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FEATURE STORY
Searching For Aliens On Earth
Carol Tang
![diatom](../images/astrobiology_life.jpg)
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Diatoms are among the most widely distributed
organisms on earth, yet lack a means to get around on their own.
Scientists are studying diatom distribution to learn how small lifeforms
might travel through space.
photo: CAS Special Collections
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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.
![spring at Cuatro Cienegas](../images/cuatro_cienagas.jpg)
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An
astonishing number of species have evolved in the waters of Cuatro
Cienegas. The unique geochemical cocktail in each pool may have
fueled this explosion of diversity.
photo: Carol Tang
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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.
Astrobiology At
The Academy |
Could extreme habitats on Earth tell us whether
and where life exists on other planets?
This question is the theme of the Academy’s new
Astrobiology exhibit. Over the coming months, sections of the exhibit
will be changed to feature life in other extreme ecosystems. Visitors
will learn about the extremophiles that inhabit hydrothermal vents
and hot springs as well as the broad and exciting nature of this
newly-minted science.
There are videos of a hydrothermal vent chimney
spewing noxious gases through the seafloor, surrounded by giant
mussels, crabs, and tubeworms. A section of an actual vent chimney
and a sample of the mineral ores that formed inside are also on
display. Astrobiologists will use this evidence of undersea life
as a model of potential vent communities on Europa, a moon of Jupiter
with an ocean covered with ice.
One case houses the youngest sample of seafloor
on display in the world. Pulled from the Pacific Ocean, it formed
less than a year ago. At the push of a lever, a plate tectonics
exhibit brings alive the static images of textbooks and demonstrates
how seafloors continue to form.
One of the most valuable items in the exhibit
is a piece of the Murchison meteorite famed for harboring amino
acids from outer space. Containing the building blocks of life,
this shiny black rock could provide an explanation for how life
has been transported within our galaxy. Close by lies a chunk of
yellowish jarosite, the mineral that helped confirm Mars was once
wet. Rare on Earth, it is restricted to hot springs and acid lakes.
While rocks provide important clues to environmental
conditions of the past, the exhibit also has examples of Earth’s
living extremophiles. The desert pupfish on display belong to one
of the most endangered species in the world, limited to a single
hot spring in Death Valley. These animals come from Steinhart Aquarium’s
captive breeding program, which is helping to keep this species
alive.
The exhibit will certainly rouse interest in this
new area of science and encourage us to look beyond the familiar.
The expanded knowledge gained from extreme environments may just
provide the clues needed for discovering life beyond Earth.
Maria Pegoraro
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Carol Tang is Chair of the Educational Programs
and a Research Associate at the California Academy of Sciences.
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