Sea stars thrive in the tide pools of the Olympic Coast National Marine
Sanctuary, one of the most dynamic and chemically complex water environments
in the world. ©NOAA, Nancy Sefton
Scientists have been thinking about life on other worlds for a very, very
long time. Of course, the first question we have to answer is, "What is
life?" Since we know of only one planet that supports life, we start by
looking at where life is found here on Earth.
All life on Earth—from bacterium to blue whale, slime mold to sequoia,
and everything in between—needs one thing: liquid water. In this
essay, we'll talk a little about that watery thread that binds all Earthly
life, and a lot about how—and where else—it might exist in
our Solar System.
Water: the elixir of life
Why do scientists think that life as we know it requires liquid water?
The reasons are complex and manifold, but it boils down to the physics
and chemistry of being alive. Living things are systems that grow, sustain,
and reproduce themselves with an unmatched combination of complexity and
accuracy. Those functions require a medium that can contain (in solution
or suspension) a huge variety of complicated molecules; that can rapidly
transport those molecules from one part of an organism to another; that
is plentiful enough to be easily obtained and replenished; that can hold
large amounts of energy as a reservoir for physical and chemical activity;
and that can serve as a place where incredibly complicated reactions can
occur, yet is itself not particularly physically or chemically active.
That's a tall order for any substance. But liquid water can do it all.
In primary school, we all learned that water flows easily from place to
place, covers 70% of Earth's surface, and is the so-called "universal solvent."
In secondary school, we studied other important characteristics of water:
it has a high heat capacity (which means that it can absorb or release
relatively large amounts of thermal energy - heat—with only a modest
change in temperature); it's a key participant in a myriad of chemical
reactions; and it has a neutral pH, the balance point between acid and
base. What organism could ask for more?
Water serves more life-supporting roles on an ecosystem level. Thanks
to its unusual molecular structure, it's the only known non-metallic substance
to expand when it freezes. (That's why ice cubes and icebergs float; frozen
water is less dense than liquid water.) Also, frozen water will melt if
you press hard enough on it, even if the temperature stays below freezing.
(That's partly why glaciers flow: a liquid layer below the surface lets
the solid ice above it slide slowly along.) Put these two properties together.
If a body of water is deep enough, it can be frozen at the surface and
yet remain liquid far below, providing an insulated, protected environment
in which a great range of physical and chemical reactions can occur.
Liquid water can exist under diverse conditions. This cross-section illustrates
the location of Lake Vostok, the largest known subglacial lake, which is
buried more than two miles beneath the Antarctic ice. It remains liquid
because of geothermal heating and pressure from the ice above it, which
also insulates the water. ©NSF, Nicolle Rager-Fuller
The presence of liquid water, then, signals both the likelihood that reactions
essential to life can occur, and the possibility that an ecosystem can
be sustained. That's why the search for life starts with the search for
Follow the water
So let's look around for places in space where the right conditions could
combine to make liquid water possible. If a planet orbits its host star
too closely, the temperature is too high and the planet's water content
vaporizes. Too far from the host star, and the planet's water content freezes.
In other words, conditions on the planet must allow the temperature to
stay within the 100° C (212° F) range of liquid water. In our Solar
System, that makes Venus too hot and the surface of Mars too cold. Farther
from the Sun it's even colder, but Jupiter and Saturn's gravitational fields
create internal tides in the moons around them, possibly generating enough
heat to sustain liquid water, almost certainly in Jupiter's moon Europa.
|The Habitable Zone
Earth is the only planet in our Solar System that falls within a range
of temperature, size, and atmospheric thickness that allows for liquid,
solid and gaseous water to coexist. ©AMNH
Beyond our Solar System, the search has just begun for other "Goldilocks
planets"—not too hot and not too cold—where liquid water might
persist. In April 2007, for example, astronomers found what appears to
be a slightly-larger-than-Earth-size planet in the right orbit around a
very dim star: Gleise 581, 20.4 light-years away in the direction of the
Once we identify where water might be present, we can search for it in
several ways. The simplest way is direct imaging: taking pictures of the
surface. Large, featureless expanses that are darker than surrounding surface
features can suggest the presence of seas and oceans. (Galileo Galilei
used this technique on the Moon in 1609, noting the presence of maria
or seas. However, he was mistaken; those seas were bone dry.) Reflection
spectroscopy is more difficult but more precise: since we know which wavelengths
of light are absorbed by water, when we look with telescopes at how any
surface absorbs sunshine, we can detect the presence of water ice from
that reflected light. The Lunar maria reflect like basaltic rock, not ice.
