Whether we are alone in the universe
is one of the oldest questions humans have pondered.
_______________________________________________
For most of history, it has belonged squarely
in the provinces of religion and philosophy.
In recent decades, however,
scientists also have been attracted
to the problem in increasing numbers.
Fifty-one years ago,
a young astronomer by the name of Frank Drake
began sweeping the skies with a radio telescope
in the hope of stumbling across a message
from an alien civilisation.
Thus began SETI
-- the Search for Extraterrestrial Intelligence --
an ambitious enterprise to survey
thousands of sunlike stars
in our neighbourhood of the Milky Way galaxy
for any signs of artificial radio traffic.
When SETI began in 1960,
it was regarded
as quixotic at best,
crackpot at worst.
"A quest of the most adverse odds,"
was the way distinguished biologist
George Simpson expressed it.
The prevailing opinion among scientists
was that life was the result of a chemical fluke
so improbable it would be unlikely
to have happened twice in the observable universe.
"Life seems almost a miracle," wrote Francis Crick,
the co-discoverer of the structure of DNA.
It was echoed
by another Nobel prizewinning biologist,
Jacques Monod, in a bleak assessment:
"Man at last knows that he is alone
in the unfeeling immensity of the universe,
out of which he emerged only by chance."
In one of the most astonishing shifts of scientific fashion,
the consensus today is that the universe is teeming with life.
Christian de Duve, the Belgian-born biologist
and another Nobel prizewinner,
has gone so far as to call life a "cosmic imperative",
believing it is "almost bound to happen" on any Earth-like planet.
And where there is life, intelligence may eventually follow.
In the 1990s, US space agency NASA
created an astrobiology institute
to co-ordinate the burgeoning program
of research into the origin, distribution
and evolution of life in the universe.
Although wary of directly funding SETI, NASA
nevertheless enthusiastically embraces it
under the broader astrobiology umbrella.
Despite this sea change in thinking,
there is still not a shred of evidence
for any life beyond Earth,
intelligent or otherwise.
Instead of the hoped-for
clamour of interstellar messages,
there is only an eerie silence.
Why, then, the upbeat assessments from so many scientists?
Part of the reason
is the spiralling number of planets
being discovered orbiting other stars.
Hundreds have been detected
using modest ground-based telescopes,
and still more from a customised satellite named Kepler.
Planets are not imaged directly
but inferred through the way they blot out light
as they cross the face of the parent star
or cause a detectable wobble in the star from the tug of gravity.
Although Earth-like planets
are harder to detect and remain elusive,
estimates suggest that our galaxy alone
may contain more than one billion.
So there is plenty of real estate for life.
It is a fallacy, however,
that habitable is the same as inhabited.
To be sure, a planet should be
reasonably like Earth to support life,
at least life as we know it.
But that is far from sufficient.
The problem
concerns the origin of life
and the actual probability
that it will emerge
on a typical Earth-like planet.
A century and a half ago,
Charles Darwin explained
how life has evolved
across billions of years
from simple microbes
to the richness and complexity
of the biosphere we see today.
But he pointedly left out of his account
how life got going in the first place.
"One might as well speculate
about the origin of matter," he quipped.
Unfortunately,
the science of biogenesis
has progressed very little since.
We still have no idea
of the pathway that led
from non-living chemicals
to the first living cell.
In fact, we may never have
a blow-by-blow account of life's origin;
it happened so long ago
that all traces must have been obliterated.
But to answer the question
about the prevalence of extraterrestrial life
we merely need to know whether the transition
from non-life to life is probable or improbable.
Carl Sagan, the charismatic American astronomer
and ever the optimist, believed that life must arise easily
because it started on Earth rather quickly.
Our planet was born 4.5 billion years ago
amid a disc of gas and dust swirling around the proto-sun.
But for hundreds of millions of years
it was pounded by giant asteroids,
some big enough to boil the oceans
in the aftermath of their impacts.
Yet there is good evidence from Western Australia
that by 3.5 billion years ago life was already well established.
Sadly, Sagan's argument is flawed.
