David Warmflash and
Most scientists have long assumed that life on Earth is a homegrown
phenomenon. According to the conventional hypothesis, the earliest
living cells emerged as a result of chemical evolution on our planet
billions of years ago in a process called abiogenesis. The
alternative possibility--that living cells or their precursors
arrived from space--strikes many people as science fiction.
Developments over the past decade, however, have given new
credibility to the idea that Earth's biosphere could have arisen
from an extraterrestrial seed.
scientists have learned that early in its history our solar system
could have included many worlds with liquid water, the essential
ingredient for life as we know it. Recent data from NASA's Mars
Exploration Rovers corroborate previous suspicions that water has at
least intermittently flowed on the Red Planet in the past. It is not
unreasonable to hypothesize that life existed on Mars long ago and
perhaps continues there. Life may have also evolved on Europa,
Jupiter's fourth-largest moon, which appears to possess liquid water
under its icy surface. Saturn's biggest satellite, Titan, is rich in
organic compounds; given the moon's frigid temperatures, it would be
highly surprising to find living forms there, but they cannot be
ruled out. Life may have even gained a toehold on torrid Venus. The Venusian surface is probably too hot and under too much atmospheric
pressure to be habitable, but the planet could conceivably support
microbial life high in its atmosphere. And, most likely, the surface
conditions on Venus were not always so harsh. Venus may have once
been similar to early Earth.
Moreover, the expanses of interplanetary space are not the
forbidding barrier they once seemed. Over the past 20 years
scientists have determined that more than 30 meteorites found on
Earth originally came from the Martian crust, based on the
composition of gases trapped within some of the rocks. Meanwhile
biologists have discovered organisms durable enough to survive at
least a short journey inside such meteorites. Although no one is
suggesting that these particular organisms actually made the trip,
they serve as a proof of principle. It is not implausible that life
could have arisen on Mars and then come to Earth, or the reverse.
Researchers are now intently studying the transport of biological
materials between planets to get a better sense of whether it ever
occurred. This effort may shed light on some of modern science's
most compelling questions: Where and how did life originate? Are
radically different forms of life possible? And how common is life
in the universe?
Philosophy to the Laboratory
To the ancient philosophers, the creation of life from nonliving
matter seemed so magical, so much the realm of the gods, that some
actually preferred the idea that ready-made living forms had come to
Earth from elsewhere. Anaxagoras, a Greek philosopher who lived
2,500 years ago, proposed a hypothesis called "panspermia" (Greek
for "all seeds"), which posited that all life, and indeed all
things, originated from the combination of tiny seeds pervading the
In modern times, several leading scientists--including
British physicist Lord Kelvin, Swedish chemist Svante Arrhenius and
Francis Crick, co-discoverer of the structure of DNA--have advocated
various conceptions of panspermia. To be sure, the idea has also had
less reputable proponents, but they should not detract from the fact
that panspermia is a serious hypothesis, a potential phenomenon that
we should not ignore when considering the distribution and evolution
of life in the universe and how life came to exist specifically on
In its modern form, the panspermia hypothesis addresses how
biological material might have arrived on our planet but not how
life originated in the first place. No matter where it started, life
had to arise from nonliving matter. Abiogenesis moved from the realm
of philosophy to that of experimentation in the 1950s, when chemists
Stanley L. Miller and Harold C. Urey of the University of Chicago
demonstrated that amino acids and other molecules important to life
could be generated from simple compounds believed to exist on early
Earth. It is now thought that molecules of ribonucleic acid (RNA)
could have also assembled from smaller compounds and played a vital
role in the development of life.
In present-day cells, specialized RNA molecules help to build
proteins. Some RNAs act as messengers between the genes, which are
made of deoxyribonucleic acid (DNA), and the ribosomes, the protein
factories of the cell. Other RNAs bring amino acids--the building
blocks of proteins--to the ribosomes, which in turn contain yet
another type of RNA. The RNAs work in concert with protein enzymes
that aid in linking the amino acids together, but researchers have
found that the RNAs in the ribosome can perform the crucial step of
protein synthesis alone.
In the early stages of life's evolution,
all the enzymes may have been RNAs, not proteins. Because RNA
enzymes could have manufactured the first proteins without the need
for preexisting protein enzymes to initiate the process, abiogenesis
is not the chicken-and-egg problem that it was once thought to be. A
prebiotic system of RNAs and proteins could have gradually developed
the ability to replicate its molecular parts, crudely at first but
then ever more efficiently.
This new understanding of life's origins has transformed the
scientific debate over panspermia. It is no longer an either-or
question of whether the first microbes arose on Earth or arrived
from space. In the chaotic early history of the solar system, our
planet was subject to intense bombardment by meteorites containing
simple organic compounds.
