Certain Big, Charged Molecules Are Universal to Life on Earth. Can They Help Detect It In The Far Solar System?

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This article of mine, slightly tweaked for Many Worlds, first appeared today (July 6)  in Astrobiology Magazine,  www.astrobio.net

NASA’s Cassini spacecraft completed its deepest-ever dive through the icy plume of Enceladus on Oct. 28, 2015. The spacecraft did not have instruments that could detect life, but missions competing for NASA New Frontiers funding will — raising the thorny question of how life might be detected. (NASA/JPL-Caltech)

As NASA inches closer to launching new missions to the Solar System’s outer moons in search of life, scientists are renewing their focus on developing a set of universal characteristics of life that can be measured.

There is much debate about what might be considered a clear sign of life, in part, because there are so many definitions separating the animate from the inanimate.

NASA’s prospective missions to promising spots on Europa, Enceladus and Titan have their individual approaches to detecting life, but one respected voice in the field says there is a better way that’s far less prone to false positives.

Noted chemist and astrobiologist Steven Benner says life’s signature is not necessarily found in the presence of particular elements and compounds, nor in its effects on the surrounding environment, and is certainly not something visible to the naked eye (or even a sophisticated camera).

Rather, life can be viewed as a structure, a molecular backbone that Benner and his group, Foundation for Applied Molecular Evolution (FfAME), have identified as the common inheritance of all living things. Its central function is to enable what origin-of-life scientists generally see as an essential dynamic in the onset of life and its increased complexity and spread: Darwinian evolution via transfer of information, mutation and the transfer of those mutations.

“What we’re looking for is a universal molecular bio-signature, and it does exist in water,” says Benner. “You want a genetic molecule that can change physical conditions without changing physical properties — like DNA and RNA can do.”

Steven Benner, director of the Foundation for Applied Molecular Evolution or FfAME. (SETI)

Looking for DNA or RNA on an icy moon, or elsewhere would presuppose life like our own — and life that has already done quite a bit of evolving.

A more general approach is to find a linear polymer (a large molecule, or macromolecule, composed of many repeated subunits, of which DNA and RNA are types) with an electrical charge. That, he said, is a structure that is universal to life, and it can be detected.

As described in a recent paper that Benner’s group published in the journal Astrobiology: “the only molecular systems able to support Darwinian information are linear polymers that have a repeating backbone charge. These are called ‘polyelectrolytes.’

“These data suggest that polyelectrolytes will be the genetic molecules in all life, no matter what its origin and no matter what the direction or tempo of its natural history, as long as it lives in water.”

Through years of experimentation, Benner and others have found that electric charges in these crucial polymers, or “backbones,” of life have to repeat. If they are a mixture of positive and negative charges, then the ability to pass on changing information without the structure itself changing is lost.

And as a result, Benner says, detecting these charged, linear and repeating large molecules is potentially quite possible on Europa or Enceladus or wherever water is found. All you have to do is expose those charged and repeating molecular structures to an instrument with the opposite charge and measure the reaction.

Polyelectrolytes are long-chain, molecular semiconductors, whose backbones contain electrons. The structure and composition of the polyelectrolytes confers an ability to transfer electric charge and the energy of electronic excited states over distance. (Azyner Group, UCSC)

James Green, director of NASA’s Planetary Sciences division, sees values in this approach.

“Benner’s polyelectrolyte study is fascinating to me since it provides our scientists another critical discussion point about finding life with some small number of experiments,” he says.

“Finding life is very high bar to cross; it has to metabolize, reproduce, and evolve — all of which I can’t develop an experiment to measure on another planet or moon. If it doesn’t talk or move in front of the camera we are left with developing a very challenging set of instruments that can only measure attributes. So polyelectrolytes are one more to consider.”

Benner has been describing his universal molecular bio-signature to leaders of the groups competing for New Frontiers missions, which fill the gap between smaller Discovery missions and large flagship planetary missions. It’s taken a while but due to his efforts over several years, he notes that interest seems to be growing in incorporating his findings.

Astrobiologist Chris McKay at NASA’s Ames Research Center.  (IDG News Service)

In particular, Chris McKay, a prominent astrobiologist at NASA’s Ames Research Center and a member of one of the New Frontiers Enceladus proposal teams, says he thinks there is merit to Benner’s idea.

