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Life on other planets
Is there extraterrestrial life? No answer to this eternally
fascinating question currently exists, but astronomers have gathered a
significant amount of relevant information. Bruce Jakosky argues that we
have every reason to believe that there could be life elsewhere in the
universe. Reviewing the development of life on Earth, he considers the
likelihood of comparable processes having taken place on Mars and Venus,
on moons around Jupiter and Saturn, and on planets orbiting other
distant stars.
The argument suggesting that there is life on other planets is very
simple and straightforward. It begins with life on Earth as that is, of
course, the only life that we know about. The fossil record and the
genetic record tell us that life on Earth is very old, having originated
somewhere before 3.5 billion years ago (b.y.a.) and possibly before 3.85
b.y.a. The oldest known life forms were at once both very simple and
remarkably complex. The simple aspect is that they consisted of
microscopic single-celled bacteria and archaea. These were much less
complex than the macroscopic life forms that exist today and that have
existed for around a billion years.
Even the oldest life forms that we can find in the fossil record,
however, dating back to 3.5 b.y.a., are very complex--they are very
sophisticated organisms that relied on DNA and RNA to transfer genetic
information, on ATP to store energy in a usable and accessible form,
and, for some of them, on photosynthesis to get access to energy. These
organisms are much more complicated than we imagine the first life forms
would have been.
Life did not exist, however, for one-half to one billion years
following the formation of the Earth. The earliest half-billion years on
the Earth were marked by the continued influx of impacting objects left
over from the formation of the planets. Even today, these objects are
capable of dramatically affecting the terrestrial environment when they
collide with the Earth. Back then, when larger objects were present,
they were capable of sterilizing the Earth completely. There were an
estimated five or so impacts subsequent to about 4.5 b.y.a. that were
capable of completely sterilizing the Earth s surface.
The early environment on the Earth, therefore, was not conducive to
the continuing existence of life, and the earliest life may not have
been able to grab a foothold until sometime after around 4.2 or 4.0
b.y.a. Thus, life may have taken as little as a hundred million years,
and no more than about a half-billion years, to form once the
environment became sufficiently clement to allow it.
The rapidity with which life originated on the Earth has important
implications for the process of forming life. It tells us that the
formation of life is not a difficult event, but, rather, it is a
relatively straightforward consequence of natural events on the planet.
This is consistent with our current view that life originated through
chemical and geochemical processes, starting with organic molecules in a
wet surface or near-surface environment and using energy from some
chemical source to build them into more complicated molecules. Even
though we do not yet understand the specific processes that led to the
origin of life, we can easily see that simple chemical processes can
lead to more complicated molecules and, eventually, to life.
The ingredients for life
On Earth, we imagine that life really needed only a few key
ingredients to get started. Liquid water is one such substance. It is
difficult for us to imagine that life could exist without liquid water.
Even with this requirement, we still see that life could be widespread
throughout the universe; if we allow for the possibility that a
different liquid also could hold the key to life, then life could be
even more widely distributed.
The second ingredient is access to the necessary biogenic elements,
such as C, H, O, N, and so on. This is not a very limiting factor,
though, since we expect these elements to be very widespread throughout
the universe, to be incorporated into planets during their formation,
and to be readily available at the surfaces of geologically active
planets.
A source of organic molecules was required for the origin of life
on Earth, and presumably would be required for life elsewhere as well.
On Earth, organics could have come from one or more of several different
sources. These include the Earth s atmosphere, where they could form
from energetic processes such as lightning in a slightly reducing
atmosphere (the so-called Urey-Miller process); hydrothermal vents at
the bottoms of the early oceans, heated by the extremely active
volcanism that would have been present then, where organics could form
by a chemical slide toward equilibrium as very hot water cools off once
injected into the oceans; or from organic molecules that were present in
dust and planetesimals accreting onto the Earth. Most likely, all of
these sources contributed to the prebiotic supply of organic molecules.
Finally, a source of energy is needed to power life. The energy
causes the molecules in the environment to react, moving them out of
their natural state of chemical equilibrium. As they move back toward
equilibrium, they can release chemical energy to power other chemical
reactions, thereby providing usable energy for biota.
