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Abiogenesis
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.
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.
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?
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 cosmos.
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
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.
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.
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
spread?
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 asteroid.
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 body.
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.
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
frequently.
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
within them.
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.
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.
(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.
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.
Of
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
protect bacteria.
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
sharply increased.
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.
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.
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.
Although MARIE
includes no biological material, its sensors are designed to focus
on the range of space radiation that is most harmful to DNA.
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 atmosphere.
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.
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.
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.
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.
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Planetary
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.
