The Ancient Mars Water Story, Updated

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Rendering of Gale Lake some 3.5 billion years ago, when Mars was warmer and much wetter. The Curiosity mission is finding that Gale Crater water-changed rock is everywhere.
Rendering of Gale Lake some 3.5 billion years ago, when Mars was warmer and much wetter. The Curiosity mission is finding that rocke in Gale Crater changed by water everywhere. (Evan Williams, with data from the Mars Reconnaissance Orbiter HIRISE project)

Before the Curiosity rover landed on Mars, NASA’s “follow the water”maxim had already delivered results that suggested a watery past and just maybe some water not far below the surface today that would periodically break through on sun-facing slopes.

While tantalizing — after all, the potential presence of liquid water on a exoplanet’s surface is central to concluding that it is, or once was, habitable — it was far from complete and never confirmed via essential ground-truthing.

Curiosity famously provided that confirmation early on with the discovery of pebbles that had clearly been shaped in the presence of flowing surface water, followed by the months in Yellowknife Bay which proved geologically, geochemically and morphologically the long-ago presence of substantial amounts of early Martian water.

Some of the earliest drilling was into mudstone that looked very much like a dried up basin or marsh, and that was exactly what Curiosity scientists determined it was, at a minimum.  It took many months for Curiosity leaders to ever use the word “lake” to describe what had once existed on the site, but now it is a consensus description.

Since the presence of a fossil lake was confirmed and announced, the water story has taken something of a backseat as the rover made its challenging and revelatory way across the lowlands of Gale Crater, through some dune fields and onto the Murray formation — a large geological unit that is connected to the base of Mount Sharp itself.  And all along the path of the rover’s traverse mudstone and sandstone were present, a clear indication of ever larger amounts of water.

I spoke recently with geologist and biogeologist John Grotzinger, the former NASA chief scientist for Curiosity and now a member of the science team, to get a sense of how things had progressed for the Gale water story.  He said there was no longer any doubt that the crater was once quite filled with water.

“We have  not seen a single rock at Gale that doesn’t say that the planet was wet.  In the areas where the rover has driven, I’d be very comfortable now in saying that the surface and ground water was often present for millions to tens of millions of years.”

Gale crater mudstone
Gale Crater mudstone at the Kimberly site. (NASA/JPL-Caltech)

Grotzinger said that the depth of the lakes, basins and playas clearly varied and are not well defined, but the rover’s newest extended mission will shed some light on the issue.  That’s because it is now (four years-plus after landing)  going to be actually climbing Mount Sharp, it’s original mission goal.

This is of great interest because Mars scientists already know that ahead lies fields of hematite, sulfates and phyllosilicates (clay), all minerals identified from orbit that can only form in water.  These deposits higher up the mountain can make the case for a deeper Gale Lake, or they could tell of up-welling ground water.  But in either case, they make the case for a watery ancient Mars.

There are innumerable ways in which this Gale water story is important.  Since it has been pretty well established that Gale Crater was formed by an asteroid impact 3.8 to 3.7 billion years ago, Grotzinger said that there is some consensus around the view that the water was present at least in the 3.5 to 3.6 billion years ago range.

While those are indeed ancient times — the planet was formed about 4.5 billion years ago —  it is quite a bit more recent than what was earlier considered to be the end of the period that early Mars might have supported surface water.  In those more conventional models, by 3.5 billion years ago Mars was parched, very cold, and had only the remnants of a protective atmosphere and magnetic field.  Yet now it appears that water was common, maybe plentiful.

“Clearly,” Grotzinger said, “there has to be some rethinking about ancient Mars and water.  It used to be that watery Mars was thought of as being in the 4 billion years time frame.  That has to be revised.”

 

Curiosity arm at Murray buttes, in the Murray formation. The endless acres of mudstone are visible. (NASA/JPL-Malin & Edgett)
Curiosity arm at Murray buttes, in the Murray formation. The endless acres of mudstone are visible. (NASA/JPL-Caltech/Malin and Edgett)

This presence of substantial amounts of water as late (or later) than 3.5 billion years ago has presented a major problem for Mars climate scientists.  By their calculations, there was essentially no way that abundant surface water could be present at that time — especially because of the “faint young sun” paradox.

As first put forward by Carl Sagan and colleagues, the paradox is this:  astrophysicists know from the study of stars like our sun that they begin with some 70 percent of the luminosity they will ultimately and gradually reach, and that as a result Mars (and Earth) would have been much colder in early days than it is now.   And it’s very cold indeed now.

There has been much discussion in recent years of various ways that a greenhouse effect could have warmed Mars (and Earth) during that early period, but noting conclusive or consensus-building has been identified geochemically on the surface or in the remaining atmosphere of Mars.

What’s more, in order for Gale Lake — and no doubt many others like it – to survive for as long as it apparently did requires a water cycle to replenish the water that is lost.  This is where one of the most intriguing and controversial questions about the Mars water story comes in.

It has long been known that much of the northern section of Mars is significantly lower than the southern highlands, and that the lowlands have far fewer geological features.  These observations led to the hypothesis some time ago that there was once a large northern ocean on Mars that could replenish the lakes and rivers of the south.

Artist rendering of a possible northern ocean on Mars. (NASA/ JPL-Caltech.)
Artist rendering of a possible northern ocean on Mars. (NASA Goddard Space Flight Center.)

The possible presence of such an ocean has been studied and debated for some time, but Grotzinger said only now is he “getting more sympathetic to the notion.”  Many others are also becoming more open to being persuaded  because it is extremely difficult to explain the proven existence of large amounts of surface water elsewhere on Mars without such a big liquid source.

