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October 14, 2004

Life Beyond Earth

A question for the ages: Is the universe lively or lonely?

Philosophers dating back to the 4th century B.C.E., religious scholars in the Middle Ages, scientists in the Enlightenment and science fiction authors from all eras have tackled this question in one form or other, according to a prominent NASA astrobiologist.

“So what is different now?” asked Carl B. Pilcher, a keynote speaker at last week’s “Science 2004 – No Boundaries,” the fourth annual three-day showcase on the Oakland campus celebrating Pittsburgh’s role in science and technology.

“What’s different is that for the first time we are in a position to apply the tools of science to this question. We can use what we’ve learned about life on Earth and from other planetary environments that we know exist and are beginning to understand.”

A NASA scientist since 1988, Pilcher has served as the senior scientist for astrobiology in the Astronomy and Physics Division of the Office of Space Science at NASA headquarters since 2001. He spoke on “The Quest for Habitable Worlds and Life Beyond Earth.”

“What do we know from life on Earth? What we’ve learned in the last three decades or so is that life is remarkably hardy, remarkably resilient, and it exists in places we never thought could support life,” including sea floors, hot springs and the two driest places on Earth, Antarctica and the Atacama Desert, Pilcher said.

Life on Earth has identifiable requirements: a source of water, through not necessarily a robust source; a source of energy, such as sunlight, although the energy could be chemical, and some basic nutrients that provide fuel to sustain life.

“If you learned biology more than two or three decades ago, you were taught a taxonomy where life is divided into two basic types: prokaryotes – very simple cells, such as bacteria – and eukaryotes – single cells but more complex cells like nuclei,” Pilcher told the audience of some 250 at Alumni Hall Oct. 6. “The old system is based mostly on morphology and how organisms reproduce.”

Today’s model “tree of life,” however, “adds a molecular capability that has been developed only in the last 30 years or so, of examining the genetic material of life and understanding how that genetic material tells us about life’s diversity, and that genetic diversity corresponds to a tremendous amount of metabolic diversity.”

In looking at organisms that “eat for a living” (as opposed to organisms that photosynthesize), Pilcher pointed out, “along with animals, we tend to eat things that are high energy, with glucose as a representative [example], and we have to combine it with oxidants, in our case, molecular oxygen. With all the energy we get, it’s not surprising that we have developed a specialized metabolism, and it becomes one of the factors that has enabled us to become macro-scopic.”

However, micro-organisms may be undergoing the same process with oxidants other than oxygen, such as nitrate, ferric iron, sulfate, or a variety of other compounds, Pilcher maintained. “When you hear microbiologists say that some bugs ‘breathe rock,’ it means that micro-organisms may be using magnesia or iron as an electronic receptor the same way we use oxygen.”

In addition, there are manifold fuel options, such as hydrogen, nitrogen, methane and ammonium iron that serve as food. “So as long as a bug has the wherewithal to take a fuel and combine it with an oxidant, it’s going to derive energy from that reaction. In fact, it turns out, if you take any combination of this ‘fuel-oxidant equals energy reaction,’ you’re likely to find an organism somewhere on Earth that does that for a living,” Pilcher said. “Now we understand how bugs can be living in solid rock two kilometers underground or living in hot springs where we would have thought the requirements for life were not being met.”

Studying the history of life on Earth yields another lesson, he said. “Life on Earth is very old. [We’ve discovered] 3.5 billion-year-old stromatolytes from Australia – fossiled remains of microbial communities that formed from individual organisms. Life may actually be older than that, up to perhaps 3.8 billion years.”

But the Earth in its formative stages, some 4.5 billion years ago when it was being bombarded by space objects as it fused into a planetary mass, probably could not support life until about 3.9 – 4 billion years ago, Pilcher said.

“It looks like life was present on Earth just about as early as it possibly could have been present, which means either that life forms very, very readily or that life is so hardy that it formed at a time before we thought it possible,” he said.

The latter possibility is more likely, Pilcher said, because scientists believe that the Earth’s atmosphere did not have oxygen levels sufficient to sustain life at the time life first existed here. “It probably took the Earth 2.2 – 2.3 billion years before oxygen got to today’s level. Before that, the Earth probably was largely devoid of oxygen despite the fact that life certainly existed by 3.5 billion years ago and may have existed even earlier.”

Efforts to look outward from Earth to our solar system neighbors have been aided in recent years by NASA space exploration projects, which give scientists a closer look at nearby planets and moons. What has been gleaned also sheds some light on the “lively or lonely universe” question, Pilcher said.

