FIRST RESULTS OF THE RELATIVE HUMIDITY SENSOR ON BOARD
M2020 PERSEVERANCE ROVER
M. Hieta, J. Polkko, I. Jaakonaho, M. Genzer, A.-M. Harri, Finnish Meteorological Institute, Helsinki, Finland (maria.hieta@fmi.fi), G. M. Martínez, Lunar and Planetary Institute/USRA, Houston, TX, USA, L.
Tamppari, M. de la Torre Juárez, Jet Propulsion Laboratory/California Institute of Technology, Pasadena,
CA, USA, J. A. Rodriguez-Manfredi, Centro de Astrobiologia (INTA-CSIC), Madrid, Spain

Introduction:
MEDA HS is a relative humidity sensor for Mars
2020 Perseverance rover [1] provided by the Finnish
Meteorological Institute (FMI). MEDA HS is a part
of Mars Environmental Dynamic Analyzer (MEDA),
a set of environmental sensors provided to NASA by
the Centro de Astrobiología (CAB) at the Instituto
Nacional de Técnica Aeroespacial in Madrid, Spain
[2].
MEDA’s principal goals are to provide continuous measurements that characterize the diurnal to
seasonal cycles of near-surface environment and local environmental dust properties. The main scientific goal of the humidity sensor is to measure relative humidity of the Martian atmosphere near the
surface and to complement the previous Mars mission atmospheric measurements for better understanding of the Martian atmospheric conditions and
the hydrological cycle. Relative humidity has been
measured from the surface of Mars previously only
by Phoenix and Curiosity so MEDA HS will provide
valuable data from the near surface environment
[3,4,5].
Accuracy of the sensor calibration has been analysed in detail and assessment of sensor performance
after landing on Mars confirms that the calibration
has been successful and sensor performance is as
expected. MEDA HS has been operating on Martian
surface for over 400 sols without issues and is
providing high quality data for the science community. This presentation describes the MEDA HS sensor and the first results from M2020 mission.

MEDA humidity sensor description:
The MEDA HS is a humidity instrument which
measures relative humidity. MEDA HS is built
around the capacitive HUMICAP® sensor technology by Vaisala and the reading electronics are based
on an oscillator transducer that converts the capacitance of the sensors into frequency.
Each HUMICAP sensor head has its own temperature sensor: on-chip platinum Pt1000 platinum
resistance thermometer (PRT). Pt1000 sensors are
read by the MEDA Instrument Control Unit (ICU)
and they are used for two purposes. Pt1000 1 with 2wire measurement is used for monitoring regeneration temperature, and Pt10002 with 4-wire measurement is used for for scientific temperature meas-

urement during nominal operation. Having the reference temperature measurement in the actual
HUMICAP sensor chip is a major improvement
compared to the previous generation sensors.

Figure 1: Left: MEDA humidity sensor assembly. The
PCB and sensor heads are located inside the cylindrical
part covered with white PTFE filter to protect from dust.
Right: MEDA HS location in the remote sensing mast.
Credit: NASA/JPL-Caltech

MEDA HS operational modes:
Humidity sensor operations are controlled by ICU's
flight software. In the beginning of the mission
MEDA nominal operational cadence was to measure
autonomously around the clock, 1 hour on and 1 hour
off alternating between even and odd hours. The
nominal acquisition of HS is to get readings from all
transducer channels with 1 Hz interval. The
HUMICAP sensor heads react to the surrounding
relative humidity even when the sensor is off so after
powering on the relative humidity can be read almost
instantly. After 1 s the readings are considered reliable and during HS calibration seconds 2-5 are averaged to get the most accurate reading of the sensor.
Self heating will start to affect the readings after few
seconds.
The HS has two operational modes: highresolution interval mode (HRIM) and continuous
mode. In HRIM the HS is powered on only for 10
seconds and then powered off to avoid self-heating.
HRIM can be used with different intervals and 15
minute and 5 minute intervals are configured in
MEDA. In continuous mode the sensor stays powered on for long periods. Both HRIM and continuous
mode can be used in HS operations considering the
advantages and limitations of each mode. HRIM is
best for measurements requiring the best accuracy
but temporal coverage is better in continuous mode.

