by Rod Pyle
But why Glenelg? According to Grotzinger, it represented “the union of three interesting geological sites.” The entire region was touched by an alluvial fan, and as such would give a variety of rocks and soils from regions above, with water alteration almost ensured. But within this, Glenelg appeared to be special. Eventually called “the Promised Land” by the science team, Glenelg contained what appeared to be layered bedrock, which would be a great place to begin investigations. There was a light-toned area with well-developed layering, which would likely turn out to be sediments, and therefore close to the heart of sedimentologists like Grotzinger. There were also darker, almost-black bands of something running through the area, which were only visible to the scientists once they had eyes on the ground. Adjoining this was a region with more cratering, which could represent older material, and the third type of terrain appeared to be much like the landing site. Once at Glenelg, they would have access to all three in one spot.
Grotzinger again: “We really do think it's going to be interesting. You can see a light-toned unit [of rock], which we believe to be material that from orbit has this signature, this property, of having relatively high thermal inertia. That's the ability of a material to retain its heat. So, late in the day or at night, orbiters are able to observe this rock well because it seems to be giving off heat late into the day and night. We don't know what the reason for that is, but it's always been a bit of a beacon for us and we're getting closer and closer to it.” High thermal inertia could mean cemented sediments, and soon Curiosity would examine the region for ground truth.
While preparing for the first drive, they wanted to check out one of their most valuable noncontact instruments. ChemCam would not have to touch rocks to understand what they were composed of, and it promised to revolutionize Mars exploration—as did so much of what Curiosity carried. Rather than drive up, inspect, and “sniff” every interesting target they saw, or merely wish that they could get closer to something too high up to reach, ChemCam would allow the science team to aim the laser at rocks up to twenty-five feet away, which is real time-saver in an environment with interesting targets all over the place. As you will recall, the built-in spectrometers would then read the laser-induced incandescence off the rock to determine its composition.
On August 19 came the first chance try it out. Targeting a 2.5-inch rock they called Coronation, ChemCam was powered up for its inaugural use. Over a ten-second time span, the powerful mast-mounted laser shot the rock thirty times with a brief but powerful one-million-watt blast of light. This generated enough heat to flash burn a tiny layer of the rock that the spectrometer was able to read successfully.
Coronation was nothing particularly special as Mars rocks go—the geologists had thought it was probably a little chunk of basalt, or lava-based rock. But it was handy and sported a sufficiently clean surface to use as a good first target. The resulting spectra showed that it was in fact a basalt and that the instrument was working perfectly.
Curiosity prepared to move out, but not before Coronation gained a bit of additional notoriety. As with Curiosity itself, Coronation was given its own short-lived Twitter feed. The tweets included some typically geeky humor, this one at the expense of Mars's largest volcano, Olympus Mons: “Did you know I was born in a volcano? Basalts like me come from lava. That's why we call it Olympus Mom.” Ouch. But all's fair in educational outreach, I suppose.
After initial tests—driving, turning, and backing up—the engineers and scientists carefully examined the imprints of Curiosity's wheels in the Martian soil. Besides giving the soil-dynamics people an idea of the consistency of the dirt and hardness of the surface, the tracks were also an indication that all six wheels were working properly. After a few more baby steps, it was time to depart.
On August 22, wheels rolled and Curiosity departed Bradbury Landing…although “departed” might be a bit of an overstatement. That makes it sound like the luggage was strapped on the trunk, the bride and groom jumped in, and off Curiosity drove, cans clattering off the back. It was not quite like that. At the slow speeds the rover drivers started with, it was more like defining an embarkation upon a world journey by getting out of your recliner. But this was the commissioning drive, and with a two-year mission at minimum (this is the “primary mission,” everyone hopes for much longer), there was no reason to rush. Including the mobility tests, they moved about twenty feet the first day.
The machine moved in apparent fits and starts for almost a week, testing and evaluating both itself and the surface it was driving on. With the long back-and-forth of information coming down, and commands returning to Mars, progress was slow at first. Nobody wanted to rush things—and for certain nobody wanted to make some kind of mistake now. They had survived the treacherous landing; an error now would simply be too embarrassing.
During this time Curiosity also had a couple of major media moments. First, a laudatory statement by the NASA administrator Charles Bolden was transmitted to the rover and played back to Earth. Then, in a savvy PR move, JPL arranged for the pop singer and STEM (science, technology, engineering, and mathematics) education advocate will.i.am to do the same thing with a song he composed titled “Reach for the Stars.” Kids loved it.
In the first week of September the rover was able to make solid progress toward the first destination. During breaks in driving, controllers tested both the CheMin and SAM instruments. These tests were conducted with both instruments empty, as no Martian soil had yet been sampled. In fact, the SAM test was performed on residual Earth air, more of which remained in the sample container than had been expected. But everything worked, and that was the important thing. Reading Earth air also gave the science team verifiable readings they could compare to ground tests.
While reaching Glenelg was the primary goal, no self-respecting geologist would give up an opportunity to stop and look at an object of interest. Said object popped into view on August 20, and the location looked like a good spot to calibrate some of the instrumentation as well.