Other recent findings and insights are pointing to the existence of a northern ocean as well.  For instance, a paper by Michael Mumma and Geronimo Villanueva of the NASA Goddard Astrobiology Center published last year in the journal Science estimated that a Martian ocean once covered 19 percent of the planet.  They used ratio measurements of the presence of two variants of water — regular H2O and deuterium or “heavy water” — to conclude that vast amounts of regular water had escaped from Mars over the eons.

And just this summer a team led by Alexis Palmero Rodriguez from the Planetary Science Institute in Tuscon, Arizona found evidence of what they described as ancient tsunami waves on Mars.  If confirmed, they could help explain one of the puzzling issues surrounding a potential northern ocean — that features of a shoreline have not been detected so far.

“So, we think this is going to remove a lot of the uncertainty that surrounds the ocean hypothesis,” Rodriguez told BBC News as the tsunami finding. “Features that have in the past been interpreted as relating to an ocean have been controversial; they can be explained by several, alternative processes. But the features we are describing – such as up-slope flows including large boulders – can only be explained in terms of tsunami waves.”

Co-author Alberto Fairen of the Centre for Astrobiology in Madrid said that the team concluded that a big meteorite impact triggered the first tsunami wave about 3.4 billion years ago.

He said the wave was composed of liquid water and formed widespread backwash channels to carry the water back to the ocean.

Their work, which was published in Scientific Reports, centers on two connected regions of Mars, known as Chryse Planitia and Arabia Terra — quite far from Gale Crater.

Tsunami-born sediments (arrow) inundate the land in an upslope direction (towards bottom-right)
Possible tsunami-deposited sediments (arrow) inundate the land in an upslope direction, towards bottom-right. (Alexis Rodruigez, Lunar and Planetary Institute)

While the potential existence of an ancient Martian ocean remains the subject of hot debate, the overall Mars water story is now considered pretty firm and with major implications for the potential presence of life on the planet.  Early Mars has already been deemed “habitable” by Curiosity scientists in terms of its geochemistry and more, and the presence of lakes or ocean water on the surface for tens of millions of years (or more) could certainly provide conducive places for life to form.

So a next step for Mars science is to determine what kind of Martian minerals best preserve organic material and potential signatures of long-ago life.  This field of study is called taphonomy, and Grotzinger was at a taphonomy conference at Williams College when I spoke with him.

ohn P. Grotzinger is the Fletcher Jones Professor of Geology at California Institute of Technology and chair of the Division of Geological and Planetary Sciences.
John P. Grotzinger is the Fletcher Jones Professor of Geology at California Institute of Technology and chair of the Division of Geological and Planetary Sciences. He spent four years as chief scientist for the Curiosity mission. (NASA)

“We’re definitely turning the corner from habitability to taphonomy,” Grotzinger said.  “This is to prepare for the 2020 mission”  to Mars, during which intriguing rocks will be identified for future sample returns to Earth.

The way that exoplanets are studied now and will be in the future is, of course, quite different from what is possible on Mars.

But there are strong parallels in terms of the importance of water and understanding the atmospheric make-up and geochemistry, and there’s this widely-accepted maxim from the world of astrobiology:  if a second form of life is ever found on Mars or anywhere else in our solar system, the likelihood that life is common in the cosmos grows exponentially.

Clearly, to have life start twice in our one solar system would make the search for life in other solar systems that much more compelling and pressing.

 

 

 

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One Planet, But Many Different Earths

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Artist conception of early Earth. (NASA/JPL-Caltech)
Artist conception of early Earth. (NASA/JPL-Caltech)

We all know that life has not been found so far on any planet beyond Earth — at least not yet.  This lack of discovery of extraterrestrial life has long been used as a knock on the field of astrobiology and has sometimes been put forward as a measure of Earth’s uniqueness.

But the more recent explosion in exoplanet discoveries and the next-stage efforts to characterize their atmospheres and determine their habitability has led to rethinking about how to understand the lessons of life of Earth.

Because when seen from the perspective of scientists working to understand what might constitute an exoplanet that can sustain life,  Earth is a frequent model but hardly a stationary or singular one.  Rather, our 4.5 billion year history — and especially the almost four billion years when life is believed to have been present  — tells many different stories.

For example, our atmosphere is now oxygen-rich, but for billions of years had very little of that compound most associated with complex life.  And yet life existed.

The same with temperature.  Earth went through snowball or slushball periods when most of the planet’s surface was frozen over.  Hardly a good candidate for life, and yet the planet remained habitable and inhabited.

And in its early days, Earth had a very weak magnetic field and was receiving only 70 to 80 percent as much energy from the sun as it does today.  Yet it supported life.

“It’s often said that there’s an N of one in terms of life detected in the universe,” that there is but one example, said Timothy Lyons, a biogeochemist and distinguished professor at University of California, Riverside.

“But when you look at the conditions on Earth over billion of years, it’s pretty clear that the planet had very different kinds of atmospheres and oceans, very different climate regimes, very different luminosity coming from the sun.  Yet we know there was life under all those very different conditions.

“It’s one planet, but it’s silly to think of it as one planetary regime. Each of our past chapters is a potential exoplanet.”

 

A rendering of the theorized "Snowball Earth" period when, for millions of years, the Earth was entirely or largely covered by ice, stretching from the poles to the tropics. This freezing happened over 650 million years ago in the Pre-Cambrian, though it's now thought that there may have been more than one of these global glaciations. They varied in duration and extent but during a full-on snowball event, life could only cling on in ice-free refuges, or where sunlight managed to penetrate through the ice to allow photosynthesis.
A particularly extreme phase of our planet’s history is called  the “Snowball Earth” period.  During these episodes, the Earth’s surface was entirely or largely covered by ice for millions of years, stretching from the poles to the tropics. One such freezing happened over 700 to 800 million years ago in the Pre-Cambrian, around the time that animals appeared. Others are now thought to have occurred much further back in time. They varied in duration and extent but during a full-on snowball event, life could only survive in ice-free refuges, or where sunlight managed to penetrate through the ice to allow photosynthesis.

