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An informative but very long article.
Light of life: The hunt for
another Earth
Is anybody out there?
JUST over 400 years ago, Galileo Galilei turned his newly constructed
telescope towards Earth's moon. What he saw there reminded him of
home: a dappling of lighter and darker areas where sunlight played across
the lunar surface. As he wrote in his 1610 treatise Siderius Nuncius: "If the
sphere of the Earth were seen from a distance, when flooded with the
Sun's rays, that part of the surface which is land would present itself to
view as brighter, and that which is water as darker in comparison." The
moon, like Earth, Galileo concluded, was a world of lands and seas. And as
his contemporary Johannes Kepler noted, where there are lands and seas,
there could be life. Perhaps, indeed, life had built the moon's large,
near-perfectly circular craters.
They were by no means the last to be seduced by signs of life where no
life exists. Wherever we look beyond Earth, we are eager to spot our ilk.
William Herschel, the celebrated astronomer who discovered infrared
radiation and the planet Uranus, noted roads, towns, pyramids and regions
"tinged with green" on the moon. Early observers attributed fluctuating
dark patches on the surface of Mars to the seasonal growth of vegetation.
Venus's cloud-covered face was long thought to obscure a humid twin of
Earth with hot, lush jungles, or perhaps a warm global ocean teeming with
life.
Today we know the moon is lifeless and sterile, its blotches immense plains
of solidified magma. Mars's dark patches are dust storms; Venus's friendly
exterior obscures a choking greenhouse of sulphurous fumes. Time after
time, life's signatures have been written in disappearing ink.
That is something to chew over as we enter a new era of planetary
discovery. In less than two decades, we have spied about 600 worlds
orbiting other stars. This year alone, the Kepler space telescope
, NASA's
dedicated planet hunter, has added over 1700 candidates to the list. A few
finds from Kepler and elsewhere seem to be roughly the size of Earth and in
orbits that just might allow liquid water to exist, providing a solvent bath in
which carbon-based life could evolve and flourish. In short, it won't be long
before we spot another planet that might conceivably harbour life, if
indeed we haven't already. But would we know it if we saw it - and if so,
how?
Fittingly, it was Galileo who first showed us what a life-bearing world looks
like from afar. Not Galileo the astronomer, but Galileo the NASA spacecraft.
En route to Jupiter in 1990, it briefly turned its suite of remote-sensing
instruments - cameras and spectrometers to record the emission and
absorption of light - towards Earth from 960 kilometres away. Would it see
an obviously living planet?
Astronomer Carl Sagan and his colleagues reported that Galileo's
visible-light camera spied a contrast between darker areas and brighter
areas. Earth's absorption spectrum also contained the signatures of water
vapour and carbon dioxide. Water vapour suggests abundant surface
water, possibly in liquid form; carbon dioxide indicates a rocky planet, since
in gas-giant worlds this gas would combine with readily available hydrogen
to form simple hydrocarbons such as methane. Piecing all this evidence
together, we might then interpret the dark areas as oceans and the bright
areas as land (Nature, vol 365, p 715).
A beacon of photosynthesis
Some of those bright areas also had an odd spectral characteristic: they
efficiently absorbed visible wavelengths corresponding to the peak of the
available solar energy, but reflected less prevalent infrared. We might have
recognised this as the signature of chlorophyll in surface vegetation,
efficiently and selectively taking advantage of the light resources available
to it. Earth's plants shine a beacon of photosynthesis into space.
This was not the most compelling piece of evidence for life, however. That
came in the form of the spectral signatures of abundant oxygen and a
significant concentration of methane side by side in Earth's atmosphere.
This juxtaposition is chemically implausible. Oxygen is highly reactive; it
readily oxidises methane into water and carbon dioxide. If even just a little
methane persists in an oxygenated planet's environment, some
thermodynamically improbable process must be constantly replenishing it.
In Earth's case, that something is life. Oxygen is the metabolic by-product
of photosynthesising organisms. A vanishingly small amount of methane is
produced by slow geological processes in Earth's crust, but most of it is
the exhaust of a diverse and ancient group of microbes that reside in such
oxygen-poor Earthly environments as swamps, rice paddies and the
stomachs of ruminant animals.
All in all, the Galileo probe's evidence added up to a reasonably convincing
identification of life seen from about 1000 kilometres away. But would such
signals be recognisable over the tens to hundreds of light years that
separate us from most planets we have discovered in other solar systems?
