Curiosity Page 10
Recall that this is by far the largest and heaviest American robotic explorer to land on another world. The dimensions of the overall MSL spacecraft, in its flight configuration, is often compared to the Apollo lunar-landing program: it is fifteen feet in width, and its heat shield is larger than Apollo's. As for the rover, it weighs four times as much as the Apollo lunar-roving vehicle. The other popular comparison is that it is roughly equivalent in size to a Mini Cooper. And so forth. In short, it's the biggest, baddest boy yet on Mars.
All these comparisons vanish when you approach Curiosity in person—at that point it is just a huge, complex, and impressive machine. There are three versions that civilians like myself can see at JPL: One that is very close to the real item resides in the lobby of one of the main buildings on campus. Another two live up in the Mars Yard, the test track JPL uses to work out the kinks in an environment as close to Mars as they can get on-site. One of those, the Scarecrow, is the stripped-down, lightweight version that is tested in places like Death Valley. But stand next to any of them, and one thing pops out at you immediately: Curiosity is big. The top deck reaches to the middle of my chest. The camera mast towers over my head.
And then there are the wheels. Each of the six is the size of a small beer keg, both in diameter and in width. Of course, they weigh a tiny fraction of a keg—they are exquisitely machined to incredible tolerances.
Everything about Curiosity dwarfs previous rovers.
Its scientific capabilities are a match for its physical size. Not since the Vikings has anything this well equipped landed on Mars, and it blows that 1970s vintage machine into the weeds with its sophistication.
It's hard to encompass all the capabilities of MSL—the topic could fill another book. But let's take a look at what's under the hood. There are ten instruments, with a total mass of about 165 pounds, or fifteen times that of the MER rovers. What follows is an introductory inventory so we all know what's coming.
Fig. 10.1. CURIOSITY'S LOAD: The one-ton rover is packed with instrumentation, as indicated on this chart: the DAN and RAD instruments are located on top and on the side of the rover; ChemCam, Mastcam, and REMS are on the mast; CheMin and SAM are inside the main chassis; and the MAHLI, APXS, rock brushes, drill, and scoop are all stationed on the robotic arm. Image from NASA/JPL-Caltech.
So, on board Curiosity are the following:
CAMERAS
Most apparent is the camera mast. Both MER rovers had them, but this one is, of course, larger and more capable than what has gone before. There are two sets of traditional cameras on the mast, one with wide-angle and the other with telephoto lens systems. Early on, a zoom lens was proposed and development was begun, but it was just too difficult to pull off with the kind of reliability that was needed, so they settled for a twin fixed-lens system. The resolution of these cameras is stunning, making previous devices look like early cell-phone cameras. Both cameras have electronic memory built in, in the form of an eight-gigabyte buffer. This allows them to store over 5,500 raw images. The cameras can shoot more images at higher resolution and more quickly than their predecessors. They can also shoot 720p high-definition video footage at ten frames per second. It's not exactly cinematic speeds (a minimum of twenty-four frames per second), but it's far beyond previous efforts. Remember also that reliability is generally preferred over bells and whistles, so the fact that it has any HD-video capability is a huge accomplishment. Finally, these articulated on the mast to allow for full 360-degree pans.
There are also a number of lesser cameras on board: a total of twelve Navcams (for navigation) and Hazcams (for hazard avoidance) are mounted at all four corners of the rover. While they are generally used in far lower resolutions than the Mastcam, they have HD-capable chips inside as well.
Then there is the MAHLI instrument—the Mars Hand Lens Imager. It's called a “hand lens” imager because it is distantly related to the little magnifiers that geologists use on Earth to look more closely at rocks in the field. The MAHLI is mounted out on the robotic arm and can be placed close to anything that device can reach. At 1,600 by 1,200 pixels, it too is an HD imager. Next to the lens are some LEDs (light-emitting diodes) to illuminate rocks that are shaded or to operate at night. One set of LEDs provides regular white light and another provides UV light, which makes some minerals fluoresce. The mechanical focus mechanism can go from millimeters (almost touching the rock) to infinity, so it can also image the horizon if needed. Calibration targets are mounted nearby to check focus and color balance, including a 1909 Lincoln penny that is used for critical focus checks (the vintage penny was a favorite of one of the developing scientists).
