Undeniable (35 page)

Mars has opposite problems. The air is thin, and the surface is very cold. We can observe the ice caps of Mars with telescopes from here on Earth. There is quite a bit of regular water ice, but the enormous white arctic and Antarctic ice cap features on Mars are mostly frozen carbon dioxide (dry ice). That makes the polar places at least
−
130° C (
−
270° F). The water vapor and carbon dioxide that condense to form the ice caps are part of the Martian atmosphere. By an Earthling's standards it's not much of an atmosphere, barely 0.7 percent of the surface pressure of Earth, but it's enough to create winds and interesting weather. The
Opportunity
rover drivers from time to time direct the spacecraft to drive uphill to higher ground, where the winds and electrostatic conditions are favorable for blowing the dust off the vehicle. It's an unusual business, but one that has made remarkable discoveries.

The ongoing explorations of Mars show that the planet was once covered with lakes, streams, and expansive seas. The
Curiosity
rover practically landed in what is obviously a dry riverbed. One cannot help but wonder with all that water on Mars over three billion years ago, were there living things there? Could Martian microbes live on today, underground where they are protected from climate extremes and cosmic rays? On the open ground that our rovers are able to get to, we observe evidence of water, but nothing looks to be living there today. Keep in mind, though, that our technology is limited. The limitations can be expressed in dollars for planetary science. With our current technology and investment, we can land our exploratory spacecraft only on open areas on the Martian surface. We can't narrow the landing area enough yet for a precise touchdown point. This is a real constraint in the search for life, or evidence of life. Imagine you were to explore Earth looking for life, but your technology constrained you to land on the Great Salt Lake flats, or in the Sahara Desert. You might not see much in the way of life until you had driven hundreds of kilometers in the right direction.

The Phoenix Lander alighted upon the Martian surface in 2008. Its findings added another intriguing twist in the search for life on Mars. Phoenix landed atop a thin layer of sand or soil at the north pole. Right below the surface, just a few centimeters down, there is an enormous sheet of ice, water ice. It's apparently just below the surface for many kilometers in every direction. What if there is something living in that ice that might be akin to the half dozen or so genera of bacteria that live below the ice on our own planet? As we have seen on Earth, life is extremely tenacious once it gets started. If early Mars was sufficiently hospitable, maybe the process started there billions of years ago and never ended.

As the CEO of the Planetary Society, I often advocate for a big investment in the search for life on Mars. Suppose we built a spacecraft that could land near a valley, gulley, or gulch near the equator of Mars, a place where it might get just above the freezing point of water on a sunny Martian summer day. Then suppose we had a rover that could detach from the main spacecraft, drive over to the edge or rim of the gulley, then descend on a tether, lowering itself, like a rappelling rock climber, to an icy outcrop or exposed stratum. In the midmorning as the Sun shone directly on that ice, instruments onboard our tethered rappelling robot would look very, very closely. What if they found something still alive there? What if there are indeed Martian microbes still eking out a living way out there in the icy cold of our next neighbor?

Answering these kinds of questions is not terribly expensive, in the bigger scheme of things. Right now, the U.S. investment in planetary science is less than $1.5 billion a year. Put another way, it is less than 0.05 percent of the federal budget. That includes all the missions: Mars, Mercury, Jupiter, Saturn, and the New Horizons mission currently en route to distant Pluto. What if we upped that a billion and found life on Mars? It would be an extraordinary investment, costing barely the equivalent of an extra cup of coffee per taxpayer. With a president, a Congress, and a NASA administrator focused on such a thing, we could change the course of human history.

