by The Design
accumulated on the diodes, the REMS team can derive the aerosol optical depth, a mea-
sure of how much sunlight has been blocked from reaching the ground by particulate mat-
ter in the atmosphere. Mastcam photos of the Sun can make the same measurement more
Figure 8.7. Spectral response of the REMS UV photodiodes. “UVABC” is also known as
“UV total dose.” UVA, UVB, and UVE have provided the highest-quality data. From Smith
et al. ( 2016 ).
11 Martínez et al. (2016)
290 Curiosity’s Environmental Sensing Instruments
precisely than REMS, but the REMS UV sensor measurements are far more frequent
(multiple times per day, as opposed to the Mastcam cadence of roughly once per week).
The sensors are covered about 10% of the time by shadows (mostly from the rover mast),
but they spend most of their daytime unobstructed, and data taken in shadow are easy to
remove from the data set.
Dust is an obvious concern to an upward-pointed light sensor, therefore each of the
photodiodes is surrounded by a ring-shaped magnet that prevents Martian dust from fall-
ing in its center, working to keep the photodiodes clean. MAHLI takes a photo of the sen-
sor roughly every two months in order to monitor dust deposition on it (see Figure 8.8).
Although a lot of dust has accumulated on the magnets over time, the windows over the
sensors have remained relatively clean over the course of the mission, and have become
cleaner when the rover has paused in windy areas, particularly during the Pahrump Hills
investigation from sol 800–900 and while driving through the Murray buttes around sol
1400 and following. Empirically, the REMS team has found the UVA, UVB, and UVE
sensors to provide the best estimates of optical depth.12
8.4.2.3 REMS Pressure sensor
The pressure sensor is located inside the REMS electronics box, itself inside the belly of
the rover. Although it is thermally isolated from the environment, the rover warm electron-
ics box has vents that allow its interior to be filled with Martian atmosphere at ambient
pressure. The pressure sensor has two transducers, one of which is designed to be more
stable, and the other of which is designed to be more responsive to rapid pressure changes.
Either provides good measurements, though, so the two transducers provide redundancy
in the experiment design. Each has two electrodes, with the distance between the elec-
trodes changing as a result of changes in pressure. That changes the capacitance of the
transducer, providing a sensitive measure of pressure changes. The pressure sensors pro-
vide better readings after warming up, so measurements toward the end of the 5-minute
window of each hour are considered more accurate than those recorded in the
beginning. 13
8.4.3 REMS on Mars
The loss of two of the wind sensor boards on Boom 1 (identified with orange tags in
Figure 8.6) during landing was catastrophic to the wind experiment. REMS recorded data from the forward-facing wind sensor on Boom 2 since landing, but because the rover has
been facing primarily south throughout the surface mission, and the prevailing wind has
come mostly from the north, virtually all of the wind measurements have been
12 Smith et al. (2016)
13 Gómez-Elvira et al. (2012)
8.4 REMS: Rover Environmental Monitoring Station 291
Figure 8.8. All MAHLI images of the REMS ultraviolet sensor to sol 1500. More images were taken on sols 1552, 1614, and 1675. NASA/JPL-Caltech/MSSS.
contaminated by rover hardware being in the way. The first board on Boom 2 failed on sol
1485, and the other two on sols 1491 and 1504. 14
Apart from the issues with the wind sensor, the REMS instrument package has been
performing reliably, sol after sol, recording measurements of Mars’ weather. Figure 8.9
summarizes some of the REMS data for most of the mission, sols 0–1514.
14 Ashwin Vasavada, personal communication, email dated April 17, 2017
292 Curiosity’s Environmental Sensing Instruments
Figure 8.9. REMS air temperature and pressure minima and maxima for sols 0–1514 give an illustration of the continuity and density of the data set. The air temperature repeats reliably (within a narrow band of variation) year over year. Seasonal pressure variations have been consistent, with a notable secular decrease in pressure with time, caused by Curiosity’s increasing elevation. Seasons (gray text) refer to the southern hemisphere, where Curiosity landed. Note that sol numbers for each season go a bit into the future, beyond the data set.
