Quick, small and innovative: That’s the kind of mission NASA is seeking, and that’s what the Mars Scout program is all about. NASA’s past missions to Mars–Mariner, Viking, Pathfinder–were mostly large-scale exploratory ventures, with sometimes a decade or more separating launches. Mars Scout missions, scheduled to begin in 2007, can be launched much more frequently and cheaply. Best of all, the program allows small groups of scientists to propose very specific missions on subjects that address cutting-edge questions in their fields. For the first Scout launch, more than 25 proposals were submitted. Two years later, the competition is down to these four. The final decision will be made this summer.


The most conservative–and potentially cheapest–Scout proposal, called Phoenix, would recycle instruments that were intended for the 2001 Surveyor Lander but not used. It would also make use of the 2001 Odyssey Lander, which has been tested but never launched. Using a robotic arm, Phoenix would dig a trench and retrieve soil samples, analyzing them for signs of microbial life. It would also study near-surface ice and water vapor in the region.


SCIM (Sample Collection for Investigation of Mars) would be the first craft to make a 1.75-billion-mile round-trip flight from Earth to Mars. Two round solar panels would power its instruments (left). After folding its panels, SCIM would enter the planet’s atmosphere (below), pass through one of Mars’ huge dust storms and gobble up some 1,000 cubic centimeters of dust, gas and other samples. Scientists drool over the prospect of working directly with such samples–the abil-ity to deliver them to Earth
is SCIM’s chief advantage.


Many orbiters have surveyed Mars from afar, but ARES (Aerial Regional-scale Environmental Survey) would take a much closer look. In the thin Martian atmosphere, its spring-loaded wings would automatically unfold from its fuselage, allowing it to glide just 1 mile above the surface. Video cameras, a spectrometer and a magnetometer would take measurements of water vapor and gases. The magnetometer would also search for Mars’ most ancient rock (older than any on Earth), which, unlike much of the planet, is
magnetically charged.


Among the best places to look for life on Mars: anywhere there’s volcanic activity. Mars has many volcanoes, including Apollinaris Patera (inset). Marvel (Mars Volcanic Emission and Life) will search for an active one that might heat an underground hydrothermal system–a warm water environment that could harbor life. The craft’s two spectrometers would be so sensitive, says principal investigator Mark Allen, that “if you had just three cows anywhere on Mars, we would be able to detect the amount of methane they added to the atmosphere.”