September 2006 (cover story)Link
There are many strange landscapes in the solar system, but perhaps none stranger than that of Titan, Saturn’s largest moon. Deserts blanket Titan for hundreds of miles, rippling with wind-sculpted dunes that rise more than 300 ft. Images taken by the Cassini spacecraft over the past two years also reveal riverbeds sculpted by liquid methane, canyons, and what appear to be a volcano and a shoreline. When Cassini dropped the Huygens probe onto Titan’s surface in 2005, the 701-pound craft landed in a substance with the consistency of wet sand. Shrouding it all is a smoggy, orange-hued atmosphere 10 times thicker than Earth’s and made up of complex organic molecules.
“Titan is so cool,” says Peter Ward, who leads NASA-funded astrobiology research at the University of Washington. “Titan is the most exciting place in the solar system astrobiologically. It has the most exciting chemistry set in our solar system by far. If there’s life on Titan, it’s alien life--really alien life.”
But finding microorganisms on Titan--or anywhere in the universe--is no easy task. Titan has carbon-based molecules, for example, which is one of the necessary ingredients for life as we know it. But the recipe may be different there than it is here on Earth.
“Methane plays the same meteorological role on Titan as water does on Earth. So what would life look like if it drank a glass of methane in the morning, rather than a glass of Florida orange juice?” asks molecular biologist Steven Benner, a distinguished fellow at the Foundation for Applied Molecular Evolution. No one knows, but it is one of many questions intriguing astrobiologists.
Over the past few years, spacecraft such as Cassini have provided an unprecedented look at alien landscapes, boosting the search for life throughout the solar system to new levels. But according to Benner and other scientists, some of the most insightful research is taking place right here on Earth. Life has been found in the unlikeliest of habitats, from South Pole snow to hot springs in Yellowstone. “I’ve always been amazed that it’s so hard to go any place on this planet where there’s energy and water and not find life,” Benner says. “It’s everywhere.”
Understanding the conditions in which life can thrive here helps to shed light on where it might survive elsewhere. Just how dry, cold, hot, ancient and unorthodox can life get? The answers will help determine whether we have extraterrestrial neighbors--and just where in the solar system, or beyond, they might be.
How Did Life Begin on Earth?
On July 3, 2005, the Deep Impact spacecraft released an 820-pound probe into the path of the comet Tempel 1, which was traveling at 23,000 mph. Upon collision, the probe blasted a crater into the comet’s surface, sending a stream of debris flying through space. Deep Impact’s cameras snapped pictures of the carefully choreographed event, and scientists have been poring over the images ever since. They hope to find in the cosmic crash some clues about how life first formed on Earth. A better understanding of how microbes got a foothold here may help scientists identify other planets with the right mix of conditions.
There is one common thread to life on Earth that scientists know for certain: All life here is composed of the same basic building blocks. All proteins are made of compounds known as amino acids. All genes are made of molecules known as nucleotides, which are attached to a backbone made of phosphate and a sugar called ribose.
The big unknowns are when and how those compounds got here in the first place. Life must have emerged after the planet’s birth 4.55 billion years ago and before the oldest indisputable evidence of fossilized microbes, which date back about 3.45 billion years. Sometime during that billion-year window, some of life’s ingredients may have been carried to early Earth by comets like Tempel 1. The scientists studying Deep Impact have already found that the plume of material ejected from the crater contains an abundance of organic molecules--suggesting the comet carries a substantial amount of these critical substances.
The raw materials necessary for life could also have emerged on Earth. Two years ago, Benner and his colleagues produced ribose, which helps form the backbone of DNA, under the sort of chemical conditions that might have existed in deserts on the young planet.
The final piece of the puzzle is how the building blocks became organized into simple living things. Some researchers think that ocean waves may have delivered water rich in organic compounds to tidal flats, where the pounding surf and baking sun acted as a biochemical reactor. Others suspect that life arose in the muck surrounding midocean ridges, where minerals and other energy-rich chemicals spew out of cracks in the Earth’s crust.
“Think of it like a big jigsaw puzzle. We’ve emptied all the pieces out of the box, and we’ve put a few together,” says Bruce Runnegar, scientific director of the NASA Astrobiology Institute. “We’ve got a few pieces of the sky and bits along edges, but we don’t have the whole picture.”