The MARDI, Mars Descent Imager, is mounted to the bottom front of the rover. It had a short life—it was used only while the spacecraft was descending toward the Martian surface. The images it acquired—MARDI also utilizes a 1,600 by 1,200 pixel imager—would show the motions of Curiosity as it descended from the upper atmosphere, as well as give a progressively closer view of the landing site and its terrain as the rover neared touchdown. The camera was turned on shortly before the heat shield was popped off. It should be noted that funds for this device were “descoped,” or trimmed, by NASA during the development process. But Mike Malin, the owner of the company that builds many of the Mars cameras, whom we met in Death Valley, contributed his own finds to finish the project. That takes the term commitment to a whole new level.
Fig. 10.2. EYES OF CURIOSITY: A self-portrait of the top of the camera mast taken by the MAHLI camera on the robotic arm. The large optic in the top box is ChemCam, the smaller, square lens sets below are Malin's Mastcams. Image from NASA/JPL-Caltech.
The final camera is called ChemCam. This is easily the most exotic of the cameras, at least in terms of how it functions. They are all amazing devices, but it is how this one goes about taking pictures that astounds. It is also mounted on the camera mast, but in the housing beside the camera is a powerful laser. In a twist on science fiction, NASA took a death ray to Mars. The device fires laser blasts powerful enough to vaporize rock, in packets of fifty or so nanosecond bursts, burning a small trail of holes in the target rock each time it is used. The telescopic camera next door sees the resultant light flash and converts that to spectral readouts, which tells scientists what the rock is composed of. Its accuracy is impressive, and it can be used from a distance of up to about twenty-five feet. The Chem in its name refers to the fact that it can discern the chemical constituents of the rocks it vaporizes. ChemCam was codeveloped by the United States and France.
Incidentally, also on the mast is the Rover Environmental Monitoring Station, or REMS. It measures various aspects of Martian weather, including humidity, temperature, air pressure, wind speed, and ultraviolet radiation. This was the only component damaged during the landing, when one of two wind-speed detectors was smacked by a small rock upon touchdown. But it still provides valuable data with a bit more effort on the part of the ground controllers. The instrument was provided by Spain and Finland.
ROBOTIC ARM
The next most obvious component is the large robotic arm below the camera mast and on the front of the rover. It is far larger than previous rover arms (no surprise here), and this size increase was required to accommodate the multitude of instrumentation it would carry. At the end of the arm is a large turret, about the size of the top of a barstool, that rotates the various instruments into position for use. The arm can extend up to seven feet, and the turret alone weighs 75 pounds—three times as much as the entire Sojourner rover from 1997.
The turret sports a variety of tools, including the aforementioned MAHLI instrument. In addition to this are:
The Alpha Particle X-ray Spectrometer (APXS): This instrument, contributed to the mission by the Canadian Space Agency (the same folks who brought you the Canadarm on the space shuttle and International Space Station), has flown in one form or another on every NASA Mars rover to date. When held near a rock, it bombards it with high-energy particles, then reads
the signal that results from that irradiation. From this signal, the spectrometer can ascertain what elements are in the target. Among other things, it can tell whether or not the rock was ever exposed to weathering, water, and the like.
Scoop and Brush: For gathering rock and soil samples, there is a scoop on the turret. Nearby is a wire brush called the Dust Removal Tool or DRT (dirt, get it?), that can clean a rock prior to investigation by other instruments.