Again, there are several possible sources of energy, including
sunlight (especially the energetic ultraviolet light that could have
penetrated all the way to the Earth s surface in the early periods
before there was significant ozone), lightning in the atmosphere, or
geochemical energy obtained from geothermal heat in water circulating
through hydrothermal vents. All of these energy sources were available,
probably in abundance. There is no need for the energy sources that
drove the earliest life to be the same as those that power life today;
thus, the complicated chemical mechanisms that drive photosynthesis did
not have to be present in the original life.
We expect that, under these conditions, the formation of life was
relatively straightforward. We also expect that life could originate and
continue to exist any place where similar environmental conditions are
met. This could mean elsewhere in our solar system, or on planets around
other stars. A search for life, therefore, is almost tantamount to a
search for the basic environmental conditions in which life could exist.
Life on Mars
Elsewhere in our own solar system, we immediately think of Mars as
a possible abode for life. There is abundant geologic evidence on the
martian surface to indicate that liquid water has played an important
role in shaping the surface throughout time. The evidence suggests that
water was relatively stable at the martian surface during the first
half-billion years recorded in its geology (from about 4.0 to 3.5
b.y.a.). If correct, this might suggest that life could have originated
on Mars surface at that time.
Subsequent to 3.5 b.y.a., however, there also is abundant
geological evidence for the continued presence of water. At this later
time, the water was not stable as a liquid at the surface, except
perhaps intermittently. Rather, water was present deep within the crust
and was released to the surface in catastrophic floods only
occasionally. Within the crust, however, the water would have been
available to support either an origin of life or its continued existence
if it had originated earlier. In addition, within the crust there was an
abundant source of energy from the volcanic activity that has persisted
throughout most or all of martian history, and from chemical weathering
of the minerals comprising the crust.
Life could have originated at the surface on early Mars or in the
deep subsurface at any time, and life could exist today. If life is
present today, it likely would be either deep beneath the surface where
water could exist as a liquid (several kilometers deep, perhaps) or
exposed at the surface in any transient vents where hot, volcanically
heated water is released at the surface.
Although there is some evidence to suggest that there might be
fossils from organisms within meteorites from Mars, this evidence is
very controversial and is not yet generally accepted. Significantly,
even if this meteoritic evidence is wrong, the basic argument regarding
the possibility of life on Mars will not change. This is true even
though the meteorite findings appear to have reinvigorated the interest
in searching for life on Mars.
After Mars, other suggestions for an abode for life become more
speculative. Life could have arisen on early Venus, when the Sun was
dimmer, temperatures were lower, and the planet might not yet have
undergone a transition to the present thick, hot, greenhouse atmosphere.
Of course, any evidence of an early Venusian biosphere would have been
long since obliterated.
The moons of Jupiter and Saturn
Life also could exist on Europa, a satellite of Jupiter, living in
a possible ocean of water that may lie buried beneath the surface
covering of water-ice. There, melting of the ice would result from tidal
heating generated by Jupiter tides, triggered by gravitational
interactions with Io as they both orbit around Jupiter. If there were an
ocean, tidal heating and decay of radioactive elements would provide a
substantial source of geothermal energy that might be tapped by living
organisms.
Although there are exciting images from the Galileo spacecraft that
suggest that liquid water has been present beneath the surface of
Europa, there is no certain evidence for the existence of an ocean. Life
conceivably could exist on Io, as well; there, abundant energy is
available through the tidal heating, although there is no evidence for
water of any sort.
Life also might have existed on Titan, a satellite of Saturn. This
is much more speculative, because temperatures today are much too low to
allow plausible life forms to exist. There might have been liquid water
early in Titan s history, however, with the heat to melt the abundant
water-ice being provided by large impacts during the end of the
satellite s formation. Even without active biology Titan represents an
interesting exobiological laboratory, where organic chemical processes
occur even today in a manner similar to what might have occurred on the
early, prebiotic Earth.
Life beyond the solar system
As we move outside of our own solar system, the prospects for
finding environments suitable for life become still more speculative. As
of today, we do not know of a single planet around another star that
provides an appropriate habitat for life. However, this does not mean
that we have no information on the subject.
A theory of how planetary systems form as a natural byproduct of
the formation of stars has been developed. This theory is based strongly
on the conditions that we see in our own solar system. However, it also
is based on astronomical observations of star-forming regions in the
galaxy, interstellar clouds of gas and dust, actual disks of gas and
dust that occur around young stars, and, now, direct detections of giant
planets and brown dwarfs around other stars. As a result, there is
strong reason to believe that this theory might be more general than if
it were based only on our own solar system.
