NASA / JPL-Caltech / MSSS

A close-up of the turret on the end of the Curiosity rover's robotic arm shows the Alpha Particle X-Ray Spectrometer front and center on Sol 32 of the mission (Sept. 7-8). The picture was taken by Curiosity's Mastcam imaging system and shows Martian soil in the background.

NASA's Curiosity rover has rolled out its X-ray analyzer and picked up its first elemental signatures of Martian material — in the form of unexpected traces of sulfur and chlorine detected on a calibration target from Earth.

"These are our first Martian solids data," said the University of Guelph's Ralf Gellert, principal investigator for the Alpha Particle X-Ray Spectrometer, or APXS.

The accidental debut of the APXS comes amid the monthlong Mars mission's transition from engineering tests to full-on science observations.

Just a day's worth of robotic-arm commissioning remains, mission manager Jennifer Trosper told reporters today during a teleconference. The characterization period has lasted just one day longer than the minimum timetable. That's "not bad," Trosper said, with intentional understatement.

"After characterization is over, the engineering team doesn't let go of the keys completely, but we kind of loosen our grip," she said.

The $2.5 billion rover mission will rely on 10 instrument suites to unravel Martian mysteries over the next two Earth years — with a big question at the top of the list: Did ancient Mars have the chemical constituents to support life? To delve into that mystery, Curiosity has two onboard laboratories that will eventually sample soil, ground-up rock and even the atmosphere — but the APXS, which is mounted at the end of Curiosity's robotic arm, will also play a role.

The APXS is designed to shoot X-rays at a target rock, and then read the reflected energy to determine which elements are present. That information can help scientists decide which rocks merit further examination with other instruments.

A piece of New Mexico basalt was mounted on the rover before its launch last November, to serve as a calibration target for the detector. The first reading was taken on Monday, which was the mission's 35th Martian day, or "sol." Gellert told reporters that most of the detected elements were in line with what was found before Curiosity's cruise to Mars, but there were extra peaks for sodium and chloride. Those apparently reflected the composition of "tiny grains" of Martian sand that were thrown onto the target, perhaps during the Aug. 5 landing, Gellert said.

The detector also picked up the presence of argon in the Martian atmosphere. The sensitivity of the readings showed that "the instrument really works perfectly," Gellert said.

NASA / JPL-Caltech / MSSS

A sample of basaltic rock from a lava flow in New Mexico serves as a calibration target on the front of the Curiosity rover. This picture of the 1.4-inch-wide (3.5-centimeter-wide) target was taken by Curiosity's Mars Hand Lens Imager on Sept. 9.

NASA / JPL-Caltech / Univ. of Guelph

This graphic shows data obtained by the APXS instrument from its calibration target. The peaks indicate the abundance of various elements, including argon (Ar) from the Martian atmosphere, zirconium (Zr) from the instrument itself, and sulfur and chlorine (S and Cl) from flecks of Martian soil.

NASA / JPL-Caltech / MSSS

An image captured by Curiosity's Mars Hand Lens Imager from a distance of 8 inches (20 centimeters) shows the open inlet where powdered rock and soil will be funneled down for analysis in the rover's CheMin laboratory. The entrance of the funnel is about 1.4 inches (3.5 centimeters) wide and covered wtih a mesh screen for filtering purposes.

NASA / JPL-Caltech / MSSS

The MAHLI camera took this picture of its calibration target, which includes a 1909 penny. MAHLI's principal investigator, Ken Edgett, called attention to flecks of Martian material just under Abraham Lincoln's ear and beneath the first "9" in the 1909 date.

Bits of Martian material were also seen in pictures captured by the rover's Mars Hand Lens Imager, or MAHLI. The principal investigator for that camera experiment, Ken Edgett of Malin Space Science Systems, pointed out two flecks that were visible on a one-cent coin mounted onto the rover as a calibration target. One bit was 200 microns in size, while the other was 100 microns. (In comparison, a human hair is roughly 100 microns wide.)

"Our first MAHLI close-ups of sand on Mars actually came from the penny that we sent," Edgett said.

Watching a Martian mini-eclipse
Future observational campaigns won't be so accidental: Sometime today, Curiosity's high-resolution Mastcam camera is scheduled to look up at the sun and try to catch the transit of one of Mars' moons, Phobos, across the solar disk. There are a couple of additional opportunities over the next few days to make such transit observations, but after that, it'll be another Earth year before the moons transit again. Curiosity's predecessors, the Spirit and Opportunity rovers, made similar observations of mini-eclipses as seen from the Martian surface more than once.

Once the engineering tests are finished, Curiosity is due to "drive, drive, drive" toward its first major destination, a geologically interesting area that's been nicknamed Glenelg, Trosper said. That spot is where three different types of Martian geology come together, and studying the area could provide insights into how the planet has changed over the course of billions of years.

Getting to Glenelg could take weeks: It's located roughly a quarter-mile (400 meters) from Curiosity's landing site, and the rover has been traveling a maximum of 30 to 40 meters per day. Trosper said the mission's science team wants to stop along the way to give Curiosity's instruments a good workout with a real Martian rock.

'Contact science' ahead
"We will drive until the science team finds that rock, and then we will stop and position the rover to do contact science with APXS and MAHLI," she said.

Joy Crisp, the mission's deputy project scientist, said the target would probably be a big chunk of fine-grained basalt, the most common type of rock on Mars. "That's likely to be what we find that will suit our needs for that first use of contact science," Crisp said. "That rock will have to be big enough to push on it with the arm."

Crisp said that scientists weren't quite ready to "stick our necks out" and discuss the composition of the gravelly soil seen under the rover's wheels. That would have to wait until "probably next week," she said. Eventually, the science team will get a precise reading for soil composition, by scooping some of it up and dropping it into the SAM (Sample Analysis at Mars) and CheMin (Chemistry and Mineralogy) laboratories on the rover. Later, the rover will bring out its drilling equipment and do some serious probing of Martian rock. But that'll take a while, Trosper said.

"We need to do scooping activity in a sandy area," she said. "It's on the order of a months-type time frame for drilling."

It's good to know that Curiosity's instruments are ready to go. Check out this animated GIF image of the SAM laboratory's access doors being opened and closed on the rover's deck while the Navcam imaging system watches:

Update for 5 p.m. ET: NASA's Guy Webster emailed a few additional details on the plan to watch a Martian mini-eclipse: "The observation plan includes about 300 Mastcam exposures during the course of the transit.  They might all be received as thumbnails, but probably only a small subset as full-frame images, due to downlink priorities." Let's hope Curiosity isn't clouded out. (Just kidding...) 

More about Mars:

• Mars rover takes a peek at what lies beneath
• Rover ready for rocks after a month on Mars
• Gallery: 11 amazing things the rover can do
• Curiosity snaps its own profile picture
• Cosmic Log archive for the Mars mission

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.