Scientists have long believed the planet's caps contain frozen water, but those findings stemmed from indirect evidence, such as analyzing temperature data or detecting traces of hydrogen, according to the Associated Press.
European scientists used infrared images captured by the orbiter Express, which is circling Mars' south pole, to detect vapors of water molecules.
"You look at the picture, look at the fingerprint and say this is water ice," Allen Moorehouse, the project's manager of spacecraft operations, told the AP. "This is the first time it's been detected on the ground. This is the first direct confirmation."
Findings from the National Aeronautics and Space Administration's Mars Global Surveyor in 2001 convinced experts that the polar caps contained water ice close to the surface of Mars through photographic images. Express made the first detection of a chemical signature of water ice.
Although Mars appears to contain a significant amount of water ice below the surface, there has been no evidence of liquid water, which scientists say is essential for life. NASA's rovers were sent to areas that scientists suspect may have once contained liquid water.
Express will reach its final orbit on Jan. 28, though instruments have been turned on since Jan. 5, according to the European Space Agency. The orbiter released a rover, Beagle 2, which has not been heard from since its scheduled landing on Mars on Dec. 25.
NASA's Spirit rover ran into its own trouble this week when it failed to send data to mission scientists for two days. NASA said the rover briefly resumed transmitting "limited data" to Earth Friday morning, giving scientists hope that Spirit's problems can be analyzed and fixed.