Drill: Perhaps the most remarkable part of the turret's instrumentation in terms of never having been done before is the drill. It's called the Powder Acquisition Drill System or PADS, and it works in concert with the Collection and Handling for Interior Martian Rock Analysis or CHIMRA. The entire combo is fancy nomenclature for a percussion drill that beats and grinds rocks into powder, then collects those grains, transports them into a couple of small chambers, and retains them for deposition into the body of the rover where the analysis stations are. Each of these samples is about the volume of a baby aspirin. If it sounds simple, it's not. The development and perfection of the drill and collection mechanism was a huge endeavor on its own, and the engineers spent a lot of sleepless nights working on it right up until launch. More on the drill and collection mechanism later—it's a fascinating story all on its own.
Now for the instruments on board the rover's main body. Hold on, because this gets complex.
We'll start with the easy ones first.
DAN
The Dynamic Albedo of Neutrons instrument: This device, provided by the Russian Space Agency, sends neutrons into the soil beneath Curiosity and then measures the return signal. Using this method, water content as low as a tenth of a percent can be measured as far as six feet below the surface.
RAD
The Radiation Assessment Detector instrument: This passive radiation detector measures radiation, that's it. It was turned on during MSL's cruise phase between Earth and Mars to measure radiation in the deep-space environment, and it continues to function on the surface of the red planet. Its primary purpose is to ascertain whether human voyagers to Mars would be able to survive the radiation levels on the way to the red planet and while on the surface. So far the answer seems to be yes, but with conditions. They will need a lot of shielding of one kind or another, both during the trip and while on Mars.
Now we get to the really juicy stuff:
SAM
The Sample Analysis at Mars instrument. SAM is one of two scientific superheroes of this mission. This machine takes up almost half the science payload of Curiosity, and with good reason—it's the costar, along with CheMin, of the show. It contains chemical and analytical equipment that not long ago would have filled a moderately large university laboratory, and it helps MSL to deserve the “laboratory” part of its name. SAM's primary chore is to find various forms of carbon and lighter elements associated with life, such as hydrogen, oxygen, and nitrogen. It does so with three subinstruments. Inside it contains a mass spectrometer, which separates elements and compounds by their mass for measurement and identification. SAM also houses a gas chromatograph, which heats rocks and soils until they turn into vapor, then analyzes the gasses as they escape. Finally, it has a tunable laser spectrometer. This incredible machine can measure the isotopes, or atomic numbers, of specific elements, such as carbon, hydrogen, and oxygen. All of SAM's instrumentation is highly accurate and has collectively raised the bar for Martian exploration.
CHEMIN
The Chemistry and Mineralogy X-Ray Diffraction instrument: Another remarkable powerhouse of analysis, CheMin looks at soil or drilled rock samples to determine types and the amounts of minerals within. The abundance of certain minerals helps to understand the environment of ancient Mars—for example, olivine persists where there is little water, hematite forms in wet environments. These are just two examples; many other minerals can be examined.
What is so ingenious about CheMin is the way it analyzes these minerals. Once the fine, powdered material is delivered to CheMin by the robotic arm, it is funneled into a small glass container. An x-ray beam is shot through the sample, and how that beam interacts with the sample is very instructive. Some of the high-energy waves are absorbed and fluoresce in a wavelength that tells the scientists what substance has intercepted it. The x-rays also react with the crystalline structure in the sample and become diffracted, and the angle and amount of this diffraction can identify elements within.
MEDLI
Finally, bringing up the rear, is a largely unsung device among the much-heralded instrument package, the MSL Entry, Descent, and Landing Instrument. Not to be confused with the MARDI camera, this device also had a short life, functioning well before MARDI did—it was active only during the entry and early landing phase of the mission, measuring pressures and temperatures and also allowing those on the ground to infer the exact direction of flight. MEDLI was mounted on MLS's heat shield.
This remarkable suite of investigative instruments gives Curiosity the analytical power of, quite literally, a science laboratory on Mars. That's one reason the name of the mission was changed from Mars Smart Lander to Mars Science Laboratory—same initials, but a vastly different meaning. The capabilities of a large and well-equipped analytical laboratory have been shrunk to the size of a coffin, placed in a chassis, and put on wheels.