“The science team has had an interest for some time now as we're driving across the plains to find a rock that looks like it's relatively uniform in composition to do some experiments between ChemCam, which we've been acquiring a lot of data with, and APXS, which we haven't used yet on a rock,” said Grotzinger. “Both of those instruments could make a measurement and there could be differences between the measurements because one is measuring at a small scale (ChemCam) and one is measuring at a larger scale (APXS). These rocks that we drive by on the plains here that look dark, they probably have basaltic composition. That's a familiar material to us,” and therefore a perfect baseline to calibrate the two instruments to one another.
The rock was about the size of a tissue box and roughly pyramidal in shape (don't get excited, conspiracy theorists…the resemblance is slight at best and it does not have the Eye of Providence—as seen on the US one-dollar bill—on the side). The science team named it Jake Matijevic in honor of a JPL engineer who died shortly after Curiosity landed. The attribution touched everyone involved with the program.
It took four days to complete the activities at Jake. To get a calibration between ChemCam and the APXS instrument readings, they used both those devices on the rock.
Grotzinger commented on the activity: “The hope is [that] we can analyze this rock and then do a cross comparison between the two instruments,” he said. “Not to mention it's just a cool-looking rock there, sitting out on the plains with almost pyramidal geometry, so that's kind of fun as well.” A little whimsy in science never hurts.
When the evaluations were done, both ChemCam and the APXS found surprising, and matching, data from the rock. It was unique among the Martian rocks seen to date: an igneous rock, high in the mineral feldspar and lower than expected in magnesium, iron, and nickel. The feldspar indicated that it had been formed in the presence of water, but this was a volcanic rock, and it was closer in composition to terrestrial igneous rocks than anything previously seen. T
his composition pointed to some ancient and large-scale process involving subsurface, high-pressure volcanic activity and water. These types of rocks are rare even on Earth, and when found here on this planet, they tend to be in oceanic island environments like Hawaii. It's an interesting mental exercise to extend those possibilities to Mars.
Over the next few sols they drove and drove, slow but steady. Then on sol 39, an object came into view that was a showstopper. It was named Hottah and was just under four hundred feet from Bradbury Landing.
Now, if you stumbled over this rocky formation on the way across, oh, say Death Valley (no, I am not obsessing about that awful place…I'm not, I'm not), as a civilian, you'd probably not think of it as anything other than a toe-stubber. A geologist would see it as an interesting indicator of water flow in the past. But on Mars? For the geologists, it was like coming across a rusted Chevy, albeit a bit more expected.
To be fair, they had seen something similar a couple of sols earlier that they named Link, but it was not nearly as exciting—or cool looking—as Hottah. And of course they were both named after geological sites of historical note in Canada. You can assume that naming convention from here on unless otherwise noted.
When I saw Grotzinger a day or two later at a media event, he looked (a) like he hadn't slept for some time (they were still in the thick of Mars Time) and (b) like the challenges to his circadian rhythms did not matter one bit. While as calm as usual on the outside, there was a sense of adrenaline coursing underneath the surface. “Hottah looks like someone jackhammered up a slab of city sidewalk, but it's really a tilted block of an ancient streambed,” he said in a September 27 press conference. “In some cases, when you do geology, a picture's worth a thousand words.” He was pretty damn happy.
That was an apt statement for something as cool looking as Hottah. While I have said it would look downright plain on Earth, even the geological laymen among us could see that this was spectacular. It was a broken and tilted slab made up of water-transported rocks, pebbles, and sand, ranging from dust-sized to as large as a golf ball, sticking up at an angle. These pebbles had been rounded off by long-distance movement—they looked like tiny river rocks, which they were. I'd seen a lot of sedimentary layers in various geology field trips in college, and this was an immediately familiar sight. The bits were far too large to be moved here and eroded by wind, this was deposition by water. It was a nice moment.
The science team's assessment was that this conglomerate layer—a bunch of sand and pebbles rolled along until smooth—had been formed by a rapidly flowing stream, perhaps knee- or waist-deep. While water had been inferred in many locations on Mars and sampled as ice near the poles, this was the first close-up observation of this kind—of the direct result of erosion and deposition by a flowing stream.
Gorgeous as it was, however, it was not the Grail: “A long-flowing stream can be a habitable environment,” Grotzinger said. “But it is not our top choice as an environment for preservation of organics. We're still going to Mount Sharp, but this is insurance that we have already found our first potentially habitable environment.”
Individual members of the science team would continue to pore over the images of Hottah, taken from a distance by Mastcam's telephoto lenses, but it was time to continue the trek. Suffice it to say that the results were interesting and that flowing water was a comparative slam dunk. There's better to come.
The next stop on Curiosity's tour of a once-wet Mars was named Rocknest. The area was made up of loose soil and appeared to be a prime target for the first sampling activity of the scoop on the robotic arm. The rover proceeded to a part of Rocknest they called Ripple, which was exactly that, a rippled bit of sandy soil apparently deposited by wind. It would provide nicely sifted fine grains.
