Origin of Life on Planet Water - Index
Spontaneous Generation - The Primeval Soup
Panspermia - Life Hitched a Ride to Earth from Space
Evolution Pathway and Links to the Climate and Atmosphere
Evolution of Planet Water
The First Forms of Life
Evolution of Life as We Know It
How Plants Change the Atmosphere
=>> Is There Life Elsewhere ?
Is There Life Elsewhere?
The Earth is our only known example of a planet that contains life, and we can use it to
understand the conditions necessary for life and the possibilities for life elsewhere.
We only know one form of life on one planet. It remains to be seen whether the conditions
on Earth that are necessary for life here and necessary for life elsewhere.
We can only presume so. Time also complicates things both in terms of the suitability
of conditions and the occurrence of life.
Perhaps we should restrict ourselves to the notion of 'Life as We Know It' - for there may
be many forms of life out there, which we may find hard or impossible to recognize.
We only know our life on Planet Water.
A related question is: If we wound back the clock and started at the beginning again, would
life develop on Planet Water and what form would it take? This affects our assessment of
how likely life is out there. Similarly if we had one million planets with conditions like
that when earth developed here, how many would develop life as we know it?
There are two major camps on this issue:
=> The 'It is Easy' Mob
=> The 'It is Hard' Mob
These groups are at the ends of the extremes of views about life elsewhere.
The 'It is Easy' Mob
The 'It is Easy' Mob argue that there is a lot of evidence to suggest that life is everywhere
waiting to be found. Some of their arguments are:
# The geologic record tells us that life on Earth began very quickly after it became
possible for life to exist, therefore it must be easy to form or gets viable seeds
from space. The last "Earth-sterilizing" giant impact event probably occurred
somewhere between about 4.4 and 4.0 billion years ago. Fossil microscopic cells
and carbon isotopic evidence suggest that life was widespread 3.5 b.y.a. and
that it may have existed before 3.85 b.y.a. Thus, after it became possible for
life to exist, no more than about a half billion years passed, and possibly only
one or two hundred million years, before life was widespread. This short time period
suggests that the origin of life is a relatively easy process, and that it is
a straightforward consequence of natural chemical reactions in a geologically
active environment. One hundred millions years is still a very long time.
# The organic molecules found on meteorites, including those from Mars, contain
amino acids similar to those in living things on earth.
This suggests that the building blocks for life, if not the seeds of life itself,
are everywhere, and that these materials or seeds are raining down on planets
like ours.
# There are so many stars out there with so many planets that this makes highly unlikely
things such as liquid water and all the precursors for life as we know it becomes
a sure thing. If you multiple a very small probability by a very large number the
answer is '1' - i.e. bound to happen somewhere.
The 'It is Hard' Mob
The 'It is Hard' Mob argue that life as we know it is so dependent on the pathways,
linkages with atmospheric conditions and many other factors that are random or hit
or miss happenings without a defined path or end-point. The history of life on earth has
many extinction episodes and chance events that would seem to be very difficult to
generate again here or elsewhere.
Some of their arguments are:
# It seems that liquid water is a highly unlikely thing for a planet - Water is an obligate
requirement for life.
# The step from organic molecules - the 'Primeval Soup' to living things is a massive one.
It requires a quantum leap in terms of organization and the ability to replicate and
provide the information needed to generate all the components of a primitive cell.
We have no real idea about how this could occur.
# The linkages between the atmosphere and climate on earth and living things are tightly
intertwined. The pathways in the evolution of these things are also strongly linked.
It seems that it would be very difficult to duplicate these relationships and
time sequence of events required for these components. Primitive life forms
are confined to isolated anaerobic patches, because oxygen kills them.
Likewise all the aerobes on the planet, including ourselves would perish in
the primitive Earth atmosphere when life originally developed.
# Life could exist elsewhere, but it is highly likely that it would be very different
form of life and perhaps unrecognizable.
Clearly whichever stance is taking the fundamental precursor of WATER is required for life.
# As we consider the possible occurrence of life elsewhere in our solar system, we can try
to generalize the environmental conditions required for life to originate or to exist:
# Liquid water is required. Although other liquids, such as methane or ammonia, might
serve the same purpose, water is likely to have been much more abundant and widespread.
# Access to the biogenic elements is required, as building blocks out of which to
construct life. On Earth, these include C, H, O, N, S, and P, for example, among the
two dozen or so elements that play a role in life. Although life elsewhere might not
use exactly the same elements, we would expect it to use many of them. For example,
life on Earth utilizes carbon (over silicon, for example) not because it is abundant
but because it is so versatile in forming chemical bonds. Carbon also is readily
usable, since it can exist as CO2, which is available as a gas or dissolved in water;
SiO2 cannot exist in great abundance in either form and would be much less accessible.
Given the ubiquity of carbon-containing organic molecules in the universe, we expect
Carbon and water to play a role in life anywhere.
# Finally, a source of energy is required, in order to sustain the order of living
things. On Earth today, most of life's energy comes from the Sun, through
photosynthesis. However, chemical sources of energy suffice, and would be more
readily available for early life. These sources would include geochemical energy
obtained from hydrothermal systems associated with volcanism, or energy from
chemical weathering of minerals at or near a planet's surface.
