Emily Lakdawalla
Page 36
galactic cosmic rays have energies ranging from about 10 to 20 MeV.) It generates 13.4
million neutrons with each pulse, all within a period of about 2 microseconds. It can be
operated with a single pulse, but usually generates 10 pulses per second.
After an active pulse it can take several milliseconds for neutrons of different energies
to leak out of the surface. The detectors count up the arriving neutrons over time, produc-
ing a “die-away curve,” a graph of the number of counts with respect to time since the
8.3 DAN: Dynamic Albedo of Neutrons 283
Figure 8.5. Location of the DAN instrument components on the rover. Top photo: NASA/JPL-Caltech release PIA14257. Lower left: PIA15181. Lower right view with belly pan removed: NASA/JPL-Caltech, annotated by Emily Lakdawalla.
284 Curiosity’s Environmental Sensing Instruments
pulse for each detector. Stacking die-away curves from many pulses improves the signal-
to- noise ratio of the DAN data. The most commonly used DAN active observation includes
20 minutes of 10-hertz pulsing, or about 12,000 total pulses. The rover usually takes rear
Hazcam images during a DAN active observation to document the kind of material under-
neath the DAN instrument at the time.
The detector is expected to count roughly 10 neutrons returning from each pulse, or
about 100 counts per second during 10-hertz pulsing. The amount of neutrons that leaks
varies by a factor of a few, depending upon how much hydrogen and other neutron absorb-
ers are present in the surface. For comparison, the continuous neutron emission from the
MMRTG produces about 25 and 10 counts per second in the lead- and cadmium-shielded
detectors, respectively. During cruise, DAN detected higher counts of 35 and 15 per second
as a result of exposure of the spacecraft to galactic cosmic rays. Once on Mars, the back-
ground increased even further because of the response of the surface of Mars to galactic
cosmic rays. Thus the background is comparable in magnitude to the dynamic contribution
from the neutron generator. The DAN team uses the tenth-of-a-second delay between pulses
to measure the background, which can be removed from the results of active surveys.
Turning DAN active neutron counts into estimates of subsurface hydrogen abundance
requires mathematical modeling. The DAN team performs simulations with a large set of
models for the subsurface. The models all begin with a typical Martian soil composition
(based on APXS measurements from the Mars Exploration Rover mission). They also
assume a rate of incoming cosmic radiation, which is dependent upon the density of the
atmosphere above the rover at the time of the measurement, so they incorporate REMS
data on the atmospheric pressure at the time of the active DAN measurement. They allow
other model parameters to vary. Some models are homogeneous ones, in which the total
abundance of hydrogen and chlorine are allowed to vary. Other models are two-layer ones,
in which chlorine is held constant but the amount of hydrogen is allowed to vary in upper
or lower layers, and the layer thickness also allowed to vary. The models spit out die-away curves, and then the DAN team performs a least-squares analysis to find which set of
model parameters best fits the observed die-away curve.
Two things limit the lifetime of the active scanning capability of the DAN experiment.
The pulsing neutron generator has a warranted lifetime of 10 million pulses, so Curiosity
can expect to perform about 1000 typical active observations over its lifetime. It is not
likely that Curiosity will hit this limit, though, because DAN’s neutron generator also has a clock time limit of about 3 years after launch. Helium generated by the neutron- generating pulses and the decay of its tritium target eventually ruins the vacuum inside the ion accelerator. Similar hardware on Earth has lasted anywhere from 2 to 6 years, with a median
lifetime of 3 years, before failing. If the neutron generator fails, DAN will be less capable of estimating the depth of any subsurface layering that might be present, but it can still be used in passive mode to watch for subsurface variations.
8.3.3 Using DAN
DAN operates most of the time in a passive mode, its two detectors counting up neutrons
any time the rover’s computer is awake. The counts are typically binned every 20 seconds,
producing a nearly continuous record of the number of neutrons hitting the detectors,
including along rover traverses.
8.4 REMS: Rover Environmental Monitoring Station 285
Because of the neutron generator’s limited lifetime, the DAN team desired active scans
to be performed frequently during rover traverses. Initially, most drives of any length
included two to four mid-drive DAN active scans. But the 15-minute length of each DAN
observation traded directly against drive distance, so the strategy of frequent DAN obser-
vations was abandoned after sol 403. Instead, there is most commonly one DAN active
observation per drive sol. The observation may be performed after the end of a drive sol,
between drive sols (its most typical location during restricted-sol periods), or before the next sol’s drive. DAN cannot be used in active mode at the same time as some other rover
activities because of the neutrons it generates. Examples include ChemCam observations,
CheMin analyses, and driving or arm motion.
There were nearly 500 DAN active experiments performed over the course of the mis-
sion up to sol 1417. DAN has operated throughout the mission with no significant gaps in
coverage; nearly every rover stop is documented with a DAN active measurement. DAN
active measurements have fed back into tactical planning. DAN measurements of abun-
dant thermal neutrons on sol 991, combined with unusual ChemCam measurements of
rocks in the same area, led to the drilling of the high-silica target Buckskin below Marias pass on sol 1060. DAN had the opportunity to experiment on silica-rich materials at the
Greenhorn and Lubango sites on sols 1144 and 1329.
