The Mars 2020 Rover In-Depth
NASA's next major mission to the Red Planet will store samples for eventual return to the Earth
Posted by Van Kane
27-07-2013 13:05 CDT
NASA’s planned Mars 2020 rover likely will both continue the astrobiological exploration of Mars begun by the Curiosity rover and provide stepping stones to the next stages of Martian exploration.
Two weeks ago, NASA’s Mars 2020 rover Science Definition Team (SDT) delivered its report recommending the science goals for the mission. Probably to the surprise of no one, the team recommended essentially the same science goals as had several previous SDTs on what NASA’s next mission to Mars should do. Like the Curiosity rover currently on Mars and the planned European and Russian ExoMars rover mission, the 2020 rover will look for clues as to whether Mars ever contained the conditions to enable life and whether traces of life or pre-biotic chemistry remain.
The mission will also provide an important transition to the next phases of Mars exploration by caching samples that could eventually be returned to Earth and testing technologies for future human and robotic missions.
The challenge the SDT faced was how to do all of this on a budget (~$1.5B) that, with inflation, may be just somewhat more than the half cost of the initial Curiosity rover. To fit within the budget, a key tradeoff would have to be made that will make the 2020 rover less capable in a key respect than the Curiosity and ExoMars rovers.
The 2020 rover will be enabled by the substantial investment NASA made in the design of the Curiosity rover and its entry, descent, and landing system. NASA and the Jet Propulsion Laboratory that built the Curiosity rover also retain substantial stockpiles of spare parts and engineering expertise that can be used in rebuilding substantial portions of the spacecraft.
I’ve read many press summaries of the SDT’s recommendations, which tend to focus on the proposed caching of samples for possible return to Earth. If NASA follows through, this will be the first concrete step towards a goal that Mars scientists have made their top priority for decades. However, the caching is just one aspect of the proposed mission.
To understand the full promise of this mission, I’ll go through each of the SDT’s proposed goals for the mission (which closely parallel those NASA asked the SDT to consider). First, though, the proposed mission implementation to meet those goals makes more sense with some background on how rovers are used as scientific platforms.
In the course of driving several kilometers, a rover will pass by thousands of potential spots for more detailed examination. Each of those examinations, though, can take days to weeks complete. There is a tradeoff between driving distance, and hence number of locales that can be explored and the number of spots where in-depth data can be gathered.
To make the best trade possible between these two goals, the mission team employs a hierarchy of scientific assessments with those at the top taking the least time and those at the bottom the most. First, the team uses orbital observations to select the locales it would like to visit. Then as the rover arrives at each locale (and also during the drives between them), the rover’s remote sensing instruments are used to get the “big picture.” From these images, the science selects a small number of targets for the next level of investigation by the contact instruments. As the name implies, these instruments are placed in contact with a rock or soil sample target. Instruments in this class include the microscopic imagers and alpha-particle X-ray spectrometers carried by the MER Spirit and Opportunity rovers and the Curiosity rover. For a still smaller number of extremely interesting targets, the time is taken to collect a sample. On the Curiosity rover and the planned 2018 ExoMars mission, these samples are delivered to highly sophisticated analytical instruments inside the rover for detailed measurements. The 2020 rover will collect samples and place them in a cache, which may eventually be returned to Earth for more detailed measurements than can be made within a rover. (The MER rovers do not have the capability to collect samples.)
The SDT described their proposal for the 2020 mission in terms of fulfilling four goals, and I’ll present their recommendations for each of those four goals.
Goal A: “Explore an astrobiologically relevant ancient environment on Mars to decipher its geological processes and history, including the assessment of past habitability.”
Every NASA landed Mars mission – except the 1996 Pathfinder mission that focused on technology demonstration – has had the goal of exploring Mars’ past and current potential for life or pre-biotic chemistry. For the MER rovers, the goal was simply to determine whether water – an essential ingredient for life – was present at the surface early in Mars’ history. The Curiosity rover is exploring Gale Crater to examine soils from many eras of Martian history to determine whether or not environments for life existed and to determine whether biosignatures of past life remain. The 2018 ExoMars rover will explore another site on Mars for its astrobiology potential.
The SDT has proposed that the 2020 rover continue the strategy and pursue astrobiology as the mission’s defining goal. Its proposed strategy breaks into two parts. The first is to have the rover carry a suite of instruments capable of a exploring site’s geologic history in-depth with an emphasis on how that history affected the possible presence of past life.
The team proposes that the rover carry multispectral cameras for obtaining images and an imaging spectrometer for analyzing composition across entire sites. These instruments would provide the context to interpreting each locale’s history as well as allowing the science team to select specific targets for more detailed exploration.
