Washington, Aug 11 (IANS): NASA's Curiosity rover has sent "the most clearly visible" images of Martian clouds that resemble Earth's ice-crystal cirrus clouds.

Clouds moving in the Martian sky have been observed previously by Curiosity and other missions on the surface of Mars, including NASA's Phoenix Mars Lander in the Martian arctic nine years ago, but new images of the clouds are the the most clearly visible so far from Curiosity, NASA said.

Curiosity landed five years ago this month about five degrees south of Mars's equator.

"It is likely that the clouds are composed of crystals of water ice that condense out onto dust grains where it is cold in the atmosphere," said Curiosity science-team member John Moores of York University, Toronto, Canada.

"The wisps are created as those crystals fall and evaporate in patterns known as 'fall streaks' or 'mare's tails'. While the rover does not have a way to ascertain the altitude of these clouds, on Earth such clouds form at high altitude," Moores said.

Researchers used Curiosity's Navigation Camera (Navcam) to take two sets of eight images of the sky on an early Martian morning in July.

For one set, the camera pointed nearly straight up. For the other, it pointed just above the southern horizon.

Cloud movement was recorded in both and was made easier to see by image enhancement.

A midday look at the sky with the same camera the same day showed no clouds, NASA said.