Figure 2: Measuring cadence of different measurement
modes of the MEDA HS. High accuracy HRIM measurements have been used during the peak RH hours of
the sol.

MEDA HS maintenance regeneration:
Regeneration heating can be performed periodically
to remove any possible contaminants from the sensor
head. The HUMICAP sensor heads are heated to
160°C - 170°C which also removes the CO2 absorbed
in the sensor polymer. This operation causes a temporary recovery effect in the RH readings for few
sols when the daytime RH rises and nighttime RH
drops. We recommend not using 2 sols right after
each regeneration at all, sols 3-10 can be used with
increased uncertainty. After 10 sols the readings can
be used normally. Very first regeneration of the HS
was in sol 63 and we don’t recommend using HS
data before that. Following regenerations have been
performed on sol 74 and 180 so the data between the
sols 63-80 and 180-190 are compromised.
MEDA HS data products:
The main data products of the MEDA HS are the
local relative humidity, local sensor temperature and
water vapour volume mixing ratio (VMR). The derived relative humidity is the average of both sensor
heads. Local temperature is sensor temperature and
measured from Pt10002 sensor integrated in
HUMICAP sensor head. Relative humidity is given
relative to ice. The sensor temperature is not the
same as local atmospheric temperature so it needs to
be taken into account when using the relative humidity data. The HS data is archived and published in the
Planetary Data System (PDS) for further use of the
scientific community.

Figure 3: Example of daily relative humidity and sensor
temperature from sols 109-115. Only high accuracy
measurements are included.

Daily relative humidity cycle and nighttime
VMR:
MEDA has been measuring the environmental parameters around the sol day and night. An example
of daily relative humidity and sensor temperature can
be seen in Figure 3. Relative humidity reaches its
maximum value in dawn before sunrise when the
temperature is lowest. After sunrise the RH drops
close to zero. While HS does show these low daytime values, the humidity is below the accuracy of
the sensor and the readings are not scientifically useful.
VMR is derived from RH readings only when the
relative humidity is high enough (for example >2%
RH is a good limit) to give meaningful readings. Figure 4 shows an example of nighttime VMR measurements and Figure 5 shows maximum RH up to sol
299.

Figure 4: Example of nighttime VMR values derived from
RH. During day the relative humidity is close to zero and
the relative uncertainty is large and would lead to very
large uncertainties in daytime VMR values.

Figure 5: Maximum relative humidity values measured by
MEDA HS up to sol 299. RH is referenced to sensor internal temperature.

Seasonal changes:
Perseverance rover has been on the surface of Mars
for over 400 sols and seasonal changes begin to show
in the MEDA HS data. Figure 6 shows the changing
daily cycle of relative humidity and Figure 7 the
evolution of nighttime VMRs. The landing was during the spring season and VMR started to rise towards the summer. Gaps in the data are due to operational limitations like the solar conjuction. For meteorological predictions for Jezero crater see for exam-

ple

Pla-García

(2020)

[6].

Figure 6: First 299 sols of nighttime VMR values provided by MEDA HS
.

Figure 7: First 299 sols of daily relative humidity profiles
provided by MEDA HS.

Conclusions:
MEDA HS on board the M2020 rover provides relative humidity readings of the near surface atmosphere. MEDA HS has operated flawlessly after integration to Perseverance rover, during cruise and finally after landing. The first measurements from the
surface of Mars were as expected and the first regeneration cycles of the sensor heads has been performed successfully. A comprehensive measurement
uncertainty analysis has been performed for the sensor and the results show that HS is providing high
quality data from the Martian surface to provide important meteorological observations and to support
MEDA and other M2020 investigations.
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