In theory, the answer is yes. Indeed, NASA's Hubble and Spitzer space
telescopes have already obtained crude atmospheric spectra for a handful
of exoplanets. That, however, has largely been through happy chance:
such observations are limited to hot, extremely large gas giants that are
precisely aligned to move periodically across the faces of their stars as
seen from Earth. This "transit" configuration allows starlight to shine
through the planet's puffy, extensive upper atmosphere, revealing spectral
features.
The Kepler telescope uses the transit method to detect much smaller,
cooler planets more akin to Earth. But it isn't nearly powerful enough to
collect the trickles of starlight that filter through these diminutive planets'
atmospheres. Typically, it can measure their sizes and orbital
characteristics, but little else.
That situation is unlikely to change any time soon. "Kepler is finding worlds
that look really intriguing, but there are no missions in the pipeline to follow
up on these initial discoveries," says Victoria Meadows, a planetary
scientist at the University of Washington in Seattle. Capitalising on what
we might see will require an alternative approach.
Meadows heads the Virtual Planetary Laboratory (VPL), a network of
scientists brought together by NASA, which aims to provide just that. Its
goal is to use computing muscle to quantify the astrophysical, atmospheric
and geological factors that influence whether a planet can harbour life, and
model how these factors change a habitable planet's appearance over
time. We can then zoom out from these modelled virtual worlds and see
how they would appear viewed from light years away, at all angles and at
all stages in their history. The aim, says Meadows, is to create a "Swiss
army knife" of tools that, when presented with data on a new planet from
Kepler or elsewhere, will tell us something about its likelihood of bearing
life.
There's one type of habitable planet we find easiest to model. A
cornerstone of the VPL project is the "Earth in a box" - a sophisticated
three-dimensional mock-up of our home world which can be viewed from all
possible vantage points and under different solar illuminations. In a paper
published in June this year, Meadows and her colleagues compared this
model with actual space-borne observations and showed that it reproduces
our planet's fluctuating brightness and light spectrum to within 3 to 5 per
cent accuracy (Astrobiology, vol 11, p 393).
The advantage of this virtual home is that it is mobile: we can drag our
twin to anywhere in the cosmos to see what we would make of it there. "It
allows us to explore what Earth would look like if it were being observed
from very far away as it orbited around its star," says Meadows.
So how would it look? For a start, it would appear blue. That fits with our
self-image as an ocean planet, but in fact it has less to do with the sea's
azure hue and more with why the sky looks blue: molecular gases such as
oxygen and nitrogen in Earth's atmosphere preferentially scatter the bluer
portions of sunlight, colouring our view up and out - and also from outside
looking in. The VPL's modelling suggests that, by measuring the ratio of
certain wavelengths of ultraviolet and visible light reflected from an alien
planet, we might detect this scattering from very far away. That would be
a tantalising hint that at least one other similarly lively world abides
beneath a friendly blue sky (Astrophysical Journal, vol 729, p 130).
Leaps of faith
With luck we might still see oceans. If we just happen to observe a distant
Earth-like planet from an angle so that the side facing its host star is
almost entirely turned away from us, we will see a thin arc of illuminated
surface, similar to a crescent moon. As an ocean-covered region rotates
into this planetary sliver, light will bounce off the water's flat, mirror-like
surface, increasing the sliver's reflectivity and glinting the signal of oceans
across the light years (arxiv.org/abs/1008.3864). An analogous shift in
contrast between a planet's thick atmosphere and an airless lunar surface
might also reveal a large moon like ours as it orbited across a planet's face.
That would be a promising sign of habitability: our unusually large satellite
is a crucial factor in correcting Earth's otherwise chaotic axial tilt, and
helps ensure stable climatic conditions in which life can thrive.
All this is very well, but just because life might plausibly be somewhere
does not mean it is. Relying too much on finding characteristics similar to
those of our own planet invites the same leaps of faith about the existence
of life that we have been prone to in the past - with the same likelihood of
a fall. That is why VPL's modelling aims to aid the more direct approach of
looking for the by-products of life in planetary atmospheres.
Even without the added problems of viewing over many light years, this is
a fraught business. The most obvious signatures of life are spread out
through the spectrum, with water vapour and oxygen at visible and
near-infrared wavelengths and methane, carbon dioxide and oxygen's
molecular cousin, ozone, at longer infrared wavelengths. That makes it
difficult to observe all of them with any one telescope.
