Mars analog studies in Yellowstone

I recently returned from a week in Yellowstone, studying hot springs as an analog of possible environments on Mars and the early Earth.  To the casual eye, hot springs look devoid of life, but really they are teeming ecosystems of thermophilic bacteria.  Phylogenetic studies suggest these organisms are closely related to the earliest life on Earth; and Mars is known to have had extensive volcanism in its past (perhaps persisting to the present day?), including the largest volcano in the solar system, Olympus Mons.  If life ever existed on Mars, we might find evidence around ancient thermal vents.

High temperature pool ( > 90 deg C), a biofilm coats the interior
 surface, despite the near boiling temperature.

The problem is, would we know what to look for?  And if we found anything, could we be absolutely sure it was once living?  Evidence for the earliest life on Earth consists of microscopic fossils found in the Apex chert, roughly 3.5 billion years old.  However, the biogenicity of these structures is a subject of scientific debate.  Sure, the shapes resemble contemporary bacteria... but morphology alone is an insufficient biosignature.  Our understanding of the processes that lead to preservation of microstructures is incomplete. Hence, studies in contemporary mineralizing environments can lead to insight that will help distinguish abiotic pseudofossils from bona fide microfossils.

Outflow channel from the spring, all the white is silica,
all the colours are bacterial communities.

For several days, I stared at scenes like this, particularly interested in the white film of silica developing over the orange "bubblemat" -- or Phormidium.  The water from the hot spring carries dissolved silica; as the water evaporates, the silica is left behind, forming new layers of rock.  Anything present can become encased, leaving fossil traces behind.

Phormidium, or bubblemat

Phormidium is a type of cyanobacteria, responsible for the oxygenation of our atmosphere.  It grows in thick microbial mats, held together by extracellular polymeric substances (EPS) secreted by the bacteria.  The products of its respiration become trapped as air bubbles within the mat.  A thin layer of heated water flows over the surface of the mat, but the air bubbles can force part of the mat to crest the water level, initiating silica deposition.

Silica layer, at 45x magnification

I have a microscope attachment for my camera/phone, and was able to snap a few images while still in the field.  I set the lens on a mostly dry, smooth white surface of silica, and was pleasantly surprised to find filaments and air bubbles -- capturing the initial stages of microfossil preservation.


Hopefully, studies like this will help inform our understanding of the earliest life on Earth, and our search for life on Mars.

What can astrobiologists study here on Earth?

The emerging science of astrobiology seems to be lacking its primary object of study.  It seems a little premature to talk about the scientific study of aliens, when we haven't found any yet, and can hardly be considered a space-faring species ourselves.  Despite a name hinting of "space life," the scope of astrobiology includes more than just extraterrestrials.  It is the study of life within a universal context, and Earth is indeed part of the universe.  Earth-based life is the only form of life we currently know of, and in order to make predictions about the potential for life elsewhere, astrobiologists study this data point of one.  Especially...

Extremophiles -- organisms that live in extreme environments, like hot springs or dry Antarctic valleys.  (although.. the name 'extremophile' highlights our bias, as a hot spring environment can hardly be considered extreme to the organisms dwelling there.)  By studying life that thrives in environments normally considered hostile, scientists have expanded our awareness of where we might find life.  Extremes of temperature (both hot and cold), pressure, salinity, acidity, radiation... life on Earth can cope with a wide variety of conditions.  The most extreme survivors tend to be bacteria, but tardigrades are remarkable little animals that have even survived exposure to outer space.  Some extremophiles live in environments similar to what can be found in locations on Mars.  This raises the possibility that if life evolved on an ancient, wetter and warmer Mars, it might still exist there, thriving in a vast ocean of subsurface ice.

Biosignatures -- What constitutes evidence of life?  Before we can find proof of life on other planets, we need to know what we're looking for.  Most of the life on Earth, for most of its history, has been microbial.  From this, we can assume there will be more single-celled life in the universe, making detection somewhat more difficult.  Even the evidence for the earliest life on Earth is still debated, because microbes don't leave obvious bones in the ground.  Microfossil structures have that resemble contemporary bacteria have been found in sedimentary deposits dating back 3.5 billion years; but morphology is not a sufficient indicator of biogenicity.  Processes leading to the preservation of structures on the microscopic scale are not well understood, and what looks like an ancient cyanobacteria could be an abiotic artifact.  Even if Mars is dead now, it might have once harbored life.  To plan future missions and prevent overstating any evidence, astrobiologists attempt to define what makes for clear indicators of past life.

