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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The Search for Exoplanet Life Goes Broad and Deep

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The scientific lessons learned over the centuries about the geological, chemical and later biological dynamics of Earth are beginning to enter the discussion of exoplanets, and especially which might be conducive to life. This is an artist's view of the young Earth under bombardment by asteroids, one of many periods with conditions likely to have parallels in other solar systems. (NASA's Goddard Space Flight Center Conceptual Image Lab)
The scientific lessons learned over the centuries about the geological, chemical and later biological dynamics of Earth are beginning to enter the discussion of exoplanets, and especially which might be conducive to life. This is an artist’s view of the young Earth under bombardment by asteroids, one of many periods with conditions likely to have parallels in other solar systems. (NASA’s Goddard Space Flight Center Conceptual Image Lab)

I had the good fortune several years ago to spend many hours in meetings of the science teams for the Curiosity rover, listening in on discussions about what new results beamed back from Mars might mean about the planet’s formation, it’s early history, how it gained and lost an atmosphere, whether it was a place where live could begin and survive.  (A resounding ‘yes” to that last one.)

At the time, the lead of the science team was a geologist, Caltech’s John Grotzinger, and many people in the room had backgrounds in related fields like geochemistry and mineralogy, as well as climate modelers and specialists in atmospheres.  There were also planetary scientists, astrobiologists and space engineers, of course, but the geosciences loomed large, as they have for all Mars landing missions.

Until very recently, exoplanet research did not have much of that kind interdisciplinary reach, and certainly has not included many scientists who focus on the likes of vulcanism, plate tectonics and the effects of stars on planets.  Exoplanets has been largely the realm of astronomers and astrophysicists, with a sprinkling again of astrobiologists.

But as the field matures, as detecting exoplanets and inferring their orbits and size becomes an essential but by no means the sole focus of researchers, the range of scientific players in the room is starting to broaden.  It’s a process still in its early stages, but exoplanet breakthroughs already achieved, and the many more predicted for the future, are making it essential to bring in some new kinds of expertise.

A meeting reflecting and encouraging this reality was held last week at Arizona State University and brought together several dozen specialists in the geo-sciences with a similar number specializing in astronomy and exoplanet detection.  Sponsored by NASA’s Nexus for Exoplanet Systems Science (NExSS), NASA Astrobiology Institute (NAI) and the National Science Foundation,  it was a conscious effort to bring more scientists expert in the dynamics and evolution of our planet into the field of exoplanet study, while also introducing astronomers to the chemical and geological imperatives of the distant planets they are studying.

Twenty years after the detection of the first extra-solar planet around a star, the time seemed ripe for this coming together — especially if the organizing goal of the whole exoplanet endeavor is to search for signs of life beyond Earth.

 

Our vast body of knowledge about the formation, processes and evolution of Earth will become increasingly important in the exoplanet field as new generations of instruments make different and more precise kinds of measurements possible. Using Earth dynamics as a guide, those measurements will be made into models of what might be occurring on the exoplanets. The artist rendering of exoplanet Upsilon Andromedea g by Ron Howard.
Our vast body of knowledge about the formation, processes and evolution of Earth will become increasingly important in the exoplanet field as new generations of instruments make different and more precise kinds of measurements possible. Using Earth dynamics as a guide, those measurements will be made into models of what might be occurring on the exoplanets. The artist rendering of exoplanet Upsilon Andromedae g is by Ron Howard, Black Cat Studios.

Ariel Anbar, a biogeochemist at ASU, was one of the leaders of the meeting and the call for a broader exoplanet effort.

“The astronomical community has been pushing hard to make very difficult measurement, but they really haven’t been thinking much about the planetary context of what they’re finding.  And for geoscience, our people haven’t thought much about astronomical observations because they are so focused on Earth.”

“But this makes little sense because exoplanets open up a huge new field for geoscientists, and the astronomers absolutely need them to make the calls on what many of the measurements of the future actually mean.”

What’s more, the knowledge of researchers familiar with the dynamics of Earth will be essential when planet hunters and planet characterizers put together their wish lists for what kind of instruments are included in future telescopes and spectrographs.  For instance, a deep knowledge would be useful of the Earth’s carbon cycle, or what makes for a stable planetary climate, or what minerals and chemistry a habitable planet probably needs.

And then there are all the false positives and false negatives that could come with detections (or non-detections) of possible signatures of life.  The search for life beyond Earth has already had two highly-public and controversial seeming detections of extraterrestrial life — first by the Viking landers in the 1970s and the Mars meteorite ALH84001 in the mid 1990s.  The two are now considered inconclusive at best, and discredited at worst.

The risk of a similar, and even more complex, confusing and ultimately controversial, discovery of signs of life on an exoplanet are great.  The Arizona State workshop debated this issue at length.

President’s Professor at ASU’s School of Earth and Space Exploration and Department of Chemistry and Biochemistry.
Ariel Anbar, President’s Professor at ASU’s School of Earth and Space Exploration and School of Molecular Sciences. He hopes that the drive to understand exoplanets will push his field to develop a missing general theory for the evolution of Earth and Earth-like planets.

