Looking for Life on Mars and Beyond
Abigail A. Salyers
An ActionBioscience.org original article
Abigail A. Salyers, PhD, is
professor of microbiology at the Department of Microbiology, University of
Illinois
Many people who watched the TV images coming back from the two
Mars rovers this year were probably disappointed. The bleak-looking, arid
desert they saw could not possibly support life. Or could it? Before explaining
why scientists are more optimistic than ever about finding life on Mars, it’s
worth reviewing what we know about life on Earth.
Was there life on early Earth?
Earth was a barren landscape millions
of years ago.
If space travelers had visited
Earth during the first 3.5 billion years of its 4-billion-year existence, they
would have seen large expanses of water, with smaller expanses of land, but no
plants, insects, or animals. Moreover, if the spaceship had arrived during the
first 2 billion years of Earth’s existence, its measuring devices would have
reported that there was virtually no oxygen in the atmosphere and no ozone
layer to protect the surface from UV radiation.
Early Earth was teaming with microbial
life.
Based on these observations, the
space travelers might well have concluded that there was no life on Earth. If
so, our hypothetical visitors would have been spectacularly wrong. In fact, the
Earth was teeming with microscopic life—tiny creatures called bacteria and archaea. Scientists believe that these microbes were the
first forms of life on Earth. After about 2 billion years, bacteria and archaea were joined by eukaryotic microbes. Only within the
last few hundred million years—an eye blink in Earth’s history—would forms of
life large enough to be seen with the unaided eye start to appear.
The activities of these early
microbes and their descendants have direct relevance for us today.
Microbes played a key role in
developing Earth’s life support systems.
·
Scientists now
know that those microbes, especially the bacteria and archaea,
formed the Earth’s life-support system. Their activities made it possible for
the more complex forms of life we see today to evolve. Microbes also do most of
the recycling of dead things, so that life can continue.
·
A type of
photosynthetic bacteria called cyanobacteria(originally misnamed blue-green algae) put the first
molecular oxygen in the atmosphere about 2 billion years ago, raising the
oxygen level to about 10% of what it is today, allowing the ozone layer to form
and oxygen-using creatures to evolve.
·
Much later, plants
took over the bulk of the job of producing oxygen, but they can do this only
because of organelles called chloroplasts, which evolved from cyanobacteria. Given Earth’s history, it makes sense to
look for microbial life on Mars.
What is a “microbe”?
Microbes consist of a variety of
organisms, including fungi.
To most scientists, and for
purposes of this article, the term “microbe” means any creature capable of
reproducing itself that is too small to be seen with the unaided eye. This
includes bacteria, archaea, fungi, protozoans, algae, and (possibly) viruses.
Archaea are microbes that look
like bacteria but have now been found to form a separate domain of life, the
other two domains being bacteria(single-celled
microorganisms lacking a nucleus) and eukaryotes(all
nucleated organisms, e.g., fungi, protozoa, plants, animals). There are two
caveats to this definition:
Archaea, bacteria, and
eukaryotes are the three main branches of the tree of life.
·
Some scientists place viruses in a separate category because they
are parasitic on cells capable of reproducing themselves, such as bacteria,
fungi, and mammalian cells. Other microbes, with a few exceptions, are capable
of reproducing themselves without this type of aid. Many of these, but not all,
reproduce by dividing (called “binary fission”).
·
Some microbes, including bacteria and fungi, are large enough to
be seen without a microscope. These exceptions are included under the term
“microbe” because they are closely related to other microbes that are not
visible to the unaided eye.
Microbes have greater diversity than
any other life form on Earth.
It is difficult to come up with
an airtight definition of microbe because, as scientists now know, microbes are
an incredibly diverse collection of creatures. There is far more genetic
diversity in the microbial world than in the plants, insects, or animals—a fact
that is not surprising when you consider that microbes had a 3 billion year
evolutionary head start. To get an idea of what this genetic diversity means,
consider this: If insects (which are sometimes called, incorrectly, the most
diverse group of living things) were sorted into species using the criteria
employed by scientists who study bacteria, archaea,
or other microbes, there would be one, or at most two or three, species of
insects. The diversity of microbes is also evident in the variety of
environments in which they live: volcanoes, clouds, sulfuric acid mine
drainage, hydrothermal vents, deep subsurface rocks,
Life requires water, and Mars had it
at one time.
How likely is there to be life on Mars?
Because water is essential for
life as we know it, the discovery of recent geochemical evidence of water on
Mars, some of which may still be present, was an exciting finding for
biologists. To learn whether life could exist in a barren landscape such as
that seen on the surface of Mars, where any water present is mostly present in
the frozen state, some microbiologists have journeyed to a part of Earth that
resembles Mars in some respects: the polar deserts of
·
This region of
·
The hole in the ozone layer that has developed over
·
The level of radiation encountered in
Is there life in the polar
desert of
The atmosphere on Mars is much thinner
than Earth’s.
A major difference between the
environment found in the high deserts of
A more troubling feature of the
Martian atmosphere is the very low level of nitrogen (N2). On Earth,
N2 makes up 78% of atmospheric gases. On Mars it only composes 3%.