Lyons is the principal investigator for one of the newer science teams selected to join the NASA Astrobiology Institute (NAI), an interdisciplinary group hat calls itself “Alternative Earths.”

Consisting of 23 scientists from 14 institutions, its self-described mission is to address and answer these questions: How has Earth remained persistently inhabited through most of its highly changeable history?  How has the presence of very different kinds of lifeforms been manifested in the atmosphere, and simultaneously been captured in what would become the rock record? And how might this approach to early Earth help in the search for life beyond Earth?

“The idea that early Earth can help us understand other planets and moons, especially in our solar system, is certainly not new,” Lyons said.  “Scientists have studied possible Mars analogues and extreme life for years.  But we’re taking it to the next level with exoplanets, and pushing hard on the many ways that conditions on early Earth can help us study exoplanet atmospheres and habitability.”

The importance of this work was apparent at a recent workshop on biosignatures held by NASA’s initiative, the Nexus for Exoplanet System Science (NExSS.)  As Earth scientists, Lyons and his group are expert at finding proxy records in ancient rocks that hold information important to exoplanet scientists (among others) want to know.

Those proxy fingerprints occur as elemental, molecular, and isotopic properties preserved in rocks that correspond to ancient characteristics in the ocean or atmosphere that can no longer be observed directly.

“We can’t measure the pH in ancient oceans, and we can’t measure the composition of ancient atmospheres,” Lyons said.  “So what we have to do is go to the chemistry of ocean and land deposits formed at the same time and look for the chemical fingerprints locked away and preserved.”

At the exoplanet biosignatures workshop, Lyons was struck by how eager exoplanet modelers were to learn about the proxy chemicals they could profitably put in their models for clues about how distant planet atmospheres might form and behave.  It’s clear that no single element or compound will be a silver bullet for understanding whether there’s life on an exoplanet, but a variety of proxy results together can begin to tell an important story.

chromium
The element chromium and its isotopes have become important proxies for the measurement of oxygen levels in the atmosphere of early Earth and have led to some revised theories about when those concentrations jumped.  Understanding the potential makeup of early Earth’s atmosphere and oceans is a pathway to understanding exoplanets.

“We told them about the range of things they should be modeling and, wow, they were interested.  I was thinking at the time that ‘you guys really need us — and vice versa.'”

Some of the researchers most intrigued by potentially new geochemical proxies from the University of Washington’s Virtual Planetary Laboratory,  They’ve been a pioneer in modeling how different atmospheric, geological, stellar and other factors characterize particular kinds of planets and solar systems and their possibilities for life.

In keeping with the growing connection between exoplanet and Earth science, Lyons just brought one of the VPL top modellers, Edward Schwieterman, to UC Riverside for a postdoc as part of the Alternative Earths project.

Among his initial projects will take the new data being generated by the Alternative Earths team about the atmosphere and oceans of early Earth, and model what would happen on a planet with that kind of atmosphere if it was orbiting a very different type of star from our own.

“It’s a direct use of early Earth research on exoplanet studies, and is exactly the kind of work we plan to do be doing,” Lyons said. “Eddie is the perfect bridge between the lessons learned from early Earth and their implications for exoplanets.”

Banded iron formations Karijini National Park, Western Australia. The layers of reddish iron show the presence of oxygen, which bonded with the iron to form a rust-like iron oxide. These formations date most commonly from the period of 2.4 to 1.9 billion years ago, after the Great Oxidation Event.
Banded iron formations at Karijini National Park, Western Australia. The layers of reddish iron point to an early ocean poor in oxygen and rich in dissolved iron. These formations date most commonly from the periods just before and right after the Great Oxidation Event, which spanned from about 2.4 to 2.0 billion years ago. Their distributions over times and their chemical properties are key proxies for the tempo and fabric of the earliest permanent oxygenation of Earth’s atmosphere.

Lyons, along with colleagues Christopher Reinhard of Georgia Tech and Noah Planavsky of Yale and other members of their Alternative Earths team, are especially focused on an effort to understand Earth’s atmosphere—as tracked in the rock record—over the eons and especially the levels of oxygen present.

The concentration of oxygen in the atmosphere is now about about 21 percent and, by some estimates, reached as high as 35 percent within the past 500 million years.

In comparison, early Earth had but trace amounts of oxygen for two billion years before what is called the Great Oxidation Event—when marine O2-producing photosynthesis outpaced reactions that consumed O2 and allowed for the beginnings of its permanent accumulation in the atmosphere.  Estimated to have occurred 2.4 billion years ago, it began (or was part of) an oxidizing process that led to ever more complex life forms over the following one to two billion years.

Timothy Lyons, distinguished professor of geobiochemistry at the University of California, Riverside. He is also the principal investigator of a National Astrobiology Institute project xxx.
Timothy Lyons, distinguished professor of biogeochemistry at the University of California, Riverside. He is also the principal investigator of a National Astrobiology Institute project “Alternative Earths:  Explaining Persistent Inhabitation on a Dynamic Early Earth.

There is a spirited scientific debate underway now about whether that “Great Oxidation Event” triggered permanently high levels of oxygen in the atmosphere and the oceans, or whether it began an up and down process through which the presence of oxygen was quite unstable and still well below current levels until relatively recent times.