Our planet will become uninhabitable
in less than a billion years when the sun
swells up as it starts to run out of fuel.
Unless life had started quickly,
intelligent beings like us
may never have had time
to evolve to ask questions
like "Are we alone?"
before biology got snuffed out.
It is therefore impossible to argue,
from a sample of one selected
by our very existence,
that life will always pop up
readily on earthlike planets.
Some people pin their hopes
for settling the matter
on scientists making life in the lab.
In 1952 the University of Chicago chemist
Harold Urey persuaded a student, Stanley Miller,
to try to recreate the conditions
of the early Earth inside a flask.
Miller sparked electricity
through a mix of common gases and water,
and found that his "primordial soup"
could make the building blocks of proteins
within a few days.
For a while, Miller's experiment
looked like the first step on the road to life.
Unfortunately subsequent steps
have proved a lot harder,
and most experiments
in pre-biotic chemistry
seem to lead to blind alleys.
The fundamental problem
is that even the simplest living cell
is already so fiendishly complex
it's hard to imagine how it could
have arisen simply from more
of the same Miller-Urey processes.
It is worth contrasting Miller's experiment
with the work of a new breed of "synthetic biologists"
such as Craig Venter, who helped sequence the human genome.
Venter and his colleagues have succeeded
in making novel microbes by inserting
customised DNA into existing living cells.
This work is often misrepresented
as life created in the proverbial test tube,
but it is very far from that.
Rather, synthetic biology
seeks merely to redesign existing life
by modifying its genetic instructions,
not to make life from scratch out of basic chemicals.
The latter prospect is a very long way off,
and even were it to succeed, it would still
fall short of proving that life arises readily.
It is one thing to make life in a laboratory
with all sorts of fancy equipment
operated by an intelligent designer,
quite another for it to emerge spontaneously
from a grubby sludge on the sea bed
-- or wherever it first began.
The popular belief that life starts easily
in earthlike conditions would immediately
be verified if we were to find a second sample of it.
Many astrobiologists think Mars offers a possibility.
The red planet has been a favourite abode for life
since the astronomer Percival Lowell thought
he had glimpsed canals on its surface
and H.G. Wells wrote War of the Worlds in 1898.
In the 1970s NASA
decided to put the matter to the test
by sending two spacecraft called Viking
to land on the Martian surface,
each equipped with experiments
to sniff out microbes in the soil.
The landers found a freeze-dried desert
of highly oxidising dirt bathed
in deadly ultra-violet radiation
and protected by a very thin atmosphere.
In spite of the hostile conditions,
one of the experiments
gave a strikingly positive result.
Designed by the organic chemist Gil Levin,
it used an ingenious technique
to detect if any bugs in the dirt
were eating a nutrient broth,
by looking for radioactively tagged
carbon dioxide being given off as waste.
The experiment worked repeatedly
at both landing sites,
but returned null results
when the dirt was strongly heated,
as might be expected if Martian microbes
were killed by the elevated temperature.
To this day, Levin maintains
that he found life on Mars,
but most astrobiologists are sceptical,
partly because the other Viking experiments
were negative or ambiguous,
and partly because one cannot rule out
reactive soil chemistry mimicking the results.
Even if Mars is a dead planet today,
it may not always have been so.
Survey photographs
from a series of high resolution orbiters
show a dramatic landscape sculpted by liquid water.
Rivers, flood plains,
gullies, lakes and shorelines
are conspicuous in the topography.
On-the-ground analysis
reveals water ice in the polar caps,
and fluvial features in the rocks.
Clearly, in the far past
Mars was warmer
and wetter than today,
probably the result
of massive greenhouse warming
from an early thick atmosphere
of carbon dioxide, now
mostly leaked away into space.
Hopes remain high that Mars once hosted life,
though probably no more complex than bacteria.
In 1996 US president Bill Clinton
electrified the world's press
when he stood on the White House lawn
and announced that NASA had evidence of life on Mars
in the form of a meteorite found in Antarctica,
containing blobs that resemble tiny bacteria.
From time to time,
Mars takes a hit by an asteroid or comet
with enough force to propel rocks into space.
