source NewScientist
1. 1976, The Viking Mars landers detect chemical signatures indicative of life
2. 1977, The unexplained extraterrestrial "Wow!" signal is detected by an Ohio State University radio telescope
3. 1996, Martian "fossils" are discovered in meteorite ALH80041 from Antarctica
4.
2001, A more rigorous estimate of the "Drake equation" suggests that
our galaxy may contain hundreds of thousands of life-bearing planets
5.
2001, The red tinge of Jupiter's moon Europa proposed to be due to
frozen bits of bacteria, which also helps explain the mysterious
infrared signal it gives off
6. 2002, Russian scientists argue that a mysterious radiation-proof microbe may have evolved on Mars
7. 2002, Chemical hints of life are found in old data from Venus probes and landers. Could microbes exist in Venusian clouds?
8. 2003, Sulphur traces on Jupiter's moon Europa may be the waste products of underground bacterial colonies
9. 2004, Methane in the Martian atmosphere hints at microbial metabolism
10. 2004, A mysterious radio signal is received by the SETI project on three occasions - from the same region of space
1. 1976, The Viking Mars landers detect chemical signatures indicative of life
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.
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2. 1977, The unexplained extraterrestrial "Wow!" signal is detected by an Ohio State University radio telescope
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.
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3. 1996, Martian "fossils" are discovered in meteorite ALH84001 from Antarctica
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.
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4.
2001, More rigorous calculations connected to the 1960s "Drake
equation" suggests that our galaxy may contain hundreds of thousands of
life-bearing planets
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.
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5.
2001, The red tinge of Jupiter's moon Europa proposed to be due to
frozen bits of bacteria, which also helps explain the mysterious
infrared signal it gives off
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.
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6. 2002, Russian scientists argue that a mysterious radiation-proof species of microbe may have evolved on Mars
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.
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7. 2002, Chemical hints of life are found in old data from Venus probes and landers. Could microbes exist in Venusian clouds?
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.
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8. 2003, Sulphur traces on Jupiter's moon Europa may be the waste products of underground bacterial colonies
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.
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9. 2004, Methane in the Martian atmosphere hints at microbial metabolism
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.
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10. 2004, A mysterious radio signal is received by the SETI project on three occasions - from the same region of space
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.
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