Early Habitable Environments and the Evolution of Complexity Principal Investigator - David J. Des Marais

Mineralogical Traces of Early Habitable Environments

NEWS AND HIGHLIGHTS                              /  Science Investigations  /  NASA Missions  /

Tori Hoehler, Lead Co-Investigator

Co-Investigators: Richard Morris, Allan Treiman, David Blake, Linda Jahnke, David Des Marais
Researcher: Michael Kubo
Ames Post Docs: Niki Parenteau, Jennifer Kyle, Tom Bristow, Sanjoy Som
Collaborators: Roger Summons, Bo Barker Jorgensen, Kenneth Stedman, Sherry Cady,
Dawn Cardace



Team members were invited to present their research at the NASA Astrobiology Institute (NAI) Executive Council meeting held on January 17-18, 2013, at Ames Research Center.

Principal Investigator, Dave Des Marais, talked about the overall theme of the Ames Team research, "Early Habitable Environments and the Evolution of Complexity," and explained the four approaches to understanding the origins of life.

Tori Hoehler focused on understanding the interplay between "Rocks and Life" and how this interaction contributes to habitability. He talked about optimizing the use of CheMin as a tool for characterizing the habitability of Mars. Tori further expanded on the Coast Range Ophiolite Microbial Observatory (also know as The McLaughlin Drilling Project) and the Josephine Ophiolite Project which provides a natural lab for understanding mineral-based constraints on water, energy, temperature, pH (mineralogy -- habitability).

(January 2013)


Chlorella cell with PBCV1 particlesThe preservation potential of viruses within the microbial record has undergone limited investigation despite viruses being the most abundant biological entities on Earth. Their small size and absence of a metabolism has led to the hypothesis that they lack unique biological signatures, and the potential to become preserved. In order to establish a baseline for research on virus biosignatures, we have initiated laboratory research on known lipid-containing lytic viruses. PRD1 is ~65nm in diameter and replicates in Salmonella typhimurium LT2. PBCV1 is 190nm in diameter and replicates in freshwater Chlorella NC64A. 400 ppm silica solution (final concentration) was added to the respective virus stocks and was sampled over the course of one year. Control microcosms of virus without silica, host only, and host plus virus in the presence and absence of silica were also sampled. Samples were collected immediately after silica addition then periodically up to one year.

The infectivity of PRD1 was strongly affected by the presence of silica and decreased by three orders of magnitude within one month then remained stable for the duration of the experiment. PBCV1 infectivity remained relatively constant. For both viruses, the presence of host cellular debris removed more virus particles than the presence of silica. Lipid analysis using gas chromatography-mass spectroscopy found that free fatty acids were produced in systems with virus present, and that the PRD1 lipids remained relatively stable over the course of time, whereas PBCV1 lipids moved towards saturation of the fatty acid methyl esters. PBCV1 particles and PBCV1 plus host were shown to be particularly interesting due to the presence of a sphingolipid which was only present in small amounts in the host system. The virus particles were also shown to contain sterols, which are known to be strongly associated with sphingolipids. The presence of sterols is especially interesting as sterols are known to be more resistant to degradation within modern systems and are known biomakers within the microbial record. Though virus biosignature research is in its incipient stages, the data suggest that virus lipid signatures are preserved under laboratory conditions and may offer the potential for contribution to the organic geochemical record.


Raindrops In Rock: Clues To A Perplexing Paradox

By Richard Harris

Raindrops in RockThe late astronomer Carl Sagan presented this paradox to his colleagues: We know the sun was a lot fainter two billion years ago. So why wasn't the Earth frozen solid? We know it wasn't because there's plenty of evidence for warm seas and flowing water way back then. The question is still puzzling scientists. But new clues to that paradox come from an unlikely source: fossilized raindrops, from 2.7 billion years ago. Back then, the Earth had no trees or flowers or animals birds or fish. But it did have volcanoes. And it did rain. You can see evidence of that in a remarkable fossil, found in South Africa. It records an ancient downpour. "So it rained 2.7 billion years ago on volcanic ash," says scientist Sanjoy Som. "The ash was covered by a very thin but very resistant layer of ash, and all that was again covered by more volcanic ash." That ash turned to rock. And even today, you can still see the little rims around the small impact craters the raindrops created. "So they're flawlessly preserved, which is surprising for rocks that are that old." Som was getting his Ph.D. at the University of Washington, and his advisor suggested that Som examine those fossilized raindrops to see what he might learn about the ancient atmosphere. "Because I have background in aeronautical engineering I found that challenge super exciting and jumped on it," says Som, who is now at NASA's Ames Research Center.

The scientists knew that the size of raindrops would tell them how thick the atmosphere was back then. Raindrops fall more slowly through thick air than through thin air. So the thicker the air, the more gently they will fall to earth, and the smaller the craters will be. And the raindrop craters they found were similar to those you get on dusty lava today. So Som and his colleagues conclude that the atmosphere back then was a lot like it is today, maybe a bit thinner, but surely no more than twice as dense.