Graphs courtesy Javier Gómez-Elvira.
8.5 REFERENCES
Hassler D et al (2012) The Radiation Assessment Detector (RAD) investigation. Space Sci
Rev 170:503–558, DOI: 10.1007/s11214-012-9913-1
Hassler D et al (2013) Mars’ surface radiation environment measured with the Mars
Science Laboratory’s Curiosity rover. Science, DOI:10.1126/science.1244797
8.5 References 293
Gómez-Elvira J et al (2012) REMS: The environmental sensor suite for the Mars Science
Laboratory rover. Space Sci Rev 170:583–640, DOI: 10.1007/s11214-012-9921-1
IKI Laboratory for Space Gamma Spectroscopy (2011) Russian neutron detector DAN for
NASA’s Mars Science Laboratory landing rover. http://l503.iki.rssi.ru/DAN-en.html.
Accessed 21 May 2014.
Martínez G et al (2016) Likely frost events at Gale crater: Analysis from MSL/REMS
measurements. Icarus 280:93–102, DOI: 10.1016/j.icarus.2015.12.004
Matthiä K et al (2016) The Martian surface radiation environment – a comparison of mod-
els and MSL/RAD measurements. J Space Weather Space Clim 6:A13, DOI: 10.1051/
swsc/2016008
Mitrofanov I et al (2012) Dynamic Albedo of Neutrons (DAN) experiment onboard
NASA’s Mars Science Laboratory, Space Sci Rev 170:559–582, DOI: 10.1007/
s11214-012-9924-y
Mitrofanov I et al (2014) Water and chlorine content in the Martian soil along the first
1900 m of the Curiosity rover traverse as estimated by the DAN instrument, J. Geophys.
Res. Planets 119:1579–1596, DOI: 10.1002/2013JE004553
Pla-Garcia J et al (2016) The meteorology of Gale crater as determined from rover envi-
ronmental monitoring station observations and numerical modeling. Part I: Comparison
of model simulations with observations. Icarus 280:103–113, DOI: 10.1016/j.
icarus.2016.03.013
Rafkin S C R et al (2014) Diurnal variations of energetic particle radiation at the surface of Mars as observed by the Mars Science Laboratory Radiation Assessment Detector,
J. Geophys. Res. Planets, 119:1345–1358, DOI: 10.1002/2013JE004525
Rafkin S C R et al (2016) The meteorology of Gale Crater as determined from Rover
Environmental Monitoring Station observations and numerical modeling. Part II:
Interpretation. Icarus 180:114–138, DOI: 10.1016/j.icarus.2016.01.031
Smith M et al (2016) Aerosol optical depth as observed by the Mars Science Laboratory
REMS UV photodiodes. Icarus 180:234–248, DOI: 10.1016/j.icarus.2016.07.012
Vasavada A et al (2017) Thermophysical properties along Curiosity’s traverse in Gale
crater, Mars, derived from the REMS ground temperature sensor. Icarus 284:372–386,
DOI: 10.1016/j.icarus.2016.11.035
Zeitlin C et al (2016) Calibration and Characterization of the Radiation Assessment Detector (RAD) on Curiosity. Space Sci Rev 201:201–233, DOI: 10.1007/s11214-016-0303-y
; 9
Curiosity’s Chemistry Instruments
9.1 INTRODUCTION
Curiosity has four instruments that study the chemistry of Martian materials. Two of them
focus on elemental abundances. ChemCam is a remote sensing instrument, able to detect
the elemental composition of a rock or soil from a distance of up to 7 meters by shooting
it with a laser, a technique deployed in space for the first time on Curiosity. The Alpha
Particle X-Ray Spectrometer (APXS) is a contact science instrument to examine the com-
positions of rocks and soils reached by the arm, and has a long Martian heritage.