The Origins of Life -- one of the greatest ponderables of all time!  Science has yet to advance a complete theory regarding the origins of life on Earth.  The theory of evolution covers the speciation of life once it has developed, but does not address the question of life's origins.  Darwin wrote to a colleague about the possibility of life forming in a "warm little pond" initiating speculation about the primordial soup, and prebiotic chemists have since tried to recreate the recipe, without success.  The most famous experiment occurred in the 1950's, when Stanley Miller was able to synthesize some of the building blocks of life, including amino acids, from simple molecular precursors.  Currently, the RNA World hypothesis seems the main contender for the origins of life, positing simple life forms of ribonucleic acid, before the development of proteins and DNA (deoxyribonucleic acid).  An understanding of the conditions necessary for the origins of life on Earth can help guide the search for extraterrestrial life, by predicting where the transition from chemistry to biology might also have occurred.

NASA defines astrobiology as "the study of the origins, evolution, distribution, and future of life in the universe."  It is the study of the extent of life in the universe, which includes life on Earth.  Indeed, the study of Earth-based life forms the foundation of how to look for alien life, even if we have yet to find any.

Maybe we're all aliens...

Did life necessarily start on Earth?  What if we were seeded here from another world?


The origin of life on Earth remains an unresolved question.  Fossil evidence suggests the presence of life almost as soon as the planet cooled enough, following a period known as the Late Heavy Bombardment when inner solar system was a veritable shooting gallery of asteroid impacts.  The short amount of time between planet-sterilizing collisions large enough to boil off the oceans and the emergence of life has suggested to some that the early Earth was seeded with life.  This notion that life got its start elsewhere before coming to Earth is known as 'panspermia'.  Despite seeming somewhat fanciful, several prominent scientists have written seriously about the idea -- including Lord Kelvin, Svante Arrhenius, and Francis Crick, one of the co-discoverers of the structure of DNA.  


Panspermia comes in a few different varieties, depending on the mode of transportation.  The most commonly considered being ballistic panspermia, in which microorganisms catch a ride inside a meteorite to another world.  A large enough impact can blast small chunks off a planetary surface into space, where it might drift for several hundred thousand to millions of years before colliding with another planet.  This process of exchange is well documented, as several meteorites have been identified as originating on Mars.  The most famous of these being ALH84001, a Martian meteorite found in the Allan Hills region of Antarctica in 1984.  This small rock rocketed into infamy in 1996 when President Clinton announced the discovery of "microfossils" inside, as seen with an electron microscope.  The scientific community has gone back and forth over the ultimate nature of the shapes, whether biogenic or not, such that there is no consensus.  One upshot of the debate, additional studies have made the case that ballistic panspermia is at least theoretically possible... exactly how likely is still unknown.

Supposed nano-fossils seen in the Martian meteorite ALH84001

Another flavor can be described as diffusive panspermia, in which small spores drifting in the wind reach the upper atmosphere, then continue into space, which drift along solar winds until settling on a new habitat and taking hold.  Spores are reproductive structures well adapted to extended survival in unfavorable conditions, and are released in the billions upon billions of spores by plants, bacteria, fungi, algae, etc..  Given astronomically long odds as a means of interplanetary distribution, this seems almost ridiculously unlikely; but the possibility can't be completely ignored either.


Complicating both of these methods of interstellar distribution are the harsh conditions and vast distances of space.  Ultraviolet light and other cosmic radiation is highly destructive to DNA and the complex molecules of life.  Then there's surviving the ultracold temperature of 3 K (temperature in Kelvin, named for Lord Kelvin, mentioned above as a proponent of panspermia; also -270 ºC or -454 ºF), in the vacuum of space.  Plus, the difficulty of remaining viable after the unimaginably long durations required to travel such vast distances.  Directed panspermia gets around these complications by proposing an intelligent transfer of life between planets by extraterrestrials in spaceships.  As with other theories that fall under the auspices of pseudo-science, this hypothesis provides an explanation without offering any realistic means of testing -- indeed, the lack of evidence is often hailed as definitive proof.  That said... if humanity ever colonizes Mars, directed panspermia will become a reality; and thus it probably exists among any interplanetary species, if any exist.


