Over the years many hints have emerged that there might be life beyond Earth. New Scientist looks at 10 of the most hotly-debated discoveries.
Tests performed on Martian soil
samples by NASA's Viking landers hinted at chemical evidence of life.
One experiment mixed soil with radioactive-carbon-labelled nutrients and
then tested for the production of radioactive methane gas.
The test reported a positive result.
The production of radioactive methane suggested that something in the
soil was metabolising the nutrients and producing radioactive gas. But
other experiments on board failed to find any evidence of life, so NASA
declared the result a false positive.
Despite that, one of the original
scientists - and others who have since re-analysed the data - still
stand by the finding. They argue that the other experiments on board
were ill-equipped to search for evidence of the organic molecules - a
key indicator of life.
Read more: Did Viking missions see life on Mars?
In August 1977 an Ohio State
University radio telescope detected an unusual pulse of radiation from
somewhere near the constellation Sagittarius. The 37-second-long signal
was so startling that an astronomer monitoring the data scrawled "Wow!"
on the telescope's printout.
The signal was within the band of
radio frequencies where transmissions are internationally banned on
Earth. Furthermore, natural sources of radiation from space usually
cover a wider range of frequencies.
As the nearest star in that direction
is 220 million light years away, either a massive astronomical event -
or intelligent aliens with a very powerful transmitter would have had to
have created it. The signal remains unexplained.
Read more: 13 things that do not make sense
NASA scientists controversially
announced in 1996 that they had found what appeared to be fossilised
microbes in a potato-shaped lump of Martian rock. The meteorite was
probably blasted off the surface of Mars in a collision, and wandered
the solar system for some 15 million years, before plummeting to
Antarctica, where it was discovered in 1984.
Careful analysis revealed that the
rock contained organic molecules and tiny specs of the mineral
magnetite, sometimes found in Earth bacteria. Under the electron
microscope, NASA researchers also claimed to have spotted signs of
"nanobacteria".
But since then much of the evidence
has been challenged. Other experts have suggested that the particles of
magnetite were not so similar to those found in bacteria after all, and
that contaminants from Earth are the source of the organic molecules. A
2003 study also showed how crystals that resemble nanobacteria could be
grown in the laboratory by chemical processes.
Read more: Death knell for Martian life
In 1961 US radio astronomer Frank
Drake developed an equation to help estimate the number of planets
hosting intelligent life - and capable of communicating with us - in the
galaxy.
The Drake equation multiplies together
seven factors including: the formation rate of stars like our Sun, the
fraction of Earth-like planets and the fraction of those on which life
develops. Many of these figures are open to wide debate, but Drake
himself estimates the final number of communicating civilisations in the
galaxy to be about 10,000.
In 2001, a more rigorous estimate of
the number of life-bearing planets in the galaxy - using new data and
theories - came up with a figure of hundreds of thousands. For the first
time, the researchers estimated how many planets might lie in the
"habitable zone" around stars, where water is liquid and photosynthesis
possible. The results suggest that an inhabited Earth-like planet could
be as little as a few hundred light years away.
Read more: Other Earths
Alien microbes might be behind
Europa's red tinge, suggested NASA researchers in 2001. Though the
surface is mostly ice, data shows it reflects infrared radiation in an
odd manner. That suggests that something - magnesium salts perhaps - are
binding it together. But no one has been able to come up with the right
combination of compounds to make sense of the data.
Intriguingly, the infrared spectra of
some Earthly bacteria - those that thrive in extreme conditions - fits
the data at least as well as magnesium salts. Plus, some are red and
brown in colour, perhaps explaining the moon's ruddy complexion. Though
bacteria might find it difficult to survive in the scant atmosphere and
-170°C surface temperature of Europa, they might survive in the warmer
liquid interior. Geological activity could then spew them out
periodically to be flash frozen on the surface.
Read more: Bacterial explanation for Europa's rosy glow
In 2002 Russian astrobiologists claimed that super-hardy Deinococcus radiourans evolved on Mars. The microbe can survive several thousand times the radiation dose that would kill a human.
