Mars Global Surveyor passed a milestone earlier this month when its 100,000th image was added to NASA's online image gallery for the mission. The total number of photographs is now more than twice the combined quantity supplied by the Viking orbiters in the 1970s.

More interesting is the detail included in the new images. One is said to be among the most detailed views ever provided of the Red Planet. Another reveals new clues about a mysterious "Inca City" on Mars.

In 1972, Mariner 9 returned images that showed ridges on Mars that intersect and appear to rise from layered material near the south pole. The puzzling features were dubbed Inca City, and their origin has never been understood.

Scientists thought they were sand dunes, possibly old ones that had been buried, hardened, and then exposed again. Other researchers figured they might be dikes of molten rock.

A new image from the Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) reveal the ridges to be part of a larger circular structure that's about 53 miles (86 kilometers) across.

"It is possible that this pattern reflects an origin related to an ancient, eroded meteor impact crater that was filled-in, buried, then partially exhumed," according to scientists at Malin Space Science Systems, which operates the camera for NASA. "In this case, the ridges might be the remains of filled-in fractures in the bedrock into which the crater formed, or filled-in cracks within the material that filled the crater. Or both explanations could be wrong."

Mars loves a good mystery.

Puzzling Martian landscapes are fun to explore. But all scientific eyes cast on Mars in recent years have had one major purpose: looking for water.

Two new images provides some of the best detail ever of gullies that could represent water flowing on the Martian surface -- not yesterday, but perhaps recently in geological terms.

The photos show gullies on the walls of two craters thought to have been carved out long ago by asteroids or comets. Both craters sit in the in the Newton Basin in the Sirenum Terra region of Mars. The pictures have been colorized using colors obtained from other MGS photos.

One photo is among the highest-resolution ever obtained from Mars, according to Malin scientists. It resolves the surface to 5 feet (1.5 meters) per pixel.

Objects the size of school buses can be seen in the full size image, the scientists say, though there is no evidence for any such transportation devices on the planet.

Interestingly, all of the gullies in the crater originate at one specific layer. They may have been formed by the release of groundwater from that layer sometime in the past, say the proponents of the water-on-Mars idea.

The other picture tells a similar story, but it also shows wintertime frost on the crater wall, and dark-toned sand dunes on the floor.

Mars Global Surveyor, launched Nov. 7, 1996, has been orbiting Mars since Sept. 12, 1997. The mission has studied the entire Martian surface, atmosphere, and interior. A total of 112,218 images had been officially archived by NASA as of earlier this month.

Scientists say MGS has returned more data about the Mars than all other missions combined. However, other missions -- now and in the future -- will surely generate discoveries that go beyond what MGS has been capable of finding.

Ironically, after all the groundwork laid by MGS (and the Viking missions) showing water might once have flowed on the Red Planet, it was NASAs Odyssey orbiter, just a few weeks after arriving, that found the first solid evidence for vast amounts of water ice near the surface of present-day Mars.

In 2004, a pair of rovers is due to arrive at the Red Planet. Their onboard geologists tools have the potential to answer important questions about the composition of the surface. And if there are strong signs of past life, scientists say the rovers might even have a chance of noticing.