快猫短视频

A LIFELESS planet, a cold world stirred only by dust storms raging above dry
red plains. That was the bleak picture of Mars painted by the Viking spacecraft,
which failed to find signs of life there in the late 1970s. But some scientists
now believe that Viking could have missed vital clues. Accumulating evidence
suggests that Mars may once have been a more hospitable world鈥攁 warm
planet with running water and bubbling hot springs. Could life have emerged in
this pleasant climate more than 3.5 billion years ago? Could it even still
flourish in isolated spots, far from Viking鈥檚 gaze?

The search for life on Mars has begun again in earnest. And there is a chance
that the question could be resolved within the next decade. A series of NASA
spacecraft are set to depart for Mars, starting in November with the Mars Global
Surveyor. This spacecraft will map the planet鈥檚 surface in unprecedented detail.
The programme will culminate in 2005 with a mission to bring samples of the
Martian surface back to Earth (see 鈥淩eturn to the Red Planet鈥�). The main goals
are to find out much more about how the climate of Mars has changed over time,
and whether it has ever been home to living organisms.

NASA will begin by looking for evidence of liquid water鈥攖he hallmark of
a hospitable climate and a vital resource for all life on Earth. Today, Mars is
too cold to have liquid water on its surface. But the chances are that in its
early history, Mars was warmer and was host to flowing water. The Viking mission
may already have uncovered its telltale traces.

Viking images show that the Martian terrain falls neatly into two types. One
is heavily cratered and dates back more than 3.5 billion years to the early
Solar System. The other type of terrain is younger, and reveals the geological
processes that have acted since then. The differences between the two types of
surface suggest that very early in Mars鈥檚 history, running water may have shaped
the landscape. Craters on the young surface are fairly well defined. But on the
oldest surfaces, erosion has filled in the larger craters and smoothed out the
landscape; old craters smaller than about 15 or 20 kilometres across have
disappeared completely. A few of those that are only partly eroded show gullies
on their rims, which could only have been carved by running water.

The valleys on Mars also suggest that water flowed on the young planet. The
old terrain is scored by branching networks of valleys, similar to those forged
by river tributary systems on Earth. These are definitely the work of water,
flowing at or very near to the surface. However, the surface water must have
disappeared later on鈥攖here are no valley networks on younger surfaces,
except near some of the younger volcanoes.

But how could the water have vanished? While most of it could well have
escaped into space, the surface features suggest that some water filtered
underground. Some of the younger terrain shows large channels that appear to
erupt from under the surface. These are similar to catastrophic flood channels
found on Earth, for instance in the east of Washington state. Flood channels
formed there near an ice-dammed lake, when the ice melted and the lake suddenly
drained during the last ice age.

The Martian flood channels suggest that there is still lots of water beneath
the surface. In some places, they are hundreds of kilometres wide and over a
thousand kilometres long. In 1979, Michael Carr of the US Geological Survey in
Menlo Park estimated that the amount of water necessary to carve the channels is
the equivalent of a layer of water several tens of metres deep covering the
entire surface of the planet. This suggests that there may still be a layer of
water up to half a kilometre thick in the crust.

Primordial soup

Mars might well have been a watery planet, but did it have all the
ingredients for life? American chemist Stanley Miller developed the traditional
story of life鈥檚 beginnings about 40 years ago. He suggested that life began as
an organic 鈥渟oup鈥� produced by lightning-induced chemical reactions in the
primordial atmosphere. However, in the late 1970s, Jack Corliss of Oregon State
University put forward an alternative. He suggested that life formed in an
environment that had an internal source of energy, abundant dissolved minerals
and protection from changes in the climate鈥攊n other words, a hot spring.
The most primitive bacteria are all 鈥渉yperthermophilic鈥�, thriving only in the
boiling water found near springs. So life clearly existed in these conditions
very early in its history.

Over the past decade, a lot of intriguing evidence that hot springs were once
common on Mars has come to light. Hot springs are often associated with
volcanoes, which were vigorously active on Mars in the past. In fact, over the
past 3.5 billion years volcanic material has resurfaced the planet鈥檚 entire
northern hemisphere. In 1991, Ronald Greeley at Arizona State University in
Tempe suggested that the total amount of volcanic rock on Mars may be equivalent
to a global layer about 5 kilometres thick.

The heat from underground magma could easily have fuelled hot springs like
those we see on Earth. One hint of this comes from valleys in the flanks of some
younger Martian volcanoes. In 1989 and 1990, Virginia Gulick of NASA鈥檚 Ames
Research Center in Mountain View, California, and Vic Baker of the University of
Arizona in Tucson suggested that they are most likely to have been carved by
water. Heating by the magma would have forced the water to rush up from beneath
the surface.

