Mars has an atmosphere, although thin... so why haven't we taken advantage of that fact and sent reconnaissance vehicles that can't get stuck in petty sand dunes?

some advantages:
1. they would be solar powered (same as the rovers)
2. constant flying would prevent dust from collecting on solar panels (which i heard was a big problem for the rovers for a while)
3. they would not have to contend with land obstacals
4. they would probly be cheaper than satelites

some disadvantages
1. a much more complex guidance system than current land rovers (would have to stay on the sunny side of mars)
2. higher possiblity of failure
3. no sun = crash.... burn.... (because carrying batteries is too heavy, atleast the ones big enough to sustain flight)
4. part wear/tear is probly greater

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yes, as soon as I posted I thought of utilizing a blimp, glad to see someone else thought of it too... but yeah, an airborne craft wouldn't be able to analyze rocks as well, but it would be much cheaper to produce than rovers...

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Like this?

NASA Ares



Space.com New Mars Eagle Has Flown, And Landed, in Oregon Test



NASA Quest: Planetary Flight



Planetary Society: Mars Balloon Flies Again

NASA: Balloons on Mars

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I read somewhere about using a nuclear pebble-bed reactor that (I believe) would use CO2 as propellant. If I find the links to this I'll post them here, but it's been awhile.

I've always been into airplanes, and I like the idea of an airborne Mars explorer, but I think a Mars UAV is going to be a lot of trouble.

First off, Mars is so far off that human piloting is out of the question. The craft will need a lightweight, robust and flexible AI that can handle pretty much the whole show. UAV aircraft, like the Predator and Global Hawk, have limited autopilot provisions, but still require a lot of human direction, and they're right here on Earth.

Also, keep in mind that the thinner the atmosphere, the harder it will be to operate an aircraft. The top of Everest is about one-third that at sea level and it's next to impossible to get a helicopter up there. The pressure on mars is one one-hundredth that at sea level, and even with the lower gravity, this is still very harsh. Simply flying straight and level will be a trial.

On top of this, the Mars aircraft will need to be VTOL, and will have to operate out of harsh terrain. As stated before, a traditional helicopter will likely not work. A Harrier-style system springs to mind, but the Harrier is less than ideal. It operates using a high-powered jet that expends plenty of fuel, and even then has to carry a limited amount of reaction mass (in the form of water) to boost its vertical thrust. It can only hover for around thirty seconds before the reaction mass runs out and it crashes. The failure rates for all kinds of VTOL are awful even here on earth, and they're operating on runways and out of open clearings at 100% atmospheric pressure, with trained and experienced human beings at the controls.

Once aloft, your craft will need huge wings and a lot of power, and even then you'll have to make the craft incredibly lightweight, restricting what you can bring along. Your aircraft and autopilot will also have to deal with Mars' incredible planetary-scale dust storms. Earth's relatively placid storms stymy even the best of human pilots.

All of this will also require a very large amount of amazingly efficient, lightweight, and long-lasting power. The Sojurner rover travelled at a maximum speed of 5 centimeters per second--and it was covered in solar panels.

A mars blimp would solve a lot of these problems. However, it would also have the same weight restrictions, and it would be even more sensitive to dust storms and rough terrain--even the smallest hole would ruin your project entirely.

I think we ought to just use rovers, myself...