Summary: This five-part debate will cover a variety of topics prompted by the hypothesis of "Rare Earth," a book by Peter Ward and Donald Brownlee that suggests complex life may be unique to Earth. The focus on microbial life continues today in Part 2 as the moderator asks where we can expect to find life in our solar system and beyond.
This five-part debate will cover a variety of topics prompted by the hypothesis of "Rare Earth," a book by Peter Ward and Donald Brownlee that suggests complex life may be unique to Earth.
On Monday, Part 1 wrangled with the question of whether life could originate and exist anywhere except on Earth. The general consensus was that simple (microbial) life, at least, may be common in the universe. The focus on microbial life continues today in Part 2 as the moderator asks where we can expect to find life in our solar system and beyond. The moderator is Michael Meyer, the NASA senior scientist for astrobiology.
Michael Meyer: If there is life out there -- either microbial or complex -- where can we expect to find it?
Peter Ward: Life might have originated on Mars and Europa early in the solar system’s history (and may live there still). Many of us think that, at best, we’ll find evidence that life once existed on Mars and may or may not have started on Europa.
My guess is that the Earth is the only place in the solar system where there is existent life -- but we might expect to find a rich fossil record of extinct life on Mars.
Of all planets beyond the Earth, Mars is by far the best known. It has been poked, prodded, examined and measured by a variety of Earth- and space-borne instruments, including those many that have successfully and unsuccessfully either landed or crashed on the surface of the red planet. An enormous amount of information now suggests that early in its history, while our Earth was still a chaotic and uninhabitable world of magma oceans and unceasing asteroidal impacts, Mars may have been a benign world, of equable temperatures and almost planet-spanning oceans. It may, as well, have been a world with an atmosphere that included oxygen.
All of these factors lead to an inescapable conclusion – that the early Martian conditions would have been favorable for the development of life. Some scientists have even suggested that life arose on Mars, and then was transported to Earth.
For several hundred million years or more these benign conditions may have lasted, and in that time span evolution could have worked wonders. Perhaps the first geologists sampling Martian sedimentary rocks older than 4 billion years in age will find not only the fossil remains of bacteria, but also the remains of more complex organisms.
Perhaps the fossils of animals will be found.
What would that scene be like: the swing of a rock hammer against a Martian outcrop, splitting a piece of ancient Martian shale, and the heart-stopping joy of finding a mollusk look-alike or the bones of a fish-equivalent? Yet even if life did attain such a rapid rise in complexity on Mars, it did not last, for Mars as an environment for life died early.
Even as bacteria on Earth were readying for the rush to higher grades of life, Mars was dying or was already long dead – assuming that life originated there at all. On Mars, the oceans seeped back into the planet or were lost to space, the oxygen in the atmosphere bound itself to rocks, and life died out.
David Grinspoon: I agree with the belief that Mars is currently lifeless. My impression that Mars today is dead is derived from the stale atmosphere (no signs of biological disequilibrium yet discerned) and the lack of internally driven geological activity. I think that to support a biosphere over billions of years, a planet needs more than isolated pockets of water.
Don't get me wrong -- I am a big proponent of Mars exploration. No matter what we find there, we will learn a lot. And if Mars is lifeless, this gets us off the hook because there won’t be any difficult ethical choices about human activities there. But all opinions about life elsewhere are just that. We need to go and look.
Donald Brownlee: If no evidence for life is found on Mars, then the formation of life probably is neither easy nor common in the solar system. We already have seriously negative results from asteroidal meteorites.
There are now over 30,000 asteroidal meteorites in captivity, and none of them show compelling evidence of alien life. Many of these rocks came from bodies that were much richer in water, carbon, and nitrogen than Earth, and many had warm and wet interiors that lasted for millions of years. Life apparently did not form in the asteroids. Presumably this is because asteroids did not have the right environments even though they did have the right building materials.
Creation of life apparently needs a richer diversity of disequilibria than can be found inside wet organic-rich interiors of asteroids. Probably what is needed is something akin to environments that occurred on early Earth and hopefully other planets as well.
David Grinspoon: We need to keep an open mind for possible bio-signs in unexpected places as we explore the entire solar system and beyond. If we relax our (understandable) attachment to "life as we know it," other intriguing possibilities become worthy of our consideration.
For a planet to foster the origin of life and maintain the necessary conditions, I believe that the most important requirement is a planet with continuous and vigorous geological activity over billions of years. Watery conditions are needed for our kind of life, but any chemical environment where complexity can flourish might do, and we don't know enough about planets and about chemical evolution to place good limits on these environments.
Although my hunch is that currently Mars is lifeless, I am still holding out for Venus: nice conditions in the clouds, energetic flows, strange UV absorbing pigments, unexplained particle populations, etc., if you don't mind a little acid. Europa, and possibly Titan or Io, also may harbor life.
[Titan is a moon of Saturn; Io is moon of Jupiter.]
Frank Drake: In places like Io and Titan, we may find the first evidence of other biochemistries that are beyond our powers of prediction. I am a little on the pessimistic side with regards to Io -- it has no substantial atmosphere.
But Titan! Wow! A prodigious organic chemical factory, some kind of solvent, even an atmosphere. It sounds better than primitive Earth. Sure, it is very cold there, but chemistry still happens easily if more slowly at Titanian temperatures. Could it be that one creature's arctic clime is another creature's balmy tropical island?
Don Brownlee: My prediction is that the nearest alien neighbors live in feces and food scrap left on the Moon by the six Apollo missions. Even though it’s been three decades, there is a good chance that hearty bacteria live and can reproduce inside encapsulated small damp places and survive the monthly cycles of heat and cold as well as the effects of solar flares, ultraviolet light, and hard vacuum.
If born-on-the-Moon organisms are not living in food scraps (and worse) there are probably dormant terrestrial organisms trapped inside vast numbers of components -- wire harnesses and tape interfaces that are parts of the lunar lander, back packs, surface experiments, rover, etc. Somewhere out there is Allan Shepard’s unsterilized golf ball, which is likely to carry a small zoo of terrestrial microorganisms. Beyond our Moon, my great hope is that microbial life or at least fossil evidence for its prior existence will be found on Mars, Europa, or some other solar system body.
If we find life elsewhere in our solar system, and show that it is not a distant cousin of terrestrial life, this will greatly support the idea that formation of life is easy and commonplace, given the right environmental conditions.