Reliable models that can be applied to many environments could help sort
out some of this confusion. A typical task for the VPL's researchers might
be to compute how light from a star different in size, temperature, and
luminosity from our sun will affect a planet in a habitable orbit, by heating
it more or less, or through creating and destroying compounds in the air or
on the surface - for example turning molecular oxygen, O2, produced by life
into ozone, O3.
"These sorts of modelling efforts will tell us things like how important it is
to look for oxygen versus ozone, or where the best spectral region is to
look for water, or which star types are really worth looking at," says
Charles Beichman of the NASA Exoplanet Science Institute in Pasadena,
California. "In each case, we want to look for whatever gives us the most
leverage for learning exactly what kind of planet we're studying."
History being what it is, the need to identify false positives looms large.
Oxygen is a case in point. Our near neighbour Venus possibly once had an
ocean billions of years ago, until plentiful sunlight and its CO2-rich
atmosphere contrived to spark a runaway greenhouse effect that boiled off
its seas. Ultraviolet sunlight would then have gradually broken these
airborne water molecules down into oxygen and hydrogen. The lighter
hydrogen would have drifted off into space, but the oxygen would have
persisted for many millions of years, gradually bonding with the surface
rocks. Catch such a planet at the right time, and its hellish landscape
would be veiled beneath an inviting facade of water vapour and breathable
oxygen.
A similar deception could take place on a frozen, geologically inert world
rather like modern Mars, but slightly more massive and with a more
substantial atmosphere. There, for lack of volcanic gases and fresh rocks
with which to react, the oxygen from any sublimating water ice might build
up essentially indefinitely - a sign of life where none exists. "For an
Earth-sized planet in the middle of the habitable zone of a sun-like star,
perhaps most of the time atmospheric oxygen will be a biosignature," says
Meadows. "But outside of that narrow space we have to be very careful."
Earth's own history shows that even more caution is necessary before
interpreting an absence of oxygen as an absence of life. Though life is
thought to have sprung up relatively quickly on Earth, probably within
Earth's first billion years, it took a further billion years before oxygen
reached appreciable atmospheric levels, about 2.4 billion years ago. Before
that, methane from the ancient, anaerobic microbial biosphere would have
been the most readily observable atmospheric biosignature. But methane
alone is no cut-and-dried indicator of life: it can be made in small amounts
through lifeless interactions of hot water and rock deep within a planet, as
indeed it is on Earth. That methane could slowly seep out into an
atmosphere and, in the absence of oxygen to mop it up, linger there. That
leaves us with a conundrum. "We've only had an oxygenated atmosphere
for half of our planet's lifetime," says Meadows. "So what the heck do we
look for besides methane in an anoxic environment?"
According to Tori Hoehler, an astrobiologist at NASA's Ames Research
Center in Moffett Field, California, the answer to that question might come
from seeing life in terms of its function as a chemical catalyst. "Methane in
Earth's atmosphere represents the fact that life could gain chemical energy
by putting carbon dioxide and hydrogen together," he says. But that is not
a one-step process - and the intermediate products vented from known
anaerobic metabolisms might give us new clues as to what to look for on a
distant planet.
Meadows and her team have taken this idea on board and focused on how
"organosulphur" compounds such as methanethiol or dimethyl sulphide,
which are produced in great quantities by some anaerobic microbes on
Earth, might manifest themselves on a faraway planet. "We identified fluxes
that could come from life at the surface, had them interact with the
planet's atmospheric chemistry and ultraviolet light from the parent star,
estimated their atmospheric lifetimes, and generated the resulting spectra,"
says Meadows.
What emerged was a surprise. Methanethiol and dimethyl sulphide were
plentifully produced, but the action of ultraviolet light rapidly knocked off
their methyl groups, which combined in pairs to form ethane gas. Ethane,
while colourless and odourless, has a clear infrared spectroscopic signature
and a significant atmospheric lifetime. A possible signature of an anoxic,
but living, planet, the researchers concluded, is a high atmospheric ratio of
ethane to methane (Astrobiology, vol 11, p 419).
That is a significant result, as this ethane build-up should be particularly
potent in planets around cool, diminutive stars called M-dwarfs. For a
planet in such a situation to have liquid water, it would orbit practically
cheek by jowl with its stellar host - well within Mercury's distance from the
sun - making it easier to discover through the gravitational wobble that its
tight orbit would induce on the star. These could be the first living worlds
we discover beyond our solar system - although the wrenching
gravitational forces and intense ultraviolet radiation on such planets might
make even familiar life forms difficult to recognise.
