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(CNN) -- A spacecraft orbiting Mars has captured images of retreating and advancing polar frost clinging to circular craters, dramatically documenting changing seasons on the red planet.

NASA released the images this week, snapped recently by the Mars Global Surveyor, a space agency probe that has taken surface pictures since it began orbiting the red planet in 1997.

Spring has taken hold in the north, where frost layers have been retreating since May. And autumn has arrived in the south, where creeping patches of ice herald a six-month martian winter.

The top two images are from northern latitudes. The craters are Lomonosov on the left and an unnamed one on the right. The latter has a patch of ice on the floor that will likely persist through the summer, NASA said. The bottom two images are of craters in the south, Barnard on the left and Lowell on the right.

(CNN) -- A European Space Agency robot ship has become the first to fly over the south pole of the Sun twice, ESA said this week. The nearest star has changed considerably since the Ulysses probe first visited six years ago.

Then the sun was settling into a long period of quiet known as solar minimum. Now the sun's 11-year activity cycle is peaking, powered by turbulent magnetic fields below the surface.

By comparing measurements made over the solar pole in 1994 with the new data, scientists hope to learn more about the complexities of the sun.

"The sun is the only star we can study at close quarters," said Ulysses project scientist Richard Marsden in a statement. "We need to get to know it in all its moods."

In early 2001, the probe will swing back down to the ecliptic plane on its return to high northern latitudes in October.

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