. The graphite’s blend of carbon isotopes hints at biological origins, although the finding is — once again — hotly debated. “Are there other explanations than life? Yeah, there are,” Bell said. “But this is what I would consider the most secure evidence for some sort of fossil or biogenic structure.”
“X-ray of 4.1-billion-year-old zircon dark spots made by carbon deposits”
If the signals in the ancient rocks are true, they are telling us that life was everywhere, always. In almost every place scientists look, they are finding evidence of life and its chemistry, whether it is in the form of fossils themselves or the remnants of life’s long-ago stirrings. Far from fussy and delicate, life may have taken hold in the worst conditions imaginable. “Life was managing to do interesting things at the same time Earth was dealing with the worst impacts it’s ever had,” said Bill Bottke, a planetary scientist at the Southwest Research Institute in Boulder, Colorado. Or maybe not. Maybe Earth was just fine. Maybe those impacts weren’t quite as rapid-fire as everyone thought. We know Earth, and everything else, was bombarded by asteroids in the past. The moon, Mars, Venus and Mercury all bear witness to this primordial pummeling. The question is when, and for how long. Based largely on Apollo samples toted home by moonwalking astronauts, scientists came to believe that in the Earth’s Hadean age, there were at least two distinct epochs of solar system billiards. The first was the inevitable side effect of planet making: It took some time for the planets to sweep up the biggest asteroids and for Jupiter to gather the rest into the main asteroid belt.
The second came later. It began sometime between 500 and 700 million years after the solar system was born and finally tapered off around 3.8 billion years ago. That one is called the Late Heavy Bombardment, or the lunar cataclysm. As with most things in geochemistry, evidence for a world-rending blitz, an event on the hugest scales imaginable, is derived from the very, very small. Isotopes of potassium and argon in Apollo samples suggested bits of the moon suddenly melted some 500 million years after it formed. This was taken as evidence that it was blasted within an inch of its life. Zircons also provide tentative physical evidence of a late-era hellscape. Some zircons do contain “shocked” minerals, evidence for extreme heat and pressure that can be indicative of something horrendous. Many are younger than 3 billion years, but Bell found one zircon suggesting rapid, extreme heating around 3.9 billion years ago — a possible signature of the Late Heavy Bombardment. “All we know is there is a group of recrystallized zircons at this time period. Given the coincidence with the Late Heavy Bombardment, it was too hard not to say that maybe this is connected,” she said. “But to really establish that, we will need to look at zircon records at other localities around the planet.” So far, there are no other signs, said Aaron Cavosie of Curtin University in Australia.
“Craters on the moon have been taken as evidence for the Late Heavy Bombardment, but reassessments of the geological evidence from Apollo moon rocks casts doubt on whether asteroid bombardments during Hadean era were as severe as thought”
In 2016 Patrick Boehnke, now at the University of Chicago, took another look at those original Apollo samples, which for decades have been the main evidence in favor of the Late Heavy Bombardment. He and UCLA’s Mark Harrison reanalyzed the argon isotopes and concluded that the Apollo rocks may have been walloped many times since they crystallized from the natal moon, which could make the rocks seem younger than they really are. “Even if you solve the analytical problems,” said Boehnke, “then you still have the problem that the Apollo samples are all right next to each other.” There’s a chance that astronauts from the six Apollo missions sampled rocks from a single asteroid strike whose ejecta spread throughout the Earth-facing side of our satellite.
In addition, moon-orbiting probes like the Gravity Recovery and Interior Laboratory (GRAIL) spacecraft and the Lunar Reconnaissance Orbiter have found around 100 previously unknown craters, including a spike in impacts as early as 4.3 billion years ago. “This interesting confluence of orbital data and sample data, and all different kinds of sample data — lunar impact glass, Luna samples, Apollo samples, lunar meteorites — they are all coming together and pointing to something that is not a cataclysmic spike at 3.9 billion years ago,” said Nicolle Zellner, a planetary scientist at Albion College in Michigan. Bottke, who studies asteroids and solar system dynamics, is one of several researchers coming up with modified explanations. He now favors a slow uptick in bombardment, followed by a gradual decline. Others think there was no late bombardment, and instead the craters on the moon and other rocky bodies are remnants from the first type of billiards, the natural process of planet building. “We have a tiny sliver of data, and we’re trying to do something with it,” he said. “You try to build a story, and sometimes you are just chasing ghosts.”