The young Earth could have also received
more complex molecules with enzymatic functions, molecules that were prebiotic but part of a system that was already well on its way to
biology. After landing in a suitable habitat on our planet, these
molecules could have continued their evolution to living cells. In
other words, an intermediate scenario is possible: life could have
roots both on Earth and in space. But which steps in the development
of life occurred where? And once life took hold, how far did it
Scientists who study panspermia used to concentrate only on
assessing the basic plausibility of the idea, but they have recently
sought to estimate the probability that biological materials made
the journey to Earth from other planets or moons. To begin their
interplanetary trip, the materials would have to be ejected from
their planet of origin into space by the impact of a comet or
While traveling through space, the ejected rocks or dust
particles would need to be captured by the gravity of another planet
or moon, then decelerated enough to fall to the surface, passing
through the atmosphere if one were present. Such transfers happen
frequently throughout the solar system, although it is easier for
ejected material to travel from bodies more distant from the sun to
those closer in and easier for materials to end up on a more massive
Indeed, dynamic simulations by University of British Columbia
astrophysicist Brett Gladman suggest that the mass transferred from
Earth to Mars is only a few percent of that delivered from Mars to
Earth. For this reason, the most commonly discussed panspermia
scenario involves the transport of microbes or their precursors from
Mars to Earth.
Image: NASA LANGLEY
RESEARCH CENTER; TONY BRAIN Photo Researchers, Inc. (inset)
NASA's Long Duration Exposure Facility carried spores of the
bacterial species Bacillus subtilis (inset) in orbit for six years.
Researchers found that a thin aluminum cover was enough to shield
the spores from damaging ultraviolet radiation, enabling 80 percent
of them to survive.
Simulations of asteroid or comet impacts
on Mars indicate that materials can be launched into a wide variety
of orbits. Gladman and his colleagues have estimated that every few
million years Mars undergoes an impact powerful enough to eject
rocks that could eventually reach Earth.
The interplanetary journey
is usually a long one: most of the approximately one ton of Martian ejecta that lands on Earth every year has spent several million
years in space. But a tiny percentage of the Martian rocks arriving
on Earth's surface--about one out of every 10 million--will have
spent less than a year in space.
Within three years of the impact
event, about 10 fist-size rocks weighing more than 100 grams
complete the voyage from Mars to Earth. Smaller debris, such as
pebble-size rocks and dust particles, are even more likely to make a
quick trip between planets; very large rocks do so much less
Could biological entities survive this journey? First, let us
consider whether microorganisms could live through the ejection
process from the meteorite's parent body.
Recent laboratory impact
experiments have found that certain strains of bacteria can survive
the accelerations and jerks (rates of changes of acceleration) that
would be encountered during a typical high-pressure ejection from
Mars. It is crucial, however, that the impact and ejection do not
heat the meteorites enough to destroy the biological materials
Planetary geologists formerly believed that any impact ejecta with
speeds exceeding the Martian escape velocity would almost certainly
be vaporized or at least completely melted. This idea was later
discounted, though, following the discovery of unmelted, largely
intact meteorites from the moon and Mars. These findings led
H. Jay Melosh of the University of Arizona to calculate that a small
percentage of ejected rocks could indeed be catapulted from Mars via
impact without any heating at all.
In short, Melosh proposed that
when the upward-propagating pressure wave resulting from an impact
reaches the planetary surface, it undergoes a 180-degree phase
change that nearly cancels the pressure within a thin layer of rock
just below the surface. Because this "spall zone" experiences very
little compression while the layers below are put under enormous
pressure, rocks near the surface can be ejected relatively
undeformed at high speeds.
Next, let us consider survivability during the entry into Earth's
atmosphere. Edward Anders, formerly of the Enrico Fermi Institute at
the the University of Chicago, has shown that interplanetary dust
particles decelerate gently in Earth's upper atmosphere, thus
avoiding heating. Meteorites, in contrast, experience significant
friction, so their surfaces typically melt during atmospheric
passage. The heat pulse, however, has time to travel a few
millimeters at most into the meteorite's interior, so organisms
buried deep in the rock would certainly survive.
Over the past five years a series of papers by one of us (Weiss) and
his colleagues analyzed two types of Martian meteorites: the nakhlites, a set of rocks blasted off Mars by an asteroid or comet
impact 11 million years ago, and ALH84001, which left the Red Planet
four million years earlier.
(ALH84001 became famous in 1996 when a
group of scientists led by David McKay of the NASA Johnson Space
Center claimed that the rock showed traces of fossilized
microorganisms akin to Earth's bacteria; a decade later researchers
are still debating whether the meteorite contains evidence of
Martian life. - click below images to enlarge)
By studying the magnetic properties of the meteorites
and the composition of the gases trapped within them, Weiss and his
collaborators found that ALH84001 and at least two of the seven nakhlites discovered so far were not heated more than a few hundred
degrees Celsius since they were part of the Martian surface.
