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.
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.
Other instrument types that were left off include dedicated near infrared spectrometers similar to those used on the orbiters (CRISM, OMEGA-like) which could be used to ground truth the major mineralogical identifications these instruments have demonstrated.
ReplyDeleteAlso, noticeably absent is an APXS instrument similar to that flown on Pathfinder, MER rovers, and Curiosity.
As it stands, it looks like the payload will be very good at interrogating very small portions of rocks, but will lack the ability to understand broader swaths of the area around it. This may be a critical flaw given the lack of mobility of a rover and the low number of outcrops that will be studied.
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