For such worlds and more familiar ones, the modelling initiative is slowly
producing a long list of esoteric processes that could both help and hinder
the identification of life. "We are revealing the complexity behind initially
simplistic assumptions about habitable planets and how to find them," says
Meadows.
Ultimately, though, that can only take us so far without the right data.
NASA's James Webb Space Telescope, slated for launch no earlier than
2018, could probably obtain low-resolution atmospheric spectra for a few
potentially habitable M-dwarf planets as they transit across the faces of
their stars. Besides that, we are likely to be left guessing about what the
light of life will look like. "The VPL and similar projects are going to help
guide the design of future telescopes by telling us what astrobiologists
really want to look at," says Beichman.
Whether those telescopes are built is another matter. Just imagine,
though, some day seeing with imperfect vision blurry pricks of blue
scattered through the sky, some winking with oceans, a few blanketed
with what could be the breath of life. Who wouldn't want to take a closer
look?
Not in my backyard?
As we begin to consider what signs of life might come from other solar
systems (main story), intriguing possibilities still exist closer to home.
• Mars A few billion years ago, Mars was a lot more like Earth, with a
clement climate and abundant liquid water. Today, liquid water might exist
in subsurface reservoirs. Mars orbiters and remote-sensing instruments on
Earth have also detected faint whiffs of methane seasonally wafting from
the depths - signs, perhaps, of life that has long since retreated to a safe
and hidden refuge.
• Venus This near neighbour was also once a much more amenable
environment for life. Close examination of Venus's stifling clouds today has
revealed mutually reactive hydrogen sulphide and sulphur dioxide, as well
as carbonyl sulphide, a substance produced on Earth chiefly by biotic
processes. That nourishes a controversial theory that microbes could live
on high in the planet's atmosphere, shielded from harmful ultraviolet rays
by protective sulphur rings.
• Europa If evidence from NASA's Galileo mission is anything to go by, this
moon of Jupiter boasts a deep salty ocean beneath its icy crust, which is
itself filigreed with fissures and veined with organic compounds (picture,
above). If the crust is not too thick, those energy-rich organic compounds
might channel nutrients into an aquatic biosphere.
• Enceladus In 2005, the Cassini orbiter spied active geysers pluming from
near this Saturnian moon's south pole, providing evidence of warm pockets
of liquid water just beneath the surface. It is unclear whether life could
have arisen there - but any microbe vacationing from Earth would find
conditions there quite hospitable.
• Titan The Cassini mission's observations of Saturn's largest moon reveal
not one but two promising abodes for life. Titan's crust appears to float on
a vast ocean of liquid water and ammonia in which life not that dissimilar to
Earth's could perhaps exist. Meanwhile, something seems to be depleting
the hydrocarbons ethane and acetylene on the moon's frigid surface. That
might indicate exotic carbon-based life with a hydrogen-fuelled metabolism
that uses liquid methane rather than water as a solvent. Could this be a
first glimpse of a "second genesis" of life on a completely different basis to
Earth's?
Exotic possibilities
Just as oxygen, methane and other gases can suggest the presence of a
biosphere hungry for chemical energy (main story), the prevalence of easily
metabolised, energy-rich materials in a warm, rocky planet's atmosphere
might constitute an "anti-biosignature" - evidence for a world bereft of life.
According to planetary scientist Yuk Yung of the California Institute of
Technology in Pasadena, finding an abundance of something easily
digestible like hydrogen in such a planet's spectrum would be equivalent to
seeing masses of windblown paper currency billowing through a city's
streets. In both cases, the chances are good that nobody lives there.
This, of course, at least partially assumes the primacy of carbon-based
chemistry. The signatures of life built out of strange things like silicon or
magnetised plasma or neutrinos, or organisms using energy sources such as
radioactive decay or matter-antimatter annihilation, are extremely
ill-defined. In large part that is because it is unclear how such creatures
would perform many of the activities that underpin our definitions of life,
such as metabolism, growth, reproduction and evolution.
An alternative possibility that does not depend on biology is to look for
inadvertent "technosignatures" of life. Taking cues from Earth's own recent
history, when artificial chlorofluorocarbon (CFC) refrigerants proved
remarkably adept at disrupting the photochemistry of our atmosphere's
ozone layer, perhaps we might seek on other worlds the spectral
signatures of gases believed to be produced solely through technology and
engineering. It might be a bleak identification: such artificial substances
could endure as mute testaments to life, regardless of its biochemistry,
long after a civilisation had burned itself out.