Planets are thought to form from the collapsing gas and dust that
eventually become a star. As the cloud collapses due to the pull of its
own gravity, it will begin to spin faster due to the conservation of
angular momentum. Because it is spinning, not all of the material can
collect into a single central ball that becomes the star. Some of the
matter will stay behind as a disk around the protostar; this disk
consists of dust grains and gas, in orbit around the newly formed star.
PRIVATE The dust will begin to accumulate into larger objects,
first by sticking together due to electrostatic forces, and later by
gravitationally attracting other nearby objects. Eventually, these
planetesimals become large enough to accumulate into a small number of
individual protoplanets, each of planet-sized proportions. Only the
rocky material can accumulate at the relatively high temperatures that
occur close in to the star.
Farther out, where temperatures are cooler both from the lesser
compression of the protoplanetary disk and from the greater distance
from the central star, water-ice also will condense and accumulate.
The greater mass available due to the presence of water-ice allows more
massive planetary cores to accumulate. These then can begin to attract
the gas that also resides in the disk. The gas accumulation then allows
giant planets, similar to our own Jupiter and Saturn, to form.
These processes are thought to be relatively general, allowing the
formation of planetary systems that might look much like ours. Numerical
simulations of the formation of rocky planets suggest that our inner
solar system might be typical, consisting of a small number of planets
in well-spaced orbits.
If so, this suggests that habitable planets might be relatively
common--there will be a significant likelihood of finding a planet at
just the right distance to allow liquid water to exist. Moreover, a
relatively wide range of distances from the central star would allow
this. In our own solar system, the habitable zone might extend from
almost as close in to the Sun as Venus to almost as far away from the
Sun as Mars. There is at least one habitable planet in our own solar
system, and possibly as many as three or four more that might have been
habitable at one time or might still be habitable today.
Planets beyond the solar system
Significantly, we are now able to detect planets that are orbiting
other stars, and we are finding that they are relatively abundant. For
the most part, we cannot yet detect Earth-sized planets, only
gas-giants. It is hard to estimate what fraction of stars might have
planets. While as many as half of the young stars have protoplanetary
disks that may lead to planets, less than 10 percent of the more mature
stars that have been examined seem to have gas-giant planets.
Unfortunately, the statistics for Earth-like planets are not known and
cannot be determined from the available information; even for
gas-giants, such a small number of stars have been examined that the
statistics may not be reliable yet.
The planets that we are detecting are providing new information on
how planetary systems evolve. For example, gas-giant planets have been
discovered that are much closer in to their star than was expected.
These almost certainly would have to have migrated in toward the star
from farther out, a process that would have devastating results for any
terrestrial planets.
Given that planets do exist, however, we imagine that there must be
abundant rocky planets, and that many of these will be within their
star s habitable zone. This means that liquid water probably will be
abundant on planets in our galaxy. If life really is able to form as
easily as we think it can, under the proper conditions, then it is
likely that life is rampant throughout the galaxy. Of course, life is
much more likely to take the form of bacteria-like organisms than of
larger, more complex organisms.
While we expect that evolution will occur on other planets, and
that more complicated forms of life could exist, it seems most likely
that life on other planets will be like the simplest life forms on
Earth--those that have existed for the longest time and in the most
varied environments, and those that may be the most abundant forms of
life on Earth--bacteria.
Does this mean that intelligent life does not exist elsewhere in
the galaxy? It is hard to say. On the one hand, some scientists suggest
that increased intelligence offers such a tremendous benefit to an
organism that it must be a highly likely outcome of evolution on any
planet, given sufficient time. On the other hand, intelligent life on
Earth is the outcome of a random series of evolutionary processes, and
there appears to be no natural imperative either toward more complex
organisms or toward more intelligent organisms. So it may be that
intelligence is a rare phenomenon.
Whether we are speaking of bacterial life or intelligent life,
however, we have insufficient evidence today to know for certain whether
extraterrestrial life exists. Although the present discussion is based
on solid observations of life on Earth and the nature of the universe,
the application to the question of life elsewhere so far is purely
theoretical. It is only through the continued exploration of our home
planet, our solar system, and our universe that we can hope to find
fundamental solutions to the questions surrounding the existence of
life.
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