Trying to Solve the Riddle of the Rock
When the world learned last summer that scientists had found possible evidence of ancient life on Mars, it let out a collective gasp at the awesome prospect.
Now scientists have begun piecing together perhaps the most formidable forensic puzzle ever. Can they prove that the microscopic forms found on an extraterrestrial meteorite are bodies of long-dead beings, rather than “globs of Martian goo,” as one astrophysicist put it.
Unraveling the true identity of the Martian visitors will require expertise from biology, chemistry, physics, geology and astronomy. Researchers are sifting through the evidence to answer questions never faced before:
What would the corpse of an alien life form buried in rock for 3.6 billion years look like?
What traces, if any, would it leave behind?
Beyond that, scientists are worried that they might be blinded by humanity’s egocentric view of what life can be.
“Everything we know about life we learned from Earth,” said NASA Ames Research Center astrophysicist Christopher McKay. “We have to be careful about jumping to conclusions.”
Some think that linking the tiny globs to life is already too big a leap. The scientific community is divided on the question.
“I don’t know a better explanation for what we’re seeing,” said chemist Richard Zare, whose Stanford University lab identified the rock’s organic (carbon-based) compounds.
However, UC Berkeley biologist Norman Pace says some of his colleagues are so skeptical that they dismiss the sausage-shaped forms on the rock as “turds.”
UCLA ancient life expert William Schopf is the “official skeptic” brought in by NASA for “balance” at its Aug. 7 news conference announcing the find. Schopf, who discovered the oldest known life forms on Earth, says his own experience makes him wary: These don’t look like any fossils he has seen.
“I personally hope they’re right,” he said of the Mars rock researchers. “But if I used the same evidence to argue for life on Earth [at 3.6 billion years ago], I’d be laughed out of the business.”
Even those convinced that the fossil-like forms are the real thing admit that their claim may be impossible to prove. At best, they say, they can prove that the forms can’t be anything else. They can build a case and present it to their peers. If the verdict goes against them, they could stand to lose their most valuable asset: their scientific reputations.
The first chunks of the rock to reach Zare’s lab traveled under pseudonyms, like Hollywood stars. “They [NASA scientists] called them Mickey, Minnie and Goofy,” Zare said. NASA had mixed the pieces with ordinary meteorites, he explained, so chemists could be as objective as possible in their tests.
The now-famous rock known as Alan Hills 84001 had a well-traveled history even before it reached NASA. Believed to have formed on a warm, wet Mars about 4.5 billion years ago, it apparently got knocked off the red planet by a crash-landing asteroid and spent 16 million years looping around the sun.
About 13,000 years ago, scientists say, the chunk of Mars got sucked in by Earth’s gravity and fell in Antarctica, where it was found 12 years ago. This theory about the remarkable odyssey of ALH84001 is generally accepted by researchers. Speculating about the once-living baggage it may have carried, however, is far more risky to a scientist’s reputation.
Zare was nervous about the whole business of studying meteorites. In physics and chemistry, he said, experiments can be reproduced. But a meteorite is a one-of-a-kind sample. Besides, he said, one doesn’t generally find scientists in chemistry labs who are interested in exploring space.
Even now, Zare’s lab does not have specific government funding for studying the Mars rock, and he thinks he would get turned down if he asked. “The scientific community is conservative,” he said. “It would say, ‘This is a harebrained proposal.’ ” So he pays for the work out of other meteorite research funds.
Yet Zare’s role has been central: His lab has identified and mapped the rock’s organic compounds thought to be fossil traces of life.
He has a great deal riding on the outcome. “There is a possibility I’ll be destroyed by it,” he said. “Science progresses like the turtle, by sticking out its neck, but I don’t want to be turtle soup.”
Right now, Zare is looking for traces of one of life’s primary building blocks: amino acids, the chemical compounds that make up proteins. But progress is slow. “Research is like poetry,” he said. “It’s hard to tell how fast you can do it.”
These particular chips of Martian haiku come from NASA’s Johnson Space Center sealed in vials, wrapped in foil and zipped into plastic bags for protection. They are placed in a cold silver cylinder and hooked up with tubes and wires like a patient in intensive care. Blue and green laser lights flash as the experiment proceeds.
The equipment is surgically clean and continually tested for possible contamination. “Most of the time is spent testing for [contaminants] . . . to make sure it’s not car exhaust from the street, or something from the air, or something it picked up in Antarctica or Johnson [Space Center],” Zare said.