Light of life: The hunt for another Earth - space - 28 September 2011 - New Scientist
Light of life: The hunt for
another Earth
Is anybody out there?
JUST over 400 years ago, Galileo Galilei turned his newly constructed
telescope towards Earth's moon. What he saw there reminded him of
home: a dappling of lighter and darker areas where sunlight played across
the lunar surface. As he wrote in his 1610 treatise Siderius Nuncius: "If the
sphere of the Earth were seen from a distance, when flooded with the
Sun's rays, that part of the surface which is land would present itself to
view as brighter, and that which is water as darker in comparison." The
moon, like Earth, Galileo concluded, was a world of lands and seas. And as
his contemporary Johannes Kepler noted, where there are lands and seas,
there could be life. Perhaps, indeed, life had built the moon's large,
near-perfectly circular craters.
They were by no means the last to be seduced by signs of life where no
life exists. Wherever we look beyond Earth, we are eager to spot our ilk.
William Herschel, the celebrated astronomer who discovered infrared
radiation and the planet Uranus, noted roads, towns, pyramids and regions
"tinged with green" on the moon. Early observers attributed fluctuating
dark patches on the surface of Mars to the seasonal growth of vegetation.
Venus's cloud-covered face was long thought to obscure a humid twin of
Earth with hot, lush jungles, or perhaps a warm global ocean teeming with
life.
Today we know the moon is lifeless and sterile, its blotches immense plains
of solidified magma. Mars's dark patches are dust storms; Venus's friendly
exterior obscures a choking greenhouse of sulphurous fumes. Time after
time, life's signatures have been written in disappearing ink.
That is something to chew over as we enter a new era of planetary
discovery. In less than two decades, we have spied about 600 worlds
orbiting other stars. This year alone, the Kepler space telescope
, NASA'sdedicated planet hunter, has added over 1700 candidates to the list. A few
finds from Kepler and elsewhere seem to be roughly the size of Earth and in
orbits that just might allow liquid water to exist, providing a solvent bath in
which carbon-based life could evolve and flourish. In short, it won't be long
before we spot another planet that might conceivably harbour life, if
indeed we haven't already. But would we know it if we saw it - and if so,
how?
Fittingly, it was Galileo who first showed us what a life-bearing world looks
like from afar. Not Galileo the astronomer, but Galileo the NASA spacecraft.
En route to Jupiter in 1990, it briefly turned its suite of remote-sensing
instruments - cameras and spectrometers to record the emission and
absorption of light - towards Earth from 960 kilometres away. Would it see
an obviously living planet?
Astronomer Carl Sagan and his colleagues reported that Galileo's
visible-light camera spied a contrast between darker areas and brighter
areas. Earth's absorption spectrum also contained the signatures of water
vapour and carbon dioxide. Water vapour suggests abundant surface
water, possibly in liquid form; carbon dioxide indicates a rocky planet, since
in gas-giant worlds this gas would combine with readily available hydrogen
to form simple hydrocarbons such as methane. Piecing all this evidence
together, we might then interpret the dark areas as oceans and the bright
areas as land (Nature, vol 365, p 715).
A beacon of photosynthesis
Some of those bright areas also had an odd spectral characteristic: they
efficiently absorbed visible wavelengths corresponding to the peak of the
available solar energy, but reflected less prevalent infrared. We might have
recognised this as the signature of chlorophyll in surface vegetation,
efficiently and selectively taking advantage of the light resources available
to it. Earth's plants shine a beacon of photosynthesis into space.
This was not the most compelling piece of evidence for life, however. That
came in the form of the spectral signatures of abundant oxygen and a
significant concentration of methane side by side in Earth's atmosphere.
This juxtaposition is chemically implausible. Oxygen is highly reactive; it
readily oxidises methane into water and carbon dioxide. If even just a little
methane persists in an oxygenated planet's environment, some
thermodynamically improbable process must be constantly replenishing it.
In Earth's case, that something is life. Oxygen is the metabolic by-product
of photosynthesising organisms. A vanishingly small amount of methane is
produced by slow geological processes in Earth's crust, but most of it is
the exhaust of a diverse and ancient group of microbes that reside in such
oxygen-poor Earthly environments as swamps, rice paddies and the
stomachs of ruminant animals.
All in all, the Galileo probe's evidence added up to a reasonably convincing
identification of life seen from about 1000 kilometres away. But would such
signals be recognisable over the tens to hundreds of light years that
separate us from most planets we have discovered in other solar systems?