For most of the contact science, the SDT is proposing that the rover carry a new generation of instruments. Current Mars rover contact spectrometers measure mean composition over an approximately two centimeter contact area. The new generation of instruments under development can make composition measurements for spots as small as a tenth of a millimeter. With that resolution, the contact spectrometers would make dozens to hundreds of measurements across the contact area.
If you take a close look at soils and most rocks, you’ll see that most are composites of many fragments that each have their own geological story to tell. The contact spectrometers that are likely to be proposed for the 2020 rover will be capable of exploring each of those fragments individually. (The SDT also proposes that the rover carry an imaging microscope to study the morphology and texture of each contact area.)
The remote sensing and contact instruments listed above are included in the baseline recommendations and are expected to be affordable at the low end of the expected budget (~$90M to $125M) for the science instruments. If the budget becomes plusher, the SDT recommends two additional instruments to study the shallow subsurface beneath the rover. A ground penetrating radar would detect subsurface rock and soil layers, providing better context for understanding the geology exposed at the surface. A gamma ray spectrometer would measure the composition of soil in the upper few centimeters and could alert scientists to interesting substances just below the upper veneer of soil.
A capable instrument suite enables scientific exploration; the rover still must be delivered to a location that orbital instruments show might have been a location for life or pre-biotic chemistry. A number of such locations are known, and more are being searched for. However, these sites often lie within rough terrains with just a small area free of large rocks that would end the mission should the rover be unlucky enough to land on one. The 2020 rover mission will inherit the precision landing system developed for the Curiosity rover that reduced the area of the landing ellipse to a fraction of what it had been for previous landers.
The SDT recommends shrinking that ellipse further to allow more landing sites to be considered. On past missions, the parachute has opened at the earliest possible time during the descent. For the 2020 descent, the SDT recommends that the entry system have the ability to vary the time of opening based on its estimate of its position relative to the landing zone. This relatively simple enhancement could reduce the size of the landing ellipse by 25% to 50%.
Many potentially interesting astrobiology sites on Mars lack any area the size of a landing ellipse free of large rocks or dangerously steep slopes. A second enhancement the SDT asked NASA to consider is terrain recognition navigation (TRN) that would enable the lander to compare images of the landing area stored on board with real-time images taken during the descent. This capability would allow the descent system to determine its actual location and steer free of hazardous terrain in the moments of final descent.
Goal B: “Assess the biosignature preservation potential within the selected geological environment and search for potential biosignatures.”
Much of the scientific attraction of Mars comes from its preservation of ancient surfaces and rocks that might retain records of conditions that could have led to life or even records past life itself. The second proposed goal for the 2020 mission would have the rover actively assess whether biosignatures could have been preserved and to search for those biosignatures.
What would be a biosignature? If we were extremely lucky, it might be fossilized layers from algae-like micro-organisms visible to the cameras. More likely, it would be the alteration of rock or soil chemistry in a way that would be best explained by complex organic chemistry or the actions of life. Again, if we were lucky, it could be the presence of organic matter preserved for billions of years.
The rover would seek biosignatures using all of the instruments listed above and with one or two instruments that would be selected for their ability to detect organic material. The Viking and Phoenix landers and the Curiosity rover (and the future ExoMars rover) have relied on sophisticated analytical instruments such as mass spectrometers to detect organic molecules. These instruments are capable of much more sophisticated measurements than is possible with contact instruments that must operate directly in the harsh Martian environment. Analytical instruments have soil samples delivered to them where they can be analyzed with numerous techniques and altered through heating or wetting to release gasses or induce chemical reactions.
Unfortunately, the budget for the 2020 rover does not include funding for analytical instruments. Instead, the SDT proposes that the rover carry one or two spectrometers capable of detecting the presence organic matter. The authors of the SDT report are clear that the strategy they propose will result in the 2020 rover having significantly less capabilities to analyze potential organic material than other missions with analytical instruments.
The SDT points out a compensating new capability for the 2020 rover: sample caching. As described below, if the cache is eventually returned to Earth, terrestrial laboratories could perform far more sophisticated measurements than would ever be possible on a rover or lander.
Goal C: “Demonstrate significant technical progress towards the future return of scientifically selected, well-documented samples to Earth.”
Returning a carefully selected set of Martian samples has long been a goal of the Mars science community. The last planetary Decadal Survey ranked a mission to select and cache a set of samples for future return to Earth as its highest priority large mission for the coming decade. A follow on mission would collect the samples and take them into Martian orbit, and a third mission would retrieve the samples from that orbit and bring them back to Earth.
The President’s Office of Management and Budget balked at beginning a sequence of missions that in combination could cost $6B to $8B. They agreed to the 2020 rover mission to continue thein situ exploration of Mars and to demonstrate technical progress towards future robotic and manned missions, including caching samples.
The SDT concluded that the difference between demonstrating the technical capability to select and cache samples and actually leaving a returnable cache would be minimal. They recommend the rover assemble a sample cache of up to 31 to 38 five centimeter long core samples of rock and soil acquired by the rovers drill. Once collected, the cache would be placed on the surface for a future mission to collect in a few years or even a few decades.