8.3.4 Anomalies
The three-year expected lifetime of DAN’s neutron generator ran out at the end of 2014,
but DAN continues to operate normally. In October 2016, it showed the first signs of deg-
radation. To the relief (and surprise) of the DAN team, the instrument returned to nominal
operations afterward. Still, the generator can be expected to fail any day. The DAN and
rover operations teams have collaborated on a workaround to allow DAN to continue
operations as the neutron generator ages, and they monitor its health regularly.
8.4 REMS: ROVER ENVIRONMENTAL MONITORING STATION
One of the signal accomplishments of NASA’s Mars program has been the continuous
monitoring of Martian weather and climate since the beginning of Mars Global Surveyor’s
science mission in 1998. The data set permitted scientists to develop general circulation
models for Mars’ atmosphere like those that have been developed for Earth. But moving
from global to smaller-scale models depends on surface meteorological data that has been
historically scarce. REMS is designed to gather the wind, temperature, moisture, and pres-
sure data necessary to constrain small-scale weather models for Mars. It also studies the
ultraviolet radiation that penetrates Mars’ atmosphere, which can have harmful effects on
organic molecules.7
7 The main paper describing REMS is Gómez-Elvira et al. (2012). Post-landing articles summarizi
ng REMS performance and results are Pla-Garcia et al. (2016), Smith et al. (2016), and Vasavada et al.
(2017)
286 Curiosity’s Environmental Sensing Instruments
The principal investigator of the REMS experiment is Javier Gómez-Elvira of the
Centro de Astrobiología (CSIC-INTA) in Spain. Most of the REMS components were
provided by the Centro de Astrobiología, except for the pressure sensor located within the
rover body and humidity sensor on Boom 2. They were provided by the Finnish
Meteorological Institute, who also contributed the pressure sensors on the Viking and
Phoenix landers.
8.4.1 Introduction: Gale weather
Gale is in a complicated weather regime, where global, regional, and local weather pat-
terns are all important, and the patterns change on daily and seasonal timescales.8 There
are three main global wind patterns. In the “daily thermal tide,” the Sun heats air on half of Mars, making the atmosphere expand and so decreasing its surface pressure, while
cooling air on the night side has higher pressure. Air flows from the high to low pressure
areas, generating a current that wants to flow perpendicular to the terminator, toward the
sunlit side and away from the night side of Mars. In the “hemispheric dichotomy slope
flow,” Mars’ global topographic dichotomy also drives winds, which flow upslope (north
to south) during the day and downslope (south to north) overnight. Finally, there is a sea-
sonal pattern driven by the heating and cooling of Mars’ poles: air rises at the hot pole and sinks at the cold pole near the solstices, and rises from the equator and sinks at the poles near the equinoxes. All of these effects combine to make complex patterns of winds that
change diurnally and seasonally.
Where is Gale in all of this? It’s close to the equator and has high elevations to the south but also has Elysium Mons to the north. At some times of year (like the southern autumnal
equinox at Ls=0), all the different circulations add up to produce practically no predicted wind in the Gale region. By contrast, near southern summer solstice at Ls=270, Gale is
predicted to experience strong winds blowing from north to south all day and night.
However, Curiosity may feel few of these global currents. Gale is a deep hole in the
ground, surrounded by high walls and containing a tall mountain in its center, and gener-
ates its own weather. During the day, wind blows upslope, toward the crater walls and the
mountain peak. At night, winds flow downslope. Curiosity landed near the deepest part of
the crater, and over the course of the mission it has moved from a wind regime controlled
by the crater walls to one controlled by the mountain.
Martian calendar information relevant to the REMS experiment is summarized in
Table 8.1. For more information on the Martian calendar, see section 3.2.
Table 8.1. Mars seasonal events and correspondence with Earth dates and Curiosity sols.
Mars Spring equinox
Summer solstice
Autumnal equinox
Winter solstice
year
(Ls = 0°)
(Ls = 90°)
(Ls = 180°)
(Ls = 270°)
31
Sep 13 2011
Mar 30 2012
Sep 29 2012 / sol 53
Feb 23 2013/sol 196
32
Jul 31 2013 / sol 350
Feb 15 2014 / sol 543
Aug 17 2014 / sol 722
Jan 11 2015/sol 865
33
Jun 18 2015 / sol 1018
Jan 03 2016 / sol 1212
Jul 04 2016 / sol 1390
Nov 28 2016/sol 1533
34
May 05 2017 / sol 1687
Nov 20 2017 / sol 2059
May 22 2018
Oct 16 2018
8 Rafkin et al. (2016)
8.4 REMS: Rover Environmental Monitoring Station 287
8.4.2 How REMS works
REMS consists of four units: two booms mounted on the mast, an ultraviolet sensor
mounted on the deck, and an electronics box located inside the rover (Figure 8.6). The electronics box contains a pressure sensor. Both Boom 1 and Boom 2 have wind and air
temperature sensors. Boom 1 has a ground temperature sensor, and Boom 2 has a humidity
sensor. REMS is a relatively autonomous instrument, gathering data 10 times per second
for the first 5 minutes of every hour, all the time. The science team also inserts several
longer periods (an hour or more) of continuous 10-times-per-second data collection into
every sol of activity, rotating the periods around the Martian clock to cover all times of day over a period of several sols. MAHLI is routinely used to monitor the condition of the
ultraviolet sensor, and occasionally to view the booms.
