On the cover of a past issue of Science is a picture of Saturn’s icy moon Enceladus. If you look closely, you’ll notice that one half of the surface is rough, pock-marked with craters, while the other half is comparatively smooth and, as far as I can tell, completely devoid of craters. This type of texture delineation is not uncommon in the solar system, the dark spots on our moon result from the less reflective material that erupts from beneath the surface as a result of volcanic activity—together with the rough, more reflective areas, it’s what makes the “man in the moon.” Like our moon, scientists at NASA speculated that portions of Enceladus’ surface had been smoothed over by ice flows originating from beneath the surface, a process called cryovolcanism. The complete lack of craters on the smooth side told the scientists that the volcanic activity occurred relatively recently on the geological timescale as even the most recent craters had been covered up—not a single one remained. In March of 2005 Cassini, the spacecraft that has been buzzing around the Saturnian system since 1 July 2004 beaming data back to Earth including these breathtaking images, detected a strong outflow of ions emanating from Enceladus’ south pole. Cassini then took aim at the ion flow and, in July 2005, passed through the streaming gas at an altitude of just 168 km above Enceladus’ surface. There’s a good possibility that the plume is made of water, originating from a liquid water reservoir located beneath the surface.
To witness active extraterrestrial geology is exciting enough, and Enceladus joins Jupiter’s moon Io as the only two bodies in the solar system—besides the Earth—upon which surface geological activity has been witnessed. But what’s most exciting to me is the possibility that goes along with the existence of subterranean water. Just as scientists postulate took place here on Earth approximately 4.5 billion years ago, the basic ingredients of water, energy and organic molecules might have also broiled together in an Enceladusian version of the “primordial soup” with the identical eventual result as occurred on Earth: the emergence of life.
If we follow the “water + energy + organic molecules = life” formula then Enceladus becomes only one of three locations in the solar system in which there’s hope of finding extraterrestrial life. Mars, with its colossal canyons presumably carved eons ago by running water and water ice at its north pole, is one possible location. Another is Jupiter’s moon Europa that, like Enceladus, is covered entirely by ice but displays evidence for recent geological activity, which again implies a heat source and activity beneath the surface (Both Mars and Europa have captured the imaginations of ET seekers, Mars in 1997 when the controversial discovery of the ALH84001 Martian meteorite containing possible fossil evidence for microbial life and a couple decades earlier when Europa was the cradle of life chosen by the unseen monolith-dispensing aliens in the Arthur C. Clarke classic “2010: Odyssey Two”.). In fact missions have already been drawn up by NASA to investigate the presence of life on both Mars and Europa, and I’m sure Enceldaus will be similarly targeted if indeed a heated water source beneath its surface is confirmed.
I can think of no other finding than the discovery of life elsewhere from Earth that would be more transforming to, not only how we view our universe, but also to the way in which we view ourselves. Since the dawn of time we’ve looked to the heavens in search of meaning, and ever-present is the question, “Are we alone?” What would it mean to finally find an answer?
For one, we would for the first time be able to make a comparison. For me there are two questions I find incredibly interesting: How inevitable is life, and how inevitable is our form of life? Both of these questions can only be helped along with the discovery of extraterrestrial life. Both questions are useless in the absence of other life. So let’s suppose we do find life.
The first question is one of abundance. How abundant is life across the universe? Has it only occurred on this one planetary body among over one-hundred comparable planetary bodies in our solar system, in orbit around this one star of 200 to 400 billion stars in the Milky Way galaxy, a galaxy that is clustered closely with approximately 30 other galaxies which, in turn, is just one of many clusters in the Virgo Supercluster that is comprised of some 2000 member galaxies?