(I have an irrational fear of scorpions.  They're evil, and I want nothing to do with them.  In my moments of levity, I like to rant about scorpions being on Earth as a result of unintentional panspermia.  Scorpions are like the Norway rat of spacefaring civilizations.  I mean look at them: scorpions are aliens, and they shouldn't exist here.  They're an invasive species.  The only possible explanation is that they were stowaways on a flying saucer, and a visiting intelligence accidentally left them here millions of years ago.)


As fascinating it might be to consider panspermia, it ultimately feels unsatisfying as a hypothesis regarding the origins of life, as it merely deflects the question.  If life got here from elsewhere... how did it get started elsewhere?  Prebiotic chemists study the conditions of early Earth, and try to cook up an analogue prebiotic soup to explain the origin of life.  Out of this study has come a softer version, what might be described as contributive panspermia.  Many of the building blocks of life are fairly common in the universe.  Carbonaceous chondrites, meteorites containing organic compounds, probably contributed ingredients to the prebiotic soup, including amino acids and polycyclic aromatic hyrdocarbons.  An experiment simulating cometary ice formation produced lipids, which can spontaneously form vesicle structures resembling a cell membrane when dissolved in water.  The delivery to Earth of extra-terrestrially derived molecules could have been an essential step in the origin of life.

What about life as we don't know it?

The search for extraterrestrial life seems focused on life that is similar to Earth-based life: with strategies like 'Follow the Water,' and looking for carbon containing compounds.  What about life as we don't know it?

The main reason behind expecting aliens to be similar to Earth life, is that we know carbon-based life works.  Given how little we know about the universe at large, it's reasonable to imagine "strange" forms of life could evolve, completely different than life as we know it.  But without examples, any assumptions are mostly speculative.  How do you look for something when you're not exactly sure what you're looking for?

At first glance, it seems easy to distinguish what is alive from what is not.  Most definitions of life consist of a list of qualities -- mobility, growth, reproduction, etc. -- and anything possessing these qualities is alive.  Loose interpretation of qualitative lists can potentially identify non-living processes as alive; after all, fire consumes resources and reproduces itself.  Borderline cases confuse matters further.  Viruses do not possess a metabolism of their own and must hijack the cellular machinery of a host in order to replicate.  Depending on who you ask, viruses can be considered alive or not; there is no consensus on this basic question.  Lacking a realistic definition of life makes it difficult to predict what other forms of life may or may not be possible.

Silicon-based life is sometimes mentioned as an alternative to carbon.  Chemically, this substitution is entertained as possible because silicon falls directly beneath carbon on the periodic table, so the two elements have similar reactivities.  Carbon and silicon both have four electrons in their outermost shell, and tend to form bonds with four neighboring atoms.  It seems doubtful silicon life could exist as a direct analogue of carbon-based life however.  A primary waste product of our metabolism is carbon dioxide, which normally exists as a gas.  The equivalent, silicon dioxide, or sand, is a little more difficult to excrete at standard Earth temperature and pressure.  If it exists, any silicon life would probably exist in conditions alien to anything we could reasonably survive; perhaps a permanently molten surface too close to it's host star.


Earth life depends on water, such that a cell can be roughly described as a bag of water.  At a basic level, more important for life than water is the existence of a liquid solvent.  Alternate solvents for the chemistry of life have been considered, which would be necessary on worlds where water does not exist as a liquid.  Ammonia is normally a gas on Earth, but is a liquid at temperatures where water is a solid.


In my mind, the most exciting astrobiological target within our solar system is Titan, as it posses the most potential for strange biochemistry.  Titan is the largest moon of Saturn, and is very cold far from the Sun.  All the water is locked up as ice, hard as granite.  But there is a liquid cycle, with lakes of methane and ethane -- which we burn as natural gas on Earth.  The Cassini probe recently spotted seasonal rain on Titan.  Theoretical models have described the potential for life utilizing acetylene as a primary food source, and some calculations suggest we should see more acetylene at the surface of Titan.  That doesn't automatically imply strange alien life forms exist on Titan... but it's one possible answer out of the four or five easiest explanations.


Lacking clear examples of alternative forms of life, we primarily strive to find worlds that could harbor the one kind of life we know for sure exists in the universe.  Carbon based life in a water solvent, like us.

Is there life on Mars?