In theory, the answer is yes. Indeed, NASA's Hubble and Spitzer space
telescopes have already obtained crude atmospheric spectra for a handful
of exoplanets. That, however, has largely been through happy chance:
such observations are limited to hot, extremely large gas giants that are
precisely aligned to move periodically across the faces of their stars as
seen from Earth. This "transit" configuration allows starlight to shine
through the planet's puffy, extensive upper atmosphere, revealing spectral
features.
The Kepler telescope uses the transit method to detect much smaller,
cooler planets more akin to Earth. But it isn't nearly powerful enough to
collect the trickles of starlight that filter through these diminutive planets'
atmospheres. Typically, it can measure their sizes and orbital
characteristics, but little else.
That situation is unlikely to change any time soon. "Kepler is finding worlds
that look really intriguing, but there are no missions in the pipeline to follow
up on these initial discoveries," says Victoria Meadows, a planetary
scientist at the University of Washington in Seattle. Capitalising on what
we might see will require an alternative approach.
Meadows heads the Virtual Planetary Laboratory (VPL), a network of
scientists brought together by NASA, which aims to provide just that. Its
goal is to use computing muscle to quantify the astrophysical, atmospheric
and geological factors that influence whether a planet can harbour life, and
model how these factors change a habitable planet's appearance over
time. We can then zoom out from these modelled virtual worlds and see
how they would appear viewed from light years away, at all angles and at
all stages in their history. The aim, says Meadows, is to create a "Swiss
army knife" of tools that, when presented with data on a new planet from
Kepler or elsewhere, will tell us something about its likelihood of bearing
life.
There's one type of habitable planet we find easiest to model. A
cornerstone of the VPL project is the "Earth in a box" - a sophisticated
three-dimensional mock-up of our home world which can be viewed from all
possible vantage points and under different solar illuminations. In a paper
published in June this year, Meadows and her colleagues compared this
model with actual space-borne observations and showed that it reproduces
our planet's fluctuating brightness and light spectrum to within 3 to 5 per
cent accuracy (Astrobiology, vol 11, p 393).
The advantage of this virtual home is that it is mobile: we can drag our
twin to anywhere in the cosmos to see what we would make of it there. "It
allows us to explore what Earth would look like if it were being observed
from very far away as it orbited around its star," says Meadows.
So how would it look? For a start, it would appear blue. That fits with our
self-image as an ocean planet, but in fact it has less to do with the sea's
azure hue and more with why the sky looks blue: molecular gases such as
oxygen and nitrogen in Earth's atmosphere preferentially scatter the bluer
portions of sunlight, colouring our view up and out - and also from outside
looking in. The VPL's modelling suggests that, by measuring the ratio of
certain wavelengths of ultraviolet and visible light reflected from an alien
planet, we might detect this scattering from very far away. That would be
a tantalising hint that at least one other similarly lively world abides
beneath a friendly blue sky (Astrophysical Journal, vol 729, p 130).
Leaps of faith
With luck we might still see oceans. If we just happen to observe a distant
Earth-like planet from an angle so that the side facing its host star is
almost entirely turned away from us, we will see a thin arc of illuminated
surface, similar to a crescent moon. As an ocean-covered region rotates
into this planetary sliver, light will bounce off the water's flat, mirror-like
surface, increasing the sliver's reflectivity and glinting the signal of oceans
across the light years (arxiv.org/abs/1008.3864). An analogous shift in
contrast between a planet's thick atmosphere and an airless lunar surface
might also reveal a large moon like ours as it orbited across a planet's face.
That would be a promising sign of habitability: our unusually large satellite
is a crucial factor in correcting Earth's otherwise chaotic axial tilt, and
helps ensure stable climatic conditions in which life can thrive.
All this is very well, but just because life might plausibly be somewhere
does not mean it is. Relying too much on finding characteristics similar to
those of our own planet invites the same leaps of faith about the existence
of life that we have been prone to in the past - with the same likelihood of
a fall. That is why VPL's modelling aims to aid the more direct approach of
looking for the by-products of life in planetary atmospheres.
Even without the added problems of viewing over many light years, this is
a fraught business. The most obvious signatures of life are spread out
through the spectrum, with water vapour and oxygen at visible and
near-infrared wavelengths and methane, carbon dioxide and oxygen's
molecular cousin, ozone, at longer infrared wavelengths. That makes it
difficult to observe all of them with any one telescope.