Other telltale signs of hot springs on Mars have, surprisingly enough, been
found on Earth. They come from around a dozen meteorites known as the SNC
meteorites, after the initials of three places where they were found on Earth.
Most of these are fragments of basaltic lava, some of which fell to Earth during
the past century, and most planetary scientists believe that they came from
Mars. The lava crystallised within the past 1.3 billion years, so the meteorites
must once have been part of a rocky planet that partly melted late in the
history of the Solar System. Only Mars or Venus could have retained enough heat
to do this. The meteorites also contain trapped gases that have exactly the same
composition as the Martian atmosphere. They were probably chipped out of the
Martian surface by a collision with a large asteroid.

The SNC meteorites contain the minerals typically found near hot springs on
Earth. Veins of calcite, which often form when hot water flows through cracks in
rocks on Earth, run through some of the meteorites. And one of the rocks
contains a mineral called iddingsite. Comparison with rocks on Earth suggests
that the iddingsite formed when hot water circulated through the rock. In 1993,
Allan Treiman of the Lunar and Planetary Institute in Houston, Texas, confirmed
that both the calcite and iddingsite must have formed on Mars rather than on
Earth鈥攖hese minerals are embedded deep inside the meteorites, where
terrestrial water could not have penetrated.

More hints of hot springs come from the ratios of stable isotopes in the
Martian atmosphere, measured by the Viking spacecraft and by observations from
Earth. The isotope ratios are remarkably similar to those in the planet鈥檚 crust.
快猫短视频s believe that this may be explained by the presence of circulating hot
water on Mars.

Researchers have compared the amounts of certain pairs of isotopes鈥攆or
instance, hydrogen and its heavier isotope deuterium鈥攁nd found that there
is an excess of the heavier isotope compared to terrestrial values and to
estimates of the composition of Mars鈥檚 early, warmer atmosphere. This suggests
that the lighter isotope has escaped into space, which it would do more easily
than its heavier partner.

Then, in 1994, Laurie Leshin and her colleagues at the California Institute
of Technology measured the deuterium-to-hydrogen ratios within different
minerals in the SNC meteorites. They were up to five times greater than the
value found on Earth, but were remarkably close to the ratio in the present-day
Martian atmosphere. To keep the balance in the crust and in the atmosphere the
same, gases must have been continuously exchanged right up to the present
day.

Oxygen isotope studies also point to a mixing of the atmosphere into the
crust, and the easiest way for isotopes to move in this way is within
circulating hot water. This occurs in the hydrothermal systems at 鈥渕id-ocean
spreading centres鈥� on Earth, where faults create cracks in the crust so that
magma below can heat the seawater. This process is partly responsible for
setting the isotope ratios in oceans.

Alternatively, occasional improvements in the Martian climate during the past
several billion years, rather than hot springs, might have driven the mixing of
isotopes to give the same result. Periods of warmer temperatures would have
allowed some of the ice to melt. Surface water could then have filtered into the
crust, carrying with it the atmospheric gases.

There is indeed some evidence, admittedly weak, that the usually harsh
Martian climate might occasionally have given way to milder conditions. This
comes from young, possibly glacial features on the Martian surface. Jeffrey
Kargel of the US Geological Survey in Flagstaff points out that these are less
than 3 billion years old, so they must have formed after the early warm period.
Only a later warm spell could have allowed the glaciers to move over the surface
and then melt.

Hot spells

So if there was a warm period, what caused it? It could have resulted from an
influx of carbon dioxide which warmed the atmosphere through a greenhouse
effect. This CO₂ could have come from the polar ice caps, its release
triggered during periods when the planet鈥檚 polar axis tilted more towards the
Sun than it does today. At the moment, Mars鈥檚 equator is tilted by 25掳 from
the plane of its orbit round the Sun. However, its tilt varies unpredictably,
and could have increased to as much as 60掳. At this extreme, the temperature
increase at the poles could have been enough to melt several kilometres of polar
ice.

My own calculations last year, and those of Michael Mellon at NASA鈥檚 Ames
Research Center in California more recently, suggest that the ice caps could
have contained fifty times as much CO₂ as there is in the atmosphere.
Released into the atmosphere, this could raise the average temperature of the
planet by about 20 掳C. However, it is unlikely that even this temperature
rise would have been enough to allow a stable cycle of water moving in and out
of the surface without freezing. So hot springs are still the most likely
mechanism for the gas exchange.

Hot springs were probably most common early on, when the planet was warmer,
water flowed on the surface and there were plenty of active volcanoes to provide
the heat. But could there be any springs still bubbling today? Some volcanoes
may still be active on the cooling planet. Although none has been spotted, we
know from the ages of some inactive volcanoes that they were active within the
past half billion years鈥攚hich is relatively recent in geological terms. So
Mars may still host the odd volcanic eruption that brings hot springs to life.
If so, the springs would be the ideal place to search for life.

