It’s Halloween, when we are on high alert for the unknown that prowls in the shadows. I still shiver when I think of a Halloween night, many harvest moons ago, when a ragged, blue-wigged stranger showed up to trick-or-treat. The creature communicated by grunts and stomps, terrified my sister Gail and me…and turned out to be our mother.
What other creatures lurk in the vastness of space, unfamiliar and perhaps terrifying (or not), but still part of our family which we call life?
“Are we alone?”, arguably one of humankind’s oldest questions. As they looked at the heavens, the ancients imagined other worlds populated by gods and mortals. Friends or foe? Like us or unimaginable creatures?
In 2019 we still ask this same question, but for the first time in human history, we can approach this scientifically. We know so much more about the diversity and inner workings of the tens of millions of species of creatures with whom we share planet Earth. To complement our knowledge about life and its existence in our terrestrial environment, we have conducted the grand experiment of sending life to other celestial bodies in the form of the NASA’s Apollo astronauts who have walked and lived on the moon, and soon to return with women for the Artemis program. But our robotic surrogates have journeyed far beyond, to conduct experiments for us in the atmosphere and on the surface of planets, moons, asteroids, and comets in our own solar system. NASA’s Voyager 1 spacecraft entered interstellar space on 25 August 2012. The Breakthrough Foundation Starshot program is contemplating a flyby to a nearby planet …in another solar system.
How can we actually find what waits beyond earth, possibly as close as our nearest planetary neighbors? Like any hunter, we have to know what to look for. We have a clear idea of what a deer looks like, but how do we find what we don’t know? We have two options: seek enlightenment from the diversity of life on Earth, past and present. This diversity does seem incredible, with organisms ranging in size from a micrometer or two, to the giants of the Age of Dinosaurs, metabolisms that allow organisms to live off the carbon dioxide and nitrogen in the atmosphere (looking at you, cyanobacteria) to those of us who require plants, fungi and animals to sustain us. Many, but not all of us breathe oxygen, why not use iron or sulfur as terminal electron acceptors? We (life) have conquered temperatures ranging from the frigid climes of the Antarctic to the geysers well above the boiling point of water. We live in acid, base, high radiation, no radiation, high salt, low salt, and so on. And some so-called polyextremophiles inhabit more than one extreme, for example the boiling acid hotsprings of Yellowstone National Park.
This diversity seems incredible until you look “under the hood” and realize that all life uses the same operating systems of the same DNA-based system, with proteins and RNA encased in lipid bilayers, all compounds based on organic carbon. Why not use a different alphabet for life? A mineral or protein coating? What other possibilities are feasible? The burgeoning field of synthetic biology is the art of the possible, the toolkit to create and examine other possibilities, and someday – perhaps soon – create a life form.
Of course all this assumes that we can even agree on a definition for life.
So, armed with the knowledge of life on Earth, and what we have created, we go forth to fertile hunting grounds, which to a first order approximation, we assume need liquid water as a solvent, and a source of energy to power the biosphere. These constraints suggest the potential for life in our own neighborhood is high. Sure, today the surface of Venus is hellishly lead-melting hot, the hottest planetary surface in our solar system, but perhaps as recently as 500 million years ago it was much more like us, several billion years into the time that Earth was inhabited. Our sun, which over time burns ever hotter, would have triggered a runaway greenhouse. As Carl Sagan suggested in 1967, perhaps a Venusian biosphere existed and now seeks refuge in the clouds 50-60 km off the surface where it is far more clement, even today.
On our other side is Mars, a planet with water ice in abundance and transient liquid water today, but was likely to have liquid water, or at least brine slurries, in the past responsible for the surface features that clearly resemble dried stream beds. Most recently, small amounts of organic carbon and fixed nitrogen have been detected, so Mars remains on the high suspect list for extinct, if not extant, life.
Ceres – the largest object in the asteroid best – is now known to have water ice. Better yet, several moons of Jupiter, most notably Europa, is known to be an ice-covered ball with indications of organic carbon and a liquid water ocean. The tiny moon of Saturn, Enceladus, has graciously provided a series of geysers in its southern hemisphere spouting the liquid water that lies beneath, water that NASA’s Cassini spacecraft determined to have the sort of molecules that warm the heart of an astrobiologist, the organic carbon precursors used by life on Earth.
But there is another moon of Saturn, Titan, that resembles an organic chemistry lab gone awry. The surface has lakes of the simple organic solvents ethane and methane, with more complex organics and frozen water rocks on the surface. And the hint of liquid water beneath the surface. Perhaps the breeding ground for a different type of life, or much like ours once you scratch the surface.
No self-respecting astrobiologist would have discussed Pluto, until right before NASA’s New Horizon’s space craft arrived and the words “Pluto” and “ice” were heard in the same sentence. And then there are all the icy trans-Neptunian objects. Such a rich neighborhood we live in, and if our theories of the probability of the origin of life based on certain physical and chemical conditions are true, perhaps we also live in a solar system that has fulfilled the promise of its fertility multiple times.
And if that is true even in our own little solar system, one of perhaps a 100 billion in the Milky Way Galaxy, itself one of perhaps 100 billion galaxies in the universe. Even though many may not be habitable as they may, for example, be too close to a stellar nursery, surely we are not alone.
But perhaps we are? We will never know in our lifetime.
With all that unknowns in life that might frighten us into action, from poverty and war to economic and environmental challenges, why should the hunt for extraterrestrial aliens even concern us? Because we are humans, and the curiosity that drove our ancestors Millenia ago to look up and wonder still exists in their descendants.
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