A study of martian rocks shows underground water persisted for prolonged periods in the first billion years of the planet's existence, European scientists say.

The European Space Agency's Mars Express orbiter has been studying rocks blasted out of impact craters, revealing material that once lay hidden beneath the surface.

The Mars Express and NASA's Mars Reconnaissance Orbiter focused on craters in the Red Planet's ancient southern highlands, called Tyrrhena Terra, to learn more about the history of water in this region, a release from ESA's Paris headquarters reported Wednesday.

Focusing on the chemistry of rocks embedded in the crater walls, rims and central uplifts, scientists say they've identified 175 sites bearing minerals formed in the presence of water.

"The composition of the rocks is such that underground water must have been present here for a long period of time in order to have altered their chemistry," lead study author Damien Loizeau said.

While the material brought to the surface by impacts appears to have been in close contact with water, there is little evidence for rocks on the surface having been altered by water, scientists said.

"Water circulation occurred several kilometers deep in the crust some 3.7 billion years ago, before the majority of craters formed in this region," study co-author Nicolas Mangold said.

Impact craters are natural windows into the history of a planet's surface, and the deeper the crater the further back in time its history is revealed, scientists said.

"The role of liquid water on Mars is of great importance for its habitability and this study using Mars Express describes a very large zone where groundwater was present for a long time," Olivier Witasse, ESA Mars Express project scientist, said. (c) UPI