Looking beyond the Earth, then, two planets in our solar system show strong evidence
for having had environmental conditions suitable for an origin of life at some time
in their history - Mars and Europa (for this purpose, we will consider Europa to be a
planet), and perhaps Venus.
Mars today is not very hospitable. Daily average temperatures do not rise much above
220 K, some 53 K below the freezing point of water. Despite this, there is good
evidence that liquid water has been present at the surface in the past and probably is
present within the crust today.
Networks of dendritic valleys on the oldest martian surfaces look like those that form
on Earth from the runoff of water.
However, recently it has been suggested that these features may have been produced by
liquid carbon dioxide rather than water.
The source of water could have been either precipitation from the atmosphere or
"sapping" (involving release from a crustal aquifer), but liquid water looks to have been
involved.
The dendritic nature of the valleys suggests that they formed gradually, indicating that
water was much more stable at the surface then than it is today.
In addition, ancient impact craters larger than about 15 km in diameter have been
heavily degraded. They do not show the usual ejecta blankets, raised rim, or central peaks,
and some partly eroded craters have gullies on their walls that look water carved.
Craters smaller than about 15 km have been removed entirely. The simplest explanation is
that the craters were eroded by surface water and, again, that water must have been more
stable then.
Although not yet well understood, the atmosphere could have been thicker during the
earliest epochs, 3.5 to 4.0 b.y.a., with correspondingly greater greenhouse warming
raising the temperatures and allowing liquid water to be stable.
Subsequent to 3.5 b.y.a., there is evidence for abundant water within the crust.
Catastrophic flood channels occurred throughout geologic time and originate from
beneath the surface. Liquid water should exist below a depth of a few kilometers,
where geothermal heating would raise temperatures to the melting point of ice.
Clearly, Mars may have had liquid water available throughout time, at the surface early
in history, and within the crust throughout history.
Mars also has had abundant sources of energy throughout time. Volcanism has supplied
heat throughout geologic time, from the earliest epochs up to the very recent past,
as have impact events. Additional energy can come from weathering of volcanic rocks;
oxidation of iron within basalt, for example, releases energy that organisms can use.
The abundance and availability of the biogenic elements at the martian surface completes
the requirements for life. Given the presence of water and energy, it is plausible that
Mars has had an independent origin of life.
Even if an origin of life did not occur on Mars, life still could be present there.
Just as rocks from the martian surface have been ejected into space by impacts,
only to fall on the Earth as martian meteorites, rocks from Earth could be similarly
transported to Mars. Should they contain organisms that survive the journey, and
should they land in suitable martian habitats, the bacteria could survive.
In fact, life could have originated on Mars and been transplanted subsequently to the
Earth!
Europa seems an unlikely place for life. A satellite of Jupiter, it is a little bit
smaller than the Earth's Moon, and its surface is covered with almost pure ice.
However, heating of the Europa interior by the combination of radioactive decay and
tidal heating may raise the temperature to above the melting point of ice at
relatively shallow depths. Since the layer of H2O at the surface is 150-300 km thick,
a global, ice-covered ocean of liquid water may exist.
Recent images of the Europa surface from the Galileo spacecraft suggest the presence of
at least transient pockets of liquid water. Globally, the surface appears covered with
long grooves or cracks. At smaller scale, these quasi-linear features show detailed
structures that suggest local ice-related tectonic activity and infilling from below.
At the smallest scale, blocks of ice are present. By tracing the grooves that
criss-cross them, it is clear that the blocks have moved with respect to the larger mass.
If there is liquid water at all, then it is most likely to be present at the interface
between the ice and the underlying rocky interior. The rocky center is likely to have
had volcanic activity, perhaps at a level similar to that of the Earth's Moon;
the Moon had abundant volcanism until about 3.0 b.y.a. The volcanism within the rocky
core would provide a source of energy for possible life, as would weathering of minerals
by reactions with the water.
Thus, Europa has all of the ingredients for an origin of life. Of course, there
likely has been less chemical energy available on Europa than on Mars, so any life
would not be expected to be abundant.
Mars and Europa are the only places in our solar system that we can identify today as
having (or having had) all of the ingredients necessary for an origin of life.
Venus
Why are Venus and Earth so different? If the Earth were moved to Venus' distance from
the Sun, it would heat up, more water vapor would evaporate into the atmosphere,
and the additional water would add to the greenhouse warming.
Venus may not always have been so inhospitable, however. Four billion years ago, the
Sun emitted about 30 % less energy than it does today. With less sunlight,
the boundary between clement and runaway climates may have been inside Venus' orbit,
and Venus may have had surface temperatures only 100 degrees C above the Earth's
current temperature.
Life could survive quite readily at those temperatures. As the Sun heated up,
Venus would have warmed gradually until it would have undergone a catastrophic
transition to a thick, hot atmosphere. There is no reason why Venus could not have
had an origin of life several billion years ago, and that all evidence of a biosphere
has since been obliterated by high temperatures and geologic activity.
As the Sun continues to heat up, we might expect that Earth will undergo a similar
catastrophic transition only a couple billion years from now!
It is clear that the ingredients for an origin of life have been present throughout
time and may be present today within our solar system. At one time or another,
four planets may have had the necessary ingredients to have had an origin of life.
We need to go there and search for traces of life on these planets and elsewhere.
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