Mars

When we break things down to the simplest pieces, our body is made of particles: electrons, and quarks that are inextricably tied up in protons and neutrons. The protons and electrons are held together by nuclear and electromagnetic forces to form atoms. Atoms can be classified into elements, based on how many protons and electrons they have. The lightest elements, hydrogen, helium, and lithium formed in the Big Bang. The heavier ones formed in denser places, the cores of stars. That much of us was once part of a star in one sense is astonishing to think. However, once you get past that, you realize that almost every element on Earth, and on other planets, was once part of a star.

What makes life remarkable is that the elements that make it up have organized themselves into molecules. And not just any molecules. Even relatively complex molecules, like the 10 atom molecule acetone, have been found in interstellar space. The building blocks of our life are much more complex molecules, proteins, containing thousands of atoms.

Two pieces of chemistry are fundamental to the processes by which proteins might form and evolve. First, the backbone of all complex, biological molecules is carbon. Carbon has four electrons that are free to initiate bonds with anywhere between two and four other atoms at any given time. This allows carbon to form the complex molecules that are the building blocks of life. Carbon is common throughout the universe, so it does not limit where life might form. (The only other common element with four electrons that can participate in forming molecules is silicon. However, is less likely for two electrons in silicon to form a bond with one atom, making it less suitable for forming complex molecules.)

On Earth, the other key chemical ingredient is water. Water is unusual in that it is an asymmetric molecule, and the oxygen atom and the two hydrogen atoms are slightly oppositely charged. This allows water to form weak bonds with itself and with other molecules. The bonds that form between water molecules allow water to remains a liquid at temperatures and pressures when most other molecules are either gases or solids. The weak bonds that water can form with other molecules allows those molecules to be dissolved in water. Once dissolved, those molecules can interact with other molecules. In many cases those molecules will simply trade atoms, but in some cases, more complex molecules will form. It is thought that the building blocks of life --- first amino acids, then proteins, then self-replicating molecules like RNA --- formed in the water that covered the early Earth.

The chances of forming complex, self-replicating molecules by sticking random elements together are low. The question is, How small (or, for the optimist, how large) are those chances? That is not something that can be calculated with a computer, as of yet. It would be easy to calculate if the molecules in a simple cell had to all end up at the right place at the right time as a fluke of pure chance. However, this is almost certainly not what happened.

Instead, we believe that all of the random changes were shaped by natural selection to produce more viable self-replicating molecules, catalytic reactions, and eventually cells. Natural selection works as follows. All of the changes to a self-replicating molecule that stop it from reproducing will disappear. All of the changes that are benign will be kept, but they will only be propagated if luck is on their side. However, if a change is made that helps the self-replicating molecule reproduce more effectively, that change will start to take over because the new molecule has an advantage over all the other ones. Therefore, although beneficial changes are rare, the fact that they are beneficial means that they will usually survive, and eventually be added to with further beneficial changes. This interplay between random chance and the natural selection makes evolution work. However, this is also makes it hard to compute probabilities that life would evolve. Currently, it would take as long to model evolution on a computer as it would to wait for it to happen --- probably at least a billion years. Therefore, we turn to the natural world to gain insight into how often life evolves, and ask the related question, How likely is life to evolve on other planets?

Right now, we have one example of a planet on which life has evolved, Earth. This one planet tells us very little about how often life is formed, because if life hadn't formed here, we wouldn't be here to notice that there was no life. Life on Earth only tells us that life formed at least once. We need another example, and that is where Mars (and potentially some of the moons around Jupiter and Saturn) comes in. The above image shows the surface of one of our nearest planetary neighbors, Mars, with a gully that appears similar to ones on Earth that have been carved by water. This is just one of many images of Mars that have been taken over the last 50 years that suggest that Mars might once have looked much like Earth. Although we no longer believe that Mars has civilizations like those on Earth, it is nonetheless thought that when the Sun was younger and hotter, the conditions on Mars might have been allowed life to form. If Mars has life, it would tell us that life is likely to form wherever simple a condition is met --- that the planet has liquid water.

As a result, scientists are determined to discover whether Mars has conditions that are habitable to any of the life that we have found on Earth. All life on Earth requires liquid water, so that is the big thing we are looking for on Mars. NASA's Mars Polar Lander has found evidence for water ice just under the surface of the Martian poles, which suggests that liquid water might exist elsewhere on the planet (and that it almost certainly did in the past). If Mars does have water, the next question is, Did life manage to evolve? None of our missions to Mars so far have been equipped well enough to answer that question beyond a shadow of a doubt. However, such missions are being planned.

Of course, there is a chance that life could form on other ways, in a sea of hydrocarbons instead of one made of water (hence the interest in Saturn's moon Titan), or even out of molecules based on Silicon instead of Carbon (although Silicon seems to be unable to form molecules as complex as Carbon can).