Lyons and Reinhard are of the “boring billion” school, arguing that oxygen levels did not head continuously upwards after the Oxidation Event, but rather stayed relatively stable and still very low for most of a billion and half years after the Great Oxidation Event and continued to challenge O2-requiring life—for almost a third of Earth history.

This would be primarily an Earth science issue if not for the fact that oxygen — on its own and in conjunction with other compounds — is among the most prominent and promising biosignatures that exoplanet scientists are looking for.

Christopher Reinhard, Georgia Institute of technology. (Brad...
Christopher Reinhard of Georgia Institute of Technology and Alternative Earths project. (Ben Brumfield/Georgia Tech.)

In fact, not that long ago, it was widely accepted that a discovery of oxygen and/or ozone in the atmosphere of a planet pretty much proved, or at least strongly suggested, the presence of some sort of biology on the planet below.  That view has been modified of late by the identification of ways that free oxygen can be formed abiotically (without the presence of photosynthesis and life), potentially producing false positives for potential life.

While the field is a long way from an active search for direct, in situ fingerprints of life on exoplanets light years away, oxygen and its relationship with other atmospheric gases remains a lodestar in thinking about what biosignatures to search for. The technology is already in place for characterizing the compositions of very distant atmospheres.

And this is where, for Lyons, Reinhard and others, things get both interesting and complicated.

For more than a billion years before the Great Oxidation Event Earth demonstrably supported life.  It consisted mostly of anaerobic microbes that did just fine without oxygen, but in many cases needed and produced methane, an organic compound with one carbon atom and four hydrogens.

So if an exoplanet scientist from a distant world were to search for life on Earth during that period via the detection of oxygen only, they would entirely miss the presence of an already long history of life.  Searching for a potentially large-scale presence of methane might have been more productive, though that is a source of rigorous debate as well.

 

An image of a rock with fossilized stromatolites, tiny layered structures from 3.7 billion years ago that are remnants from a community of microbes. Found in a newly melted part of Greenland, Australian scientists reported in the journal Nature that the stromatolites lived on an ancient seafloor at a time when Earth's skies were orange and its oceans green. They describe the stromatolites as perhaps the oldest fossil found so far on Earth. (Allen Nutman/University of Wollongong)
An image of a rock with fossilized stromatolites, tiny layered structures from 3.7 billion years ago that are remnants from a community of microbes. Found in a part of Greenland new exposed by melting glaciers, Australian scientists reported in the journal Nature that the stromatolites lived on an ancient seafloor at a time when Earth’s skies may well have been orange and its oceans green. They describe the stromatolites as perhaps the oldest fossil found so far on Earth, although chemical suggestions of life may extend further back in time . (Allen Nutman/University of Wollongong)

Because both oxygen and methane can be formed without life, a current gold standard for detecting future biosignatures on exoplanets is the presence of the two together.  As a result of the way the two interact, they would remain in an atmosphere together only if both were being replenished on a substantial, on-going scale.  And as far as is now understood, the only way to do that is through biology.

Yet as described by Reinhard, the most current research suggest that oxygen and methane were probably never in the Earth’s atmosphere together at levels that would be detectable from afar.  There is some evidence that Earth’s atmosphere held a lot of methane in its early times, and there has been a lot of oxygen for the past 600 million years or so, but as one grew in concentration the other declined — and during the “boring billion” both were likely low.

“So we have a complicated situation here where using the best exoplanet biosignatures we have now, intelligent beings looking at Earth over the past 4.5 billion years would not find a convincing signature of life for most, or maybe all, of that time if they relied only on co-occurrence of oxygen and methane,” Reinhard said.  Yet there has been life for at least 3.7 billion years, and those beings studying Earth would have come up with a very false negative.

Lyons insists this is should not be a source of pessimism in the search for life on exoplanets, instead it is a “call to arms for new and more creative possibilities rather than the lowest hanging fruit.” It’s a challenge “to help us sharpen our thinking in a search that was never going to be easy.”

And the best test bed available for coming up with different answers, he said, may very well be the many different Earths that have come and gone on our planet.

 

 

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Earth: A Prematurely Inhabited Planet?

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A schematic of the history of the cosmos since the Big Bang identifies the period when planets began to form, but there's indication of when life might have started. Harvard's Avi Loeb wants to put life into this cosmological map, and foresees much more of it in the future, given certain conditions. ( NASA)
A schematic of the history of the cosmos since the Big Bang identifies the period when planets began to form, but there’s no indication of when life might have started. Harvard’s Avi Loeb wants to add life into this cosmological map, and foresees much more of it in the future, given certain conditions. ( NASA)

The study of the formation and logic of the universe (cosmology) and the study of exoplanets and their conduciveness to life do not seem to intersect much.  Scientists in one field focus on the deep physics of the cosmos while the others search for the billions upon billions of planets out there and seek to unlock their secrets.

But astrophysicist and cosmologist Avi Loeb — a prolific writer about the early universe from his position at the Harvard-Smithsonian Center for Astrophysics– sees the two fields of study as inherently connected, and has set out to be a bridge between them.  The result was a recent theoretical paper that sought to place the rise of life on Earth (and perhaps elsewhere) in cosmological terms.

His conclusion:  The Earth may well be a very early example of a living biosphere, having blossomed well before life might be expected on most planets.   And in theoretical and cosmological terms, there are good reasons to predict that life will be increasingly common in the universe as the eons pass.

By eons here, Loeb is thinking in terms that don’t generally get discussed in geological or even astronomical terms.  The universe may be an ancient 13.7 billion years old, but Loeb sees a potentially brighter future for life not billions but trillions of years from now.  Peak life in the universe, he says, may arrive several trillion years hence.

