Could Opportunity Find Life on Mars?
Interview with Andy Knoll: Part II
Henry Bortman had the chance to talk with Andrew Knoll, a science team member for the Mars Exploration Rover missions and Fisher Professor of Natural History at Harvard University. See Part I
Astrobiology Magazine (AM): One of the intriguing aspects of Rio Tinto as a research site is that even though the water in the river is highly acidic, there are bacteria living in it. When you look at the ancient hematite deposits in that region, do you see fossil bacteria?
|Acidophilic demateaceus fungi (black fungi) from Rio Tinto.
Credit: Dr. Ricardo Amils Pibernat
Andrew Knoll (AK): Yes, you do. In fact, one of the things that attracted me to work with my Spanish colleagues was not that it's an oddball environment today. While it's kind of fun to be interested in life on the environmental fringes today, most life - and much of what you can learn about biology today - comes from ordinary organisms living in ordinary circumstances. That's where 99 percent of the diversity of life is.
On the other hand, there's a great question that can be asked at Rio Tinto. We can see the processes that formed the Rio Tinto iron deposits going on today; we can see the chemical processes; we can see what biology is in the environment. But the real question that one wants to keep in mind when thinking about Meridiani is: What, if any, signatures of that biology actually get preserved in diagenetically stable rocks?
|Water sample from the river in which different eukaryotic cells (Heliozoa, diatoms, dinoflagelates) and prokaryotes (much more smaller) can be seen.
Credit: Dr. Ricardo Amils Pibernat
One is that. If you were lucky enough to have access to a microscope - this would probably be at a resolution beyond what you could hope for from the microscopic imager - you could see individual microbial filaments that have been beautifully preserved. So that's the first good news is that diagenetically stabilized iron can retain a microscopic imprint of biology.
The better news is that there are two features of biology that get preserved in the more eyeball-level textures in these rocks.
One is that you sometimes get little bubbles forming because of gas emanation from metabolism. And some of those will actually roof over with iron minerals and can be preserved through diagenesis. And that's pretty much true through most sedimentary rocks that we find in the geologic column. You can get preserved gas spaces and those gas spaces are invariably associated with biological production of gases.
AM: How invariably?
AK: In our experience on Earth, it's pretty much 100 percent. What you'd want to ask is: What processes other than biology might give rise to gases within a sediment on a planet? That's something that you can do experiments on. I don't know that anyone's bothered to do them on this planet. Because, frankly, biology is so pervasive that that's the main game in town, anyway. But one could do the experiments.
The other thing, which I feel even more strongly about, is that many times, where there are microbial populations, they form these beautiful groups of filaments that just string out across the surface. They almost look like the mane of a horse. Now the great thing is that, when minerals are deposited in these environments, they actually nucleate on these strings of filaments, and you get beautiful sedimentary textures that, again, look like the mane of a horse.
|Iron stromatolites in winter (Berrocal). In the case of the Rio Tinto, "rock eating" bacteria like Leptospirillum ferrooxidans and Acidithiobacillus ferrooxidans get energy by oxidizing the ferrous iron (Fe2+) in the pyrite, turning it into ferric iron (Fe3+) (The Acidithiobacillus also get energy by oxidizing the sulfide). Because very little energy is generated in the oxidation of ferrous to ferric iron, these bacteria must oxidize large amounts of iron in order to grow. As a result, relatively little bacterial growth results in massive amounts of ferric iron precipitation.
Credit: Dr. Ricardo Amils
You can see them in Yellowstone Park, in both siliceous and carbonate-precipitating strings. If you go to places like Mammoth Springs, you can see it happening today. And if you hike into the hinterland, you can see ancient examples of that, beautiful signatures preserved in the rock.
|Andrew Knoll."[For astrobiology] everything we know about life in the universe comes from life on Earth." Image Credit: querysoft.es
In Rio Tinto, you can see iron depositing on these filaments; and in the 2 million year old terraces, you can see these filamentous iron textures. And there, again, I know of no process other than biology that could form those. So that's truly something to keep your eyes out for whenever you're looking at a precipitated rock on Mars.