A fraction of this ejected debris
eventually lands on Earth;
Clinton's meteorite was one of these rocks,
purportedly containing fossilised microbes.
However, after years of follow-up studies,
the evidence for ancient life
inside the meteorite is very tenuous.
Unless we get lucky and find
clearer evidence in another meteorite,
the best hope for detecting traces of life on Mars
rests with a sample return mission.
The plan is to send a spacecraft
that will garner a rag-bag of rocks
and convey them back to Earth for detailed analysis.
Unfortunately this ambitious scheme
has stalled for lack of funding
and exaggerated concerns
that the harvested rocks
might harbour living microbes
that could trigger a killer plague
or some other calamity
if exposed to Earth conditions.
Although the prospects
for extant life on the Martian surface,
if not quite zero, are nevertheless slim,
it remains possible that deep underground
pockets of microbes may still eke out a living
in briny aquifers warmed by the planet's internal heat.
Sketchy evidence for seasonally varying gases
exuding from the permafrost could point
to methane-producing subsurface organisms
like those in similar locations on Earth.
In the longer term,
a future manned expedition to Mars
may offer the best hope for settling the issue.
Even if we did obtain
irrefutable evidence for microbes on Mars,
it would not of itself prove that life
is a cosmic imperative, enjoying multiple origins.
The problem concerns
the traffic of Martian rocks to Earth,
which has been going on
throughout the history of the solar system.
Cocooned inside a rock,
protected from the harsh conditions of space
and shielded from deadly radiation,
a microbe could probably survive
the journey between the two planets,
even if it took millions of years.
It is very likely that if life did get going on Mars
billions of years ago when conditions were still favourable,
then it will have spread to Earth in this manner.
Indeed, it is possible
that life on Earth started on Mars
and came here inside Martian meteorites.
And just as Mars
may have seeded Earth with life,
so Earth may have seeded Mars with life,
because our planet too suffers
impacts that fling rocks into space.
The natural cross-contamination of Earth and Mars
via traded rocks complicates the story of life's origin.
If we do find evidence for life on Mars,
it may be the same as earth life
and point to a single common origin.
The search for a second genesis of life
may not require anything as expensive
and challenging as a Mars expedition, however.
No planet is more earthlike than Earth itself,
so if life does start up readily in earthlike conditions,
wouldn't it have begun many times on Earth?
Could there be traces of a second sample of life right here?
Biology textbooks claim that all life on Earth
is descended from a common ancestral form.
Evidence cited includes
many identical features in life's basic machinery,
such as the universal genetic code
that implements the instructions
contained in the four-letter alphabet of DNA.
But while it is true that all life so far studied
seems to be related, we cannot be sure
that fundamentally different forms of life
may yet be discovered.
The vast majority of terrestrial organisms are microbes,
and we have only just scratched the surface of the microbial realm.
You can't tell by looking what makes a microbe tick
-- you have to study its biochemical innards.
It is entirely possible that,
intermingled with bacteria
and other microbes that lie
on the same tree of life as you and me,
are some microbial life forms
that are radically different
-- so different that they belong
not just to another branch
on the known tree of life,
but to a separate tree altogether,
with an independent origin.
In other words,
it would be life,
but not as we know it.
The idea that Earth hosts
more than one form of life,
while highly speculative,
has nevertheless gained
some traction in recent years,
and is often dubbed
the "shadow biosphere",
or Life 2.0.
Easiest to spot would be
if Life 2.0 occupied
a habitat beyond the reach
of the hardiest organisms of known life.
And known life boasts
some pretty bizarre representatives.
Microbes have been found
living near deep ocean volcanic vents
that thrive in temperatures above 120C.
Others tolerate extremes
of salt, acidity, alkalinity or radiation.
Nevertheless, all these "extremophiles"
are adaptations of known life.
If microbes were found living at, say, 180C,
these would stand out as candidates for Life 2.0.
Much harder would be if the shadow biosphere
interpenetrated the familiar biosphere.