“For example, there is a tremendous amount of evidence that Mars was once very wet and there may be places there with water even today,” he said.

Among the clues supporting a belief in water on Mars are the Martian gullies, thought to be caused by fluids, most likely water, and the widespread patterned layering observed on Mars, suggesting water-borne pulses such as those observed occurring naturally on Earth, Pilcher said.

“Here’s another planet in the solar system, once probably warm, and almost surely wet with a lot of standing water. So the basic ingredients for life are present all over the place: water, the sun as the energy source, and we know from meteorites that the fuel is there. The conditions necessary to support life on Earth were present at least at one time on Mars.”

And Mars is not the only candidate for potential life-sustaining conditions, Pilcher pointed out. “Europa, one of Jupiter’s moons, is covered with ice, but there is reason to believe that there is a very extended liquid water ocean under the frozen covering. If life exits at the bottom of Earth’s oceans, there are ways one can imagine to get the necessary energy source to this sub-surface ocean, particularly if it’s not that far down. Could there be life on Europa? This is speculative, but it’s certainly worth speculating about.”

Similarly worth pondering is Titan, one of Saturn’s moons, where scientists have discovered that the atmosphere resembles Earth’s in that it is mostly nitrogen, Pilcher said. “But where Earth’s atmosphere has oxygen, at least today, which is a product of biology, Titan has methane in abundance, about 2 percent. The sunlight hits the atmosphere of Titan, which is as large as Mercury, and breaks the methane molecules up and forms a photo-chemical haze,” not unlike smog, preventing scientists from seeing the surface very well, Pilcher said.

But an upcoming NASA project, the Huggins probe carried by the Cassini space craft, will be launched below Titan’s atmosphere to record images of the surface. The probe is expected to start sending pictures back to Earth in January. “Titan is not a moon that people think is likely to harbor life, although we’ve been surprised practically every time we’ve gone to a new environment,” Pilcher said.

As for worlds beyond the solar system, Pilcher said, more sophisticated techniques to locate and characterize planets ensure that scientists will have plenty of candidates for life-yielding environments to examine.

Of the 130 planets identified so far, nearly all are Jupiter-sized or larger. But scientists are better able to identify smaller bodies as planet-hunting techniques improve.

“So what do we look for? If you see an O-zone, which means a planet has 10-20 percent oxygen, we can reason that there is not just physics and chemistry going on; there could be biology,” Pilcher said. “Is that the ultimate indicator? No, because a planet could be teeming with life and not have oxygen, and that is probably the case with early Earth.”

What other indicators are scientists seeking?

“Our metabolic diversity on Earth makes us look at what else is life doing if it isn’t generating oxygen,” Pilcher said. “In two examples [of life sustained without oxygen], sulfur respiration, that is a cataclysm where an organism uses sulfur the way we use oxygen and combines it with hydrogen to produce hydrogen sulfide, and methanic genesis, which is the combination generally of carbon dioxide and hydrogen yielding methane. The problem is that these also can be produced non-biologically, so if you see methane in the atmosphere it doesn’t tell you at all unambiguously that life is present.”

Countering the probabilities and possibilities of life on other planets is the so-called “rare Earth” argument, Pilcher said. “The argument goes that microbial life may be common, but animal life is rare. Animals evolved late on Earth, only 600 million years ago,” whether the process is merely time-consuming or extremely unlikely, he said.

Or, it could be that Earth’s qualities are unique in the universe.

“The moon stabilizes Earth’s axis, causing a relatively stable climate over geologic time. If a stable climate is necessary for animals to evolve, that may make them rarer,” Pilcher said.

In addition, plate tectonics, found only on Earth among the solar system planets, provide the mechanism for recycling carbon dioxide: Carbon dioxide dissolves in water, forms limestone, and shifts in the Earth’s plates cause volcanoes to spew the carbon dioxide back into the atmosphere. Plate tectonics also create continents for habitat and biodiversity options.

“And there is the Cambrian explosion, which is the sudden appearance of fossil records about 560 million years ago,” Pilcher said. “All the animal body plans that ever existed are found in that explosion, most of them are extinct, and there haven’t been any new body plans in the fossil record since. It may take a lot of coincidences to get to animal life,” let alone to intelligent civilizations, he said. “It’s an interesting argument, but I like to think it may be more a failure of imagination,” Pilcher concluded.

-Peter Hart

Filed under: Feature,Volume 36 Issue 4

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