This is a continuation of a series of posts. Each is designed to be understandable without reading the others; they do build off each other, however, so check them out starting with Part I. In the last post on this topic, I hope I got across how little we know of life’s beginnings. This planet we live on hosts the only life we know of thus far…in the whole universe! Does that mean we are most likely alone? Not really. This and the next post will look at the possibilities for life elsewhere.
It must be said that our ignorance of what the universe offers in terms of life beyond our solar system is profound. Actually, I should not be so negative, because over the past decade or so we have found out much more about our universe’s capability to host life than we knew before. This new knowledge has come in the form of the discovery of planets orbiting other star systems, along with the discovery of microscopic life in places we never thought could host life.
There is something I should mention before going further. Scientists who study this sort of thing, called astrobiologists, do not expect to find any living thing out there that is visible without the aid of a microscope. That’s because the conditions that are favorable for microscopic life (bacteria, etc.) are much broader in scope than those favorable for multi-cellular (big) life. In addition, more complex life requires a much longer period of time before it can take hold, as opposed to the shorter time required for simple one-celled life to evolve. But one thing is for certain: once we find microscopic life anywhere outside Earth, the stage is set for discovering much more complex forms.
Where We’ve Been
Humans have been probing the planets we share this sun with for a number of years now. The first exploration outside of Earth orbit happened in 1962, when Mariner 2 flew close to the thick, acid-laced cloud-tops of Venus. Then came the Apollo era, when the United States sent astronauts to the Moon and Russia managed to land a few craft on hostile Venus. We also probed Mercury with a later Mariner mission.
The twin Voyager probes, launched in the late 1970s, were our first foray to the outer planets. Voyager 1 showed us incredibly detailed close-up views of the gas giants Jupiter and Saturn. Voyager 2 flew by Uranus and Neptune as well (still the only time we’ve visited those distant planets). I recall seeing those first images of Jupiter’s clouds and Saturn’s rings in detail. I was in High School, and the details were was truly blown away. And I wasn’t the only one.
The Voyager probes are now the furthest from home that anything made by humans has ever traveled, and they win this honor by a mile! Both are well over 100 times as far from the Earth as we are from the Sun. They are also traveling by far faster than anything humans have ever built. Soon Voyager 1 will arrive at the Heliopause, which is the spherical boundary around our Sun where the solar wind ceases to be the major influence, and instead galactic forces take hold. In other words, our little travelers are about to leave our star’s neighborhood and forge a path out to the great beyond, the Milky Way Galaxy.
The U.S. has been the de facto explorer of the Moon and Mars, along with the outer planets, while the Russians picked the most hostile rocky planet on which to land a probe – Venus. The European Space Agency has also been active more recently, designing the probe that landed on Saturn’s moon Titan. While Mars garners the most attention, with its rovers and orbiting observatory, two U.S. probes are busy elsewhere. Messenger is finishing up its mission at Mercury and Cassini continues to orbit Saturn and its zoo of moons.
Life on Mars?
In the 1970s we landed for the first time on Mars. The lander was called Viking. Along with incredible photography, the non-mobile lander dug up a small sample of soil and analyzed it for life. Although it looked initially like the results might turn out positive, the soil was found to be completely hostile to life. Since then, a series of Martian orbiters and rovers have found abundant evidence of water on Mars. The only problem? This water appears to have last flowed billions of years ago. (Intriguingly, there is some evidence of periodic eruptions of liquid water from below ground, even recently.) We have yet to find fossil evidence of past life on Mars, and the planet’s current condition appears to be as hostile to life as we ever thought.
Life Beyond Mars: It’s the Moons
Since both Venus and Mercury are much too hot to hold liquid water, even in the distant past, the next good place (beyond Mars) to look for life are the many moons of Jupiter and Saturn. Europa, one of Jupiter’s moons, has been known for years to have a liquid ocean beneath a thick ice cap. The moon is heated by enormous tidal forces created by the nearby gas giant, so this probably means that hot vents discharge into the sea. On Earth, undersea hot springs host entire ecoysystems, so it stands to reason that Europa could hide similarly-powered concentrations of life clustered around hot vents beneath its ice cap. Also, Enceladus, a moon of Saturn that also has a subsurface ocean, represents an excellent habitat for life.
Although other moons around Jupiter and Saturn are thought to contain liquid or partly liquid interiors, arguably the most intriguing place to search for life is Titan. Titan, orbiting Saturn, is the Solar System’s second largest moon. It’s larger than Mercury and not much smaller than Mars. There is a lot of methane on Titan, much of it liquid because of the frigid temperatures. In fact, methane on Titan might serve the same role as water does on Earth. We have observed features like river valleys and lakes on Titan, but instead of being filled with water they are filled with liquid methane.
Why is this interesting for life? Because prior to the emergence of oxygen, Earth was a planet rich in methane as well. Early life on Earth relied on methane not oxygen, and in fact, these organisms are still around. There is an enormous community of micro-organisms (methanogens) living just beneath the sea floor today. In fact the methane they produce has been stored in ice formations that could, because of global warming, erupt and release into the atmosphere, greatly accelerating global warming. Many scientists think Titan could be revealed to operate much as Earth did billions of years ago, with microscopic life very similar to those early Earth days.
The fact is, although we have made great strides in understanding how likely it is to find life within this solar system of ours, we are just now scratching the surface. Life on Mars has by no means been ruled out, and the moons of the outer solar system are just now being examined. It will take a very sophisticated effort to look for life in Europa’s subterranean ocean, or across the huge and distant moon Titan, or on Enceladus. But even the discovery of a community of extremophiles (micro-organisms adapted to extreme environments) on one of these relatively nearby bodies would be a watershed moment. It would tell us that we are not alone in our neighborhood, and that life has likely gotten started in countless locations across the universe.
Next up: the incredibly diverse zoo of planets orbiting other stars.