The Russians zapped a population of
the bacteria with enough radiation to kill 99.9%, allowed the survivors
to repopulate, before repeating the cycle. After 44 rounds it took 50
times the original dose of radiation. They calculated that it would take
many thousands of these cycles to make common microbe E.coli as resilient as Deinococcus.
And on Earth it takes between a million and 100 million years to
encounter each dose of radiation. Therefore there just has not been
enough time in life's 3.8 billion year history on Earth for such
resistance to have evolved, they claim.
By contrast, the surface of Mars,
unprotected by a dense atmosphere, is bombarded with so much radiation
that the bugs could receive the same dose in just a few hundred thousand
years. The researchers argue that Deinococcus's ancestors were flung off of Mars by an asteroid and fell to Earth on meteorites. Other experts remain sceptical.
Read more: Tough Earth bug may be from Mars
Life in Venus' clouds may be the best
way to explain some curious anomalies in the composition of its
atmosphere, claimed University of Texas astrobiologists in 2002. They
scoured data from NASA's Pioneer and Magellan space probes and from
Russia's Venera Venus-lander missions of the 1970s.
Solar radiation and lightning should
be generating masses of carbon monoxide on Venus, yet it is rare, as
though something is removing it. Hydrogen sulphide and sulphur dioxide
are both present too. These readily react together, and are not usually
found co-existing, unless some process constantly is churning them out.
Most mysterious is the presence of carbonyl sulphide. This is only
produced by microbes or catalysts on Earth, and not by any other known
inorganic process.
The researchers' suggested solution to
this conundrum is that microbes live in the Venusian atmosphere.
Venus's searing hot, acidic surface may be prohibitive to life, but
conditions 50 kilometres up in the atmosphere are more hospitable and
moist, with a temperature of 70°C and a pressure similar to Earth.
Read more: Acidic clouds of Venus could harbour life
In 2003, Italian scientists
hypothesised that sulphur traces on Europa might be a sign of alien
life. The compounds were first detected by the Galileo space probe,
along with evidence for a volcanically-warmed ocean beneath the moon's
icy crust.
The sulphur signatures look similar to
the waste-products of bacteria, which get locked into the surface ice
of lakes in Antarctica on Earth. The bacteria survive in the water
below, and similar bacteria might also thrive below Europa's surface,
the researchers suggest. Others experts rejected the idea, suggesting
that the sulphur somehow originates from the neighbouring moon Io, where
it is found in abundance.
Read more: Vital clues from Europa
In 2004 three groups - using
telescopes on Earth and the European Space Agency's Mars Express
orbiting space probe - independently turned up evidence of methane in
the atmosphere. Nearly all methane in our own atmosphere is produced by
bacteria and other life.
Methane could also be generated by
volcanism, the thawing of frozen underground deposits, or delivered by
comet impacts. However, the source has to be recent, as the gas is
rapidly destroyed on Mars or escapes into space.
In January 2005, an ESA scientist
controversially announced that he had also found evidence of
formaldehyde, produced by the oxidation of methane. If this is proved it
will strengthen the case for microbes, as a whopping 2.5 million tonnes
of methane per year would be required to create the quantity of
formaldehyde postulated to exist.
There are ways to confirm the presence of the gas, but scientists will need to get the equipment to Mars first.
Read more: A whiff of life on the Red Planet
In February 2003, astronomers with the
search for extraterrestrial intelligence (SETI) project, used a massive
telescope in Puerto Rico to re-examine 200 sections of the sky which
had all previously yielded unexplained radio signals. These signals had
all disappeared, except for one which had become stronger.
The signal - widely thought to be the
best candidate yet for an alien contact - comes from a spot between the
constellations Pisces and Aries, where there are no obvious stars or
planets. Curiously, the signal is at one of the frequencies that
hydrogen, the most common element, absorbs and emits energy. Some
astronomers believe that this is a very likely frequency at which aliens
wishing to be noticed would transmit.
Nevertheless, there is also a good
chance the signal is from a never-seen-before natural phenomenon. For
example, an unexplained pulsed radio signal, thought to be artificial in
1967, turned out to be the first ever sighting of a pulsar.
Read more: Mysterious signals from light years away
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