Goal D: “Provide an opportunity for contributed Human Exploration & Operations Mission Directorate (HEOMD) or Space Technology Program (STP) participation, compatible with the science payload and within the mission’s payload capacity.”
When the 2020 rover mission was approved by the President’s office, one of the requirements was that it demonstrates technologies that would be useful for future robotic and human missions. The SDT recommended four options be considered (in priority order):
- Demonstrate the ability to capture and compress Martian air (which is primarily CO2) and extract liquefied oxygen for use as the oxidizer for the fuel for future a robotic or manned ascent stage from the Martian surface (commonly called in-situ resource utilization or ISRL). While many parts of this technology can be demonstrated on Earth, key issues of collecting CO2 under varying dust conditions, winds, atmospheric pressure, and temperatures can be best demonstrated on Mars
- Better instrument the entry and descent system to collect information on the conditions of descent and parachute performance. (The Curiosity entry and descent system collected extensive information during its landing; the proposed 2020 system would collect that information and new information.) The technologies discussed earlier to reduce the risk of landing by timing the parachute opening and using terrain recognition would also benefit future missions.
- Collect extensive weather information including atmospheric temperature profiles and atmospheric dust profiles to better understand atmospheric behavior. Collecting this information would better characterize the atmosphere and reduce risk for future landings on Mars.
- A biomarker system to “demonstrate detection of microbial contamination for future human missions.”
The mission proposed by the SDT would meet the goals set out for a caching rover as the top priority in the last Decadal Survey. It would also meet the goals recently set out by NASA for demonstrating key technologies for future missions.
At the same time, even if the samples not collected, the rover would carry out intensive geological and astrobiological exploration at a fifth site on Mars. (The Viking landers explored two sites in the 1970s, the Phoenix lander explored the ice-rich northern plains, the Curiosity rover is exploring Gale Crater now, and the ExoMars rover will presumably explore yet another site.) The new generation of contact instruments that will be ready for the 2020 rover will allow exploration of the composition of Martian soils and rocks at micro-scales that previous missions have not be able to do. This is an exciting new capability.
While the SDT report doesn’t spend much time on the possible weather station that the 2020 rover may carry, I think this would be an important addition. Scientists have long wanted to get a network of metrological stations on Mars to better explore weather patterns. In 2020, there may still be three functioning weather stations already on Mars: Curiosity, NASA’s InSight lander, and the Russian surface station planned for the ExoMars mission. A fourth station would be an important addition.
The 2020 rover is a mission that will be done on a tight budget. The highly capable analytical laboratories of the Viking and Phoenix landers and the Curiosity and ExoMars rovers would not fly on the 2020 rover. The capabilities the SDT recommends for the 2020 rover meet all the requirements previous SDTs have laid out for a rover mission focused on sample selection and caching. However, the 2020 rover would have less capability for science on Mars for characterizing organic matter and other volatiles than Curiosity or the ExoMars rover.
Of course, the need for the analytical laboratory would go away if the sample cache is returned to Earth laboratories. The 2020 rover would make the investment in the first crucial step, selection and caching of samples, of the Holy Grail of Mars science: returning samples. How long might those samples sit on the surface of Mars before being collected? I expect that that will be a question for how compelling the discoveries by the rover’s instruments are and the generosity of future governments. And it may not be American craft or only American craft that collect and return the samples. In the coming two decades, several space agencies are likely to have the technology to participate in the sample return.
A second bet being made is to leave out a deep drill such as the ExoMars rover will carry. The drill proposed for the 2020 rover will collect samples five centimeters (about two inches) in length, similar to that carried by the Curiosity rover. This may not be deep enough to get below the surface radiation that is believed to destroy organic matter at Mars (see this post). The ExoMars drill was designed with this problem in mind and will reach up to two meters below the surface. Adding a similar drill to the 2020 rover would require a substantial modification to the Curiosity rover design isn’t possible within the budget. However, if the Curiosity rover doesn’t find organic matter and the ExoMars rover does but deeper beneath the surface, then the bet on the shorter drill will look problematic.
Even with the current budget realities, though, the SDT has proposed a highly capable, exciting mission. NASA’s officials warmly received the recommendations, indicating that their final choices for the mission are likely to be similar to those recommended by the SDT, but some changes are possible.
A key decision by NASA will be on whether to fund the sample collection and caching system and the sophistication of that system. Current news reports indicate that it intends to fly this system.
The next step for the mission will be for NASA to issue a call to solicit instrument proposals this coming fall. The type of instruments NASA says it is interested in receiving proposals for is likely to be the definitive statement on the mission’s scientific goals.
For more information on the SDT’s recommendations you can read these documents.