It’s a great time to be an astrobiologist! Things are hopping out there in alien-life land.
This is still a science without a subject, as the critics used to say. No samples. “No data,” as Stephen Jay Gould once said to me (not name dropping, just reporting the facts).
But look how far the field has come: Go back just a couple of decades and we had no solid evidence of liquid water anywhere in the universe other than on our own little water world. Astrobiologists tend to be water chauvinists and carbon chauvinists (because they’re essential to life as we know it and are abundant in the universe). In the past, news stories about the detection of water in space, on distant planets, on the moon, in the atmosphere of the sun, etc., often failed to note very prominently that the water was vaporous, or frozen, and that you couldn’t exactly dip a ladle in it, much less go fishing.
Now, however, we’re getting somewhere on the water front. Evidence for a Europa ocean is mounting. One planet over, the Cassini spacecraft has been exploring the Saturn system for a decade, and has seen not only plumes of water vapor coming from the small moon Enceladus but has also generated data suggesting that there’s a Lake Superior-sized sea below the icy crust and centered at the south pole. See my story posted yesterday.
The Enceladus Sea remains a matter of interpretation more than direct detection: The scientists are working with several lines of intriguing evidence, including the plumes, but nothing you could put into a test tube. Still, this is solid enough data that we can feel comfortable taking the obvious next step, which is to speculate freely, waving our arms and pounding the table about the likelihood that this sea has fish.
I talked to a number of astrobiologists about this in the last couple of days. Their opinions vary, as you would expect. A quick rundown of the roster:
Luciano Iess, professor of aerospace engineering, University of Sapienza, Rome, and lead author of new paper in Science: He is not boosterish about life on Enceladus even though, with that interface of liquid water and rock, it probably meets the basic definition of a “habitable” environment.
“If you have water in contact with ice, no chemical reactions occur. You have only a change of state. But when you have water in contact with silicates, with rocks, then a much richer chemistry may arise. You have chemical reactions. When you have chemical reactions in the presence of energy, this is an environment that from the point of view of habitability, and pre-biotic conditions, is much more interesting.”
But he added: “Certainly to go from liquid water in contact with rock to life is a huge step…. I really think there is no life on Enceladus. Let us be clear. This is irrelevant, because potentially, if you gave enough time we cannot exclude that life may occur…. It’s impossible to exclude it. Even if it’s very unlikely.”
David Stevenson, Caltech planetary scientist, co-author of the Science paper: “Anything you say about that is necessarily speculative…. We don’t even know what’s needed for life.”
Mary Voytek, NASA’s senior scientist for astrobiology: When I asked her if she thinks there’s life on Enceladus, she said, “I don’t know, but I certainly hope so.” She added, taking the broader view, “My personal opinion is there’s water and organics pretty much everywhere. The possibility for habitable environments to have arisen is strong throughout the universe.”
Chris Chyba, Princeton professor of astrophysical sciences: “It looks very likely that there’s liquid water under there, and it seems likely to me that that ocean is habitable in the sense that we could find some microorganism that could live in it. The other big question is the origin of life at depth absent the abundant free energy we get from the sun.”
Here’s a key point: Life beyond Earth potentially needs two things, not just one. It needs an environment, ideally, where life can originate. Secondly, it needs an environment that remains habitable. The origin-of-life issue can be circumvented if life is seeded from space — many people think Earthlife first originated on Mars and was blasted into space in an impact, eventually plunging to Earth via meteorite. But it’s hard to see how a subsurface Enceladus sea could be seeded.
Chyba said that if someone gave him an exact copy of the Earth from 4.2 billion years ago, he would not know if life was certain to originate or had a chance of 1-in-100 billion. Yes, we know it happened here, but could that have been extremely flukish?
“If that’s the case for the Earth, we’re not going to be able to make that kind of assessment for a radically different environment,” Chyba said.
Carol Cleland, University of Colorado professor of philosophy: “I think it’s clear that there are probably lots of Earth-like planets out there. We’ve found super-Earths, and our instruments are getting better and better, and we’re going to find regular Earths. … I think there’s going to be life like ours out there. … I think life formed so quickly on Earth that it’s just not something that’s rare. What’s interesting and what’s probably rare is multi-cellular organisms.”
Jack Szostak, professor of chemistry and chemical biology at Harvard (Nobel laureate): “I’m sure there could be some interesting chemistry, but I think making the kind of high-energy carbon-nitrogen compounds that are probably needed, that seems difficult to me. … I think you need more energy.”
“For decades people have been talking about life sort of forming in the primordial oceans. Eventually a lot of stuff gets washed into the sea. The problem with that is it’s really dilute. High-energy compounds get hydrolyzed. I think the best way to bring things together and concentrate them and provide environments for primitive cells is basically on the surface. Shallow ponds, things can evaporate and get concentrated that way.”
Darwin’s “warm little pond,” in other words.
Chris McKay, NASA astrobiologist: He said we shouldn’t focus on the origin of life issue, and quoted Wittgenstein. “That of which we know nothing we should pass over in silence. The origin of life we know nothing about. … I tend to focus on what’s required for life to survive and grow. I don’t know what’s required for life to start.” See my story for McKay’s comments on Enceladus having the goods for habitability (water, energy, carbon, nitrogen).
Andrew Knoll, professor of natural history at Harvard: “I would be very surprised if there were fish in Enceladus. … I think we need water that’s persistent. We need a source of energy, and that can be chemical energy. … And in addition to carbon you’re going to need some amount of phosphorous and nitrogen. You also need sulfur. You need iron. You need a number of other ions in low concentrations.”
I asked Knoll: What if life was weird life, totally different from life on Earth?
His answer: “Games without rules have many possible outcomes.”
As the boodlers know, I’ve written a book on the search for extraterrestrial life and tend to focus on a couple of important data points mentioned by some of the experts quoted above:
Life began very early on Earth, which has remained a habitable, and inhabited, planet for upwards of 3.5 billion years. But the Cambrian explosion was only about
640 540 million years ago. This leads one to assume, as Cleland does, that life is common but fish are rare. If you want to build a trilobite, or even a sea anemone, or a jellyfish, much less an octopus or a dolphin or a blue whale, you need more than just water, energy, carbon, nitrogen and the various other special ingredients for life as we know it. You need a lot of time. That, at least, is what you’d infer from the Earth’s own history, where life spent 3 billion years loitering in unicellularity (is that a word??) before deciding to bust out a multicellular move.
Sure, the rules could be different out there in space. But remember what Gould said.