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FEATURE STORY

Searching For Aliens On Earth

Carol Tang

Just this past winter, the robotic rovers Spirit and Opportunity touched down on Mars and began an unprecedented exploration of our planetary neighbor. They have confirmed that Mars is a desert with a pink sky, a landscape covered in rusty dust, and that the average temperatures linger well below freezing.

The rovers have also found that Mars was probably once warmer, wetter—and, in many ways, much like early Earth. Pebbles of hematite, an iron-based mineral thought to form in liquid water, litter the landscape. And scrapings of Martian bedrock reveal a rare mineral usually found in acid hot springs.

Rover photos show layered sediments with distinct grooves. These grooves likely formed when salts fell out of solution as the water around them evaporated, forming delicate crystals within the rock. Over time, the crystals eroded away, leaving only their impressions behind. Such crystals and grooves are common in deserts on Earth. Finally, recent satellite images show intriguing gullies that appear to have been formed by springs. Some scientists think these gullies are quite young, dating back perhaps only a few million or even just hundreds of years.

Early Earth was probably much like modern Mars. Back then, the Earth’s rocks were more radioactive, noxious chemicals filled the atmosphere, and meteorites frequently fell from the skies. Yet, over the last 25 years, scientists have discovered that many kinds of life evolved in this inhospitable place. These extremophiles—creatures that “love” temperatures, pressures, and chemicals that would kill most other organisms—still look relatively unchanged today. The survival skills of these organisms helped them colonize isolated hot springs, ancient glaciers, barren deserts, and the deep darkness of the ocean floor.

Scientists are now studying life in these extreme environments in hopes of gaining hints about life on other planets. Astrobiology—the study of the origins, evolution, and distribution of life in the universe—requires expertise in all the classic sciences. Only by putting their heads together will biologists, geologists, chemists, and astronomers be able to answer the questions we have about extraterrestrial life. What are the critical conditions needed to support life? Is there life beyond our planet? Can earthbound creatures give us clues to how to find life elsewhere? And, most fundamentally, what is life?

The discovery that arguably launched the field took place in 1977. While exploring an underwater mountain range off the Galápagos Islands, geologists stumbled across an entirely new type of ecosystem. There, amid waters heated above boiling temperatures, and pressures 160 times greater than we experience on land, they encountered a community of six-foot-long tubeworms, eyeless ghost shrimp, strange shellfish, and exotic bacteria.

The scientists had found an alien world here on Earth. Instead of using the energy of the sun’s rays, the entire ecosystem depends solely on chemical energy—the first such community ever discovered. The bacteria convert the energy of sulfur gases streaming from the vents into food. The tubeworms have incorporated these bacteria into their cells. The bacteria pass their sugars to tubeworms directly, so the tubeworms have no need for stomachs or guts.

Life, we learned, could be found in unexpected places, doing unexpected things.

The discovery of the vents radically changed our view of which other planets might be habitable. The vent communities proved life doesn’t need to be close to the sun to get enough energy to survive. Photosynthesis—the system plants use to turn solar energy into food—was just one adaptation. Maybe chemosynthesis was actually the original system. When life began on the early Earth, this kind of environment was probably a lot more common than it is today.

The geologic formations around the vents may also help direct studies on Mars. These vents shoot out minerals from the Earth’s interior that precipitate out when they hit cold seawater. The minerals eventually plug the vent chimneys, forming characteristic ores. Scientists have gone back to the geologic record looking for these ores as a way to identify early hydrothermal vents. In Australia, there is a site where we think we can trace hydrothermal vents back 400 million years to the Devonian period.

California has been a center of astrobiological research from the beginning. It is home to the NASA Ames Research Center in Mountain View, the Jet Propulsion Lab in Pasadena, the Search for Extraterrestrial Intelligence Institute (SETI) in Palo Alto, and the headquarters of NASA’s Astrobiology Institute.