Figure 8.6. REMS components. Top left: Boom 1 as seen by MAHLI
(0526MH0003430000201119C00). Top right: Boom 2 as seen by MAHLI
(1572MH0006620020601062C00). Boards 2 and 3 of the Boom 1 wind sensor (orange)
failed upon landing. All three boards of the boom 2 wind sensor (yellow) failed around sol 1500. Bottom left: Mastcam mosaic of the ultraviolet sensor, imaged in the sol 1197 Mastcam self-portrait. The individual labels identify the different photodiodes. Bottom right: REMS
instrument control unit located in the body of the rover, which also contains the pressure sensor. NASA/JPL-Caltech/MSSS/Emily Lakdawalla.
288 Curiosity’s Environmental Sensing Instruments
8.4.2.1 REMS booms
The two booms are located 1.6 meters above the surface, sticking out from the mast, on
a part of the mast that does not rotate, so their orientations are fixed. Boom 2 projects
directly forward, while Boom 1 is oriented 120° clockwise from it, pointing back and to
the right side of the rover (see Figure 7.1 for their locations). The 120° separation was intended to make sure that one boom would always experience wind that did not lie in the
wake of the mast.
The wind sensors use hot film anemometry. By recording the amount of power it
takes to keep tiny titanium resistors at a constant temperature, the science team can fig-
ure out how effective the wind is at cooling the resistors, and use that to determine how
fast the wind has to be moving past the resistors. Because hot film anemometry depends
upon the temperature of the instrument, it’s best to have the resistors separated from the
hot rover as much as possible. It’s also best to have the resistors located as far as possible from anything that could obstruct the flow of the wind. The mast does disturb the motion
of the wind, so there were two booms to measure the wind blowing at different points
with respect to the mast. To keep the resistors disconnected from the boom structure,
they were mounted on little pedestals and attached to their electronics boards with
extremely thin wires. Unfortunately, two of the three boards on the wind sensor on
Boom 1 were damaged during landing, so the wind experiment has never operated as
designed.
The air temperature sensor consists of two thermistors on each boom, one at the middle
and one at the tip of the boom. The one at the tip is intended to record air temperature; the one at the middle helps to calibrate out any heating of the thermistor by heat conducted
through the boom itself. The electronic circuits that run the two booms, located at the base of each boom, must be kept above –70°C. When a sensor detects that the temperature falls
below that, it turns on its heater.
The ground temperature sensor uses three thermopiles to measure the infrared
bright-
ness temperature of the ground. It detects the temperature from an area to the right of the rover. Mastcam “clast survey” images, taken at the end of drives, cover the area of the
REMS ground temperature sensor field of view (see section 7.2.2.3). The MMRTG and its 2000 watts of waste heat are an important source of error in the REMS temperature measurements. There was concern that it might heat the air and ground around the rover,
including some of the ground within the view of the ground temperature sensor. However,
the data show little evidence of the MMRTG’s heat affecting air or ground temperature
measurements. 9
The humidity sensor employs a polymer film whose electrical properties change as tem-
perature and humidity change. The polymer film constantly responds to changes in the
environment, but humidity can only be read once the sensor receives power. Supplying
power to the sensor warms it, so the most accurate humidity measurements are the ones
made immediately after it has been powered on.10 Mars has little water in its air, but at night 9 Pla-Garcia et al. (2016)
10 Gómez-Elvira et al. (2012)
8.4 REMS: Rover Environmental Monitoring Station 289
the temperature drops low enough that relative humidity can reach as high as 70%. In places where the ground gets exceptionally cold at night (dusty places with low thermal inertia),
it’s possible that the near-ground air gets cold enough for water frost to precipitate in a very thin layer on rock surfaces.11 Curiosity has periodically looked for ground frost in the early morning after winter nights, but has so far not observed it.
8.4.2.2 REMS Ultraviolet sensor
The ultraviolet sensor has six photodiodes sensitive to different ultraviolet wavelengths.
Five of them (named UVA, UVB, UVC, UVD, and UVE and labeled in Figure 8.6) look at narrower slices of the ultraviolet spectrum, and the sixth (UVABC or UV total dose)
looks at the full ultraviolet range (Figure 8.7). By measuring the amount of light falling on the photodiodes and correcting for the solar elevation angle and amount of dust that has