As the pieces come together, they will help direct the search for life elsewhere in the solar system. Mars seems to have had warm, watery conditions some 4 billion years ago, which may mean that life could have formed there. On the other hand, if all life needs to get started is a mix of ingredients and an energy source, then it may have started on less hospitable worlds, such as Titan. Insight into the role of comets may help determine which planets in other solar systems are likely to harbor life. If comets turn out to deliver key compounds, astronomers may need to look for solar systems surrounded by healthy clouds of them.
Does Life Need Water?
Life, as we know it, needs some kind of liquid. “There can’t be life in a solid, and there can’t be life in a gas,” Ward says. In a gas, molecules are flying around so quickly that they can’t carry out the complicated chemical reactions necessary for life. In a solid, they can barely move at all. Liquid is the Goldilocks solvent for life: It’s just right, allowing molecules to wiggle and slide past each other.
Earth is a soggy place. It is covered with oceans, lakes and rivers. Its air is speckled with clouds and loaded with vapor. Water even penetrates miles into the Earth’s crust, lubricating the movements of continental plates. All life here uses water as its liquid solvent, even in deserts and deep inside rocks. But does that mean water is the only liquid capable of supporting life, or did life on this planet just take advantage of the most abundant liquid it could find?
This question lies at the most speculative edge of astrobiology. It is possible--in theory at least--that liquid natural gas or other hydrocarbons could support an exotic kind of carbon-based life. And if life were based not on carbon but on some other element, such as silicon, it could exist in still other types of liquid.
For now, the search for life is focused on finding places where liquid water exists or once existed. But, Benner thinks that astrobiologists shouldn’t narrow their sights. “How do you know this ‘follow the water’ strategy isn’t going to miss the weirder forms of life that don’t require water?” he asks. Mars is the only planet with any clear-cut evidence of liquid water; Jupiter’s moon Europa has liquid water and it seems likely that Saturn’s moon Enceladus does as well. But other bodies may have different liquids capable of supporting life. Liquid ammonia forms clouds on Jupiter. Sulfuric acid blankets Venus. Cassini has taken images of what may be lakes of liquid methane on Titan.
Can Life Exist Without Sunlight?
Most of the people who travel down into the mines of South Africa go in search of gold and diamonds. Tullis Onstott, a geomicrobiologist from Princeton University, goes for another treasure: life fueled by nuclear power.
Onstott and his colleagues gather samples of the water that leaks into drill holes and take them back to the lab, where they isolate microbes. The organisms they’ve found have been thriving more
than 3 miles below ground in a habitat uncontaminated by surface water. “Our environment is extremely isolated,” Onstott says. “It’s been isolated from the surface for tens of millions of years.”
The microbes appear to survive without sunlight by feeding on organic carbon, created by the reaction of carbon monoxide and water. For energy, they use the hydrogen produced when radioactive particles from the rocks split apart water molecules. “You’ve got nuclear power sustaining organisms down there indefinitely,” Onstott says.
This discovery increases the odds that life can exist on--or, more accurately, in--Mars or the icy moons of Saturn or Jupiter. Life on Mars could have retreated deep underground when the temperature plummeted. The outer moons were probably too cold ever to have supported surface life. Yet, scientists can’t rule out the possibility that organisms formed deep underground and survive there today.
How Hot or Cold Can Life Get?
Earth may seem hostile if you’re lost at sea or caught in a blizzard. But compared to other planets, it is wonderfully comfortable. The temperature stays relatively stable, so liquid water is available over most of its surface. A high-altitude blanket of ozone protects it from dangerous cosmic rays, yet enough sunlight reaches the surface to make photosynthesis possible. This energy allows forests and prairies to grow on land, and for billions of tons of algae to grow at sea.
In the search for extraterrestrial life, all of Earth’s luxuries can be discouraging. After all, our home planet’s particular sort of tranquility is far from the norm--at least in our solar system. Just consider our closest neighbors. Mars has no known supply of liquid surface water, no protection from cosmic rays, dust storms thousands of miles across and temperatures that drop to minus 125 F. Venus, meanwhile, is choked by carbon dioxide and can reach 864 F during the day. You wouldn’t look for mushrooms or mountain goats on either planet.
But in recent years, scientists have discovered that life can exist in remarkably extreme environments. Heat-loving organisms, known as thermophiles, can thrive in water as hot as 250 F. They can be found in hot springs, such as the ones that burble to the surface in Yellowstone National Park, and in the water surrounding ridges on the floor of the Atlantic and Pacific oceans, where molten rock pushes up from the planet’s interior. Their chemical make-up is adapted to withstand high temperatures. A special set of enzymes, for example, prevents heat from pulling apart carefully folded proteins.