The same could be said for a trip to Europa, one of the four large satellites of Jupiter. Europa is 3,100 kilometers in diameter, just a bit smaller than Earth's moon, but it is an entirely different type of world. In 2011, data from the
Galileo
spacecraft were analyzed carefully. It is clear now that there is a salty ocean of water under Europa's heavily cracked surface shell of ice. The ocean was discovered using magnetometer data; it's a sensitive electronic compass. Salt water conducts electricity, which in turn affects the magnetic field around Europa. The water has not frozen solid, because the orbital motion of Europa in Jupiter's powerful gravitational field makes the whole world squeeze and unsqueeze with each orbit. Europa maintains its liquid ocean with heat generated by mechanical distortion. It's just like the warmth you feel if you stretch an uninflated rubber balloon a few dozen times and then touch it to your lips. Try it!

Ever since that discovery, scientists and engineers have been discussing how to explore that ocean under the ice. If there really is liquid water, and it really has been kept warm enough to remain a liquid these last four and half billion years, perhaps there is something living there. Plans have been drawn up to build a spacecraft that would land on the surface of Europa. It would then deploy a mechanical or thermal drill, a penetrator tough enough and perhaps hot enough to work through up to fifty kilometers of ice. It would be tethered to the landing craft up above. It would carry instruments that would look for, what we imagine would be, signs of Europan life. That would be a thrilling mission, but a very costly one, certainly many billions of dollars. It would be enormously technically challenging. And of enormous importance, it would have to be careful not to violate science fiction's “prime directive.” To wit, we must not screw up the Europan ecosystems, if there are such things, by contaminating it with Earth microbes that hitched along for the ride.

In 2013, we discovered something exciting, something that might greatly simplify the search for Europan life. Astronomers aimed the Hubble Space Telescope at Europa and discovered plumes of water, seawater, spewing right out into space through fissures in the ice. If there are microbes, or even maybe centimeter-sized living things in the Europan ocean, they're being squirted right along with water into the blackness. A spacecraft could be designed to fly through the plume of water, capturing plenty enough of it to do microscopic and chemical assays of whatever might be living in that water. Such a mission can be flown for a small fraction of the cost of a lander with a drill and all the trouble we would have to prevent our microbes from contaminating theirs (if there are any of “them”). The proposed mission is called the
Europa Clipper
.

A similar challenge awaits at Enceladus, one of Saturn's moons. It is much smaller than Europa, just five hundred kilometers wide, but it, too, has a (small) ocean of water buried beneath a thick ice pack, and has even bigger jets that erupt from its south pole. Here is another place to look for life using a
Clipper
-style mission.

I don't know about you, but I find it easy to imagine some sort of deep space–worthy transparent plate mounted roughly perpendicular to the spacecraft's direction of flight. As the
Clipper
flew through plumes of water any living thing might end up there like insects on a windshield. (Not perfect, but it may be the best we can do; orbiting takes velocity.) Then with a microscope camera with an appropriate light source that could be trained on the plate, Earthlings would get a glimpse of what might be a living thing from an alien world. A more elaborate version could even collect a sample of Europan (or Enceladean) ice and bring it back to Earth for more detailed study—taking a lot of care not to contaminate things either way, of course. Talk about a tax investment value: This would be a groundbreaking experiment unlike any performed ever before in the history of the world.

If we go to Europa or Enceladus—or search more aggressively on Mars—we will run into a higher-level question about possible alien life: Will we be able to recognize it if we see it? The answer requires going back to what we know about the fundamentals of life, beyond its need for energy and its probable fondness for water. How would it regulate the chemical reactions that are needed for chemicals to make copies of themselves? One way that we know works is to use the chemical properties of chemicals already dissolved in water to provide the energy to move things around. But a living thing needs some way to keep different chemicals separated. Otherwise, the whole insides of a living thing would probably just mix up and come to a stop. So, we figure that it will need a container or membrane. It needs a wall to establish what's inside, and what constitutes its surroundings. In short, it might be very different on the inside, but from the outside it is likely to look like the bacteria and single-cell organisms we know so well, at least by one logical line of reasoning.