Furthermore, the fact that the nakhlites are nearly pristine rocks,
untouched by high-pressure shock waves, implies that the Martian
impact did not heat them above 100 degrees C.
Many, though not all, terrestrial prokaryotes (simple one-celled
organisms such as bacteria that lack a membrane-bound nucleus) and
eukaryotes (organisms with well-defined nuclei) could survive this
temperature range. This result was the first direct experimental
evidence that material could travel from planet to planet without
being thermally sterilized at any point from ejection to landing.
Problem of Radiation
For panspermia to occur, however, microorganisms need to survive not
only ejection from the first planet and atmospheric entry to the
second but the interplanetary voyage itself. Life-bearing meteoroids
and dust particles would be exposed to the vacuum of space, extremes
in temperature and several different kinds of radiation.
particular concern is the sun's high-energy ultraviolet (UV) light,
which breaks the bonds that hold together the carbon atoms of
organic molecules. It is very easy to shield against UV, though;
just a few millionths of a meter of opaque material is enough to
Indeed, a European study using NASA's Long Duration Exposure
Facility (LDEF), a satellite deployed by the space shuttle in 1984
and retrieved from orbit by the shuttle six years later, showed that
a thin aluminum cover afforded adequate UV shielding to spores of
the bacterial species Bacillus subtilis.
Of the spores protected by
the aluminum but exposed to the vacuum and temperature extremes of
space, 80 percent remained viable -- researchers reanimated them into
active bacterial cells at the end of the mission. As for the spores
not covered by aluminum and therefore directly exposed to solar UV
radiation, most were destroyed, but not all.
About one in 10,000
unshielded spores stayed viable, and the presence of substances such
as glucose and salts increased their survival rates. Even within an
object as small as a dust particle, solar UV would not necessarily
render an entire microbial colony sterile. And if the colony were
inside something as large as a pebble, UV protection would be
Informative as it was, the LDEF study was conducted in low Earth
orbit, well within our planet's protective magnetic field. Thus,
this research could not say much about the effects of interplanetary
charged particles, which cannot penetrate Earth's magnetosphere.
From time to time, the sun produces bursts of energetic ions and
electrons; furthermore, charged particles are a major component of
the galactic cosmic radiation that constantly bombards our solar
Protecting living things from charged particles, as well as
from high-energy radiation such as gamma rays, is trickier than
shielding against UV. A layer of rock just a few microns thick
blocks UV, but adding more shielding actually increases the dose of
other types of radiation. The reason is that charged particles and
high-energy photons interact with the rocky shielding material,
producing showers of secondary radiation within the meteorite.
These showers could reach any microbes inside the rock unless it was
very big, about two meters or more in diameter. As we have noted
above, though, large rocks make fast interplanetary voyages very
infrequently. Consequently, in addition to UV protection, what
really matters is how resistant a microbe is to all components of
space radiation and how quickly the life-bearing meteorite moves
from planet to planet. The shorter the journey, the lower the total
radiation dose and hence the greater the chance of survival.
In fact, B. subtilis is fairly robust in terms of its radiation
resistance. Even more hardy is Deinococcus radiodurans, a bacterial
species that was discovered during the 1950s by agricultural
scientist Arthur W. Anderson. This organism survives radiation doses
given to sterilize food products and even thrives inside nuclear
reactors. The same cellular mechanisms that help D. radiodurans
repair its DNA, build extra-thick cell walls and otherwise protect
itself from radiation also mitigate damage from dehydration.
Theoretically, if organisms with such capabilities were embedded
within material catapulted from Mars the way that the nakhlites and
ALH84001 apparently were (that is, without excessive heating), some
fraction of the organisms would still be viable after many years,
perhaps several decades, in interplanetary space.
Yet the actual long-term survival of active organisms, spores or
complex organic molecules beyond Earth's magne-tosphere has never
been tested. Such experiments, which would put the biological
materials within simulated meteoritic materials and expose them to
the environment of interplanetary space, could be conducted on the
surface of the moon. In fact, biological samples were carried
onboard the Apollo lunar missions as part of an early incarnation of
the European radiation study.
The longest Apollo mission, though,
lasted no more than 12 days, and samples were kept within the Apollo
spacecraft and thus not exposed to the full space-radiation
environment. In the future, scientists could place experimental
packages on the lunar surface or on interplanetary trajectories for
several years before returning them to Earth for laboratory
analysis. Researchers are currently considering these approaches.
Meanwhile a long-term study known as the Martian Radiation
Environment Experiment (MARIE) is under way. Launched by
2001 as part of the Mars Odyssey Orbiter, MARIE's instruments are
measuring doses of galactic cosmic rays and energetic solar
particles as the spacecraft circles the Red Planet. Although MARIE
includes no biological material, its sensors are designed to focus
on the range of space radiation that is most harmful to DNA.