“The really interesting aspect of this suggestion is that new technologies are now available for sequencing DNA that can be generalized to read any linear molecule,” McKay writes in an email.

In other words, they can detect any polyelectrolytes.

Other teams are confident that their own kinds of life detection instruments can do the job. Morgan Cable, deputy project scientist of the Enceladus Life Finder proposal, she says her team has great confidence in its four-pronged approach.   A motto of the mission on some of its written material is: “If Encedadus has life, we will find it.”

Morgan Cable, deputy project scientist for the proposed Enceladus Life Finder.

The package includes instruments like mass spectrometers able to detect large molecules associated with life; measurements of energy gradients that allow life to be nourished; detection of isotopic signatures associated with life; and identification of long carbon chains that serve as membranes to house the components of a cell.

“Not one but all four indicators have to point to life to make a potential detection,” Cable says.

NASA is winnowing down 12 proposals by late this year, so, Benner’s ideas could play a role later in the process as well. NASA’s goal is to select its next New Frontiers mission in about two years, with launch in the mid-2020s.

The Europa Clipper orbiter mission is tentatively scheduled to launch in 2022, but its companion lander has been scrubbed for now by the Trump administration.

Nonetheless, NASA put out a call last month for instruments that might one day sample the ice of Europa. Benner is once more hoping that his theory of polyelectrolytes as the key to identifying life in water or ice will be considered and embraced.

 

These composite images show a suspected plume of material erupting two years apart from the same location on Jupiter’s icy moon Europa. Both plumes, photographed in UV light by Hubble, were seen in silhouette as the moon passed in front of Jupiter.  Europa is a major focus of the search for life beyond Earth. (NASA/ESA/STScI/USGS)
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Curiosity Has Found The Element Boron On Mars. That’s More Important Than You Might Think

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ChemCam target Catabola is a raised resistant calcium sulfate vein with the highest abundance of boron observed so far. The red outline shows the location of the ChemCam target remote micro images (inset). The remote micro images show the location of each individual ChemCam laser point (red crosshairs) and the B chemistry associated with each point (colored bars). The scale bar is 9.2 mm or about 0.36 inches. Credit JPL-Caltech/MSSS/LANL/CNES-IRAP/William Rapin
Using its laser technology, the Curiosity ChemCam instrument located the highest abundance of boron observed so far on this raised calcium sulfate vein. The red outline shows the location of the ChemCam target remote micro images (inset). The remote micro images show the location of each individual ChemCam laser point (red crosshairs) and the additional chemistry associated with each point (colored bars).  JPL-Caltech/MSSS/LANL/CNES-IRAP/William Rapin

For years, noted chemist and synthetic life researcher Steven Benner has talked about the necessary role of the element boron in the origin of life.

Without boron, he has found, many of the building blocks needed to form the earliest self-replicating ribonucleic acid (RNA) fall apart when they come into contact with water, which is nonetheless needed for the chemistry to succeed. Only in the presence of boron, Benner found and has long argued, can the formation of RNA and later DNA proceed.

Now, to the delight of Benner and many other scientists, the Curiosity team has found boron on Mars.  In fact, as Curiosity climbs the mountain at the center of Gale Crater, the presence of boron has become increasingly pronounced.

And to make the discovery all the more meaningful to Benner, the boron is being found in rock veins.  So it clearly was carried by water into the fractures, and was deposited there some 3.5 billion years ago.

Combined with earlier detections of phosphates, magnesium, peridots, carbon and other essential elements in Gale Crater, Benner told me, “we have found on Mars an environment entirely consistent with a what we consider conducive for the origin of life.

“Is it likely that life arose?  I’d say yes…perhaps even, hell yes.  But it’s also true that an environment conducive to the formation of life isn’t necessarily one conducive to the long-term survival of life.”