While it plays out, scientists will be debating much bigger questions than early solar-system dynamics. If some of the new evidence truly represents impressions of primeval life, then our ancestors may be much older than we thought. Life might have arisen the moment the planet was amenable to it — the moment it cooled enough to hold liquid water. “I was taught when I was young that it would take billions and billions of years for life to form. But I have not been able to find any basis for those sorts of statements,” said Valley. “I think it’s quite possible that life emerged within a few million years of when conditions became habitable. From the point of view of a microbe, a million years is a really long time, yet that’s a blink of an eye in geologic time. There is no reason life could not have emerged at 4.3 billion years ago,” he added. “There is no reason.”
If there was no mass sterilization at 3.9 billion years ago, or if a few massive asteroid strikes confined the destruction to a single hemisphere, then Earth’s oldest ancestors may have been here from the haziest days of the planet’s own birth. And that, in turn, makes the notion of life elsewhere in the cosmos seem less implausible. Life might be able to withstand horrendous conditions much more readily than we thought. It might not need much time at all to take hold. It might arise early and often and may pepper the universe yet. Its endless forms, from tubemaking microbes to hunkering slime, may be too small or simple to communicate the way life does on Earth — but they would be no less real and no less alive.”
GOD DOES NOT PLAY DICE
Could aliens have created life on Earth?
by Annalee Newitz / 6/14/12
“Over 120 years ago, Kelvin shocked the British scientific community in a speech about what he called “panspermia,” where he suggested that life might have come from planets smashing into each other and sending bits of life hurtling through space. He and a few colleagues had hit upon this notion after observing the massive 1880 eruption of a volcano on Krakatoa. To be more precise, they observed the aftermath of the volcano, which completely sterilized the island.
No life was left at all. But then, within months, seedlings began to sprout and life took hold again. Where had that life come from? To naturalists of the nineteenth century, it was obvious that it had drifted there from nearby islands. Seeds and insects blown on the wind, or floating on the tides, had begun the process of re-greening the stricken landscape. This got Kelvin thinking about the origin of life on Earth. Couldn’t the same thing happen to barren planets drifting in space? Perhaps life had drifted to Earth on the stellar winds.
Today, we know that most life wouldn’t survive the trip through space. It would be bombarded by radiation and subjected to hard vacuum. But Francis Crick, who was one of the first biologists to identify the structure of DNA, suggested a way around this problem. In a 1972 paper he co-authored with biologist Leslie Orgel called “Directed Panspermia,” Crick suggested that perhaps extraterrestrials had seeded the Earth with microbes sent in specialized spaceships that would protect the microbes. This is an idea we see a lot in science fiction, including Prometheus.
Still, Crick and Orgel didn’t imagine aliens dribbling DNA into our water supply — they suggested it might have been sent out in automated probes, perhaps with a kind of “missionary zeal.” The problem, which Crick and Orgel discuss in the paper, is that it’s incredibly hard to prove this hypothesis, or even to gather evidence one way or the other. That’s why most scientists who study panspermia don’t have much to say about the directed panspermia scenario. “It’s not completely ridiculous,” Purdue geophysicist Jay Melosh told io9. “It’s fun to speculate about, but it’s not the subject of really respectable scientific research because there’s no evidence.”
That said, Melosh and many other scientists do think panspermia might be part of the solution to the mystery of how life began. Directed panspermia is simply the most unlikely version of a story that is actually quite plausible. Take out the aliens and the spaceships, and you still have many possible ways that microbes from other worlds might have made it to Earth. And if those microbes came from nearby, the panspermia scenario becomes even more plausible. Cal Tech geologist Joe Kirschvink has suggested that Mars is a likely origin for life in the solar system because it would have been habitable long before Earth was. 4 billion years ago, when Earth was still a roiling cauldron of methane and magma, Mars was a stable, cool planet covered in vast oceans.
It would have been the perfect place for microbial life to take hold. But how did that life make it all the way from the seas of Mars to the seas of Earth? Most likely, meteorites crashing into Mars would send fragments of the planet’s surface back into space — packed with millions of microbes. In fact, around the time that Mars might have been developing life, the solar system was undergoing what astronomers call the “late heavy bombardment,” a time of countless intense meteorite strikes. Purdue geologist Melosh, who has spent most of his career studying meteorite impacts, has actually done experiments where he and a team recreated what might have happened when meteorites slammed into Mars billions of years ago, sending ejecta out of the atmosphere and eventually all the way to Earth.