“We used the most conservative approaches to understanding the appearance of life in the universe, and our conclusion is that we are very early in the process and that it is likely to ramp up substantially in the future,” said Loeb, whose paper was published in the Journal of Cosmology and Astroparticle Physics.

“Given the factors we took into account, you could say that life on Earth is on the premature side.”

 

The Earth was formed some 4.5 billions years ago, and life that existed as long ago as 3.5 to 3.8 billion years ago has been discovered. Harvard astrophysicist Avi Loeb argues that life on Earth may well be "premature" in cosmological terms, and that many more planets will have biospheres in the far future. (xxx)
The Earth was formed some 4.5 billion years ago, and signs of life have been discovered that are 3.5 to 3.8 billion years old. Harvard astrophysicist Avi Loeb, with co-authors Rafael Batista and David Sloan of the University of Oxford, argue that life on Earth may well be “premature” in cosmological terms, and that many more planets will have biospheres in the far future.  This artist rendering of early Earth was created by NASA’s  Goddard Space Flight Center Conceptual Image Lab.

This most intriguing conclusion flows from the age of the universe, the generally understood epochs when stars and then planets and galaxies formed, and then how long it would take for a planet to cool off enough to form the chemical building blocks of life and then life itself.  Given these factors, Loeb says, we’re early.

In the long term, the authors determined, the dominant factor in terms of which planets might become habitable proved to be the lifetime of stars. The higher a star’s mass, the shorter its lifetime. Stars larger than about three times the sun’s mass will burn out well before any possible life has time to evolve.

Our sun is a relatively large and bright star, which is why its lifetime will be relatively short in cosmological terms (all together, maybe 11 billion years, with 4.5 billion already gone.)  But smaller stars, the “red dwarf,” low-mass variety, are both far more common in the universe and also much longer lived — as in trillions of years.

These smallest stars generally have less than 10 percent the mass of our sun, but they burn their fuel (hydrogen and helium) much more slowly than a larger star.  Indeed, some may glow for 10 trillion years, Loeb says, giving ample time for life to emerge on any potentially habitable planets that orbit them.  What’s more, there’s every reason to believe that the population of stars in the galaxy and cosmos will increase significantly, giving life ever more opportunity to commence.

Abraham Loeb, usually called "Avi," is the chairman of the Harvard Astronomy Department and xxxx CFA. Mac G. Schumer, Harvard Crimson Mac G. Schumer, Harvard Crimson
Abraham, or Avi, Loeb, is the chairman of the Harvard Astronomy Department and director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics. (Mac G. Schumer, Harvard Crimson)

As a result, the relative probability of life grows over time. In fact, chances of life are 1,000 times higher in the distant future than now.

This calculation, however, comes with a major caveat:  Scientists are sharply divided about whether or not a star much smaller than ours can actually support life.

The potential obstacles are many — an insufficient amount of heat and energy emanating from the star unless the planet is close in, the fact that red dwarf stars have powerful, luminous beginnings that could send a nearby planet into a runaway greenhouse condition that might result in permanent sterilization, and that many planets around red dwarfs would be close to the stars and consequently tidally locked.  That means that one side of the planet would always face the star and be light, while the other would continue in eternal darkness.  This was earlier considered to be a pretty sure deterrent to life.

Recent theoretical analyses of planets around these red dwarfs, however, suggests that life could indeed emerge.  It could potentially survive at the margins — where day turns into night and the temperatures would likely be stable– and also in other dayside regions were temperatures could be moderated by clouds and winds.  But no observations have been made to substantiate the theory.

Because of their relatively cool temperatures and resulting low brightness, individual red dwarfs are nearly impossible to see with the naked eye from Earth. But they’re out there.

The nearest star to our sun, Proxima Centauri, is a red dwarf, as are twenty of the next thirty nearest stars.  Data from the Kepler Space Telescope suggests that as many as 25 percent of red dwarfs have planets orbiting in their habitable zones — neither too hot nor too cold to keep liquid water from sometimes pooling on their surfaces.

 

Flares from our sun and from a red dwarf star.
Flares from our sun and from a red dwarf star.  These powerful bursts of energy and heat may be larger on the sun, but in percentage terms they are greater on many red dwarf stars. Since planets around red dwarfs are much closer than larger stars like our sun, those energy bursts might sterilize red dwarf planets. (NASA)

“I think we can and we should test these theories in the years ahead with observations,” Loeb said. “We should be able to tell if nearby low-mass stars have life around them” in the decades ahead.

And if red dwarfs can support life, then the future for life in the universe is indeed grand.

The merging of cosmological theory and astronomical observation that Loeb has in mind would indeed be unusual, but it is nonetheless consistent with the interdisciplinary nature of much of the broader search for life beyond Earth.  That effort has already brought together astrophysicists and geoscientists, astronomers and biologist.  It’s just way too big for one discipline.

An interesting sidelight to Loeb’s argument that Earth may well be among the earliest planets where life appeared and continued is that it would provide a solution to the extraterrestrial life puzzle known as Fermi’s Paradox.

It was in 1950 that renowned physicist Enrico Fermi was talking with colleagues over lunch about the predicted existence of billions of still-to-be-discovered planets beyond our solar system, and the likelihood that many had planets around them.  Fermi also was convinced that the logic of the vast numbers and of evolution made it certain that intelligent, technologically-advanced life existed on some of those planets.

It was an era of fascination with aliens, flying saucers and the like, but there actually were no confirmed reports of visitations by extraterrestrial life.  Ever, it seemed.

If intelligent life is common in the universe, Fermi famously wondered, “Then where is everybody?”

There are many potential answers to the question, including, of course, that we are alone in the universe.  The possibility that Earth might be among the very early planets with life has not been put forward before, but Loeb said that now it has been.