Theories About Earth's Atmospheric History

It's just plain cool to figure out something like that. But the journal Nature highlighted this finding because it touches on Carl Sagan's paradox -- that the early earth was warm even though the sun was young and dim. "Something was keeping the planet warm back then because the sun was fainter, yet there's plenty of evidence in the geological record of ocean sediments and river sediments," Som says. The easiest answer is that the atmosphere served as an insulating blanket, as it does today. We'd still be a frozen planet if we didn't have the natural greenhouse effect to keep us warm. But it turns out you need a much bigger greenhouse effect to compensate for a faint sun.

"You either need higher CO2 [carbon dioxide concentrations in the atmosphere] or higher CO2 in combination with other greenhouse gases like methane," says Jim Kasting at Penn State University, who has been puzzling over this paradox for years. But according to some controversial evidence from ancient rocks, there wasn't enough carbon dioxide in the air to get the job done. That's led other people to suggest that maybe the atmosphere was thick with nitrogen. Nitrogen doesn't trap heat, but if there's a lot of nitrogen in the air, it makes carbon dioxide a better insulating blanket. However, the raindrop results say the atmosphere was not super-thick back then. So you can apparently rule out the nitrogen hypothesis. The paradox remains.

There are lots of other ideas still to test, but precious little evidence from the early days of earth. And, by the way, while you're puzzling over that, here's something else to think about. The sun is still getting brighter. That is happening so slowly it can't explain our current trend of global warming. But Kasting says it does matter in the very long run. "Within about a billion years, it should be 10 percent brighter, and that could actually spell the end for earth because in our models that's enough to drive off the earth's water." And Earth will not be so much fun when the oceans have boiled away. But we have a billion years to figure a way out of that jam.

Listen to Sanjoy Som's interview with NPR.

An article by Som about fossil raindrop imprints appears in the April issue of Nature -- Air density 2.7 billion years ago limited to less than twice modern levels by fossil raindrop imprints.


By Adam Mann, UC Santa Cruz Science Communication Graduate Student

Tori HoehlerHow will we recognize life on other planets? Scientists don't yet know how to address this tricky question, but at the December 2009 meeting of the American Geophysical Union, biogeologist Tori Hoehler presented a different way of thinking about the answer when giving the Carl Sagan Lecture, Life at the Common Denominator: Mechanistic and Quantitative Biology for the Earth and Space Sciences.

Hoehler---who much like the lecture's namesake, Carl Sagan, is both a gifted scientist and communicator---feels that we are getting tantalizingly close to discovering life elsewhere in the universe. He believes that within the next few decades there will be evidence for extraterrestrial life.

As we learn more about water on Mars and new exoplanets are uncovered every day, the chances of finding life increase. But alien life will be, well, alien and identifying it could be difficult. Hoehler wants to create a schema for definitely identifying life from non-life. For him, it comes down to probability. Life, he explained, makes improbable events commonplace. The odds are slim that a group of amino acids will string themselves into a giant polypeptide chain. For this to repeatedly happen trillions of times is nearly impossible. Yet, here on Earth such events are ordinary.

How does life accomplish this wondrous task? By exploiting energy, Hoehler says. With the input of energy, improbable events become probable. Organisms on Earth use molecules such as ATP to power their endeavors. As the famed physicist Erwin Schrödinger once pointed out, life makes order out of the universe's disorder, drawing energy from the environment to keep itself alive.

Hoehler has generated equations that quantitatively model how much energy is needed for life to do its magic. Looking at the fundamentals of physics and chemistry allows him to make predictions about the habitability of different environments. He can assess a wide range of habitability scenarios for planets, from barren to highly productive, all based on the amount of energy available.

In the talk, Hoehler urged scientists to look for a many different biosignatures when searching for life outside the Earth. Instead of narrow-mindedly pursuing familiar things such as water, oxygen, and amino acids, scientists need to look at a planetary system as a whole. By considering the energy in and out, we will avoid missing something potentially groundbreaking.

Tori Hoehler's lecture can be seen on the AGU's website.

(December 2009)


Tori Hoehler - Digital Learning Network Digital Learning Network presentations were made throughout the year by Ames Team members on various exciting areas of research. The most recent talks were given by Tori Hoehler, "Microbiology: Finding Life on Other Worlds", and Nathan Bramall, "Searching for Extreme Life in Ancient Ice". Learn more about these scientists and their research by visiting our Education and Public Outreach Digital Learning Network page.

(December 2009)


Cosmic Distribution of Chemical Complexity
Disks and the Origins of Planetary Systems
Mineralogical Traces of Early Habitable Environments
Origins of Functional Proteins and the Early Evolution of Metabolism