Many bacteria can use N2 as a sole source of the nitrogen they need
for proteins, nucleic acids, and other cell components, but the low level of N2
would certainly limit the amount of microbial growth. Thus, if there is
microbial life on Mars, it is unlikely to be as abundant and as widespread as
on Earth and may thus be harder to find.
Different compositions and concentrations
of gases may exist in some areas under the Martian surface. Such a possibility
would be difficult to prove—unless it is proved indirectly the presence of life
in the subsurface regions and in greater abundance than expected.
It may be possible to find a
geological record of microbial life on Mars.
Is there a historical record of life on Mars?
What if life once existed on
Mars but has become extinct? Is it possible to find a geological record of
microbial life? At one time the question of whether there was a geological
record of microbial life on Earth would have been answered with ridicule.
Today, scientists are rethinking this question, especially in view of the fact
that microbial activities can leave a macroscopic mark on their surroundings.
Two examples of cases in which microbes on Earth have left a fossil record are
formations called “streamlets” and banded iron formations.
Ancient bacteria left a fossil record
on Earth.
Microbes are generally too small
to be seen with the unaided eye, but aggregates of microbes can be visible.
Rocklike formations called stromatolites are large
collections of filamentous cyanobacteria that are
clearly visible to the unaided eye, even though the component cyanobacteria are not. Stromatolites,
some which can be as big as a small boulder, formed when large aggregates of cyanobacteria became fossilized. There may be other cases
in which aggregates of microbes were fossilized and can be seen with the
unaided eye.
Banded rock formations show the
Earth’s early atmosphere had almost no oxygen.
Another marker of past life is
evidence of chemical activity. One byproduct of cyanobacterial
production of O2 was the formation and precipitation of iron oxides.
The result was colorful bands ranging from red to black. Geologists call these banded iron formations, or BIFs. The existence of BIFs
illustrates the principle that microbial activities can cause changes that are
evident long after the microbes are gone. Microbiologists and geologists are
now looking for other visible signs of microbial activities that might tell the
story of microbes long gone.
Traces of ancient molecules also
provide evidence of previous life.
Another approach to finding a
geological record of microbial life is to look for long-lived molecules.
Current methods for detecting microorganisms and identifying them have focused
on DNA. However, DNA is not nearly as long-lived as microbial lipids, which may
be a better diagnostic indicator of ancient microbial life than DNA. Other
structures found in bacteria survive long after the bacteria have died: Magnetotactic bacteria leave magnetite crystals in a
beads-on-a string formation. Some scientists have reported finding these in
Martian meteorites. Although the hypothesis that magnetite
crystals in meteors from Mars are evidence of microbial life on Mars has
been quite controversial, the example nonetheless illustrates ways in which
scientists are looking for new ways to identify fossil traces of long dead
microbes
Beyond Mars
Other planets and moons may have
liquid water.
Microbiologists are famous for
thinking small, but some of them are thinking very large these days—beyond Mars
to Europa, a moon of Jupiter, on which evidence of
water has also been found. Europa appears to be covered
by an ice layer that is miles thick, but there may be liquid water underneath
it. Why? The core of Europa is emitting heat and
energy, which may allow liquid water to exist despite the low temperature at
the surface of the moon.
Europa’s
conditions may be similar to Lake Vostok,
Can life exist where there is lack of
light?
Judging from what we know about
life on Earth, could life as we know it exist in the liquid water of Europa? One answer may come from a site in
·
Scientists are finding that ice cores taken from
·
The possibility exists that these microbes were simply blown into
the area from other parts of the globe and trapped in the ice.
·
Bacteria can remain viable for long periods if frozen, but they
may not be living, that is, actively dividing, in the Vostok
ice.
A problem for life existing
below the ice sheet that covers Europa is finding a
source of energy. On Earth, photosynthesis is the source of energy for much of
its microbial life. Photosynthetic bacteria and algae are at the basis of most
of Earth’s food chains. On Europa, however, it is
unlikely that enough light could penetrate through an ice layer as thick as
that thought to exist on Europa.
Bacteria flourish even in the bottom
of Earth’s oceans.
Scientists working at the bottom
of Earth’s deepest oceans, where virtually no light penetrates, are finding
that animal and microbial life nonetheless flourish there, near hydrothermal
vents where magma from the Earth’s core pumps chemical energy sources such as
sulfides into the water. The bacteria that grow there, and in turn feed the
hydrothermal vent animals, use the oxidation of sulfides to produce energy.
At first, this discovery was
viewed as evidence of a photosynthesis- independent type of growth and gave
some people hope that the core of planetary bodies like Europa
could also serve as an energy source. The problem with this argument is that
sulfide oxidation depends on O2, which is derived from
photosynthesis. Yet microbes have surprised us before with their metabolic
diversity. Perhaps there are microbial strategies of non-photosynthetic growth
that have yet to be discovered.
Should scientists be looking beyond an Earthcentric
view of life?
