Water is not unique to Earth, and though it's not abundant in our solar system, it's certainly not the rarest compound either. In the past few years, space-borne radar observations have hinted that water may exist, in a frozen state, in some rather unlikely places in our solar system, such as in the sun-shielded craters in the polar regions of Mercury and our moon. Within the last year, some rather solid evidence has recently been revealed that gives more credence to the idea that liquid water was at one time present on the surface of Mars. About six months ago, gullies and rills were imaged, from the Mars Global Surveyor, that were likely carved by running water, perhaps within the past 1,000 years.

In addition, just this week, new observations released from the Mars Global Surveyor provide evidence that Mars had lakes and maybe even seas of liquid water more than 3 billion years ago. The high resolution images show sandstone-like cliffs that appear to consist of fine grained material in a number of horizontal layers. On Earth, this would be a sure sign that the rocks are sedimentary; they formed from sediment deposited by flowing water. Different layers means that water ebbed and flowed over millions of years. This is pretty exciting since life as we know it needs water to exist and flourish.

However, there's no signature of liquid on Mars today. Some water is locked up in the north polar cap of Mars (most of the Martian polar caps are frozen carbon dioxide), and a minute amount of water vapor (< 1%) is present in the Martian atmosphere, especially when the ice caps sublimate during the Martian spring. It's thought that volcanic activity in the vicinity of the polar caps could have melted some of the ice, resulting in liquid water for a brief time before it either froze or evaporated. It has been theorized that if Mars did at one time have large bodies of water, a weakness in a natural dam that confined a lake or sea could have resulted in catastrophic flooding, scouring large portions of the Martian landscape. Additionally, the presence of groundwater has been invoked to explain some erosional features of the Martian surface. Nevertheless, without a volcanic eruption or some sort of event where Mars' interior heat is transferred to the surface, both the temperatures and atmospheric pressures found on the surface of Mars today are too low for water to exist as a liquid.

If water isn't in a liquid state on Mars, where in the solar system would conditions exist that would allow it to be fluid. In general, the outer gaseous planets are too cold, and well, gassy. Liquid water wouldn't stand a chance on Mercury or Venus. But what about the moons of the other planets? In most cases, if the body doesn't have an atmosphere, you can forget about water in the liquid phase. Neptune's moon, Triton, has a trace of an atmosphere, but no water vapor has been found. Saturn's largest moon, and the second largest satellite in the solar system, Titan, has a substantial atmosphere consisting mostly of nitrogen, and ice is present on its surface. However, because Titan, at times, is further from the Sun than is Pluto, there's little chance that liquid water ever flowed on it surface. The only one of Jupiter's moons that likely has even a trace atmosphere is Io (sulfur compound from volcanic activity), and no atmospheric gases have been identified on either Callisto, Ganymede, or Europa.

Europa is the 6th biggest moon in the solar system, having a diameter of approximately 2,000 miles, which is about 175 smaller than our moon's diameter - the 5th biggest satellite. Of Jupiter's four largest moons, Europa is the second densest but the least massive.

Even though Europa has no atmosphere, it has an icy surface that's cracked and fractured - it looks like a white billiard ball. Surprisingly, though, Europa could very well have a liquid ocean beneath its frozen exterior. How can a body that has no atmosphere, and likely never did, support an ocean? The answer actually has more to do with Jupiter than with Europa. The surface temperature of Europa is about -260 degrees F - about 200 degrees to cold for liquid water. But Europa's liquid ocean is not found at the surface, but rather about 5 miles below, and it may be 6 miles deep, based on measurements from the Galileo spacecraft made earlier this year, as it cruised to within about 200 miles of Europa's surface. Heat to keep the water liquid is probably generated by Jupiter's very strong gravity. Massive Jupiter tugging on the much less massive Europa over the eons, stretches and compresses it, and the resulting tidal flexing can give off a huge amount of thermal energy - enough to keep water liquid if its protected from the freezing cold surface temperatures.

Interestingly, the presence of a liquid ocean has been deduced from data obtained by a magnetometer (used to measure the strength and direction of magnetic fields) on the Galileo spacecraft. Jupiter's powerful magnetic field is strong enough to change Europa's magnetic orientation about every 5 1/2 hours. A drastic change such as this could be expected to drive electrical currents through a liquid ocean. Said in a different way, if a conductor, such a liquid water, is placed in a time-varying magnetic field, electric currents are induced that in turn create a secondary magnetic field, detectable with a sensitive magnetometer. This is the principle that medal detectors use to identify medal objects. The changes predicted by an inductive model, if a current-carrying fluid exists, were identified during the Galileo fly-by, and since ice isn't a good enough conductor to produce the detected signal, it's surmised that the most logical explanation is that there's an ocean of liquid water underneath the thick surface ice. But don't wax down your surf board just yet. Though tantalizing, these finding can't be verified until another mapping mission, equipped with specialized sensors, confirms Galileo's results, and the next such mission won't be launched for at least 6 more years.

For more about this see Sky and Astronomy - December 2000 issue and the Science for August 25, 2000).

07 December 2000