The enormity of the recent discovery that Mars has liquid water on its surface is difficult to grasp. Until this discovery, scientists could only speculate about the existence of flowing water in our solar system and, for that matter, the rest of the universe.

Although water is a substance we largely take for granted, biologists know it is critically important for sustaining life, or at least life as we know it. And while life on earth seems to come in an infinite number of forms, it is all based on the same 20 amino acids that make up protein using water as its solvent. Scientists have understood for quite some time that the H2O molecule has special properties that make it singularly appropriate for life.

And this is why its discovery on Mars is so exciting. But before we get carried away, the reality is that any life there would probably be visible only through a microscope. In fact, the water recently discovered on Mars is said to run at temperatures as low as minus 23 degrees Celsius . This means it is probably extremely salty.

Some scientists suggest that such water would be so brackish it would preclude any form of life. But similar comments were made less than 100 years ago about the Dead Sea, which is eight times saltier than seawater. Then, in the 1930s, an Israeli scientist discovered microbial communities living there. As for the extreme cold, scientists have discovered single-celled algae in Antarctic ice floes. And life can also exist under conditions of tremendous heat and pressure, as proven by extremophile microbes near volcanic vents on ocean floors.

All of this suggests life itself, together with water, make a loving couple. It also suggests that life, once started, can adapt to extreme conditions. And there is evidence that Mars was once a much less hostile place, with bodies of water similar to those on earth. If life once existed on Mars, it had millions or even billions of years to get used to the drier and colder conditions that prevail now.