California also has more than its share of extreme environments. Between 200 and 60 million years ago, the tectonic plate underlying the Pacific Ocean crashed into the continental margin of North America. The collision brought a rich supply of magma and minerals close to the surface. These geologic shifts also created places where methane bubbled up through the ocean floor. Such cold seeps attracted a whole ecosystem of tube worms, clams, mussels, methane-digesting bacteria, and other microbes—communities very similar to the ones found at hydrothermal vents.

Subsequent geologic activity thrust many cold seeps above sea level. Researchers Lisa White and Kristen Hepper of San Francisco State University have studied one of these fossil cold seeps at the border of Napa and Lake Counties. Their excavations are providing clues to how biological communities can be built entirely on chemical energy. “In California, we have the best continuous record of a long-lived, well-developed cold seep system,” says Hepper. “This allows us to study the evolution and distribution of these types of unique organisms.”

With the discoveries being made by the Mars rovers, hot springs are becoming hot targets for astrobiological research. The hot springs in Lassen National Park have already yielded rich astrobiological findings. The same processes responsible for Mount Lassen’s volcanic activity heat pools of acid to temperatures over 80 C. Despite these blistering conditions, Ken Stedman, a professor at Portland State University, has found both a sulfur-eating microbe known as Sulfolobus and viruses that prey on it. Sulfolobus is an archaean—a lifeform that evolved even before bacteria.

Stedman says Sulfolobus and its associated viruses are found in many hot springs throughout the world. He is also studying how these microbes can carry out the chemical reactions needed for life at temperatures lethal to most other creatures. In Yellowstone’s hot springs, he’s found another virus that may have been an ancestor to modern animal, plant, and bacterial viruses. It could tell us much about its original prey, which were likely among the first organisms to emerge on Earth.

My own astrobiology research focuses on another such extreme environment—the thermal springs of Cuatro Cienegas, in north-central Mexico. This habitat is analogous to many hot spring systems within California.

Cuatro Cienegas lies in a geological province known as the Basin and Range, which starts in California and extends east. In a process similar to those found in Death Valley and parts of Nevada, heated underground water emerges in isolated thermal springs along the desert floor. While underground, this water has often acquired unusual salts, minerals and trace elements. The water chemistry differs from one spring system to another, suggesting that they have independent water sources. Temperatures can vary greatly from about 0 C to 70 C depending on heat source and ambient temperature.

In many ways, these springs are like inverse islands—isolated pools of water amidst vast expanses of desert. And these islands, like the Galápagos archipelago or Madagascar, are ideal laboratories for studying evolution. Since 1939, American and Mexican biologists have noted the surprising number of species which live in Cuatro Cienegas and nowhere else in the world. Scientists now consider diversity one of the major characteristics of life. Some Cuatro Cienegas organisms are restricted not only to this valley, but to single pools. The Mexican government established the area as a wildlife refuge in 1994 to protect its biological diversity.

The desert and thermal springs of Cuatro Cienegas may be analogs of the environments where life originated on Earth. In the early days of our planet, the movement of tectonic plates created many places where molten magma welled up toward the surface. The pressure of the liquid rock created rifts in the ocean floor and stoked the fires of underwater hot springs. At these high temperatures, proteins and amino acids interact with one another more quickly. These conditions could be a prerequisite—or at least a catalyst—for life. Today, some of the organisms located at the base of the tree of life can only be found in extremely hot aquatic habitats. Could these heat-loving microbes be similar to the ancestors of all life on Earth? And if so, would this kind of environment also be a nursery for life on other planets?

Our studies in Cuatro Cienegas have already given us insights into how microbial and animal life become preserved in the fossil record of thermal springs. Algal mats at the site have been frozen into a type of fossil known as a stromatolite. Stromatolites are evidence of life that scientists could expect to find on Mars. Understanding how stromatolites form will help direct the investigations of future scientific missions to Mars.

The field of astrobiology has put the study of extremophiles into a new perspective and a new context. Mankind’s desire to look for life elsewhere in the universe has focused our studies on extreme habitats here on Earth. In the process, it has helped us appreciate the amazing diversity, hardiness, and adaptability of the life around us.

Carol Tang is Chair of the Educational Programs and a Research Associate at the California Academy of Sciences.