The new series of NASA spacecraft will be on the lookout for clues. The first
will go into orbit round Mars in September next year. It will seek out ancient
lakebeds, as well as hydrothermal systems that have discharged to the surface.
The orbiter will carry a camera to identify any active springs, or the small
channels and pits that might mark the sites of earlier springs. A sensitive
spectrometer is also on board to record the mineral signatures of lakes or
hydrothermal systems.

Later missions will also look for signs of life on dry land that the Viking
programme might have missed. The experiments on board the two Viking spacecraft
looked for evidence of various types of metabolism that we recognise in life
here on Earth鈥攆or instance, those using CO₂鈥攁nd drew a
blank. Combined with the complete lack of organic molecules within the topmost
surface at the landing sites, this seemed to suggest that Mars was a lifeless
desert.

But were the Viking probes looking for the wrong signs? The chemical
reactions that drive biological activity are now known to be much more diverse
than anyone suspected in Viking鈥檚 day. It is now clear that reactions involving
hydrogen, methane, sulphur or iron can sustain biological activity. Last year,
Todd Stevens and James McKinley of the Pacific Northwest Laboratory in Richmond,
Washington, discovered bacteria deep within basalt rock beneath the Columbia
River that thrive only on hydrogen (快猫短视频, Science, 28 October
1995, p 19). The hydrogen is continually produced by chemical reactions between
water and crushed basalt. An ecosystem like this would have been completely
invisible to the Viking craft.

But the final answer on whether life has ever existed on Mars will probably
only come after careful studies of Martian samples back on Earth. In 2005, NASA
plans to bring back samples from one or more sites. The exact landing spot will
be chosen at a later date, but contenders include an ancient lakebed, the
ancient highlands, or a young volcano.

But will NASA鈥檚 programme cast the net wide enough? To attack the problem
properly would mean bringing back many samples from many carefully chosen sites,
and it is not yet clear that the forthcoming missions will do this. There has
been no commitment from NASA that its craft will carry instruments such as
orbiting spectrometers that could detect the spectral fingerprints of water and
organic compounds on a large number of sites at very high resolution. And there
may not be enough time before the first sample-collecting mission in 2005 to do
the detailed analyses that will be needed to find the perfect site.

Nonetheless, this is the best chance yet to find signs of life beyond our
world. No one denies that the chances are slim. But if volcanoes on Mars still
rumble, if hot springs still bubble and if the spark of life was ever kindled
there, a sensational astronomical discovery might be just round the corner.

* * *

Return to the Red Planet

NASA鈥檚 next series of missions to Mars will begin in November this year.
First in line is the Mars Global Surveyor, a partial replacement for the Mars
Observer spacecraft that lost contact with Earth just as it reached the planet
in 1993. The new spacecraft will swing into a near-polar orbit round Mars in
September 1997.

It will carry cameras to build up a map of the whole planet鈥攖he ancient
cratered surface, the frozen polar caps, giant canyon system and volcanoes. The
craft will seek out the spectra of different minerals on the surface using an
infrared spectrometer. By firing pulses of laser light at the surface and timing
the arrival of the reflection, it will also measure the heights of mountains and
the depths of valleys.

The next mission will be the Mars Pathfinder, to be launched in December or
January. The Pathfinder lander has a small rover and a camera, and can measure
the abundances of elements. NASA hopes that this mission will prove that a cheap
spacecraft can land on the surface and send back impressive results. Although
the spacecraft is about the same size as each of the Viking landers, the
construction costs are only about a tenth of that for a Viking craft.

The next launch should go ahead in late 1998, and will include an orbiter to
study daily weather patterns on Mars. The orbiter will carry an atmospheric
sounder to analyse the behaviour of volatile chemicals in the Martian
atmosphere, and a camera to map the surface. Around the same time, a lander will
head for the high southern latitudes to explore the ice cap and investigate the
polar seasonal cycles.

NASA hopes to continue the assault on Mars over the next decade鈥攁nd
possibly beyond鈥攂y launching two spacecraft roughly once every two years.
The orbiter for 2001 is still on the drawing board, but may carry a gamma-ray
spectrometer like the one on the ill-fated Mars Observer. This will identify
surface elements, and map out the abundance of hydrogen tied up in water and
ice. A simultaneous lander mission might focus on the ancient highlands and the
evolution of volatile chemicals.

The later missions should reveal much more about the atmospheric circulation
and weather, and build up to the return of a sample of the Martian surface in
2005.