Last week, NASA’s managers announced the selection of seven instruments for its 2020 Mars rover from a pool of 58 proposals submitted by teams of scientists. Reading through the capabilities of the instruments makes them seem like technology from science fiction, complete with lasers and x-rays. However, the types of instruments that weren’t selected say almost as much about the goals and expectations for the mission as those that were. This mission will be optimized for finding the best samples to return to Earth rather than carrying out the most sophisticated science that could have been sent to Mars.
|The Mars 2020 rover will be based on the design of the Curiosity rover but will have a new instrument suite and hardware for collecting and caching samples for possible return to Earth. Credit: NASA|
For Mars, the key questions are about the earliest environments present on Mars, whether they could have enabled the development of life, and whether life or its precursors arose. Answering these questions can require devilishly subtle measurements. On Earth with the best instruments available (far, far more capable than those that could be flown to another planet), concrete answers are hard to come by and debates rage about the earliest conditions on Earth. (It doesn’t help that the active surface of the Earth has erased all but a few traces of the earliest surface, atmosphere, and ocean.)
The Mars scientific community has collectively decided that the best and perhaps only way to answer these questions is to return carefully collected samples to Earth for study in terrestrial laboratories. The primary goal the science community laid out for the 2020 rover was to enable the efficient selection of the most compelling sample set possible – so compelling that Congress will spend the additional few billions of dollars for missions to retrieve and return them to Earth.
|Placement of the just announced instruments on the 2020 rover. Credit: NASA|
To see how the instruments selected will work together towards this goal, imagine that you were sent to Mars and given the assignment to select a small set of samples to return to Earth. Because you can return only a few samples, you are under pressure to find those few special samples that can best reveal insights into the earliest history of Mars.
The first thing you are likely to do is to look to see what types of terrain and rock formations surround you. The 2020 rover will carry two Mastcam-Z cameras for this task. These cameras were originally intended to fly on the Curiosity rover currently on Mars, but weren't completed in time. Unlike Curiosity’s cameras, these will have the ability to zoom from wide angle to moderate zoom (28 mm to 100 mm, 35 mm film equivalent) and to take movies. (If still photos from Mars are cool, imagine movies.) These cameras will take color images, but unlike our eyes they will also be able to take images in twelve carefully selected bands (“colors”) in the visible and near infrared spectrum to help map subtle distinctions in composition.
The 2020 rover also will have the ability to assess the area around it in ways that our eyes never could. Like the Curiosity rover, the 2020 rover will zap rocks and soils with a laser to determine their composition. Curiosity’s ChemCam laser heats its targets sufficiently that a tiny amount vaporizes. The instrument analyzes the glow of the plasma cloud to measure the elements present.
However, if the laser hits a target with specific wavelengths of light at a lower energy, the target will “glow” in characteristic ways that reveal the mineralogy and the presence of organic molecules. (In technical terms, these are Raman and time-resolved fluorescence spectroscopy.)
(An analogy helps explain the difference between elemental and mineralogical composition. French bread, Indian naan flat bread, and tortillas, for example, have similar ingredients (they are much more similar to each other than to, say, a steak or a Greek salad). In this analogy, the ingredients in the recipes are the elemental composition, while the specific type of baked good reflecting both the proportion of ingredients and method of cooking is the mineralogy.)
The 2020 rover will carry an advanced version of ChemCam called SuperCam that will use all three types of laser analysis to provide both elemental and mineralogical analysis. In addition, it will have capabilities for mapping composition using visible and infrared spectroscopy, although no details were provided (such as whether this capability will be just for the spots targeted by the lasers or will be full images of the scenes around the rover).
The rocks and formations that Mastcam-Z and SuperCam can study, however, are only those at the surface. Geological formations often continue beneath the surface and the rocky outcrop in front of the rover may be the same or different than the outcrop viewed a hundred meters earlier in the rover’s drive. A Norwegian-supplied ground penetrating radar, RIMFAX, will map soil and rock layers up to a half kilometer below the surface with a resolution of 5 to 20 centimeters.
|Artist’s concept of how the RIMFAX surface penetrating radar will study rock formations below the surface. Credit: NASA|
To return to our analogy of you as Mars geologist, once you survey a location, you would go to specific soils or rocks that look interesting for closer examination. Similarly, the 2020 rover will carry two instruments to study small patches (approximately the size of postage stamps) in detail. Both will be contact instruments that operate once the rover’s arm has placed them against a patch of soil or a rock. (It is likely that the 2020 rover, like NASA’s previous Martian rovers, will be able to brush dust and the outer surface of rocks off to allow instruments to sample the more pristine internal rock.)
The Curiosity rover carried two contact instruments, a microscopic imager and the Alpha Particle X-Ray Spectrometer to measure elemental composition. The 2020 rover will carry two much more capable contact instruments. The PIXL instrument will measure elements using X-ray lithochemistry while the SHERLOC instrument will measure minerals using laser Raman and fluorescence spectroscopy. Both of these instruments will have their own microscopic cameras, and the SHERLOC instrument carry a near copy of Curiosity’s MAHLI microscopic imager. (MAHLI operates as both a normal camera as well as a microscopic camera. This camera, mounted on the rover’s arm, has taken the selfie pictures that show the rover on the Martian surface as well as images of the wheels and beneath the rover.)