Mars, the Red Planet, has inspired endless fascination and over forty exploratory missions (even if most have failed to reach their destination).  From Giovanni Schiaparelli's early maps of Martian canali -- interpreted by Percival Lowell as a global network of channels built by extraterrestrials -- to H.G. Wells' War of the Worlds, in which the denizens of a dying Mars invade Earth, mankind has imagined life on one of our closest celestial neighbors.


The Viking missions in the mid-1970s carried four life detection experiments to Mars, to look for signs of bacteria in Martian soil.  One of the tests reported evidence of metabolic activity, but none of the other tests could confirm; most discouraging, a GC-MS analysis found no evidence of organic compounds.  Scientific consensus deemed the seeming metabolic activity a false positive, a result of highly reactive chemicals in the soil, and Mars a lifeless planet.

Or is it?

Interest in the possibility of extraterrestrial life on Mars has been growing, especially since the unambiguous detection of water -- potentially in large amounts -- existing just beneath the surface.  Water is required for all life on Earth, and one primary strategy in the search for extraterrestrial life is 'Follow the Water.'

Satellite images revealed what appear to be water carved gullies, but debate circled around the age of these features.  Four billion years ago, Mars was wet, covered with oceans.  At roughly half the size of Earth, Mars cooled much more quickly and was not able to sustain a thick atmosphere.  The atmosphere of Mars today is too thin for liquid water to persist, any water would sublimate directly from a solid to a gas and likely escape into space.

In 2008, a robotic arm on the Phoenix lander scraped away a small area of soil and found ice.  There could well be oceans of permafrost hiding beneath a dusty red veneer, and deep subsurface aquifers with liquid water heated by geothermal processes, that occasionally bubble to the surface as geysers.  The question now is not whether water exists on Mars, but how much?

Some scientists speculate there is greater biomass existing within the Earth than on its surface.  Even in mine shafts carved a mile deep, there is life.  Bacteria buried in the rock, living in slow motion, with high heat and punishing pressures, extracting energy from chemicals normally considered poisonous.

Given what we know, it seems reasonable to suspect that if life ever managed to become established on Mars, it probably still exists there.  Conditions on Mars are not so different from what certain extremophiles here on Earth tolerate.  I imagine the first mission to drill into the surface will find a thriving Martian community of microorganisms, protected from the harsh UV conditions by a meter or two of rock.  Perhaps not the aliens promised by science fiction, but still infinitely fascinating.

The Phoenix lander also detected a high concentration of perchlorates in the Martian soil.  Recent studies with Earth soil suggest the levels of perchlorate present in Martian soil would break down any organic material under the conditions of the GC-MS experiment performed by the Viking lander.  Maybe... we did detect actually life back in the 70's, and just didn't recognize it.

Which came first, the chicken or the egg?

The chicken or egg riddle is an apt analogy when considering the origins of life.  Just as it's difficult to imagine eggs without chickens or chickens without eggs; what gave birth to the first living thing?  Evidence suggests that the earliest lifeforms on Earth were single-celled organisms that reproduce asexually by dividing in two -- but where did the first cell capable of replicating itself come from?

"Understanding the origin of life may be profitably explored by decoupling the origins of different features of life." - Leo Buss

A living system can be described as having three essential components: a metabolism, an information program, and a boundary dividing it from the environment.  All living things need to eat, to take in nutrients from the environment and generate the chemicals needed to maintain cellular function.  Life requires some way to copy itself, to transmit information about its specific chemical makeup and processes to the next generation.  The collection of metabolic function and genetic instructions are set apart from the outside environment, contained within a semi-permeable membrane.


Origins of life scientists can generally be described as "information first" or "metabolism first." (full disclosure: I work with self-replicating RNA, which puts my primary research in the information camp.)  And here we come back to the chicken and egg paradox...  How were information containing molecules capable of copying themselves without a system for extracting energy and transforming chemicals?  How was a complex set of chemical reactions able to develop without a set of instructions to orchestrate the operation?


I imagine the answer will be found in the synthesis, as that would mark the true origin of life as we know it.  In the meantime, much insight can be gained by attempting to study the systems in isolation.


There is also the notion of "membranes first," in that a separation from the outside is required to maintain the chemicals of life in high enough concentration.  Cells are mostly bags of water and the first life probably formed in water; a semi-permeable membrane allows the inside to take in nutrients and prevent useful chemicals from diffusing away.  It's difficult to imagine "naked chemicals" drifting about in a dilute primordial soup as life.