Reliable models that can be applied to many environments could help sort
out some of this confusion. A typical task for the VPL's researchers might
be to compute how light from a star different in size, temperature, and
luminosity from our sun will affect a planet in a habitable orbit, by heating
it more or less, or through creating and destroying compounds in the air or
on the surface - for example turning molecular oxygen, O2, produced by life
into ozone, O3.
"These sorts of modelling efforts will tell us things like how important it is
to look for oxygen versus ozone, or where the best spectral region is to
look for water, or which star types are really worth looking at," says
Charles Beichman of the NASA Exoplanet Science Institute in Pasadena,
California. "In each case, we want to look for whatever gives us the most
leverage for learning exactly what kind of planet we're studying."
History being what it is, the need to identify false positives looms large.
Oxygen is a case in point. Our near neighbour Venus possibly once had an
ocean billions of years ago, until plentiful sunlight and its CO2-rich
atmosphere contrived to spark a runaway greenhouse effect that boiled off
its seas. Ultraviolet sunlight would then have gradually broken these
airborne water molecules down into oxygen and hydrogen. The lighter
hydrogen would have drifted off into space, but the oxygen would have
persisted for many millions of years, gradually bonding with the surface
rocks. Catch such a planet at the right time, and its hellish landscape
would be veiled beneath an inviting facade of water vapour and breathable
oxygen.
A similar deception could take place on a frozen, geologically inert world
rather like modern Mars, but slightly more massive and with a more
substantial atmosphere. There, for lack of volcanic gases and fresh rocks
with which to react, the oxygen from any sublimating water ice might build
up essentially indefinitely - a sign of life where none exists. "For an
Earth-sized planet in the middle of the habitable zone of a sun-like star,
perhaps most of the time atmospheric oxygen will be a biosignature," says
Meadows. "But outside of that narrow space we have to be very careful."
Earth's own history shows that even more caution is necessary before
interpreting an absence of oxygen as an absence of life. Though life is
thought to have sprung up relatively quickly on Earth, probably within
Earth's first billion years, it took a further billion years before oxygen
reached appreciable atmospheric levels, about 2.4 billion years ago. Before
that, methane from the ancient, anaerobic microbial biosphere would have
been the most readily observable atmospheric biosignature. But methane
alone is no cut-and-dried indicator of life: it can be made in small amounts
through lifeless interactions of hot water and rock deep within a planet, as
indeed it is on Earth. That methane could slowly seep out into an
atmosphere and, in the absence of oxygen to mop it up, linger there. That
leaves us with a conundrum. "We've only had an oxygenated atmosphere
for half of our planet's lifetime," says Meadows. "So what the heck do we
look for besides methane in an anoxic environment?"
According to Tori Hoehler, an astrobiologist at NASA's Ames Research
Center in Moffett Field, California, the answer to that question might come
from seeing life in terms of its function as a chemical catalyst. "Methane in
Earth's atmosphere represents the fact that life could gain chemical energy
by putting carbon dioxide and hydrogen together," he says. But that is not
a one-step process - and the intermediate products vented from known
anaerobic metabolisms might give us new clues as to what to look for on a
distant planet.
Meadows and her team have taken this idea on board and focused on how
"organosulphur" compounds such as methanethiol or dimethyl sulphide,
which are produced in great quantities by some anaerobic microbes on
Earth, might manifest themselves on a faraway planet. "We identified fluxes
that could come from life at the surface, had them interact with the
planet's atmospheric chemistry and ultraviolet light from the parent star,
estimated their atmospheric lifetimes, and generated the resulting spectra,"
says Meadows.
What emerged was a surprise. Methanethiol and dimethyl sulphide were
plentifully produced, but the action of ultraviolet light rapidly knocked off
their methyl groups, which combined in pairs to form ethane gas. Ethane,
while colourless and odourless, has a clear infrared spectroscopic signature
and a significant atmospheric lifetime. A possible signature of an anoxic,
but living, planet, the researchers concluded, is a high atmospheric ratio of
ethane to methane (Astrobiology, vol 11, p 419).
That is a significant result, as this ethane build-up should be particularly
potent in planets around cool, diminutive stars called M-dwarfs. For a
planet in such a situation to have liquid water, it would orbit practically
cheek by jowl with its stellar host - well within Mercury's distance from the
sun - making it easier to discover through the gravitational wobble that its
tight orbit would induce on the star. These could be the first living worlds
we discover beyond our solar system - although the wrenching
gravitational forces and intense ultraviolet radiation on such planets might
make even familiar life forms difficult to recognise.