 

If the conclusion is correct that the Earth is most likely among the first planets to support life, then the famous Fermi Paradox could be easily resolved.
If the conclusion is correct that the Earth is most likely among the first planets to support life, then the famous Fermi Paradox could be easily resolved.

“Our view is that we’re at the very beginning of life in the universe, we’re just ramping up,” he said.  “So of course we haven’t been visited by anything extraterrestrial.”

As a congenital thinker in the very long term, Loeb also raised the issue of whether it makes sense for human life to remain on Earth and in our solar system.  The sun, after all, will run out of fuel in those remaining six billion years, will expand enormously as that occurs, and then will re-emerge as a super-dense white dwarf star.  Any biology in our solar system would have been destroyed long before that.

But Proxima Centauri, one of those very long-lived stars?

“It will be there a very long time,” he said.  “If the conditions are right, then maybe a time will come to migrate to any planets that might be around Proxima.  It’s four light years away, so it would take generations of humans to get there.  Certainly very difficult, but some day in the far future people may be faced with an alternative that’s considerably worse.”

 

 

 

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A Flood of Newly Confirmed Exoplanets

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Artist renderings of exoplanets previously detected by the Kepler Space Telescope (NASA)
Artist renderings of exoplanets previously detected by the Kepler Space Telescope (NASA)

In the biggest haul ever of new exoplanets, scientists with NASA’s Kepler mission announced the confirmation of 1,284 additional planets outside our solar system — including nine that are relatively small and within the habitable zones of their host stars.  That almost doubles the number of these treasured rocky planets that orbit their stars at distances that could potentially support liquid water and potentially life.

Prior to today’s announcement, scientists using Kepler and all other exoplanet detection approaches had confirmed some 2,100 planets in 1,300 planetary systems.  So this is a major addition to the exoplanets known to exist and that are now available for further study by scientists.

These detections comes via the Kepler Space Telescope, which collected data on tiny decreases in the output of light from distant stars during its observing period between 2009 and 2013.  Those dips in light were determined by the Kepler team to be planets crossing in front of the stars rather than impostors to a 99 percent-plus probability.

As Ellen Stofan, chief scientist at NASA Headquarters put it,  “This gives us hope that somewhere out there, around a star much like ours, we can eventually discover another Earth.”

he histogram shows the number of planet discoveries by year for more than the past two decades of the exoplanet search. The blue bar shows previous non-Kepler planet discoveries, the light blue bar shows previous Kepler planet discoveries, the orange bar displays the 1,284 new validated planets. (NASA Ames/W. Stenzel; Princeton University/T. Morton)
The histogram shows the number of planet discoveries by year for more than the past two decades of the exoplanet search. The blue bar shows previous non-Kepler planet discoveries, the light blue bar shows previous Kepler planet discoveries, the orange bar displays the 1,284 new validated planets.
(NASA Ames/W. Stenzel; Princeton University/T. Morton)

The primary goals of the Kepler mission are to determine the demographics of exoplanets in the galaxy, and more specifically to determine the population of small, rocky planets (less than 1.6 times the size of Earth) in the habitable zones of their stars.  While orbiting in such a zone by no means assures that life is, or was, ever present, it is considered to be one of the most important criteria.

The final Kepler accounting of how likely it is for a star to host such an exoplanet in its habitable zone won’t come out until next year.  But by all estimations, Kepler has already jump-started the process and given a pretty clear sense of just how ubiquitous exoplanets, and even potentially habitable exoplanets, appear to be.

“They say not to count our chickens before they’re hatched, but that’s exactly what these results allow us to do based on probabilities that each egg (candidate) will hatch into a chick (bona fide planet),” said Natalie Batalha, co-author of the paper in the Astrophysical Journal and the Kepler mission scientist at NASA’s Ames Research Center.

“This work will help Kepler reach its full potential by yielding a deeper understanding of the number of stars that harbor potentially habitable, Earth-size planets — a number that’s needed to design future missions to search for habitable environments and living worlds.”

Batalha said that based on observations and statistics the Kepler mission has produced so far, we can expect that there are some 10 billion relatively small, rocky  (and potentially habitable) planets in our galaxy.  And with those numbers in mind, she said, the closest is likely to be in the range of 11 light years away.

She said that all of the exoplanets found in habitable zones are in the “exoplanet Hall of Fame.”  But she said two of the newly announced planets in habitable zones, Kepler 1286b and Kepler 1628b, joined two previous exoplanets of particular interest either because of their size (close to that of Earth) or their Earth-like distance from suns rather like ours.

Batalha said a new and finely-tuned software pipeline has been developed to better analyze the data collected during those four years of Kepler observations.  Asked if the final Kepler catalogue of exoplanets, expected to be finished next summer, would increase the current totals of exoplanets found, she replied:  “It wouldn’t surprise me if we had hundreds more to add.”

Since Kepler launched in 2009, 21 planets less than twice the size of Earth have been discovered in the habitable zones of their stars. The orange spheres represent the nine newly validated planets announcement on May 10, 2016. The blue disks represent the 12 previous known planets. These planets are plotted relative to the temperature of their star and with respect to the amount of energy received from their star in their orbit in Earth units. (NASA Ames/N. Batalha and W. Stenzel)
Since Kepler launched in 2009, 21 planets less than twice the size of Earth have been discovered in the habitable zones of their stars. The orange spheres represent the nine newly validated planets announcement on May 10, 2016. The blue disks represent the 12 previous known planets. These planets are plotted relative to the temperature of their star and with respect to the amount of energy received from their star in their orbit in Earth units. (NASA Ames/N. Batalha and W. Stenzel)

Once the Kepler exoplanet list is updated, scientists around the world will begin to study some of the most surprising, enticing, and significant finds.  Kepler can tell scientists only the location of a planet, its mass and its distance from the host star.  So the job of further characterizing the planets — and ultimately determining if any are indeed potentially habitable — requires other telescopes and techniques.