What they came away with was the understanding that while one or two measured biosignatures from a distant planet would be enormously exciting, a deeper understanding of the planet’s atmosphere, interior, chemical makeup and relationship to its host star are pretty much required to make a firm conclusion about biological vs non-biological origins.  (Here is a link to an introductory and cautionary tale to the workshop by another of its organizers, astrophysicist Steven Desch.)

And so the issues under debate were:  Does a planet need plate tectonics to be able to support life?  (Yes on Earth, perhaps elsewhere.) Would the detection of oxygen in an exoplanet atmosphere signify the presence of life? (Possibly, but not definitively.)  Does the chemical and mineral composition of a planet determine its ability to support life? (As far as we can tell, yes.)  Does photosynthesis inevitably lead to an oxygen atmosphere?  (It’s complicated.)

All these issues and many more serve to make the case that exoplanet science and Earth or planetary science need each other.

This is by no means an entirely new message — the Virtual Planetary Laboratory at the University of Washington has taken the approach for a decade from the standpoint of astronomy and the New Earths team of the NAI from a geological standing point.   But still, its urgency and proposed reach was  quite unusual.

It is also a reflection of both the success and direction of exoplanet science, because scientists have — or will have in the years ahead — the instruments and knowledge to learn more about an exoplanet than its location.  The James Webb Space Telescope is expected to provide much advanced ability to read the chemical compositions exoplanet atmospheres, as will a new generation of mammoth ground-based telescopes under construction and (scientists in the field fervently hope) a NASA flagship mission for the 2030s that would be able to directly image exoplanets with great precision.

But really, it’s when more and better measurements come in that the hard work begins.

Transmission spectrum of exoplanet MIT
Information about the make-up of exoplanets comes largely by studying the transmission spectra produced as the planet crosses in front of its star.  The spectra can identify some of the elements and compounds present around the exoplanet. Christine Naniloff/MIT, Julien De Wit.

 

Astrophysicist Steve Desch, for instance,  believes it is highly important to know what Earth-sized planets are like without life.  Starting with a biologically dead exoplanet in the Earth-sized ballpark, it would be possible to get a far better idea of the signatures of a similar planet with life.  But that’s a line of thinking that Earth scientists and geochemists are not, he said, used to addressing.  He felt the ASU workshop provided some consciousness-raising about the kinds of issues that are important to the exoplanet community, and to the Earth scientist, too.

Scientists from the geoscience side see similar limitations in the thinking of exoplanet astronomers.  Christy Till, a geologist and volcano specialist at ASU, said that at the close of the three-day workshop, she wasn’t at all sure that exoplanet scientists have been aware of just how complex the issue of “habitability” will be.

“Our field has learned over the decades that the solid interior of a planet is a big control on whether that planet can be habitable — along with the presence of volcanoes, the cycling elements like carbon and iron, and a relatively stable climate.  These issues were not widely discussed in terms of exoplanets, so I think we can help move the research further.”

Till is relatively new to thinking about exoplanets, brought into the field by the indisciplinary ASU (and NExSS/NAI) approach. But she said it has been most exciting to have the potential usefulness of her kind of knowledge expand on such a galactic scale.

Although the amount of detailed information about exoplanets is very limited, Till (and others) said what is and will be available can be used to create predictive models.  Absent the models that researchers can start building now, future information coming in could easily be misunderstood or simply missed.

ASU geologist and assistant professor Christy Till, a relatively new and enthusiastic member of the exoplanet community. (Abigail Wiebel)
ASU geologist and assistant professor Christy Till, a relatively new and enthusiastic member of the exoplanet community. (Abigail Wiebel)

While the usefulness of geosciences is being largely embraced in the exoplanet field, there are clear caveats.  If Earth becomes the model for what is needed for life in the cosmos, then is the field falling into a new version of the misguided Earth-centric view that long dominated astronomy and cosmology?

With that concern in mind, astronomer Drake Deming of the Harvard-Smithsonian Center for Astrophysics made the case for collecting potential biosignatures of all kinds.  Since we don’t know how potential life on another planet might have formed, we also may well be unaware of what kind of signatures it would put out.  ASU geochemist Everett Shock was similarly wary of relying too heavily on the Earth model when trying to understand planets that may seem similar but are inevitably different.

And Ariel Anbar felt challenged by his more complete realization post-workshop that the exoplanets available to study for the foreseeable feature will not be Earth-sized, but will be “Super-Earths” with radii up to 1.5 times as great as that of our planet.  A proponent of much greater exoplanet-geoscience collaboration, he said the Earth science community has a big job ahead figuring out how the processes and dynamics understood on Earth would actually apply on these significantly larger relatives.

One participant at the workshop pretty much personifies the interdisciplinary bridge under construction , and he was encouraged by the extensive back-and-forth between the space scientists and the Earth scientists.

Shawn Domogal-Goldman, a research space scientist at the Goddard Space Flight Center and a leader of the NExSS group, is an expert in ancient earth as well the astrophysics of exoplanet detection and characterizing.  His view is that the Earth provides 4.5 billion years of physical, chemical, climatic and biological dynamics  that need to be mined for useful insights about exoplanets.

“For me, and I think for others, we’ll look back at this meeting years from now and say to ourselves, ‘We were there at the beginning of something big.'”

 

 

 

 

 

 

 

 

 

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