Life can tolerate brutally cold conditions, too. Jean Brenchley, a microbiologist at Pennsylvania State University, melted a chunk of ice that came from the base of a 3000-meter-thick ice pack in Greenland, where it had been sitting for at least 120,000 years. A close look with a microscope revealed organisms swimming around in the meltwater. “There were a lot of different things in there,” Brenchley says. “It was a very high population of cells and it was very diverse.”
The variety of cold-loving organisms also includes microbes that thrive in Antarctic sea ice at minus 49 F, in rock glaciers high above the Rocky Mountain tree line, and in a lake deep under Siberian permafrost. Called psychrophiles, these creatures have their own challenges and their own remarkable solutions. To keep from turning into solid ice, some produce antifreeze-like compounds that stop water molecules from linking together into crystals.
Thermophiles and psychrophiles offer different lessons to astrobiologists. Some scientists argue that thermophiles played a key role in the emergence of life on Earth. If that’s true, it is possible that life emerged on planets where hot water was present. Mars seems like a particularly good candidate because it has rock formations that geologists think were created in a hydrothermal system.
But today Mars is a cold planet, and the farther from the sun you go, the colder environments get. “There’s a whole lot more cold real estate than hot out there,” says the University of Washington’s Peter Ward. “Cold life is the new frontier.”
Right now all eyes are on Saturn’s moon Enceladus. In March, NASA released images of geysers on the moon’s surface spewing ice crystals into space--enticing evidence that the orb may have a reservoir of liquid water. In light of this discovery, some scientists suggest that other icy moons of Saturn and Jupiter may be promising homes for psychrophiles.
Does Life Need DNA?
Once scientists decide where to look for life, they must decide what to look for. It would be handy if Martians could walk up to a rover, knock on its camera and wave to scientists back at NASA. But chances are that if life exists outside of Earth in our solar system, it will be microscopic. Recognizing this probability, researchers at Carnegie Mellon University have engineered a way to detect organisms on the molecular level.
“Our idea was to build an instrument that could confirm that, in a single location, there were carbohydrates, proteins, DNA and maybe other, more complex biomarkers,” says the project’s team leader, David Wettergreen, who was given a PM Breakthrough Award last November (click here to see PM Breakthrough award winners and innovations). “If you find all of those in one spot, they’re probably associated with one organism.”
For three years, Wettergreen’s team has been testing a rover, named ZoŽ, in Chile’s Atacama Desert. One of the driest places on Earth, the Atacama supports only a smattering of bacteria, algae and lichens. ZoŽ goes about finding them by spraying the ground with dyes that bind to biological molecules, and then illuminating the dyes with high-intensity light.
So far ZoŽ has been performing well. Wettergreen is optimistic that this technology would function on Mars. However, as currently configured, ZoŽ has a weakness: It can recognize only organisms made of the same chemicals as life on Earth. And Earth’s recipe for life may not be the only magic formula.
Some scientists have been trying to create primitive life using RNA, which is the single-stranded version of DNA--the double helix that holds cells’ genetic information. Life today may have evolved from RNA-based organisms that eventually went extinct. Some scientists have speculated that such primordial organisms may still be around, finding refuge in habitats, such as extremely small pores in rocks, that have not yet been fully explored. “We don’t know if DNA-based life is indeed the only life on the planet,” Ward says. “It has not been demonstrated that there are not aliens lurking on Earth.”
Some researchers are using even more exotic molecules, such as peptide nucleic acid (PNA), to store genetic information. DNA and RNA carry their genetic information on backbones of ribose. PNA has a backbone made from peptides, the nitrogen-bearing building blocks for protein.
If life forms only using DNA, it can emerge only on planets with phosphorus, nitrogen and certain kinds of sugar. Some planets, such as Mars, may have had those ingredients at one point, while others, such as Jupiter, probably did not. If scientists can successfully create an alternative to DNA, the range of planets worth studying for life could be open wide.
Benner admits that speculating about alternative life-forms is more what you’d expect from a screenwriter for Star Trek than from a biologist. (In fact, the Enterprise crew did bump into a silicon-based creature in the original series.) But “as long as NASA is going to have a mission to boldly go,” he says, “we really do need to think about where we are going and what we are likely to encounter there.”
That analysis is already well under way. Meanwhile, the fact that our own planet’s strangest places have yielded life is raising scientists’ hopes for the existence of life on other worlds. Their mission now is to go out and find it.
Copyright 2006 Carl Zimmer