So in general terms, the investigative sequence proceeds like this:
Use the Mastcam to find interesting-looking areas or objects to investigate;
Observe the target from a distance with the Mastcam telephoto imager;
(Optional) Shoot a target with ChemCam's laser and observe the spectra of the burning rock;
Drive to a target if it's worthy of investigation;
(Optional) Use the MAHLI (microscopic imager) to look at the visual structure of the target;
Use the APXS to analyze the sample in place;
Scoop or drill the sample to get a bit of powder;
Send the sample to CheMin, SAM, or both for analysis.
The primary purpose of the mission is to search for environments on Mars that were once habitable, that is, that could potentially have supported living things. There are many forms of habitability, but as currently understood, it will involve water and will probably date back to a denser, ancient atmosphere with more oxygen and nitrogen in it. These scientific instruments work in a beautiful partnership to find the answers the mission seeks. Once you are able to understand how they work together, it's a pretty incredible ensemble.
Even with the copious amounts of data provided by the sophisticated instruments, there is still plenty of abstract thinking for the scientists to do. The ultimate answers lie not just in the observations of the instruments but also in the context in which the samples are found. The result is a geologist's dream—the ultimate characterization of a landscape in the search for the biggest prize of all, life on Mars. NASA is very clear—Curiosity is not a life-science mission, at least not in the sense that Viking was. It seeks evidence of habitable environments and organic compounds. Of course, if along the way it happens to come across a fossil or two, nobody will complain.
If one did not know better, one might be excused of thinking that the exploration of Mars is only about looking at rocks…because largely, it is. It is also one reason I tortured myself with the complete span of geology classes a foolish undergrad takes until he encounters a semester of differential equations…but that's another story. The upshot is that, for some of us, studying rocks is fun until it becomes too much work. But to the JPL folk, the end of my struggle was their point of departure, and what they know about rocks and Mars is enough to make your head hurt. Fortunately, you have me here, writing this book, to spare you from at least some of it.
The study of Mars by machines, working on its surface, is generally focused on looking for past and present evidence of water. Where there is (or was) water there can be (or could have been) life. The past composition of the atmosphere is critical, as are moisture levels, and indications of atmospheric
pressure, temperature, and many other things. Also, what elements were present? NASA uses a widely accepted acronym for these primary elements of life—CHNOPS, which stands for carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. These chemical elements seem to be essential to life in forms that we understand.
What do these past conditions and chemicals have in common? Well, some can be retained in one form or another in the geological record, and others can be inferred from what is observed. A lot of insight can be gleaned by looking at rocks, and specifically the deposition of what kind of rocks and how they changed over time. There are many reasons John Grotzinger was selected to lead MSL, and primary among them is that he is a premiere sedimentologist. He is interested in—among other things—rock and silt layers deposited over time in watery environments—and once the Pathfinder and especially the MER rovers found recurring evidence of this, he became the right man for the job.
Let's start with a look at what this field encompasses, since it has turned out to be so important for Mars exploration.
Sedimentology is concerned with the study of sediments as created by water over time. A sedimentologist will observe processes taking place in the field today, then apply that knowledge to what is found in more-ancient layered deposits. In general terms, the deeper or lower the sedimentary layer, the older it will be. And these processes appear to be universal—that is, they hold true on Mars just as they do here on Earth. So, clearly, deposition of sand and silt via water—over time and in amounts both great and small—are important to an understanding of Mars, and especially to gain a picture of the possible habitability of an area.
To further refine:
The history of Mars exploration can be characterized by a series of exciting discoveries that have dramatically overturned previously held beliefs about the planet. Until very recently, the dominantly held position within the scientific community was that while geologic and climatic conditions during Mars’ distant past may have been conducive to the potential origin and evolution of life, conditions on Mars today offer slim hope for life as we know it due to the unlikely existence of near-surface liquid water environments. However, recent results from NASA's Phoenix Lander and Mars Reconnaissance Orbiter missions suggest that present-day Mars may in fact contain a range of potential liquid water environments.