If his lab can make a map of amino acids on the rock, and if the map matches the pattern of the tube-shaped forms, it would be a powerful piece of evidence in favor of the fossil life hypothesis.
If he does not find amino acids, or if he finds them in the wrong places, then he will have to back up and “be suspicious about everything,” he said.
Even a perfect map of amino acids would not be final proof. As Zare points out, science can only prove things false.
His research could conclusively prove that the forms are not fossils, but it could never conclusively prove that they are. “The best we can do,” he said, “is exhaustive falsification.” They can prove that the forms can’t be, say, mineral deposits. They can’t prove they were life.
Indisputable proof of the latter would have to come from missions to the planet itself. For now, the researchers will have to settle for what they can wring from this rock.
The submicroscopic size of the fossil-like forms poses a major problem. Each is smaller than a cell’s nucleus; a billion would fit on the head of a pin. No one has experience taking pictures of structures so small--much less trying to analyze their chemistry.
NASA geophysicist David McKay (no relation to Christopher) struggles with the size problem as he tries to photograph the forms using newly developed transmission electron microscopes--which can magnify a sample 100,000 times or more. McKay, who works at the Johnson Space Center, is on the same team as Zare. The tricky part, he said, is getting the right speck under the microscope. He needs to zoom in on one infinitesimal section of rock--like zeroing in on an invisible target thousands of miles away. “We’re pushing the state of the art,” he said.
The fact that they are entering uncharted territory has microbiologists like Schopf worried that they won’t know what they’re looking at. For one thing, microscopes with such high magnification can see only an extremely tiny portion of a sample at a time. Even if a researcher had a whole year to examine a piece the size of a grain of sand, he said, “you wouldn’t be able to look at it all.”
More worrisome still is that such high-powered magnification is so new that scientists do not know what regular matter at 100,000 times normal size looks like--much less matter from Mars. At smaller scales, even perfectly smooth surfaces can appear rough; new structures keep emerging like ever-smaller Russian nesting dolls. “You can see structures that you didn’t know existed,” Schopf said. “It’s a whole new world.”
Inexperience with this world, he said, could easily lead to wrong conclusions.
Berkeley’s Pace has a more critical problem with size: He believes that the forms are not big enough to have been alive. An independent, living organism needs a minimum amount of DNA, and that minimum could not fit in these tiny structures, he argues.
Such doubts do not bother David McKay and his NASA colleague Everett Gibson. “Some people don’t believe in things smaller than 100 nanometers [billionths of a meter],” McKay said. “We’re reporting what we see. We’re not worried about some theoretical minimum size.
“We think they have blinders on,” Gibson said. “The theorists said you could never get a rock from Mars to Earth.”
Aside from experimental obstacles, the researchers face an even thornier question: What is life anyway? Is what we know all there is? Should we open our minds to other possibilities? If we define life too narrowly--based on life as we know it--will we miss drastically different alternative forms? Life on Earth consists mostly of a few simple ingredients: carbon and water with nitrogen, and sometimes sulfur and phosphorus.
Zare thinks biologists may be unnecessarily narrow-minded in confining their speculations to carbon-based life. After all, he points out, it wasn’t that long ago that scientists thought life needed oxygen and sunlight. Recently, researchers were shocked to discover large colonies of organisms thriving miles beneath the ocean in pitch-dark water next to boiling sulfurous vents. “They don’t use oxygen or light,” said Zare. The truth is, he said, “we really don’t know that much [about the requirements for life].”
Christopher McKay agrees. Perhaps the best definition, he proposed, is: We’ll know it when we see it. “If we get more specific [than that], there’s a danger we’ll . . . miss something.”
Earthbound researchers do not have enough information to know what life can be, he said. “We’re working with a sample of one: life on Earth. That’s like trying to learn about fruit by studying only apples,” he said. “We have to look at a lot of apples and oranges.”
But scientists do think they know what life needs to exist.
Most agree that life needs liquid water and some kind of energy source, whether it comes from the sun or from geological processes like volcanoes. Early Mars, they think, almost certainly had both flowing water and active volcanoes. Three and a half billion years ago, Mars might have been temperate and wet like Earth, Christopher McKay said. “There might have been two blue marbles” in the solar system, he said.
