Instruments aboard a European satellite orbiting Mars have confirmed the presence of a vast expanse of water ice at the planet’s south pole, overlaid in a small area by a veneer of frozen carbon dioxide, scientists reported Wednesday.

The small amount of carbon dioxide found by the Mars Express satellite eliminates one possible answer to the question of whether Mars ever had sufficient CO2 to foster an atmospheric “greenhouse effect” strong enough to have warmed the planet so liquid water could have formed on the surface — and possibly supported life.

What atmosphere Mars now has, about .6 percent as much as Earth, is mostly carbon dioxide, but not enough for a warming effect, and the Mars Express findings show that the poles are not a “carbon sink” holding dry ice that may once have been gas blanketing the planet.

“Obviously the CO2 reservoir at the South Pole is not a major CO2 reservoir,” said astrophysicist Jean-Pierre Bibring, of France’s Institut d’Astrophysique Spaciale, who led the Mars Express research team. “We’ll have to find another reservoir, if there is one.”

The European Space Agency’s Mars Express entered Mars orbit on Christmas Day, eight days before the first of two NASA rovers, Spirit and Opportunity, landed on the planet.

Early this month NASA announced that Opportunity, drilling and photographing a rock outcrop near its landing site, had found both minerals and geological features common in Earth rocks that have either been leached by groundwater or formed by sediment in ancient lakes or ponds.

A few days later — and 6,600 miles away in an ancient lake bed — Spirit drilled into a volcanic rock with fissures filled with material that may have crystallized from water, NASA announced.

The European Space Agency reported in January that Mars Express had discovered the first direct evidence of water ice at the south pole, confirming analyses and indirect observations extending back to 1984.

Bibring said Mars Express’ OMEGA instrument determined the composition of the polar ice cap by analyzing the light glinting off it at the end of Mars’ southern summer, when only permafrost remained. Results of their findings are being reported today in the journal Nature.