The foreground of this scene from the Mastcam on NASA's Curiosity Mars rover shows purple-hued rocks near the rover's late-2016 location. The middle distance includes future destinations for the rover. Variations in color of the rocks hint at the diversity of their composition on lower Mount Sharp. Credits: NASA/JPL-Caltech/MSSS
The foreground of this scene from the Mastcam on NASA’s Curiosity Mars rover shows purplish rocks near the rover’s late-2016 location. The middle distance includes future destinations for the rover. Variations in color of the rocks hint at the diversity of their composition on lower Mount Sharp. NASA/JPL-Caltech/MSSS

Another factor in the Mars-as-habitable story from Benner’s view is that there has never been the kind of water world there that many believe existed on early Earth.

While satellites orbiting Mars and now Curiosity have made it abundantly clear that early Mars also had substantial water in the form of lakes, rivers, streams and perhaps an localized ocean, it was clearly never covered in water.

And that’s good for the origin of life, Benner said.

Steven Benner and his colleagues were the first to synthesize a gene, beginning the field of synthetic biology. He was instrumental in establishing the field of paleogenetics. He founded The Westheimer Institute of Science and Technology (TWIST) and the Foundation For Applied Molecular Evolution.
Steven Benner and his colleagues were the first to synthesize a gene, beginning the field of synthetic biology. He was instrumental in establishing the field of paleogenetics. He founded The Westheimer Institute of Science and Technology (TWIST) and the Foundation For Applied Molecular Evolution.

“We think that a largely arid environment, with water present but not everywhere, is the best one for life to begin.  Mars had that but Earth, well, maybe not so much.  The problem is how to concentrate the makings of RNA, of life, in a vast ocean.  It’s like making a cake in water — all the ingredients will float away.

“But the mineral ensemble they’ve discovered and given us is everything we could have asked for, and it was on a largely dry Mars,” he said.  “So they’ve kicked the ball back to us.  Now we have to go back to our labs to enrich the chemistry around this ensemble of minerals.”

In his labs, Benner has already put together a process — he calls it his discontinuous synthesis model — whereby all the many steps needed to create RNA and therefore life have been demonstrated to be entirely possible.

What’s missing is a continuous model that would show that process at work, starting with a particular atmosphere and particular minerals and ending up with RNA.   That’s something that requires a lot more space and time than any lab experiments would provide.

“This is potentially what Mars provides,” he said,

Benner, it should be said, is not a member of the Curiosity team and doesn’t speak for them.

But his championing of boron as a potentially key element for the origin of life was noted as a guide by one of the Curiosity researchers during a press conference with team members at the American Geophysical Union Dec. 13 in San Francisco.  It was at that gathering that the detection of the first boron on Mars was announced.

Benner said he has been in close touch with the two Curiosity instrument teams involved in the boron research and was most pleased that his own boron work — and that of at least one other researcher — had helped inspire the search for and detection of the element on Mars.  That other researcher, evolutionary biologist James Stephenson, had detected boron in a meteorite from Mars.

Patrick Gasda, a postdoctoral researcher at Los Alamos National Laboratory, is a member of the Chemistry and Camera (ChemCam) instrument team which identified the boron at Gale Crater.  The instrument uses laser technology to identify chemical elements in Martian rocks.

Gasda said at AGU that if the boron they found in calcium sulfate rock veins on Mars behaves there as it does on Earth, then the environment was conducive to life.  The ancient groundwater that formed these veins would have had temperatures in the 0-60 degrees Celsius (32-140 degrees Fahrenheit) range, he said, with a neutral-to-alkaline pH.

While the presence of boron (most likely the mineral form borate, Benner said) has increased as the rover has climbed Mount Sharp, the element still makes up only one-tenth of one percent of the rock composition.  But to stabilize that process of making RNA, that’s enough.

This pair of drawings depicts the same location at Gale Crater on at two points in time: now and billions of years ago. Water moving beneath the ground, as well as water above the surface in ancient rivers and lakes, provided favorable conditions for microbial life, if Mars has ever hosted life. Credits: NASA/JPL-Caltech
A drawing of Gale Crater as it is organized now.  Water moving beneath the ground, as well as water above the surface in ancient rivers and lakes, provided favorable conditions for microbial life, if Mars has ever hosted life. A well-done animation including a second drawing showing conditions 3.5 billion years ago at Gale can be seen here.   It toggles back and forth to show how things have changed.  (NASA/JPL-Caltech)

Benner’s view of Gale Crater and Mars as entirely habitable is not new — the Curiosity team has been saying roughly the same for several years now.  But with four full years on Mars the rover keeps adding to the habitability story, and that was the central message from Curiosity scientists speaking at the AGU press conference.