The other two composition instruments form Curiosity’s analytical laboratory, ingest-
ing samples directly. CheMin is an X-ray fluorescence/X-ray diffraction instrument for
determining crystalline mineralogy, the first instrument of its kind sent beyond Earth.
Sample Analysis for Mars (SAM) is a fiendishly complex machine with an oven for heat-
ing samples to drive off gases. SAM’s manifolds and pumps can direct gas from oven or
atmosphere to a gas chromatograph mass spectrometer and a tunable laser spectrometer
for measuring molecular and isotopic gas composition.
9.2 CHEMCAM
ChemCam employs a process called laser-induced breakdown spectroscopy (LIBS) to measure
the elemental composition of the targets it zaps (Figure 9.1). When it fires its laser, it converts some of the target into plasma. A telescope gathers the light emitted by the plasma and sends it to a spectrometer. The wavelengths of the emitted light are diagnostic for some elements.
ChemCam also uses its telescope to capture high-resolution context images of the LIBS targets using its camera, the Remote Micro-Imager (RMI). Nothing like ChemCam has been sent to
Mars (or any other planet) before. Table 9.1 lists facts about the ChemCam instrument.1
1 Two papers published before the mission described ChemCam: Maurice et al. (2012) and Wiens et al. (2012)
© Springer International Publishing AG, part of Springer Nature 2018
294
E. Lakdawalla, The Design and Engineering of Curiosity, Springer Praxis Books,
https://doi.org/10.1007/978-3-319-68146-7_9
9.2 ChemCam 295
Figure 9.1. In fanciful artwork created for the ChemCam instrument proposal in 2004, ChemCam zaps a rock. By Jean-Luc Lacour for the ChemCam team.
Table 9.1. ChemCam facts.
Mast unit mass
5778 g
Mast unit dimensions
384 x 219 x 166 mm
Body unit mass
4789 g (of which 2344 g is
thermo-electric cooler)
Body unit dimensions
197 x 238 x 154 mm
Fiber optic cable mass
63 g
Fiber optic cable dimensions
5753 mm x 1.4 mm diameter
Calibration target mass
161 g
Calibration target dimensions
146 x 51 x 16 mm; 1.56 m
from ChemCam window
RMI CCD
1024 x 1024 pixels
RMI FOV
22.5 mrad
RMI IFOV
78 to 85 μrad vertical, 87 to
105 μrad horizontal
Autofocus laser wavelength
785 nm
LIBS laser wavelength
1067 nm
296 Curiosity’s Chemistry Instruments
Table 9.2. Major and minor elements detected by ChemCam and APXS.
Major elements
non-metallic elements halogens minor and trace elements
ChemCam only
oxygen
hydrogen
fluorine lithium
(lighter elements)
carbon
rubidium
strontium
barium
ChemCam and APXS sodium
phosphorus
chlorine chromium
magnesium
sulfur
manganese
aluminum
nickel
silicon
zinc
potassium
calcium
titanium
iron
APXS only
bromine copper
(heavier elements)
Like APXS, ChemCam can only sense elemental composition; it isn’t able to tell how
the elements are arranged into minerals. Many rocks on Mars have essentially the same
elemental composition (the same as basalt) but completely different mineralogy and geo-
logic histories. Still, ChemCam is valuable for searching for targets for follow-up contact science. Also, ChemCam is able to detect lighter elements that aren’t accessible to
APXS. Table 9.2 lists the major and minor elements detectable by ChemCam and APXS.
ChemCam is one of Curiosity’s international instruments. The Principal Investigator for
ChemCam is Roger Wiens of the Los Alamos National Laboratory. The Deputy Principal
Investigator is Sylvestre Maurice at the Institute de Recherche en Astrophysique et
Planétologie. Part of it (the mast unit) was built at the Centre National d’Etudes Spatiales (CNES) in Tolouse, France, while Los Alamos built the body unit. ChemCam is operated
out of Los Alamos and CNES, alternating every other week, and holding a hand-over phone
meeting every Monday. The French ChemCam team must work very late into their local
evening, on clocks usually 9 hours ahead of those in Curiosity mission operations at JPL.