Nonetheless, Kepler’s ability to give scientists a broad picture of the distribution of exoplanets — to find large numbers of them rather than, as pre-Kepler, one or two at a time — has been revolutionary.  It has also been remarkably speedy, thanks in large part to an automated system of analyzing transit data devised by Tim Morton, a research scientist at Princeton University,

“Planet candidates can be thought of like bread crumbs,” Morton said in a NASA teleconference. “If you drop a few large crumbs on the floor, you can pick them up one by one. But, if you spill a whole bag of tiny crumbs, you’re going to need a broom. This statistical analysis is our broom.”

Kepler identified another 1,327 candidates that are very likely to be exoplanets, but didn’t meet the 99 percent certainty level required to be deemed an exoplanet.

A large percentage of the newly confirmed planets are either “super-Earths” or “sub-Neptunes” — planets in a size range absent in our solar system.  Initially, the widespread presence of exoplanets of these dimensions was a puzzle to the exoplanet community,  but now the puzzle is more why they are absent in our system.

Despite the abundance of these exoplanets — which are believed to be mostly gas or ice giants — scientists are convinced there are considerably more rocky, even Earth-sized planets that current telescopes can’t detect.

 The size distribution of discovered exoplanet has been a surprise to scientists. The blue bars on the histogram represent all previously verified exoplanets by size. The orange bars on the histogram represent Kepler's 1,284 newly validated planets. (NASA Ames/W. Stenzel)
The size distribution of discovered exoplanet has been a surprise to scientists. The blue bars on the histogram represent all previously verified exoplanets by size. The orange bars on the histogram represent Kepler’s 1,284 newly validated planets. (NASA Ames/W. Stenzel)

 

The primary Kepler mission focused on one small piece of the sky — about 0.25 percent of it — and a distant part at that. It watched nonstop for transiting planets in that space for four years, watching unblinkingly at some 150.000 stars. The result has been a treasure trove of data that can then be broadened statistically to tell us about the entire galaxy.

So Kepler has revolutionized our understanding of the galaxy and what’s in it, and has proven once and for all that exoplanets are common.  But the individual planets that it has detected are unlikely to be the ones that allow for breakthroughs in terms of sniffing out what chemicals are in their atmospheres — an essential process for determining if a potentially habitable planet actually has some of the ingredients for life.

This is because Kepler was looking far into the cosmos, between 600 and 3,000 light years from our sun.  While the telescope identified almost 5,000 “candidate planets” during its four years of primary operation — and now more than 2,200 confirmed planets — the planets are generally considered too distant for the more precise follow-up observing needed to understand their atmospheres and chemical make-ups.

This work will fall to ground-based telescopes looking at nearer stars, and to future generations of American and European space telescopes using the transit method of detection pioneered by Kepler. (See graphic above.)  The next space satellite in line is NASA’s Transiting Exoplanet Survey Satellite  (TESS), which is scheduled to launch in 2017 and will focus on planets orbiting much closer and brighter stars.  The long-awaited James Webb Space Telescope, due to launch in 2018, also has the potential to study exoplanets with a precision, and in wavelengths, not available before.

NASA has begun development of the more sophisticated Wide Field Infrared Survey Satellite (WFIRST) to further exoplanet research in the 2020s,  and has set up formal science and technology definition teams to plan for a possible flagship exoplanet mission for the 2030s.  That mission would potentially have the power and techniques to determine whether an exoplanet actually has the components, or the presence, of life.

 

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Cloudy, With a Chance of Iron Rain

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Analysis of data from the Kepler space telescope has shown that roughly half of the dayside of the exoplanet Kepler-7b is covered by a large cloud mass. Statistical comparison of more than 1,000 atmospheric models show that these clouds are most likely made of Enstatite, a common Earth mineral that is in vapor form at the extreme temperature on Kepler-7b. These models varied the altitude, condensation, particle size, and chemical composition of the clouds to find the right reflectivity and color properties to match the observed signal from the exoplanet. Courtesy of NASA (edited by Jose-Luis Olivares/MIT)
Many exoplanets being discovered are covered with thick clouds, offering an opportunity to analyze their compositions but hiding the lower atmosphere and surface from measurement and view.  This artist rendering of Kepler-7b is based Kepler Space Telescope data and shows that half of the dayside of the planet is covered by a large cloud.  Statistical comparison of more than 1,000 atmospheric models show that these clouds are most likely made of Enstatite, a common Earth mineral that is in vapor form at the extreme temperature on Kepler-7b. (NASA/ edited by Jose-Luis Olivares/MIT)

 

From an Earthcentric point of view, rain of course means falling water.  We can have storms with falling dust — I experienced a few of those while a reporter in India — but rain is pretty much exclusively H2O falling from the clouds. But as the study of exoplanets moves aggressively into the realm of characterizing these distant planets after they are detected, the concepts of rain and clouds are changing rapidly.

We already know that it rains methane on the moon Titan, sulfuric acid on Venus and ammonia, helium and, yes, water, on Jupiter and Saturn.  Some have even posited that carbon — in the form of graphite and then diamonds — falls from the “clouds” of Saturn and Jupiter, but the eye-catching view is widely disputed.

Now the clouds of exoplanets large and small are being rigorously scrutinized not only because they can potentially tell researchers a great deal about the planets below,  but also because especially thick clouds have become a major impediment to learning what many exoplanet atmospheres and even surfaces are made of.  Current telescopes and spectrometers just can’t see much through many of the thick ones.