As the rover examines higher, younger layers, the researchers said they were especially impressed by the complexity of the ancient lake environments at Gale when sediments were being deposited, and also the complexity of the groundwater interactions after the sediments were buried.

“There is so much variability in the composition at different elevations, we’ve hit a jackpot,” said John Grotzinger of Caltech, and formerly the mission scientist for Curiosity.

“A sedimentary basin such as this is a chemical reactor. Elements get rearranged. New minerals form and old ones dissolve. Electrons get redistributed. On Earth, these reactions support life.”

This kind of reactivity occurs on a gradient based on the strength of a chemical at donating or receiving electrons. Transfer of electrons due to this gradient can provide energy for life.

The ChemCam instrument, with its laser zapper, identified the element boron as Curiosity climbs Mount Sharp. (NASA)
An illustration of the ChemCam instrument, with its laser zapper, which identified the element boron as Curiosity climbs Mount Sharp. (NASA)

While habitability is key to Curiosity’s mission on Mars, much additional science is being done  that has different goals or looks more indirectly at the planet’s ancient (or possibly current) ability to support life.  Understanding the ancient environmental history of Gale Crater and Mars is a good example.

For instance, the Curiosity team is now undertaking a drilling campaign in progressively younger rock layers, digging into four sites each spaced about 80 feet (about 25 meters) further uphill.  Changes in which minerals are present and in what percentages they exist give insights into some of that ancient history.

One clue to changing ancient conditions is the presence of the mineral hematite, a form of the omnipresent iron oxide on Mars.  Hematite has replaced magnetite as the dominant iron oxide in rocks Curiosity has drilled recently, compared with the site where Curiosity first found lake bed sediments.

Thomas Bristow of NASA Ames Research Center, who works with the Chemistry and Mineralogy (CheMin) laboratory instrument inside the rover, said Mars is sending a signal. Both forms of iron oxide (hematite and magnetite) were deposited in mudstone in what was once the bottom of a lake, but the increased abundance of hematite higher up Mount Sharp suggests conditions were warmer when it was laid down.  There also was probably more interaction between the atmosphere and the sediments.

On a more technical level, an increase in hematite relative to magnetite also indicates an environmental change towards a stronger tug on the iron oxide electrons, causing a greater degree of oxidation (the loss of electrons) in the iron.  That would likely be caused by changing atmospheric conditions.

It’s all part of putting together the jigsaw puzzle of Mars circa 3.5 billion years ago.

This view from the Mast Camera (Mastcam) on NASA's Curiosity Mars rover shows an outcrop with finely layered rocks within the "Murray Buttes" region on lower Mount Sharp. (NASA/JPL-Caltech/MSSS)
This view from the Mast Camera (Mastcam) on NASA’s Curiosity Mars rover shows an outcrop with finely layered rocks within the “Murray Buttes” region on lower Mount Sharp. (NASA/JPL-Caltech/MSSS)

Returning to the boron story, Benner said that the discovered presence of all the chemicals his group believes are necessary to ever-so-slowly move from prebiotic chemistry to biology provides an enormous opportunity. Because of plate tectonics on Earth and the omnipresence of biology, the conditions and environments present on early Earth when life first arose were long ago destroyed.

But on Mars, the apparent absence of those most powerful agents of change means it’s possible to detect, observe and study conditions in a changed but intact world that just might have given rise to life on Mars.  Taken a step further, Mars today could provide new and important insights into how life arose on Earth.

And then there’s the logic of what finding signs of ancient, or perhaps deep-down surviving life on Mars would mean to the larger search for life in the cosmos.

That life exists on one planet among the hundreds of billions we now know are out there suggests that other planets — which we know have many or most of the same basic chemicals as Earth — might have given rise to life as well.

And if two planets in one of those many, many solar system have produced and supported life, then the odds go up dramatically regarding life on other planets.

One planet with life could be an anomaly.  Two nearby planets with life, even if its similar, are a trend.

 

 

 

 

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