While Curiosity’s Alpha Particle-X-Ray spectrometer could measure only the average composition of the surface in front of it, both PIXL and SHERLOC will make hundreds to thousands of measurements across each surface. Each measurement point will be approximately the size of a grain of sand.
|An example of how the PIXL instrument will map elements (identified by their chemical symbol in each panel) at a fine resolution across rock surfaces. The SHERLOC instrument will map mineral and organic composition at similar resolution. Credit: NASA|
The new capability to measure composition at near microscopic resolution will be revolutionary. If you look at soils and the interiors of most rocks, you’ll find that they are composed of many smaller rocks and inclusions. By taking many fine-scale measurements, each rock or patch of soil becomes a rich story of many rock fragments that together provide clues to their individual formation and that of their larger rock or soil type.
SuperCam and SHERLOC’s laser spectroscopy will have an important capability that Curiosity lacks – they can easily identify and map the presence of organic materials. While many processes other than life can produce organic chemicals, life as we understand it requires a rich abundance of organic material. A key goal for the 2020 rover is to find biosignatures to indicate pre-biotic chemistry or life itself.
The Curiosity rover can detect organic materials through its mass spectrometer, but preparing samples for and using this instrument is a laborious process and has only been done rarely in the mission to date. In addition, the way the Curiosity’s instrument works, it must heat samples, which triggers chemical reactions with the perchlorates found in the soils, destroying the organic materials. Careful measurements have allowed scientists to conclude that the samples taken by Curiosity contain some organic materials, but we aren’t sure how much or what types. (Curiosity’s instrument has a mechanism to avoid the “perchlorate trap,” but it can be used only seven times and hasn’t been so far.)
By using lasers, the 2020 rover can find organics quickly and won’t be skunked by perchlorates, key advantages over Curiosity.
Two other instruments round out the 2020 rover’s manifest. The MEDA instrument, supplied by Spain, will monitor the weather and study the airborne dust. MOXIE will demonstrate the extraction of oxygen from the predominantly carbon dioxide atmosphere at Mars. Missions (manned or unmanned) that are to return to Earth could substantially reduce their launch weight if they could manufacture the oxidizer portion of their rocket fuel form the Martian air. The same applies to the oxygen supply to breath for any future astronauts.
How does the 2020 rover’s scientific instrument suite (MOXIE is an engineering demonstration) compare to that of the Curiosity rovers? The 2020 rover will have far superior remote sensing instruments (Mastcam-Z, SuperCam, and RIMFAX) and contact instruments (PIXL and SHERLOC) than Curiosity. This will allow this new rover to much more quickly find important samples to study and potentially cache. This is especially true for finding any rich deposits of organic material.
To locate two to three dozen samples within the mission’s lifetime on Mars, the 2020 rover will need to operate much more efficiently than the Curiosity rover has. The scientific team that defined the requirements that NASA used to select this instrument suite specifically asked for a suite of instruments simpler than Curiosity’s to speed operations. Because almost a decade has passed since Curiosity’s instruments were selected, the march of technology allows the new rover’s instruments to be considerably more capable than Curiosity’s.
So what’s left off? Ignoring the miniature greenhouse and the solar-powered helicopter proposals (either likely would have been media sensations), the 2020 mission will not have the class of laboratory instruments included in the both Curiosity and the ExoMars 2018 payloads. Performing the most sensitive measurements requires larger instruments than can fit on the robotic arm. To address this, both the Curiosity and ExoMars rovers have instrument laboratories housed within their bodies. For example, the Curiosity and ExoMars mass spectrometers can identify the specific composition of organic molecules. This is useful to separate organics created from non-biotic processes from those created from possible biotic processes. The laser instruments to be carried by the 2020 rover will be limited to more general identification of the presence of broader groups of organic molecules.
The mass spectrometer instruments proposed but not selected for the 2020 rover could have been more sensitive still than Curiosity and ExoMars’. The proposed CODEXinstrument, which would have had to be located as a laboratory instrument within the body of the rover, would have used lasers to vaporize minute quantities of material across the sample to be fed into a mass spectrometer. (By vaporizing samples, the instrument would have avoided the perchlorate problem.) The resulting measurements would have provided detailed maps of the chemistry of samples, the types of organics within it, and the age of the rock from which it came. Achieving both of the latter goals have been two of the justifications for returning samples to Earth. CODEX would have made progress towards both on Mars, although measurements made in terrestrial laboratories would be much more precise.
NASA doesn't discuss why particular instruments aren't chosen for a mission. CODEX and its kin may not have made the cut because the team of scientists that laid down the mission requirements specifically requested a simplified instrument suite. Or the reviewers may have concluded that the more sensitive measurements would not have been sensitive enough to answer critical questions about Mars. Or it could be that there wouldn't have been room in the rover or in the budget for them. The 2020 rover program has a tight budget, and the instrument suite selected will cost $130M, more than the $100M NASA had originally hoped to spend. (Curiosity’s instruments cost $180M.)
The instruments will be half of the 2020 rover’s payload. Still to come are details on the sample collection and caching system. Based on work done to date, it appears that the rover will collect and store two to three dozen sample cores that each will be about as wide as a pencil and about half as long as a new one.
The 2020 rover will carry an instrument suite optimized for efficiently finding the best sample suite at its landing site for a possible return to Earth. If those samples do make it to our world, we likely will have a revolution in our understanding of the Red Planet. If they do not, the scientific community may come to wish they had asked for a more capable instrument complement to do more sophisticated science on Mars. But life is about choices, and NASA and the scientific community have bet that the samples collected will be so compelling that funds will be made available for their return to Earth.
Either way, the instruments of the 2020 rover will be marvels much more advanced than their counterparts on Curiosity. We will get great science.