Here’s why:  The chemical compositions of many exoplanetary clouds are so profoundly different from what is found in our solar system.  Hot gas exoplanets, for instance, tend to have clouds of irons and silicates — compounds that are in a gas form on the surface (such as it is), then rise into the atmospheres and form into grain-like solids when they get higher and colder.  For some smaller exoplanets, the composition tends to be salts such as zinc sulfide and potassium chloride.

The process of identifying the make-up of different clouds is very much a work in progress, as is an understanding of how thick or how patchy the clouds may be.

On this question of exoplanet cloud cover, scientists at the University of Arizona have just published results from the first ever direct evidence of patchy clouds surrounding a directly-imaged “super-Jupiter” planet — a technical and observational step forward of some significant importance. They did it by using the Hubble Space Telescope to detect varying brightnesses in the atmosphere around the planet, signs that the cloud cover was patchy rather than blanketing the planet.

“The images showed that the brightness changes, and that means the clouds don’t cover the whole atmosphere,” said team leader Daniel Apai.  ” We can see that as the planet rotates, it becomes less bright when the clouds face forward and more bright when they do not.  The clouds have structure, like on Earth.”

The light curve for the planet studied, which is some four times larger than Jupiter, shows differences in brightness as the planet rotates. Those differences are consistent with a patchy cloud cover rather than clouds that surround the planet completely. (NASA, Hubble Space Telescope)
The light curve for the planet studied, which is some four times larger than Jupiter, shows differences in brightness as the planet rotates. Those differences are consistent with a patchy cloud cover rather than clouds that surround the planet completely. (NASA, Hubble Space Telescope)

The first author on the paper published in The Astrophysical Journal, Yifan Zhou, said that the planet, four times the mass of Jupiter, is far from its host star (or actually, it’s a failed star known as a brown dwarf.)  As a result,  the researchers could separate the light emitted by the planet and that from the host and measure brightness changes with precision.   The observation covered almost a full rotation of the planet and found that this super-Jupiter rotates in about 10 hours.

The planet is young (10 million years) and still has an atmosphere hot enough to have “rain” clouds made of vaporized sand; silicates that are turned into gases, rise and then cool down to form tiny particles similar in size to what is found in cigarette smoke. Deeper into the atmosphere, iron droplets are forming and falling like rain, eventually evaporating as they enter the lower levels of the atmosphere.

“This rain is similar to what you might see at the Grand Canyon — it rains on top but evaporates on the way down,” Apai said. “Here we have iron and silicate clouds but in principle they are not that different from our rain clouds.  Still, we have a lot to learn about how they form, why some form into multiple layers and some do not, and how they evolve.”

And how astronomers might be able to better pierce through them.

This cloud problem is related but different from the one faced by researchers trying to read the chemical compositions of exoplanet atmospheres using transit spectroscopy.  For them, not only clouds but layers of dusty soot often block the light passing through the atmospheres and picking up signatures of what compounds are present.

 

a whole different understand of clouds and rain,
In the world of exoplanets, clouds are more often like this  — and often far more weird — than anything we are familiar with.

 

Mark Marley, a research scientists at NASA’s Ames Research Center and a co-author on the paper, has been studying clouds on Earth and beyond for years, and knows the challenges they pose– especially since they appear to be everywhere in the cosmos where there’s an atmosphere.

“Clouds are just hard, and they’re a big deal.  We worry that with in future observations clouds will  limit how much we learn about the most interesting world.  We’ll be looking for oxygen or water or life and clouds could very well block our view.  A planet could be perfectly habitable, and we wouldn’t know it.”

Add to the likelihood that many exoplanets will be blanketed with the clouds is that cloud formation and behavior on Earth itself is incompletely understood.  Marley said that the biggest source of uncertainty in weather forecasting is the dynamics of clouds.

Clouds are also the often cursed bane of astronomers.  On Earth they can add greatly to the difficulty of seeing out through our atmosphere, and then light years away the difficulty of seeing into the inner atmospheres of exoplanets.

But not all is bleak.  Marley said that he tells astronomers that there’s a lot to be learned about exoplanet compositions by studying even the thick layers of blanketing clouds.  And instruments that will soon be available will be potentially much better at seeing through the exoplanet coverings.

Apai (and others) are looking especially to the James Webb Space Telescope when it launches in 2018 to pierce far better through clouds.  Its mirrors that see in the infrared will add a potential breakthrough capability, he said, allowing for deeper and far more extensive looks into exoplanet atmospheres.

In the conclusion to their paper, Apai and his colleagues predict “that Webb will help astronomers better determine the exoplanet’s atmospheric composition and derive detailed maps from brightness changes with the new technique demonstrated with the Hubble observations.”

Improvements for sure, but still a long way to go in terms of coming to terms with those clouds of silicate sands, iron, sulfuric acid, exotic salts and everything else that exoplanets send up into the sky and then rain back down to their surfaces and interiors.

An artist rendering of a "hot Jupiter" extrasolar planet orbiting very close to its host star. The planet designated HD 209458b, is about the size of Jupiter. Unlike Jupiter, the planet is so hot that its atmosphere is "puffed up." Starlight is heating the planet's atmosphere, causing hot gas to escape into space, like steam rising from a boiler. (NASA, ESA, and G. Bacon (STScI).
An artist rendering of a “hot Jupiter” extrasolar planet orbiting very close to its host star. The planet designated HD 209458b, is about the size of Jupiter. Unlike Jupiter, the planet is so hot that its atmosphere is “puffed up.” Starlight is heating the planet’s atmosphere, causing hot gas to escape into space, like steam rising from a boiler. (NASA, ESA, and G. Bacon (STScI).

 

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