For such worlds and more familiar ones, the modelling initiative is slowly
producing a long list of esoteric processes that could both help and hinder
the identification of life. "We are revealing the complexity behind initially
simplistic assumptions about habitable planets and how to find them," says
Meadows.
Ultimately, though, that can only take us so far without the right data.
NASA's James Webb Space Telescope, slated for launch no earlier than
2018, could probably obtain low-resolution atmospheric spectra for a few
potentially habitable M-dwarf planets as they transit across the faces of
their stars. Besides that, we are likely to be left guessing about what the
light of life will look like. "The VPL and similar projects are going to help
guide the design of future telescopes by telling us what astrobiologists
really want to look at," says Beichman.
Whether those telescopes are built is another matter. Just imagine,
though, some day seeing with imperfect vision blurry pricks of blue
scattered through the sky, some winking with oceans, a few blanketed
with what could be the breath of life. Who wouldn't want to take a closer
look?
Not in my backyard?
As we begin to consider what signs of life might come from other solar
systems (main story), intriguing possibilities still exist closer to home.
• Mars A few billion years ago, Mars was a lot more like Earth, with a
clement climate and abundant liquid water. Today, liquid water might exist
in subsurface reservoirs. Mars orbiters and remote-sensing instruments on
Earth have also detected faint whiffs of methane seasonally wafting from
the depths - signs, perhaps, of life that has long since retreated to a safe
and hidden refuge.
• Venus This near neighbour was also once a much more amenable
environment for life. Close examination of Venus's stifling clouds today has
revealed mutually reactive hydrogen sulphide and sulphur dioxide, as well
as carbonyl sulphide, a substance produced on Earth chiefly by biotic
processes. That nourishes a controversial theory that microbes could live
on high in the planet's atmosphere, shielded from harmful ultraviolet rays
by protective sulphur rings.
• Europa If evidence from NASA's Galileo mission is anything to go by, this
moon of Jupiter boasts a deep salty ocean beneath its icy crust, which is
itself filigreed with fissures and veined with organic compounds (picture,
above). If the crust is not too thick, those energy-rich organic compounds
might channel nutrients into an aquatic biosphere.
• Enceladus In 2005, the Cassini orbiter spied active geysers pluming from
near this Saturnian moon's south pole, providing evidence of warm pockets
of liquid water just beneath the surface. It is unclear whether life could
have arisen there - but any microbe vacationing from Earth would find
conditions there quite hospitable.
• Titan The Cassini mission's observations of Saturn's largest moon reveal
not one but two promising abodes for life. Titan's crust appears to float on
a vast ocean of liquid water and ammonia in which life not that dissimilar to
Earth's could perhaps exist. Meanwhile, something seems to be depleting
the hydrocarbons ethane and acetylene on the moon's frigid surface. That
might indicate exotic carbon-based life with a hydrogen-fuelled metabolism
that uses liquid methane rather than water as a solvent. Could this be a
first glimpse of a "second genesis" of life on a completely different basis to
Earth's?
Exotic possibilities
Just as oxygen, methane and other gases can suggest the presence of a
biosphere hungry for chemical energy (main story), the prevalence of easily
metabolised, energy-rich materials in a warm, rocky planet's atmosphere
might constitute an "anti-biosignature" - evidence for a world bereft of life.
According to planetary scientist Yuk Yung of the California Institute of
Technology in Pasadena, finding an abundance of something easily
digestible like hydrogen in such a planet's spectrum would be equivalent to
seeing masses of windblown paper currency billowing through a city's
streets. In both cases, the chances are good that nobody lives there.
This, of course, at least partially assumes the primacy of carbon-based
chemistry. The signatures of life built out of strange things like silicon or
magnetised plasma or neutrinos, or organisms using energy sources such as
radioactive decay or matter-antimatter annihilation, are extremely
ill-defined. In large part that is because it is unclear how such creatures
would perform many of the activities that underpin our definitions of life,
such as metabolism, growth, reproduction and evolution.
An alternative possibility that does not depend on biology is to look for
inadvertent "technosignatures" of life. Taking cues from Earth's own recent
history, when artificial chlorofluorocarbon (CFC) refrigerants proved
remarkably adept at disrupting the photochemistry of our atmosphere's
ozone layer, perhaps we might seek on other worlds the spectral
signatures of gases believed to be produced solely through technology and
engineering. It might be a bleak identification: such artificial substances
could endure as mute testaments to life, regardless of its biochemistry,
long after a civilisation had burned itself out.
Light of life: The hunt for another Earth - space - 28 September 2011 - New Scientist