9.2.1 How ChemCam works
ChemCam consists of four distinct pieces of hardware: the mast unit, body unit, the elec-
trical and optical cables connecting them, and a calibration target. The mast unit contains the LIBS laser, telescope, and camera, as well as electronics. The body unit contains the
spectrometer.
9.2.1.1 The ChemCam Mast Unit
Remote warm electronics box. Figure 9.2 shows the mast unit. The mast unit is located inside the remote warm electronics box, the large “head” of the mast. The mast unit must
be kept to between –40°C and +35°C for instrument health. Thermostats inside the
9.2 ChemCam 297
Figure 9.2. ChemCam Mast Unit (front and back). Images courtesy Roger Wiens.
298 Curiosity’s Chemistry Instruments
electronics box trigger survival heaters every night, when the temperature falls below
–35°C. The instrument is designed to be used at temperatures between –20°C and +20°C,
so when ambient temperatures are cold, it is warmed to –15°C in order to be used. Any
warming is performed at a maximum rate of 5°C per minute.
LIBS laser. The LIBS laser is based upon a commercial laser, but was redesigned to
reduce its mass by a factor of 10 and to make it reliable under the rigors of spaceflight. Its wavelength is 1067 nanometers, in the near-infrared. It should be able to sustain at least 20
million shots over its lifetime. Individual pulses last 5 nanoseconds, and it can fire up to 10
times per second. The laser fires through a telescope that focuses the beam diameter to
between 0.25 and 0.35 millimeters.
Remote Micro-Imager. The Remote Micro-Imager is a flight spare from the camera sys-
tem developed for the Philae lander on ESA’s Rosetta mission. (Other flight spares of the
same instrument were used on the ill-fated Phobos Grunt mission.) RMI has a 1024-pixel-
square CCD and captures black-and-white images using wavelengths of light from 450 to
950 nanometers. It has an auto-exposure feature. It can repeatedly capture images as fast
as 1 frame per second. The RMI is intended to provide context imaging for LIBS shot
points at a resolution finer than
is achievable with the right Mastcam. It has also turned out to be useful for long-distance imaging.
Autofocus laser. To focus the laser on a target, ChemCam employs a second laser, with a wavelength of 785 nanometers, for an autofocus operation. The autofocus laser is mounted
to the back of the secondary mirror. To prepare for a ChemCam LIBS operation, the team
would estimate the distance to the target using Navcam stereo ranging, passing that value
to ChemCam as a part of the instrument’s commands. The autofocus laser illuminated the
target at 638 different test focus steps around that estimated distance, and a photodiode
recorded how the intensity of the reflected light varied with focus. The intensity followed a generally bell-shaped curve. These data were sometimes noisy and the top of the curve
sometimes flat, so electronics determined the best-focus position for LIBS and imaging by
finding the axis of symmetry of that intensity curve.
Autofocus using RMI. Unfortunately, the autofocus laser failed on sol 801. The ChemCam team implemented a quick workaround within 15 days. ChemCam was commanded to
perform nine sets of LIBS observations for each observation point at a range of focal dis-
tances around the best-guess distance determined from Navcam images; only one of these
would turn out to be in focus.2 This process generated results but was wasteful in terms of time and data, and the team worked quickly in parallel to develop a new autofocus method
using the RMI. The new autofocus capability was uploaded to Mars on sol 980 and used
for the first time on sol 983. The RMI takes nine to eleven images, and performs a simple
algorithm to measure image contrast, selecting the highest-contrast image to determine the
in-focus distance and then proceeding with LIBS analysis. The method is similar to that
employed by the Mastcams and MAHLI, except that the color cameras measure image
complexity rather than image contrast to find the best-focus position. Regardless of the
method, it takes the RMI about two minutes to autofocus.
2 Peret et al. (2016)
9.2 ChemCam 299
LIBS operation. Once the instrument has determined the best-focus position for LIBS, it is ready to fire. The LIBS laser beam passes through two sets of lenses, expanding it from