My apologies if you were expecting a final answer to the origin of life.  Nobody knows.  The best scientific answer is that it probably occurred about 3.5-4 billion years ago.  There's debate over a deep hot origin, with life beginning at geothermal vents on the ocean floor, and surface origins in a warm lagoon with the tides helping mix the first chemical cycles.  The earliest fossil evidence is debatable, since single-celled organisms don't necessarily fossilize well nor preserve the same shape after 3.5 billion years buried in rock.  From phylogenetic assays, LUCA (the Last Universal Common Ancestor) is presumed to be a hyperthermophilic (high temperature dwelling) bacteria.  But nobody knows if that represents the first life on Earth, or merely what was able to survive a horrible cataclysm during the Late Heavy Bombardment of planet sterilizing meteor collisions.


I should also mention panspermia, the idea that Earth was seeded from life elsewhere.  Totally plausible in my opinion, but that just begs the question -- how did the first life on another planet come about?

Is astrobiology a real science?

How can astrobiology be considered a real science when we haven't found any aliens yet?

Even a single data point is useful information. A detailed study of Earth based life as we know it can tell us much about the potential for finding life elsewhere. Astrobiology is the study of the origin, evolution, distribution, and future of life in the universe. Let us examine each of these points in turn.

How did life on Earth begin?

Right away, we stumble upon a huge unanswered question. Science has yet to come up with a satisfactory theory of abiogenesis, or how life could have arisen from inanimate matter. Darwin's theory of evolution does not address the origins of life, merely the change of species over time. In a letter to a colleague Darwin did suggest a "warm little pond" of charged chemicals with the potential to become life. The idea of a primordial soup has been around for over a century, but no one can figure out the definitive list of ingredients.

Many researchers have attempted to simulate chemical environments that could have existed on the early Earth, the most famous being the Miller-Urey experiment in the early 1950's. A glass flask was filled with water (the ocean) and attached to another flask filled with nitrogen, methane, and carbon monoxide gasses (the atmosphere). The liquid was heated so the ocean could evaporate into the atmosphere, where an electric spark discharge simulated the energetic effects of lightning; a condenser cooled the gas mixture back to a liquid, completing the cycle. The experiment was run continuously for several weeks and accumulated a brown tar. Analysis of the tarry residue revealed amino acids and sugars necessary for life.

Current thought suggests the conditions of the Miller-Urey experiment did not accurately model the actual environment of the early Earth, but the notion of chemical evolution seems well established. Large, complex molecules can easily be synthesized from smaller precursors under prebiotic conditions. Many biologically relevant molecules have even been found in comets and meteors, suggesting the building blocks of life are fairly common throughout the universe.

A singular living unit is the cell: a membrane bound chemical system, capable of absorbing nutrients from the environment, and making a nearly exact copy of itself. The main hypotheses regarding the origins of life tend to focus on 'information first' or 'metabolism first', although the notion of 'membranes first' deserves attention. [idea for future entry... what are the differences between the competing hypotheses of life's origins?]

Whatever the sequence of events, life seems to have established itself on Earth roughly 3.5 billion years ago. The exact date is under debate, due to the difficulty in demonstrating the existence of fossilized microbes in rocks so ancient and altered. Considerable amounts of research attempt to identify definitive biosignatures in the rock record. These studies will also help future work on Mars or other rocky bodies, in the search for possible fossilized extraterrestrials.

How does life evolve?

The theory of evolution is the cornerstone of modern biology. Evolution operates on populations over time, requiring variation in the genetics between individuals and a selective pressure that grants differing degrees of reproductive success based on the underlying genetics. "Nothing in biology makes sense except in the light of evolution," according to the evolutionary biologist (and Russian Orthodox Christian) Theodosius Dobzhansky.

What evidence do we have for evolution? All life on earth uses the same four DNA nucleotides to code for the same set of twenty amino acids that make up all the proteins required for the chemistry of life. Proteins central to metabolic processes are nearly identical across all species. Correlating the sequences of homologous proteins between species has allowed the construction of a phylogenetic tree of life, and suggests a theoretical Last Universal Common Ancestor (LUCA) to every living thing on the planet. Whether LUCA represents the first life on Earth or a later genetic bottleneck is still up for debate.

