Re: Is the water on planets other than Earth freshwater?

Mercury shouldn't have any water on it, but radar images suggest that there may be some near its poles. If so, then this water must be trapped inside craters which do not ever see any sunlight - the water may have been outgassed by Mercury as it cooled as a young planet, or it may have been brough to Mercury by comets. It also is probably mixed with a lot of dirt, and may even be buried by a layer of dust. It would be ice, though, and not liquid.

Venus doesn't really have any water left, because of its proximity to the sun and its large carbon dioxide atmosphere (makes it hot enough to melt lead!). Any water would have long ago been in the atmosphere, and eventually been destroyed by UV which takes apart the molecule. Then, the hydrogen would have escaped from Venus, leaving only the oxygen, so the water could not re-form.

Even our own Moon has ice on its polar caps. This surprising discovery was found by Clementine probe, and only survives as ice in places where there is always shadow. If the ice were to be exposed to sunlight, it would immediately sublimate into water vapor (it wouldn't be liquid first) and escape the Moon's low gravity. This isn't particularly "fresh water" since it isn't liquid, and it probably has a lot of dirt mixed in with the ice. (You can read about the discovery at http://nssdc.gsfc.nasa.gov/planetary/ice/ice_moon.html)

Moving out further, we know that Mars has some water on it - we can see water vapor clouds, even though there is not much water in the Martian atmosphere. Further, it seems as though Mars may have had a lot of liquid water in the past. If so, the water must have gone somewhere. Some of it probably was lost to space due to Mars' low gravity, but because it is larger than the Moon this wouldn't happen as quickly. It's thought that much of the water on Mars is frozen into the crust, sort of like the water table on Earth, if you froze it. It's possible that near the equator, Mars may have some liquid water deep below the surface. If so, it will likely have a lot of salts in it which would help keep it liquid. However, we can't be sure whether there is liquid water beneath the surface (though there is some compelling evidence http://antwrp.gsfc.nasa.gov/apod/ap030205.html), there is also controversy about that evidence (http://science.nasa.gov/headlines/y2003/19feb_snow.htm)), or how much salt and other things would be present in the water if it does exist.

Moving out further, the Jovian planets are mostly gas - any water in their atmosphere is a gas, and hence "fresh" vs. "salt" is not really a relevant question. But, we can take a look at their moons to see what might be there. If we take the moons of Jupiter as an example, we know that Ganymede and Callisto both have water ice in their crust. (It is possible that Ganymede has a salty ocean underneath)

Europa proves an interesting satellite to look at. This one has a crust that is pretty much all water ice. It's possible that the tides (because it's so close to Jupiter) warm up Europa enough that there could be a liquid ocean underneath the exterior frozen crust. Even if that's not the case, there are very likely pockets of liquid water within the interior of Europa. The Galileo probe (http://www.jpl.nasa.gov/galileo/) found that the ice on Europa had a good amount of salt, and so these pockets would also be "saltwater". Of course, the possibility of liquid water in either an ocean or "pockets" is why Europa is a candidate for other life in our solar system.

The moons of the other Jovian planets don't really show anything other than water ice, although Titan (a moon of Saturn) is of course interesting in that it is near the triple point (where solid, liquid and gas can coexist) of methane, much like the Earth is located near the triple point of water. This is one of the biggest reasons for the Cassini-Huygens probe (http://saturn.jpl.nasa.gov/index.cfm) to investigate Titan when it arrives in July of 2004.

There is still a lot to learn about water in our solar system, where it resides, and what state it is in. Every time we send a probe to a new place, we discover something new and unexpected!