By July 1, a group of scientists will define the goals of the rover NASA will launch in 2020 to Mars. The rover will be a near twin of the Curiosity rover that is currently on Mars. (Since Curiosity is nuclear powered, it may still be operating when its sibling arrives.) The Curiosity design will ensure that the rover is highly capable. What the Science Definition Team (SDT) will determine is what its scientific goals are. From those goals, NASA will select a suite of instruments to fulfill those goals.
What I’ll attempt to do in today’s post is to discuss some of the tradeoffs that I think may be considered in selecting the science goals. I won’t attempt to discuss potential individual instrument selections – the scientific community is tremendously creative in developing instrument concepts, many of which lie outside my expertise.
The most basic question will be whether to do detailed composition analysis there, here, or both. There means on the surface of Mars, and here means returning samples to the laboratories of Earth. The Phoenix lander, Curiosity rover, and the planned European and Russian 2018 ExoMars rover will carry highly sophisticated chemical laboratories rovers (science there). However, while the instruments on those rovers are engineering marvels, they are pale imitations of the incredibly more varied and sensitive instruments in laboratories here on Earth (science here).
The Mars community has decided (formally through the last Decadal Survey) that answering the key questions about Mars requires the sophistication of Earth-based instruments. The goal identified in the Survey for the next Mars rover was to find and cache samples for eventual return to Earth. Science instruments on the rover would serve primarily to identify interesting samples to collect. The catch, though, is that returning those samples will require two additional missions costing $4-6B. In an era of shrinking US federal budgets, any samples collected may languish on Mars for a decade, perhaps several, and possibly forever
In a world of plush budgets, focusing the 2020 rover on simple sample collection would be the obvious choice (as it was for the members of the Decadal Survey in days of rosier budget forecasts). In a world of shrinking US Federal budgets, though, the SDT members may decide that equipping the rover with highly capable – but expensive ($10s of millions) – instruments may be a better choice. With this strategy, whether samples are returned or not, the rover will have conducted sophisticated analyses of rocks and soils: A guaranteed science return.
So why not just do both? Collect samples and carry a sophisticated science laboratory? The answer is a limited budget and conflicting operational strategies. The former is simpler to explain. The science instrumentation budget for the 2020 rover is expected to be $80-100M, approximately half that available for developing the Curiosity rover’s instruments. NASA’s managers have stated that the budget won’t provide the funds for developing a full suite of complex new instruments.
The conflict in the operational strategies arises from how to maximize the use of time. (While the 2020 rover may operate for many years, planners can count on only the two it is designed for.) For a caching-focused mission, the goal is to visit as many locations as possible, assess their potential for samples worthy of return to Earth, and move on quickly from the many that don’t make the cut. For a science on Mars-focused mission, the preparation and analysis of each sample requires long periods parked in one spot. (Think of the weeks Curiosity has spent parked in each of the two locations it has analyzed samples (although the process should speed up as the rover’s operators gain experience).)
For either mission strategy, two sets of instruments might be the same. The first suite will consist of remote sensing instruments that study the surrounding landscape without physically touching any of it. Cameras will serve as the eyes of geologists (and armchair explorers on Earth). The rover may carry one or more spectrometers that analyze different portions of the electromagnetic spectrum to map composition. The Spirit, Opportunity, and the ExoMars rover used or will use this approach. An alternative would be to use a laser to vaporize rock and soil surfaces to enable chemical analysis of the briefly glowing vapor as the Curiosity rover does. Whatever the instruments selected, their goal will be to select specific locations for study or sampling and to understand the geological context.
A second set of instruments would be located on the rover’s arm and would be physically placed in contact with soils and rocks to make their measurements. The Spirit, Opportunity, and Curiosity rovers carried both microscopic imagers and spectrometers to measure composition. (The ExoMars rover will not have a robotic arm and doesn't have equivalent instruments, although it will have an infrared spectrometer embedded in its drill bit.) The advantage of these instruments is that they conduct their measurements quickly, allowing fast assessments. The downside is that the types of instruments and their sophistication are limited by the need to fit on the head of the robotic arm and be exposed to the harsh Martian environment.
For the 2020 rover, though, the compositional contact instruments may be much more sophisticated than those flown to date. Previous instruments measured average composition across each sample area (1.7 cm for Curiosity). If you look closely at soils and most rocks, you’ll see that they are composed of many smaller rock fragments, each with its own story. The next generation contact images may be able to differences composition across the contact area as small as 0.5 mm or smaller.
Regardless of the science focus, the 2020 rover seems likely to carry instruments from these first two suites. Depending on the science goals, it may also carry a third suite, an analytical laboratory. These would be instruments within or mounted on the rover that receive samples delivered by the rover’s drill or scoop. These instruments can be larger, allowing for more sophisticated measurements. They can also manipulate the samples, say heating them to drive off organic molecules or wetting them to measure the resulting chemical reactions. The Spirit and Opportunity rovers were too small to carry these instruments, but Curiosity carries two. The Phoenix lander also had an analytic laboratory as will the ExoMars rover.
The range of instruments possible for an analytical laboratory is wide, and I seem to find two or three new proposals with each scientific conference that includes discussions of future Mars missions. One core instrument type is a mass spectrometer and gas chromatograph combination that can “taste” gases driven off a sample by heating a sample. This is a standard technique for measuring carbon chemistry, including organic molecules. The Curiosity rover carries one (the Sample Analysis at Mars, or SAM instrument) and the ExoMars rover will carry a more capable version.
An exciting new class of instruments that are maturing to become flight ready would perform geochronology on Mars rocks and soils to nail down their ages. Understanding the age of key events in Mars’ history, recorded in its rocks and soils, is one of the motivating goals of returning samples to Earth. The development of instruments that can be carried to Mars provides an opportunity to address key questions without the cost of returning samples.
However, NASA’s managers have already stated that the limited instrument budget for the 2020 rover will preclude development of a suite of new instruments. That would seem to favor the remote sensing and contact instruments over the more capable but also much more complex and expensive laboratory instruments. A previous science definition team that examined instruments for a caching rover called for only remote sensing and contact instruments.
Careful ‘shopping’, though, may be able to extend the budget. NASA could fly copies of the Curiosity instruments, whose development has already been paid for. It might also fly copies of one or more of the ExoMars instruments. (The ExoMars MOMA instrument itself uses a copy of much of the Curiosity’s SAM instrument.) NASA has also said it is open to instruments provided by – and very importantly, paid for – by other nations.
Editorial Thoughts: I have seen a multitude of proposals for Mars sample return over the past several decades and not one has come close to being funded. I personally am wary of flying a rover mission that focuses too heavily on sample acquisition and caching. Those samples may never reach Earth, and funds for major rover missions may come very infrequently. While planetary scientists see sample return as the necessary next step, the long history of failed sample return proposals suggests that returning rocks exciting to geologists and astrobiologists doesn’t open the public checkbook for a many billion dollar outlay. (My personal guess is that Congress will provide the funds for a sample return if a rover finds complex organic molecules suggesting past or present life. )
I will be shocked if the SDT doesn’t call for the rover to collect and cache samples in case governments come to feel generous. However, I’d also like to see one or more complex analytical instruments fly, even if they are copies of previously flown instruments. So do science there and enable science here. That would guarantee more sophisticated measurements, and the measurements they do make may show that the samples exciting to the general public as well as scientists.