Living things that can form membranes would probably have a big advantage over other molecules that can't form them. Membranes enable living things to use the attraction and repulsion of the electrons on the outside of atoms to drive or pull molecules around. Try the osmosis experiment I mentioned back in chapter 12. It applies equally well to chemical systems on other worlds.

Searching for evidence of water on other worlds is very straightforward. Searching for membranes is a much more complicated business. Let's go back to the case of Europa. Suppose we build the
Europa Clipper
, it flies through the geysers, and it brings samples back home. Even then, how would we know? How would we find cell membranes amidst the spray? One idea is to look for the kinds of atoms that we find in the membranes of cells on Earth. In terrestrial life, the characteristic elements in a membrane are carbon, nitrogen, potassium, and sodium. There's a place to start … unless Europan life came up with a totally different way to make a membrane. Then how would you find it? If you like this kind of thinking, consider becoming an astrobiologist.

The problem is somewhat easier for Mars, because it is much closer and because there is a more straightforward, Earthlike rocky surface to explore. In the coming years, we will continue to search for life there. If we could get the right instruments to some super salty slushy outcropping, we might find evidence of fossilized Martian microbes. We might even find something still alive, and observe it directly through a microscope.

Just think what it would mean if one of those distant bodies is spewing some heretofore-unknown type of life into deep space, or if life awaits sheltered under a rock on Mars. What if that life is like us? What if it's totally different? Whatever the answer, the discovery would change the way we all think about what it means to be a living thing. It will tell us about the different ways in which life can arise and evolve. It would give us, for the first time in history, concrete proof that we are not alone in the universe.

In my wilder moments of speculation, I like to go even further. I imagine that there might be something swimming in the sea under the ice of Europa—not just microbes, but big, complex organisms. If there really is a whole ecosystem in the Europan ocean, and it's been there long enough to have established something reminiscent of our Earthling multicellular sea creatures, I assume they'll be fishlike in shape. It seems to me it's even reasonable to expect any organism—fish, fowl, or fruit fly—to have its sensory organs concentrated in something like a head and its locomotion appendages somehow wired to its head, and so on.

In other words, I wouldn't be surprised to find alien creatures with body plans not too different from ours. I have no idea if I, or anyone, will get to see such a thing. But it sure would be a powerful test of our ideas about convergent evolution. What would we learn from an alien? It could be astonishing. We'd quickly find out if life necessarily needs a genetic code, a cell membrane, similar kinds of appendages, and familiar sorts of sensory organs. Nature may have possibilities we haven't even imagined. Or maybe life, like impact craters or volcanoes, tends to look pretty much the same everywhere it occurs.

These ideas would merely be arcane musings were it not for the fascination we all feel about understanding our origin. There is no more dramatic way to test, and extend, the limits of what we have learned about evolution than by searching for evidence that it has occurred on other worlds as well. The answer will drive new technologies. It will inspire future generations of scientists. And it may revolutionize both our practical and our philosophical understandings of what it means to be human.

 

35

THE SPARKS THAT STARTED IT ALL

In his expansive discussion of evolution in
On the Origin of Species
, Charles Darwin assiduously avoided contentious speculation about how the whole story began. His commentary is restricted to a single sentence near the end of the book's last chapter: “Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed.” But the question is irresistible. Where did we come from—what was the spark that lit life's fire? These days, many scientists are venturing where Darwin could not dare. Let's join them and go back to the beginning to talk about … the beginning.

Asking the big question sounds an awful lot like asking, “Is there a god who runs the show?” There is an essential difference, however. Every other aspect of life that was once attributed to divine intent is now elegantly and completely explained in the context of evolutionary science. For me, there is no reason to think that the origin of life is any different. I am open-minded, and have no problem with most religions, but religious explanations are unsatisfactory. They don't take me anywhere; you either believe them or you don't, and that's that. Scientific theories of the origin of life are open to questions, to tests, to revisions, to replacement with new and more insightful theories. One path leads to a dead halt. The other leads to thrilling, limitless forward motion.