And that’s just one corner of the universe. Given such odds, I consider it extremely unlikely that life would have arose on planet Earth and nowhere else. Like Jodie Foster’s character, Ellie, said in Carl Sagan’s “Contact,” “The Universe is a pretty big place; it’s bigger than anything anyone has ever dreamed of before. So if it’s just us, seems like an awful waste of space…”
But without evidence we’ll never know. So then, think of what it would mean to find life right here in our own backyard. If life were to be found on Mars or Europa or Enceladus that would mean life had arisen within the same solar system two to five times. Multiple occurrences of life within our little nine-planet system corner of the galaxy makes it extremely hard to argue that life in the universe, rather than the rule. Imagine that: realizing there’s a good probability that we’re not alone and that, in fact, chances are the universe is teeming with life. What would it feel like to look to the heavens then?
The other question is one of form. How inevitable is the evolutionary process that occurred on planet Earth and resulted in the biodiversity we see around us today? There are many ways to answer this question, depending on the scale at which we make analysis. The first question I’d like answered is “Is DNA inevitable?” An incredibly exciting question! Here on Earth, with extremely limited exception, all life of life’s heredity is passed by way of DNA (The few exceptions use RNA instead, a single-stranded partner of double-stranded DNA that is similar enough in structure and mechanism of heredity as to be thought of as indistinguishable when considering different modes of heredity as we are here.). Genetic inheritance by way of DNA is elegant in its simplicity and at the same time its enormous utility—four different bases code for twenty different amino acids which are the building blocks for all of life’s tens of thousands of proteins—but is it obvious? If we start out with the same raw materials do we end up with the same machine? Is random interaction pointed in the same direction every time? Are the constraints to self-replicating systems that take form in the primordial soup so stringent as to only use DNA as the mode of heredity? Again, we can think of DNA as the way to go for all forms of life on Earth. Will it be the same for life elsewhere?
And of course, regarding form, we’ll want to know: What do they look like? To make a comparison with life on Earth we should pick a representative that best characterizes life on Earth, that best embodies evolutionary progress as it has unfolded here on Earth. This crowning achievement of evolution, this supreme being, of course, is bacteria.
Don’t believe me? By any criteria of survivability we can think of bacteria is tops. Bacteria vastly outnumber every other life group in number of species, diversity of environments they’ve adapted to—from deep sea vents of 650°F and pressures 265 times that of sea level to environments the acidity of concentrated sulfuric acid to eternally dark rock worlds packed miles beneath the surface—to representation of the total biosphere. By representation I mean the proportion of total species that make up the totality of life on Earth at any given time throughout life’s history. The late evolutionary biologist Stephen Jay Gould argued that the best way to characterize a skewed statistical distribution was to take the mode—that is, the measure most represented by individuals in a distribution. Taking the average—what we normally do—is misleading when the distribution is skewed, as in the case of Bill Gates giving the per capita income of the people in Medina, Washington a disproportionate boost. It’s more accurate to characterize Medina’s livelihood by taking the salary range within which most people in Medina fall. If we do this for evolutionary change—determine what life form has numbered the highest as evolution progressed—we find that it has never changed since evidence first showed up in the earliest fossil records. The mode falls on the life form bacteria. The life form mode is bacteria from life’s appearance at 4.5 billion years ago and it remains that today. This unchanging characterization of life since its appearance is what Gould refers to as the modal bacter. Gould claims that humans are simply a statistical consequence, variation to the modal bacter, outliers on the curve, freaks of nature (He rejected the notion of evolutionary “progress.”).
But what’s right for us might not be right for them. Gould proposes that if we start life over here on Earth a thousand times then the products at the end of the curve—the creatures with the highest degree of complexity such as humans and, more generally, mammals—would be different a thousand times. The chances of humans arising was very slim the first go ‘round, and most likely would never occur again in identical fashion. But I wonder if the same could be said for bacteria, that single great representative of life on Earth. Would the modal bacter be law elsewhere? Again, how inevitable was it really?
Finding life on Mars, or Europa or Enceladus would provide these questions I’ve mentioned with another dimension within which to maneuver; a third dimension from which we can distance ourselves from the flat, two-dimensional confine of knowing only ourselves as example for life. The questions we’ve been asking ourselves for eons would finally be given breath; they would be given data, and the questions would be transformed from those of philosophy to those of philosophy and science.
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