We shouldn’t think of alien life
strictly in terms of life on Earth.
The approach to looking for life
described above is based on the idea that life elsewhere in the universe would be
like life on Earth and would evolve in the same way. Science fiction
enthusiasts, for example, have long fantasized about creatures composed of
molecules based on silicon rather than carbon. Although scientists have tended
to be highly skeptical of this view, for good reason, some scientists believe
that it is important at least to consider the possibility that notions of life
based on what we know might cause us to miss evidence of life. This concern
about limiting the search for life to an Earthcentric
approach has caused some scientists to ask the bigger question: What features
of life are independent of assumptions about being Earthlike? Some traits of
life have been suggested by scientist Ken Nealson and
others:
Scientists suggest four criteria for determining
what may be life beyond Earth.
·
Life forms move
independently of external forces. This cannot be a central requirement for all
forms of life because, even on Earth, there are many organisms that are not
motile.
·
Living organisms
have edges. There should be some kind of covering that separates a living
organism’s interior from its exterior.
·
The composition of
living things is complex. Living beings should contain a complex mixture of
chemical compounds.
·
Life depends on
chemical reactions that cannot occur spontaneously under the conditions where
they are found to occur. On Earth, many microbes make a living by catalyzing
reactions, such as the oxidation of minerals or the production of methane, that
either do not occur abiotically or occur at a much
lower rate.
We learn more about Earth in our
search for alien life.
What if scientists fail to find life on other planetary bodies?
If scientists fail to find
evidence for current or past life on Mars, Europa, or
other planets and moons in our solar system, all is not lost. The time, energy,
and money spent on studies to develop life-searching strategies have already
given us, and are still giving us, new information about Earth.
·
Scientists are learning that only a fraction of the diversity of
life on Earth is known, especially the staggering diversity that exists in the
microbial world.
·
New metabolic activities, which are being discovered as a result
of astrobiology studies, may well yield new industrial processes.
·
Astrobiology studies have reignited interest in the age-old
question: What is life?
So, in a sense, we are going to
Mars in order to study Earth.
Is it safe to bring Mars samples back to Earth?
Popular fiction often presents alien
life as a threat to Earth.
In 1970, a novel entitled The Andromeda Strain became a
bestseller and was subsequently made into a blockbuster movie. In the movie, a
mysterious microbe was brought to Earth by a space probe. The microbe killed
people by turning blood to powder. Initially, it appeared to be unstoppable.
Fortunately for human life on Earth, the microbe mutated into a form that no
longer feasted on blood but instead was satisfied with rubber. Although the
premise of the movie was wildly improbable, it created a fear in the public
mind that extraterrestrial specimens might introduce a new and unstoppable
plague to the Earth.
Previously unknown microbes can be
dangerous to humans.
Could a microbe that evolved in
a place where there are no humans possibly cause infection in humans? From our
experience here on Earth, we know that the answer to this question is Yes. There have been many examples of human infections
caused by bacteria or viruses that have come out of soil or water, locations
where they had not had much or any prior contact with humans, or that were
previously known to infect only nonhuman animals. Examples are Legionnaire’s
disease, a lung infection caused by a bacterium that normally resides in water,
and AIDS, a viral infection that probably originated in monkeys and later
jumped to humans who were hunting and eating monkeys.
Alien microbes may find it difficult
to infect humans.
Countries with space programs must
prevent introductions of foreign microbes.
The low temperatures found on
Mars and Europa make it unlikely that an organism
that evolved in either of those locations would do very well at the much higher
temperature found in the human body. It is impossible to be sure of this,
however, since microbes have so often surprised scientists with their metabolic
diversity. At least we can say that an Andromeda-strain-type organism is highly
unlikely to be a virus because viruses that infect humans require mammalian
cells to carry out their life cycles.
The desire to make sure that
infectious organisms are not brought to Earth from other planets has led to an
international treaty that requires all of the countries that have a space
program to have a planetary protection officer who leads efforts to make sure
that no such importations occur. Currently, the planetary protection officer
for the
Safeguards are being put in place to
avoid contamination.
Dr. Rummel
does more than protect the Earth from extraterrestrial invaders. He is just as
concerned about damage going in the other direction: contamination of other
planets by Earth organisms. In the early days of space travel, scientists did
not realize that microbes from Earth could survive the conditions of vacuum,
cold, and radiation that anything on the surface of a spaceship would
experience. Nor did they consider seriously that astronauts might leave Earth
organisms behind after a visit. Now that the danger is appreciated, NASA is
taking many special precautions to make sure that there is no further
contamination of the planets its space probes visit.
Author’s notes:
» The dates given here for the origin of life on Earth are a
consensus of estimates from diverse sources. Different estimates can differ by
as much as 200 million years.
» Information on the composition of the atmospheres of Mars and other planets
can be obtained on the Internet or from general books on astronomy. Estimates
from NASA or web sites of university scientists tend to be the most accurate.
Some of the figures are given as percentages of the total and some as actual
concentrations of the gas, so different tables may at first appear to be quite
different even though they agree with each other.