In that case Life 2.0 microbes
might be all around us,
unidentified for what they are,
and unresponsive
to standard biochemical analysis.
If we did find Life 2.0 here on Earth,
it would greatly boost
the search for life in the universe,
because it would be unlikely
that life would have started twice
on one earthlike planet
and not at all on all the others.
Meanwhile, we should expand our search for ET
beyond looking for customised radio messages beamed at Earth.
Any signature of alien technology
would serve to answer the question
of whether we are alone in the universe.
One possibility is to look for radio or optical beacons.
An advanced civilisation might build a beacon
to sweep the Milky Way every few months or years,
perhaps as a monument to its greatness,
a means of attracting attention, or even as a warning.
A radio beacon would show up as a transient pulse,
or series of pulses, repeated periodically.
Bursts of radio waves from deep space have been detected,
but without follow-up to see whether they are repeated,
their origin remains unknown.
We could also look for signs
of large-scale astro-engineering.
All technology has an impact
on its environment; for example,
global warming from human industry
could be detected from light years away.
A very advanced alien community
might have left an imprint,
not just on its planetary environment,
but on its astronomical neighbourhood too.
Telltale signs
could be artificial structures,
the depletion of resources
or the accumulation of waste,
all of which might be detectable
through changes in the light from the parent star.
An even more exciting,
but yet more speculative possibility,
is that one or more alien civilisations
has spread beyond its home planet out into the galaxy,
most likely through robotic probes or self-reproducing machines.
It is conceivable our own corner or the galaxy
has been visited, an idea popular
among UFO enthusiasts and science fiction writers.
When might this have happened?
At 4.6 billion years, our solar system
is about a third of the age of the galaxy;
stars and planets existed long before Earth formed.
If intelligent life is indeed common,
then there may be planets
that hosted advanced civilisations billions of years ago.
If one of these civilisations sent spacecraft to the solar system,
there is no reason it would have been in the recent past.
Most likely it was a very long time ago -- say, 100 million years.
That raises the fascinating question of what physical traces,
if any, would survive for 100 million years, even if they were right here on Earth.
Three possibilities come to mind.
Nuclear waste, perhaps from a nuclear-powered craft,
if dumped on Earth or the moon would still be detectable
100 million years later in form of decay products.
Large-scale mines or quarries,
though buried beneath overlying rock strata,
would show up in geological surveys.
On the moon they might be visible
from simple photographic surveys
of the sort now being carried out
by NASA's Lunar Reconnaissance Orbiter.
Most tantalising of all,
products of alien biotechnology,
such as tinkering with the genomes
of terrestrial organisms or even
the creation of a non-competing
shadow biosphere of Life 2.0 organisms,
could be found by microbiologists today.
My favourite is the message in a bottle,
created in the form of an engineered series of letters
etched into microbial genomes, which might show up
during routine gene sequencing research.
Although all these extreme ideas,
entertaining though they may be,
almost certainly represent a wild goose chase,
they may nevertheless be pursued for very little money.
For example, genomes are being sequenced anyway
and it costs nothing to do a computer search for anything fishy.
And cost is the dominating factor
when doing highly speculative science.
The jewel in the crown of mainstream SETI
is a dedicated bank of smallish radio telescopes
part-paid for by Paul Allen, the co-founder of Microsoft.
Sadly, in spite of his $US31 million investment,
the project lacks a partner to fund the $US2m ($1.86m)
a year running costs, so the Allen Telescope Array
has been hibernated to await better financial times.
SETI is undeniably a long-shot, but it is one worth undertaking.
Looking for alien civilisations is really,
in the words of its founder Frank Drake,
a search for ourselves, who we are
and how we fit into the great cosmic scheme of things.
It is a subject that compels us to address
the great questions of existence What is life?
What is intelligence? Is the universe bio-friendly?
What is the destiny of mankind?
A society that is too mean-spirited
to spare a minuscule fraction of its resources
to reflect on its place in the cosmos
is a society with an unpromising future.
No hay comentarios:
Publicar un comentario
COMENTE SIN RESTRICCIONES PERO ATÉNGASE A SUS CONSECUENCIAS