Our mostly dry planetary neighbors once had lots of water -- what does that imply for us?

We already knew about Venus. We had our suspicions about Mars. Now we’re sure.

Our two closest solar system neighbors once had oceans — planet-encircling, globe-girdling, Earth-like oceans. But waterbearing planets are fragile. Venus didn’t have the right stuff and lost her oceans to space. We have the smoking gun. And now we know that Mars, also, poor Mars, couldn’t hold on. Mars has lost to space at least 80 percent of all the water it once had.

Et tu, Earth? What about you? More to the point, what about us? Despite water’s apparent abundance, what does the future hold for the most precious material on our planet? Will we find a way to mistreat our reserve of irreplaceable water and turn our planet into a planetary desert, like our neighbors Venus and Mars? Kick the temperature up a few more notches, thanks to a runaway greenhouse effect, and the ultimate consequence of global warming could be ejecting the water from our planet.

Water on the atomic level
Let’s try our hand at interplanetary forensics. First, let me introduce you to the atomic constituents of that substance chemists call H2O, which most of us more commonly know as water. The H represents the atom hydrogen. The O represents the atom oxygen. The number two after the letter H tells us that a single molecule of water is composed of two hydrogen atoms and one oxygen atom.

In order to enter the world of CSI: Solar System, we need to understand the structure of atoms in a bit more detail. Hydrogen is hydrogen because its nucleus has one positively charged proton, which is orbited by one negatively charged electron. The nucleus, however, can also include one neutron, which lacks a charge. Even with one neutron, the atom still has a positive charge in the nucleus of +1. It’s therefore still hydrogen, but with one critical difference: it is much heavier, about twice as heavy, in fact, thanks to the additional neutron.

Chemists call this kind of heavy hydrogen deuterium. Deuterium behaves identically in chemical reactions to regular hydrogen; it’s just heavier. Remember that H2O molecule? When made with a deuterium atom, it’s an HDO molecule. It would taste the same, and it would provide the same sustenance to your flowers and gerbils, but it would weigh more.