To survey areas of the planet that remain unknown, researchers affiliated with the privately financed German Mars Society in Munich are proposing a different kind of explorer: a vehicle inspired more by dirigibles like the Hindenburg than by land-rover predecessors. Projected to reach Mars in 2009, the balloon craft, named Archimedes, would hover much closer to the planet's surface than a satellite, snapping crisp, full-color images similar to those that an Earth photographer might take from a helicopter.

In an hourlong descent to the ground, the balloon craft would use an array of sensors to take temperature, wind and humidity readings. These data, collected at a variety of altitudes, would provide the raw material scientists need to understand Martian weather patterns.
........................................................................................
A helium-filled balloon craft, he reasoned, could obtain a more comprehensive profile of the planet's surface and atmosphere than any other vehicle. Best of all, the project would be cheap enough — under $2 million — that he could pursue it with money from private donors instead of depending on government financing.

NY Times

Interesting concept, but I have a couple of questions: why helium and not hydrogen? No worries of explosions in Mars' CO2 atmosphere, and hydrogen would provide greater buoyancy.

Also, why just a descent then - stop? Seems to me it shouldn't be too hard to have a system that could stay aloft at a relatively constant altitude. Or is the atmosphere just too thin?

NASA has plans for a balloon that would float for 100 days or more:

Balloons have been flying for decades in Earth's stratosphere, which has an atmosphere as thin as that on the surface of Mars. Conventional stratospheric balloons have lifetimes limited to a few days because of the daily heating and cooling of the balloon. Helium superpressure balloons, currently under development for the Ultra Long Duration Balloon (ULDB), will fly more than 100 days and perhaps as long as a year. Smaller superpressure balloons carrying payloads of only a few kilograms have already flown for as long as a year.

But still using helium instead of hydrogen.