Even so, Martians may very well be real and living comfortably in secluded parts of the red planet, scientists are beginning to believe.
These hidden residents of Mars are envisioned not as large and complex organisms, but as tiny specks of life smaller than a
pinprick that flourish deep underground in the wet and more temperate regions of the planet's hot interior.
By nature, such a microbial realm would have eluded the pair of robotic probes that landed on Mars two decades ago and found no
signs of life on the dry surface.
The denizens of the Martian deep, if they exist, may be hard to
find and disappointing to some because they probably resemble
the scum around a shower stall rather than the freaks of science
fiction.
Even so, experts say, the discovery of extraterrestrial microbes
would be a watershed in science.
Finding life on another planet would shed light on the mystery of
how it started on Earth, especially if, as some analysts believe,
Martian life evolved earlier.
The discovery would also help figure the odds of life arising
elsewhere in the universe.
A renewed hunt for life on Mars, especially microbes, is to begin
later this year. Driving the exploration are recent findings that
early Mars was hotter and wetter than previously believed and
that microbes love such environments.
Since the Mars landings, microbes on Earth have been found
thriving in places, like seabed volcanoes, that are extraordinarily
hot, dark, deep and deadly to all other forms of life.
Moreover, genetic studies have shown that microbes living in
these extreme environments are most closely related to the first
forms of terrestrial life, suggesting that evolution began in a
hothouse, as Mars might have been some four billion years ago.
"We're in a different world," said Dr Michael Carr, a scientist with
the United States Geological Survey, who led some of the
Mars-probe analyses two decades ago.
"Our understanding of biology has advanced so much in the past
20 years. The probability that life could have started on Mars is
greatly increased."
Dr Jack Farmer, a Mars specialist at the National Aeronautics
and Space Administration, echoed that judgement. "We now know
that water was abundant at the beginning, and probably still is,
below the surface," he said.
"So you have to ask, why not life? What are the chances? I give it 50-50."
Later this year, two rockets carrying Nasa payloads are to blast
off for Mars to inaugurate a potent new round of international
exploration. In the next decade or so, America, Europe, Russia
and Japan are planning to send as many as 20 missions, if enough
financing materialises. The main goals of the exploratory push are
to find water and life.
Even the discovery of fossil microbes or the biochemical
forerunners of life would be an extraordinarily precious find,
scientists say.
On Earth, wind, rain, erosion and geological tumult over billions
of years have erased most clues to what things were like in the
beginning. Mars is different.
"Much of its surface is ancient and might have a record of what
went on in its earliest history," said Dr Michael Meyer, head of
Nasa's programme to find extraterrestrial life.
"Exploring it might give us a window into the first billion years of
our solar system and the origin of life, on Earth as well as Mars."
Far from the warming rays of the Sun, the rust-coloured planet is
about half the size of Earth and has long generated debate about
the existence of extraterrestrial life.
In 1877, Italian astronomer Giovanni Schiaparelli described its
surface as covered with canals, seas and continents, setting off an
international uproar.
In 1976, two Viking spacecraft photographed the planet and
landed on its surface, finding a world of stark contrasts. There
were giant craters and extinct volcanoes and a canyon as long as
the width of the US. Close up, Mars resembled an earthly desert
of rocks and windblown sand.
But the orbiting cameras found much evidence that water once
flowed over its surface, cutting deep channels and lakes.
Samples that the landers took of the Martian soil underwent a
battery of tests but showed no unambiguous signs of life. So
scientists with lingering hopes set their sights on more elaborate
tests in the future.
Back on Earth after the landings, a quiet revolution shook the
foundations of biology as scientists began to find microbes
thriving in odd places at temperatures up to 113 deg C -- hotter
than boiling water. They were discovered in terrestrial hot
springs, in volcanic vents under the sea, in deep hot oil reservoirs
and in solid rock, kilometres down.
Dr Carl Woese of the University of Illinois stunned the scientific
world by announcing that some of the heat-loving microbes
constituted a third superfamily of life, distinct from that of
bacteria and that of plants and animals. The newly-recognised
group, called Archaea, was linked to Earth's earliest known life.