As we have shown, panspermia is plausible theoretically. But in
addition, important aspects of the hypothesis have made the
transition from plausibility to quantitative science. Meteorite
evidence shows that material has been transferred between planets
throughout the history of the solar system and that this process
still occurs at a well-established rate. Furthermore, laboratory
studies have demonstrated that a sizable fraction of microorganisms
within a piece of planetary material ejected from a Mars-size planet
could survive ejection into space and entry through Earth's
But other parts of the panspermia hypothesis are harder
to pin down. Investigators need more data to determine whether
radiation-resistant organisms such as B. subtilis or D. radiodurans
could live through an interplanetary journey. And even this research
would not reveal the likelihood that it actually happened in the
case of Earth's biosphere, because the studies involve present-day
terrestrial life-forms; the organisms living billions of years ago
could have fared much worse or much better.
Moreover, scientists cannot quantify the likelihood that life exists
or once existed on planets other than Earth. Researchers simply do
not know enough about the origin of any system of life, including
that of Earth, to draw solid conclusions about the probability of
abiogenesis occurring on any particular world. Given suitable
ingredients and conditions, perhaps life needs hundreds of millions
of years to get started. Or perhaps five minutes is enough. All we
can say with any certainty is that by 2.7 billion years ago, or
perhaps several hundred million years earlier, life-forms were
thriving on Earth.
Because it is not possible at this time to quantify all the steps of
the panspermia scenario, investigators cannot estimate how much
biological material or how many living cells most likely arrived at
Earth's surface in a given period. Moreover, the transfer of viable
organisms does not automatically imply the successful seeding of the
planet that receives them, particularly if the planet already has
If, for example, Martian microbes arrived on Earth after life
independently arose on our planet, the extraterrestrial organisms
may not have been able to replace or coexist with the homegrown
species. It is also conceivable that Martian life did find a
suitable niche on Earth but that scientists have simply not
identified it yet. Researchers have inventoried no more than a few
percent of the total number of bacterial species on this planet.
Groups of organisms that are genetically unrelated to the known life
on Earth might exist unrecognized right under our noses.
Ultimately, scientists may not be able to know whether and to what
extent panspermia has occurred until they discover life on another
planet or moon. For example, if future space missions find life on
the Red Planet and report that Martian biochemistry is very
different from our own, researchers would know immediately that life
on Earth did not come from Mars. If the biochemistries were similar,
however, scientists might begin to wonder if perhaps the two
biospheres had a common origin.
Assuming that Martian life-forms
used DNA to store genetic information, investigators could study the
nucleotide sequences to settle the question. If the Martian DNA
sequences did not follow the same genetic code used by living cells
on Earth to make proteins, researchers would conclude that
Mars-Earth panspermia is doubtful. But many other scenarios are
possible. Investigators might find that Martian life uses RNA or
something else entirely to guide its replication. Indeed,
yet-to-be-discovered organisms on Earth may fall into this category
as well, and the exotic terrestrial creatures might turn out to be
related to the Martian life-forms.
Whether terrestrial life emerged on Earth or through biological
seeding from space or as the result of some intermediate scenario,
the answer would be meaningful. The confirmation of Mars-Earth
panspermia would suggest that life, once started, could readily
spread within a star system. If, on the other hand, researchers find
evidence of Martian organisms that emerged independently of
terrestrial life, it would suggest that abiogenesis can occur with
ease throughout the cosmos.
What is more, biologists would be able
to compare Earth organisms with alien forms and develop a more
general definition of life. We would finally begin to understand the
laws of biology the way we understand the laws of chemistry and
physics--as fundamental properties of nature.
Worlds in the Making: The Evolution
of the Universe. Svante Arrhenius. Harper, 1908.
The Structural Basis of Ribosome
Activity in Peptide Bond Synthesis. P. Nissen, J. Hansen, N.
Ban, P. B. Moore and T. A. Steitz in Science, Vol. 289, pages
878-879; August 11, 2000.
Risks Threatening Viable Transfer of
Microbes between Bodies in Our Solar System. C. Mileikowsky, F.
A. Cucinotta, J. W. Wilson, B. Gladman, G. Horneck, L. Lindegren,
H. J. Melosh, H. Rickman, M. Valtonen and J. Q. Zheng in
Planetary and Space Science, Vol. 48, Issue 11, pages 1107-1115;
Martian Surface Paleotemperatures
from Thermochronology of Meteorites. D. L. Shuster and B. P.
Weiss in Science, Vol. 309, pages 594-600; July 22, 2005.
Origins of the Genetic Code: The
Escaped Triplet Theory. M. Yarus, J. G. Caporaso and R. Knight
in Annual Review of Biochemistry, Vol. 74, pages 179-198; July