NASA's one-ton Curiosity rover celebrates its first anniversary on Mars this week.

In just one Earth-year of its scheduled two-year mission, the roving geochemistry lab has helped assess the red planet's habitability, while serving as a guide for potential future missions. (See: "Mars Gets Its Close-Up.")

Since its harrowing touchdown on Mars a year ago in Gale crater on August 5, 2012, PDT (August 6, 2012, EDT), Curiosity has been one busy explorer—returning more than 190 gigabits of data and collecting and analyzing samples from two rocks over terrain roughly a mile (1.6 kilometers) in size.

Hundreds of scientists and engineers have been working on the billion-dollar mission; among them is Ralf Gellert, the lead scientist for the X-ray chemical sensor aboard Curiosity. (Related: "Curiosity Finds Evidence for a Habitable Ancient Mars.")

"Working on a Mars rover is very different from doing science experiments in the lab on Earth," he said. "[We've been] coping with all the data and trying to optimize and streamline operations."

Where the Lander Landed

Curiosity has spent nearly six months at a region known as Yellowknife Bay. The area's rocky outcrops have provided opportunities for drilling, sampling, and analyzing the chemistry of the powdered samples taken from the Mars surface. While the rover is not equipped to detect any signs of life itself, it has offered up clear evidence of a past environment suitable to support microbial life. (Watch video of Curiosity's "Seven Minutes of Terror.")

"None of us dared to dream of such a smooth landing and incredible performance from our scientific payload," said Ashwin Vasavada, deputy project scientist for the rover mission. "The landing site also turned out to be much richer scientifically than we had expected."

The rover is currently in the midst of a year-long journey toward the mission's primary target—the base of Mount Sharp, some five miles (eight kilometers) away.

Mount Sharp is a three-mile-high (five-kilometer-high) mountain rising up from the floor of the crater. It caught the eye of Mars researchers years ago, thanks to close-up photos taken by orbiting spacecraft. Curiosity will now investigate its exposed lower layers, which appear to be sedimentary and may have formed under wet conditions.

"Even from a few miles away, it is a gorgeous and huge mountain, with canyons and buttes carved by wind and water. It's so tantalizingly Earthlike; we can't wait to get there," said Vasavada. (Watch video of the Mars rover Curiosity.)

Here now is a rundown of the top five discoveries in the Curiosity's first year on Mars.

1. Discovery of an ancient, habitable environment. At Yellowknife Bay, the rover found evidence that an intermittent lake was once present, with fresh water and other chemical ingredients that life requires.

The finding of clay minerals, says Gellert, means almost neutral—neither acidic nor basic—water flowed in the area at some point in Martian history.

"[A big surprise] was to find clays near the landing site in an area that doesn't show signs of clay minerals from orbit," said Gellert.

2. Evidence of an ancient streambed. Curiosity found a rock called a conglomerate—made up of many rounded, small pebbles cemented together. This type of rock forms on Earth when water flows about knee deep.

"These show that for an extended time a stream must have been flowing to form these features," said Gellert. (Related: "Meet One of Mars Rover Curiosity's Earthbound Twins.")

3. Radiation measurements to aid human exploration. Curiosity measured the natural cosmic and solar radiation on its journey from Earth to Mars.

"By measuring the radiation from within a spacecraft capsule, Curiosity was in the same environment that human explorers to Mars will face," said Vasavada.

Based on some of the initial findings from the rover's instruments, it turns out that the radiation arriving from both interstellar space and the sun will pose a significant challenge for future Mars astronauts. In just the nine-month voyage to the red planet, Curiosity absorbed more radiation than NASA astronauts are allowed to be exposed to over their entire career. (Related: "Psychological Challenges of Manned Mission to Mars.")

4. Lack of methane in Mars' atmosphere. In the last few years, both Earth-based telescopes and Mars orbiters have seemed to detect methane on Mars—a gas that could be a result of present-day biological activity or signify other non-biological processes.

But Curiosity's experiments have so far detected very little methane within Gale crater—in fact, some 600 times less than the amount in Earth's atmosphere.

So while it appears that the conditions on ancient Mars may have been conducive to life at one point, today's environment may just be too harsh for life to exist.

5. Diversity of rocks in Gale crater. Before even getting to the ultimate destination of Mount Sharp, scientists were surprised that Curiosity found diverse types of rocks—from volcanic to mudstones to rocks with cracks filled with mineral veins. Each of these rock types tell the story of a different environment at some point in Mars' past.

Scientists expect this rich diversity to increase even more when Curiosity gets to the multilayered Mount Sharp.

"With one habitable environment in the bag, our job becomes to place it in the context of the other geology within Gale crater in order to understand where it fits into the overall evolution of this region of Mars, and maybe even the planet as a whole," said Vasavad.

"Ultimately, we'd love to make a fundamental contribution to the understanding of how Mars' climate changed dramatically in its early history."