This process is sometimes called “ballistic panspermia,” or “lithopanspermia,” because it depends on rocks being ejected into space. To recreate one part of this process in their experiments, Melosh and his team shot a bacteria-covered rock with an aluminum projectile moving at 5.4 km per second, and the shattered chunks flew over a kilometer. The bacteria survived the trauma of what Melosh and his team called “extremes of compressional shock, heating, and acceleration.” After several of these tests, Melosh and his colleagues were certain that microbes could survive one of the most destructive parts of the ballistic panspermia journey.
“A lot [of the microbes] would die, but a lot would survive in a dormant state. Their journey would take possibly millions of years. But it’s as if atmospheres are almost designed for this transfer of life. The meteorite comes from Mars, full of microbes protected from radiation by the rock. It enters Earth’s atmosphere, and as it comes in at high speed the outside melts because of friction and gets hot, but the inside is protected just like a spacecraft capsule. The microbes inside are protected. Then the aerodynamic forces in the lower atmosphere fracture the meteorite, exposing the interior.” The rock fragments rain over the land, and the surviving microbes can take hold.
Most scientists who subscribe to this idea suggest that Mars is the likely source for a ballistic panspermia event, though Melosh isn’t ruling out Jupiter’s moon Europa either. Astronomers believe Europa harbors vast oceans beneath a thick layer of ice, and it’s very possible that a meteorite could have crashed there, sending microbe-laced chunks of rocky ice into the inner solar system. Still other scientists suggest that life could even jump from one star system to another, and a recent paper on the topic explores how this could happen in star clusters.
How did we go from lifeless puddles of chemicals to strings of self-reproducing DNA on a planet that was at the time so inhospitable? Panspermia could help explain the conditions where that life evolved.
NASA planetary scientist Chris McKay offered io9 a terrific, point-by-point explanation of why panspermia is, as he put it, “a valid scientific hypothesis” worth taking seriously:
“1. The geological evidence for the earliest life on Earth is very early, soon after the end of the late bombardment. There is good evidence for life on Earth at 3.5 billion years ago, indirect evidence at 3.8 billion. The end of the late heavy bombardment is 3.8 billion years ago.
2. The genetic evidence indicates that the last universal common ancestor (LUCA) of life could have been roughly 3.5 billion years ago (but with large uncertainties) and that LUCA was a fairly sophisticated life form in terms of metabolic and genetic capabilities.
1 and 2 together give the impression that life appeared on Earth soon after the formation of suitable environments and it appears to have come in being remarkably developed – like Athena born fully formed from the head of Zeus.
3. Rocks from Mars have traveled to Earth and the internal temperatures experienced in these rocks during this trip would not have sterilized the interiors. Thus in principle life can be carried from Mars to Earth.
4. Mars did not suffer the large Moon-forming impact that would have been detrimental to the early development of life on Earth.
3 and 4 have lead to the suggestion that Mars would have been a better place for life to start in the early Solar System and it could have then been carried to Earth via meteorites.
5. Organic molecules are widespread in comets, asteroids, and the interstellar medium.
6. Comets could have supported subsurface liquid water environments soon after their formation due to internal heating by decay of radioactive aluminum.
7. As comets move past the Earth they shed dust which settles into Earth’s atmosphere.
5, 6 and 7 have lead to the suggestion that life could have started in the interstellar medium or in small bodies such as comets and then been carried to the Earth by comet dust. So, yes panspermia is a valid scientific hypotheses and warrants further investigation.”
And as we engage in that investigation, maybe we’ll discover more than we bargained for. After all, if we owe our existence to life on other planets in our solar system, that makes a strong case for life outside it too. SETI astronomer Jill Tarter told io9 via email: “I think that intelligent life here on Earth is a proof of concept that it could exist elsewhere, but we will not know unless we search systematically and exhaustively enough to accumulate sufficient information to justify significant null result. Remember the last sentence of the 1959 Cocconi and Morrison paper [published in Nature]: “The probability of success is difficult to estimate, but if we never search, the chance of success is zero.” The search for life’s origins on Earth could ultimately lead to the discovery of extraterrestrial life, too.”
INVASIVE SPECIES on MARS
BIOSIGNATURES on VENUS