AM: And you could see these with Pancam?
AK: If you took a Pancam to Rio Tinto or Yellowstone Park, they would jump out at you. Absolutely.
AM: If it turns out that the bedrock at the Opportunity landing site is made up of sedimentary deposits, does that mean that when those sediments were laid down, there had to be liquid water around?
AK: Very likely.
AM: So if they were sedimentary, and Pancam saw some sort of texture that on Earth is indicative of biology, would that mean that Opportunity had come close to finding evidence of life on Mars?
AK: Those are big ifs, but it would be a big day.
Let's back up a second, because it gets to a little bit of philosophy about how you actually look for these things. A couple of years ago, NASA embarked on a funding campaign to essentially try and anticipate any kind of suggestively biological signature that might be found in any kind of exploration of another planet so that we wouldn't be seen to be scratching our heads.
|Close-up of famous shapes measuring 20 to 200 nanometers across in Allen Hills meteorite [ALH84001], found at Allen Hills, Antarctica, showing what has generated controversy around ancient fossilized microbial life. "Several lines of evidence suggest that the volume of a sphere about 200 nanometers across is needed to house the chemistry of a cell that has a biology familiar to us." --A. Knoll Around 28 Mars meteorites have been identified so far.
Image Credit: NASA
But the plain fact is that you can't anticipate anything you might see. So what I think is a more realistic scenario is that you do your exploration, and if, in the course of that exploration, you find a signal that is (a) not easily accounted for by physics and chemistry or (b) reminiscent of signals that are closely associated with biology on Earth, then you get excited.
What will happen then, I can guarantee you, is that 100 enterprising scientists will go into the lab and see how, if at all, they can simulate what you see - without using biology. And I think that's the right thing to do. For things where the stakes are so high, I think one wants to be as careful and sober about this as you can be. And certainly that means knowing a lot more about the generative capacity of physical and chemical processes to implant both chemical and textural signatures in a rock than we know about today.
Absent astrobiology, nobody would waste their time doing these things because, on Earth, we know that there has been biology for most of the planet's history. Biology is everywhere. Biology is pre-eminent in the signals that it imparts to sedimentary rocks. So who is going to spend five years of their time as a young scientist trying to generate a signal by abiological means that's closely associated with biology? However, you switch to Mars and there are a lot more reasons to do that kind of thing.
AM: If one of the MER rovers found a rock that seemed to contain evidence of martian biology, would NASA want go back to that spot and bring it home?
|Liquid water may have flowed over the surface of Mars in the planet's distant past. Artist conception of a delta filling a crater.
AK: You bet. Depending on what we find in Meridiani - not to prejudice what we find - it may make it either a very high-priority site for NASA to return with more sophisticated equipment and be a very top priority site for sample return; or we may write it off.
That's the whole reason for this sort of incremental work. I actually like the whole architecture of NASA's plan to go one step at a time, do each step carefully, and in step two build on what you learned in step one. It makes sense.
AM: I realize I'm asking you to speculate, here, but what do you think are the odds that Mars was once a living world?
AK: I really don't know. But everything we've learned in the last few years suggests to me that water may have been episodic rather than persistent on Mars. And that lowers the probability for biology.
If water is present on the Martian surface for 100 years every 10 million years, that's not very interesting for biology. If it's present for 10 million years, that's very interesting.
It is certainly not a given that we will find that Mars was a biological planet. Half of my brain keeps trying to throw out a percentage, and I know it's such a meaningless thing to do - I think I'm just going to not do it.
But I can tell you that one of the best chances we're going to get for a number of years to address that question is right here in the iron deposits of Meridiani.
Related Web Pages
Living on Fool's Gold
Martian Meteorites: JPL
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The Sulfuric Chemistry in the Terrestrial System of Tinto River and Europa: A comparison
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Centro de Astrobiología
Pancam- Surveying the Martian Scene
Alpha Proton X-ray Spectrometer
Mars Rover: The Owner's Manual
Reverse Robotic Origami