Just because life was possible on Mars does not mean it actually took root. To prove that it did, scientists need to concentrate their attention on deciphering the secrets of ALH84001 and its passengers.
Zare, David McKay and their colleagues base their claim of Martian life on, among other things, the presence of crystals and mineral formations that are commonly produced by Earth bacteria and of organic compounds known as PAHs in about the same locations. Finally, there are the tiny tube-shaped structures themselves.
Although both the minerals and the PAHs could have been produced by nonliving processes, the fact that they are all clustered into “hot spots” is what has led the researchers to conclude that they are traces of ancient life.
Schopf, however, feels this evidence is not nearly good enough. The main problem, he says, is that life has to have some way to separate itself from its environment. Even a skin as tenuous as a soap bubble could qualify if it lived on a completely quiet world. But on the dirty, wind-blown Earth we know, “it has to have a membrane that’s resistant,” he said.
And under that membrane it has to have an internal cavity. “There has to be room inside for stuff [like metabolism] to occur. Because that stuff is what we call living.”
Another sure sign of life is the presence of gases that are found only where life exists. For example, life on Earth manufactures oxygen and methane in abundance--so the presence of oxygen and methane in Earth’s atmosphere is a clear giveaway that life exists. “If you look at Earth and you see methane, you know that something is making it,” Schopf said. Only life can replenish Earth’s supply of oxygen and methane. Finally, life--on our planet, at least--selects certain varieties of elements to use as energy sources. Living things prefer light versions of carbon, for example, rather than heavier isotopes.
So far, the Mars rock bears no evidence of gases or versions of elements that could pin down the presence of life. Zare would argue that this doesn’t necessarily mean the forms are not ancient fossils.
“What does fossilization mean?” he asked. “It’s organic remains of things that were alive. When a vertebrate dies, you have a backbone,” he said, but scientists are not sure what soft-tissued life forms might leave behind.
But Schopf says the only evidence that will convince him will have to come from the tubular forms themselves. And he doesn’t think they look like fossils. For one thing, he points out, fossils probably would not be in such good shape. “They’re 3.6 billion years old and none of them has collapsed, none of them is broken, none is degraded, none is flattened,” he said.
Many researchers have concluded that the mystery over the forms from Mars may never be completely resolved. A first step, however, is to repeat the work done by Zare and others. “I’d like to get a second opinion,” Zare said.
He would also like to get some other samples to study. But collecting more rocks from Mars won’t be possible for some time. While an estimated 100 pounds of Mars falls on Earth each year, Christopher McKay said, only a few rocks land in the pristine environment of the South Pole, and “all the easy ones have been found.” Those that land elsewhere tend to pick up too many contaminants to be useful.
NASA is planning several missions to Mars, but prospecting on the red planet won’t yield much, researchers say, until they know more about the Martian landscape. “We might pick up the wrong handful,” said Christopher McKay, whose chemistry experiment on Martian soil now lies on the floor of the Pacific along with the rest of the Russian spacecraft that plunged to Earth last Sunday.
The first priority, Pace said, is to make good maps of Mars so future missions would know where to land.
In the interim, Christopher McKay suggests, earthlings should perhaps place an ad in some intergalactic newsletter along the lines of: “Mature sophisticated life on Earth seeking meaningful exchange with. . . .” Or even more simply: “Apples: Seeking Oranges.”
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The concentric circles in the upper left hand corner of the graph show the areas where organic compounds called PAH’s were concentrated in the Mars rock, below. The dotted circle on the rock shows places where the fossil-like forms were concentrated. While PAH’s can be produced by many nonliving processes (for example, charcoal-broiling a steak), the fact that they match up to the formations on the rock strongly suggest to some researchers that these PAHs were produced by living organism.
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BACKGROUND
Researchers at Stanford and NASA announced Aug. 7 that they had found evidence that communities of microbes lived on Mars 3.6 billion years ago. They based their claim on tiny tube-like forms discovered on a meteorite picked up in Antarctica in 1984, then misclassified--and therefore not carefully studied--for more than 10 years. Researchers believe that the rock comes from Mars because it shares the same mix of minerals as 12 other known Martian meteorites. Of those 12, several contain bubbles of oxygen with a ratio of isotopes that appears, in our solar system, only on Mars. A similar announcement--about the discovery of Martian fossils--made headlines around the world in 1961; those “fossils” turned out to be ordinary pollen and ash.