An artist's impression of the newly discovered planet orbiting the red
dwarf star Gliese 581, which is far dimmer than our Sun. The planet—one
of at least three discovered in the system—is so close to its star
that it zips around it in a mere 13 days. ©ESO
The best, and by far the most expensive, method is to send probes to investigate
surface features close up. If a planet or moon has a solid or liquid surface
to land on, a robotic lander is ideal. We'll talk now about the progress
that we've made using probes. But first, a preview: in May 2008, the Mars
Phoenix Scout mission will land near the Martian north pole, to drill down
beneath the frigid surface in search of signs of microbial life.
The search on icy moons
Models and observations indicate that other places in the Solar System
probably have liquid water; one is a large moon of Jupiter's. (You'll read
about it in this week's Mission Profile.) Fifty years ago, we thought these
moons were inactive lumps of rock like Earth's Moon, but the first flyby
mission, Voyager, established that each is unique, and that Europa's smooth
surface of water ice is even crisscrossed with fractures like those across
sea ice on Earth!
We also think that Enceladus, a moon of Saturn, contains liquid water.
Surface features such as regions with no craters (indicating recent geological
events), fissures, plains, and corrugated terrain indicate that the interior
of Enceladus may be partly liquid today. Images from the Cassini spacecraft
show a huge icy plume or geyser, which might be erupting from near-surface
pockets of liquid water warmed by tidal heating. Icy droplets from this
plume actually contribute material directly to Saturn's rings! Imagine
if there were bacteria living under the ice on Enceladus. If they were
launched into space to become part of Saturn's rings, how would we detect
The search on Mars
|Modeling Interior Structure
The density and radius of Earth's Moon closely resembles Jupiter's rocky
satellites, Io and Europa, while Ganymede and Callisto are inferred to
be icy satellites. The blue color indicates water (liquid or ice), while
the gray or brown is rock. ©NASA/JPL
So far, though, the most intense search for extraterrestrial liquid water
has been conducted on Mars. Though our Moon is closest to Earth, human
visits in the 1960s showed no water on its surface and a crust that bore
no chemical sign of exposure to liquid water. (No liquid besides water
would be stable at Martian conditions.
Water ice on Mars' surface sublimes would evaporate very rapidly because
of the low pressure of the atmosphere. But the pressure of rock might within
the planet might mean that water ice, and even liquid, are present at depth.)
Mars is the next easiest Solar System body to get to—and onto—so
that's where we're looking hard.
The possibility of life on Mars has long captured the imagination of scientists
and the general public. In the late 1800s, for example, Italian astronomers
Pietro Secchi and Giovanni Schiaparelli observed channel-like structures
on Mars, which led the American astronomer Percival Lowell to hypothesize
that these were water-bearing canals. The flames were fanned more recently
when a meteorite called ALH 84001, apparently ejected from Mars in an asteroid
or meteorite impact, was found in Antarctica in 1996. The rock crystallized
about 4 billion years ago, and contains minerals formed by water interacting
with the rock. In 1997, it was thought to contain possible fossil evidence
of bacterial life—a finding later shown to be erroneous. Nevertheless,
the search for life on Mars continues to provoke scientific inquiry and
public fascination. ALH 84001was simply a diversion; the truth lies on,
or inside, the planet Mars.
The Viking probes—orbiters and landers—made breakthrough observations
of Mars in the 1970s. They gathered a wealth of data but didn't find a
trace of life or water. Two decades later, in 1997, the Mars Pathfinder
mission, with its breadbox-size Sojourner rover, found fields of loose
rocks tilted in the same direction like those found on flood plains on
Earth. This is circumstantial evidence that water once flowed on the Martian
surface, but still nothing to hang your scientific hat on.
|Building the Evidence
NASA's Mars Exploration Program continues to find signs of the Red Planet's
watery past (from left): One key finding of the Mars Global Surveyor was
a massive gully cut into a crater, like the one shown at left, that had
appeared between orbits; the Mars Exploration Rover, Opportunity, landed
in Eagle Crater; a treasure trove of rocks deposited in shallow water;
sphere-like grains of hematite, which form in water. ©NASA
The case for water on Mars has been greatly advanced by the Mars Exploration
rovers Spirit and Opportunity, which landed on the Red Planet in January
2004. They observed features (resembling sand or sediments) that we know
are gently laid down when water is very shallow. And they detected tiny
beads of a mineral called hematite (in so-called "blueberries," though
they're not really blue), which commonly forms when iron precipitates from
water. When the water settles out and evaporates, iron oxide dissolved
in the water forms these little solid concretions.
The results from the landers have been buttressed by high-resolution photographs
from spacecraft in orbit around Mars, which show clear-cut evidence of
catastrophic flooding and signs of underground water flow. So we now know
that at one time there was water—not just water, but liquid water
and in large abundance—on the surface of Mars, and that the subsurface
may still contain stable liquid water. For a more definitive answer, stay
tuned for news from the Mars Phoenix and future missions!