In 1992, Dr Thomas Gold of Cornell University proposed that
microbes might be ubiquitous throughout the upper few
kilometres of Earth's crust, inhabiting fluid-filled pores, cracks
and interstices of rocks while living off Earth's inner heat and
chemicals. The total mass of this hidden biosphere, he calculated,
might rival or exceed that of all surface life.
"Such life may be widely disseminated in the universe," Dr Gold
wrote, "since planetary-type bodies with similar subsurface
conditions may be common as solitary objects in space, as well as
in other solar-type systems."
It is these kinds of terrestrial findings that are giving Mars its
new allure, as well as judgements that the planet was warm and
wet in its early days.
Dr Norman Sleep of Stanford University recently proposed that
Mars was once probably more hospitable to life than Earth.
Some scientists speculate that a collision billions of years ago
between an asteroid and Mars could have knocked away
microbe-bearing rocks that fell to Earth. This could have sown
the seeds of life on Earth, the theory goes -- in effect, making
humans the descendants of early Martians.
Since the surface of Mars is now largely dry or frozen, scientists
say, any micro-organisms that evolved on the planet would
probably have been forced to retreat into the interior to seek out
warmth and moisture, possibly forming thriving colonies.
"As you dig deeper, it gets warmer and eventually you're going to
hit a depth where the temperature is warm enough to melt water,"
said Dr Stephen Clifford, a geologist at the Lunar and Planetary
Institute in Houston.
Martian rock, he added, might be porous enough to have ground
water moving down to depths of about 10km.
"Water locked in the crust as ground ice or ground water could be
equivalent to a global Martian sea up to a full kilometre deep," he
said. "That's based on a lot of geological evidence."
Many scientific groups, including the American Geophysical
Union, are holding meetings to discuss the possibility that
microbes arose on Mars and might now teem inside the planet.
"If there are life forms that can live in high temperatures in great
depths on Earth, this may also be possible in Mars," Dr Karl
Stetter, a German pioneer in the study of heat-loving microbes,
told a meeting held in London this month.
Dr Farmer of Nasa, who works at the Ames Research Center
south of San Francisco, is analysing terrestrial sites that might
mimic Martian ones. For instance, his team is studying
Yellowstone National Park and its geysers and hot springs to
better understand the geological deposits and visual clues of
microbe-rich springs. The aim is to have spacecraft in orbit
around Mars spot such sites, either active or fossilised.
"There's no reason we can't find them," he said. "There's an
emerging consensus to target these kinds of deposits."
The new round of Mars exploration has three main aims -- to
search for past or present life, to understand the Martian climate
and its lessons for Earth and to search for resources that human
explorers may be able to use.
The unifying theme is water, so the early missions will focus on
finding and understanding its past and present states. Later
missions will try to pinpoint microbes, which is a far more
challenging task.
The exploration is to start with twin launchings late this year,
probably in December. One is known as Mars Pathfinder, which
is to fly directly to Mars and land on the surface in an ancient
flood plain expected to be littered with interesting rocks.
Pathfinder is to transmit images of the Martian terrain with a
colour camera, to monitor the weather and to deploy a small rover
to explore the region around the lander and to sample soil and
rocks.
The other probe is known as the Mars Surveyor. It is to go into
orbit around the planet and use a battery of six instruments to
scan the surface for a full Martian year (about two Earth ones),
seeking visual clues to water and Yellowstone-type outpourings.
Ultimately, scientists say, finding microbial life on Mars may
require drilling into the crust, a difficult venture that would
probably require the presence of humans.
"It may be that you'd have to go deep to find liquid water," Dr
Meyer of Nasa said.
Many scientists are keeping their fingers crossed, eager to
discover that humans are not alone, even if the company turns
out to be microbial slime.
"Part of the reason I got into planetary science in the first place
was to seek out life in other worlds," said Dr Clifford. "Mars is
probably the best chance we're going to have of finding it in our
own solar system."
They would not have antennae nor pointy little heads nor the kind of death rays that H.G. Wells pictured in his 'War Of The Worlds.'
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