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 SDT’s 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.
Summary of the goals for the 2020 rover
mission as envisioned by the SDT. This
and all images in this post are from the briefing
to the Mars Exploration Program Analysis Group (MEPAG) July 23, 2013
presentation by the SDT chair, Jack Mustard (Brown University). Credit: JPL
Double click on any image for a larger view.
Double click on any image for a larger view.
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 rover, these samples are delivered to highly sophisticated
analytical instruments inside the rovers for more detailed measurements. The 2020 rover will collect the 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.)
Rovers on Mars follow a hierarchical
strategy for selecting a small number of targets for in-depth exploration.
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.”
The 2020 rover would follow a
hierarchical strategy to explore a location on Mars believed to have been habitable. The definitive identification of past signs
of life is likely to require the testing of returned samples in Earth
laboratories.
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.”
Examples of potential biosignatures and
measurements the 2020 rover could make to find them.
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.”
Priorities for samples to be
collected. The E2E-iSAG was a previous
mission assessment for a rover mission focused on selecting and caching
samples.
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 the in 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.”
Concluding Thoughts
Capabilities of the 2020 rover proposed
by the SDT. Boxes without ‘+’ represent
the recommended threshold capabilities below which the mission might not
deliver sufficient value for the investment.
Boxes with ‘+’ represent highest priority additions beyond the threshold
capabilities. If more funding than the
SDT were available, the mission could be further enhanced with either more
capable instruments or additional instruments such as a weather station or additional
instruments to characterize any organic matter.
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 SDT’s
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.
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