For the first three billion years, single celled organisms ruled the Earth. Only 500 million years ago did the first multi-celled organisms emerge. A key event in the transition to multicellularity was the capture and incorporation of a previously free living organism into another, or endosymbiosis. Mitochondria are membrane bound organelles that specialize in the production ATP (adenosine triphosphate), the energetic 'currency' of life; they also contain their own DNA, which replicates independently of the DNA in the cell's nucleus. Chloroplasts are the chlorophyll containing organelles in plants responsible for photosynthesis, and carry their own DNA, like mitochondria. Genetic sequencing has identified the free living bacteria that were probably involved in these capture events. Large, multi-celled organisms require greater amounts of energy than simple bacteria, and would not have been able to flourish without these specialized endosymbiotic organelles.

What is the distribution of life on Earth?

Over the course of 3.5 billion years, living things have spread across the entire planet, filling almost every niche imaginable. Even environments once considered inhospitable have been shown to harbor thriving communities of microorganisms. Life forms that thrive in conditions outside what we would expect are broadly classified as extremophiles, or 'extreme loving'. (although, some have pointed out this term is a misnomer, since the bacteria that live in hot springs are quite comfortable in that environment -- to the bacteria, WE are the extremophiles...) Thermophiles like high temperature, some living close to the boiling point of water. Psychrophiles prefer low temperature, and can be found living within ice. Halophiles require an environment with high salinity. Piezophiles need high pressure, and live at the bottom of the ocean or in deep sea trenches.

Astrobiologists study extremophiles to determine the absolute limits of life, the window of tolerance that defines the Habitable Zone (HZ). Some off-Earth locations within our solar system are known to have conditions within the range of habitability of some extremophiles. Dry windswept valleys in Antarctica have been examined as analogous to some spots on Mars, and revealed cryptoendolithic ('hidden within rock') microbes thriving in a seemingly barren wasteland. Almost everywhere we look on Earth, we can find an organism suitably adapted to the environment.

What is the future of Earth-based life?

Death comes to all living things. Not even the Sun or the Earth are eternal. Over the next several billion years the Sun will use up it's supply of hydrogen and expand, eventually consuming the orbits of the inner planets, including Earth. Any life will have long since perished, as the gradually increasing temperatures boil off the oceans and sterilize the planet. Geologists estimate higher life has about 500 million years remaining before conditions on Earth become too extreme. That will not be the end of all life on Earth; bacteria once ruled the planet for three billion years, and it is expected they will again reign supreme long after we are gone. A billion years from now, Earth could look very similar to what it looked like a billion years ago.

If Earth is the only planet in the universe to have developed life, the event will pass unremarked upon. Another star goes nova and all record of a self-aware universe reflecting back upon itself incinerated in an instant. Personally, I doubt such a cold and empty fate.

Giordano Bruno, heretical Dominican monk, was one of the first to read Copernicus and realize the stars were other other suns like our own with other solar systems. He saw an infinite universe, filled with planets like ours, teeming with life. The first confirmed extrasolar planet was announced in 1992. As of today, July 11, 2010, there are 464 confirmed exoplanets. The Kepler mission recently announced over 700 exoplanet candidates seeking confirmation from other telescopes, only a single year into its 3.5 to 6 year long planned operation.

Within twenty to thirty years, it is expected we will have telescopes capable of directly imaging the surfaces of distant worlds, observing the weather patterns and mapping continents. As our instrumentation improves, we will undoubtedly seek evidence of extraterrestrial life; and we will probably find it.

The Earth formed roughly 4.5 billion years ago, and suffered frequent sterilizing impacts from large meteors up until the Late Heavy Bombardment, approximately 4.1 to 3.8 billion years ago. Life took hold almost as soon as the Earth was habitable, around 3.5 billion years ago, and it has proven tenacious, filling every possible niche from deep sea vents to snow capped peaks.

It seems highly likely, from studying this single example of a planet with life, that life will find a way wherever conditions are favorable. Astrobiology is the emerging synthesis of many scientific disciplines meant to address these questions. The first aliens discovered will probably be microbes, but such an announcement could still profoundly affect the perception of our place in the universe. What would it mean to look up at the stars and know, not merely suspect, that there are other worlds with life out there? We may face such a night sky within our lifetimes.