But first things first. To get to the fresh stuff, Curiosity scuffed some of the dirt by spinning a wheel in it. Then the MAHLI instrument and the APXS were used to ascertain what exactly it would be grabbing. On October 7, sol 61, the scoop grabbed a small soil sample. This and a succeeding sample were used to cleanse the sampling mechanism—the scoop and collection chamber. By taking the sandy soil and moving it around—by moving the end of the arm and vibrating the chamber—it should scrape any earthly residues out of the sample-collection system. This was critical to getting “clean” readings from the incredibly sensitive instruments in the belly of Curiosity.
While this was being performed, someone noticed something in an image of the ground nearby the rover. There was a tiny bright spot. Bright areas had been seen before by the MER rovers, but nothing quite this well defined. What could reflect light like that? Mica does, and it's one of those water-indicating minerals, but nobody really thought it would be that. In any case, it was too peculiar to pass up. The presampling activity with the arm was halted while the science team looked closer.
Fig. 23.2. RIPPLE: At Rocknest the rover gathered its first soil samples at a site called Ripple. Before the rover dug into the soil, it was commanded to drive across some soft areas to assess the soil dynamics—much can be learned just from looking at the wheel tracks. Image from NASA/JPL-Caltech/MSSS.
After some head scratching, it became pretty clear that the half-inch-long item was probably a piece of plastic that had flaked or fallen off the rover (it looked like a toenail clipping to me, which would have been way cooler than the truth). That was a disappointment, but one gets used to these things. There had been a similar experience early in the Opportunity rover's life, when it spotted a small object that looked for all the world like a rabbit head, about two inches long. And by that I mean that it literally looked like the logo of the Playboy Bunny. The next time they imaged it, the thing had moved. Now that was interesting. A spectral analysis confirmed that it was a chunk of soft material from the rover that was light enough to be blown around. Not as sexy an answer as one might hope for, but the likeliest one. Same with Curiosity's bright item: just another bit of manmade Mars litter.
With the toenail mystery behind them, the “arm drivers” gathered a third and a fourth sample of Mars dirt. These samples were placed, one after the other, into CheMin. The rest of the fourth sample was used for more shake-and-scrub of the sample-collection device, as SAM was so sensitive that it was feared there might still be contamination to remove.
Also of note, there is a little tray on the front of the rover where the sampling mechanism can drop a bit of the material before it is sent onward to the instruments within. When soil is placed on this tray, it provides a clean metal background against which the MAHLI instrument can take a nice, controlled close look at some of the sample, allowing the scientists to decide what the next step should be.
The sample for CheMin needed to be about the size of only a baby aspirin. CheMin was able to receive more than one of these samples at a time due to the clever nature of its design—it has seventy-four separate containers that rotate into position on a wheel, fifty-three of which are able to accept samples. Each of these has clear, flat, glass sides that allow the high-energy x-rays to pass through and do their work. The resulting image, as described earlier, is a diffraction pattern that shows visible signatures of various minerals.
Within a sol, the results were in from CheMin: the soil sampled at Rocknest was similar to weathered basaltic soils (of volcanic origin) found in places like Hawaii. The remainder of the sample was made up by materials like simple glass. You could not pick a less Hawaii-like place to find this soil, except perhaps Jupiter or the sun, but there you have it.
As David Bish, the coinvestigator on CheMin said in a NASA news release, “Much of Mars is covered with dust, and we had an incomplete understanding of its mineralogy.” He added, “We now know it is mineralogically similar to basaltic material…which was not unexpected. Roughly half the soil is noncrystalline material, such as volcanic glass or products from weathering of the glass.”
So what about the ongoing quest for water? “So far, the materials Curiosity has analyzed are consistent with our init
ial ideas of the deposits in Gale Crater,” Bish continued, “recording a transition through time from a wet to [a] dry environment. The ancient rocks, such as the conglomerates, suggest flowing water, while the minerals in the younger soil are consistent with limited interaction with water.” This sounds like a lot of water way back when and a lot less in the last billion or so years…which matches hypotheses generated by other investigations by the MER rovers and the orbiters.
Soon the SAM instrument stepped up to the plate. While it had not yet ingested any soil samples, it did grab a sample of ambient atmosphere. The SAM team members soon realized that they had, at last, some real data on what the heck might have happened to Mars's atmosphere so long ago that helped to turn the planet from a wet Eden to the Sahara. The concentration of heavier isotopes of elements of carbon, as compared to that found in ancient rocks, demonstrated that the lighter versions of carbon isotopes had, over time, drifted away from the planet. This and similar results later in the mission will hopefully be validated once NASA's newest Mars orbiter, MAVEN (Mars Evolution and Volatile Atmosphere), arrives late in 2014. MAVEN will try to track the present-day loss of lighter elements from the Martian atmosphere in an effort to project backward and understand what happened in prior epochs.
Finally, in early November, the SAM instrument was able to taste Martian soil. Some of the leftover CheMin sample (remember that they are very small) was fed to the waiting SAM instrument.
Another benefit of the rover's design was its ability to hang onto recently grabbed samples even as it moved off to other sites. Not only could some soil be kept in the scoop for a while but the sample chambers in the instruments could be rotated back into the active area of the machine more than once. It would take weeks, but more data would come from the Rocknest samples.
