It always comes as a surprise for my students (and for most other people) to learn that we have found water on the Sun and on the Moon and on most other planets in the solar system and in nebulae (giant gas clouds) outside of the solar system.
But how could the Sun, with a surface temperature of close to 6,000°C, have any water? And haven't we always been told that the Moon is totally dry, and that is why there is no life to be found there? To make sense of this, one must recall that "water" does not necessarily mean liquid water, it simply means H2O. This could be found in the form of vapour (as on the Sun) or ice (in polar craters on the Moon and in many other places of the solar system).
And when we say "we have found water", we do not mean that it is contained in seas or oceans, except perhaps on one of Jupiter's moons (Europa). Water is often found in tiny amounts but water is ubiquitous. In fact, for astrophysicists, there is no surprise to finding water so easily. Indeed, H2O is a very easy molecule for nature to make since oxygen has a strong "affinity" for hydrogen, and they are two of the most abundant elements in the universe. But what we are really looking for is liquid water since this is the key for the existence, or even the emergence, of life.
We have yet to find a way, theoretically or experimentally, for complex molecules, which make up the essential bricks of any living organism, to assemble, organise and metabolise without liquid water. This is because water is an exceptional medium. It is a unique conductor and regulator of heat, a universal solvent, its molecules can form weak bonds with other molecules, very important for the stability of molecules critical to life.
More practically, for the space travellers that we are, or at least intend to be, finding water on the Moon or on planets such as Mars may mean that we can one day use that for drinking (after filtering) or breathing (after converting it to O2) and as a fuel, turning these places into refuelling ports. Indeed, we should recall that rocket fuel consists of liquid hydrogen and liquid oxygen, which get mixed during the propulsion process to produce energy.
There are other places in our solar system where we have found water: Mercury, Venus, Mars, Jupiter and its moon Europa, Saturn and its moon Titan, Uranus and its satellites, Neptune and its moon Triton, and Pluto and its moon Charon, which consists mostly of ice. Interestingly, Mars, although mostly too cold for liquid-water seas to exist, does have regions and seasons when the temperature goes above zero. Moreover, it has been shown that with the planet's very low atmospheric pressure, water could exist in liquid form - although in small amounts - even at temperatures below zero Celsius.
In June 2000, Nasa's Mars Global Surveyor found puzzling gullies and gaps in the Martian surface perhaps from water seeping through from underground, estimated to be 100 to 400 metres below the surface. Those spots show up on hundreds of different images from more than 120 locations on Mars. Needless to say, such discoveries have profound implications for the possibility of finding past or present life on Mars.
Europa and Titan also deserve special attention for their aqueous environments. Europa is very cold (minus 160°C) and it is entirely covered with a smooth layer of ice, but images taken by spacecrafts over the past 15 years have shown gigantic ice rafts. Scientists have concluded that the moon's internal heat must be melting part of the ice under the surface, producing an underground ocean, similar to those seen on Earth's polar seas. Beneath its icy crust, Europa could harbour the solar system's largest ocean.
The mere possibility of oceans of water on Europa raises even more the fascinating prospect of extraterrestrial life, whether in the form of simple microbes, bacteria, algae, or even fish. Here on Earth, undersea environments near volcanoes and hydrothermal vents have been found to be rich in living organisms. As one researcher has put it: "Can an ocean of liquid water persist for 4.5 billion years and not have life?"
Nidhal Guessoum is a professor of physics and astronomy at the American University of Sharjah