Lunar and Planetary Science XXXI
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THE BEAGLE 2 ENVIRONMENTAL SENSORS: INSTRUMENT MEASUREMENTS AND
CAPABILITIES, M.C.Towner1, J.C.Zarnecki1, M.R.Leese1, M.R.Patel1, T.J.Ringrose1, B Hathi1, D.Pullan2 and
M.R.Sims2, [1]Unit for Space Science and Astrophysics, Physics Dept, University of Kent, Canterbury, Kent, UK,
mct@ukc.ac.uk [2]Space Research Centre, Dept of Physics and Astronomy, University of Leicester, University
Road, Leicester, UK
Summary: Beagle 2 is a 30kg lander for Mars,
optimised for exobiology, launching in 2003 as part
of the ESA Mars Express mission[1]. The expected
lifetime on the surface is 90 Martian days, with a
landing site near the equator. One of the instruments
on board is a suite of sensors for monitoring the local
environment, and hence help determine if life could,
or still can, exist there. The design of the suite is
strongly influenced by mass limitations and as such
consists primarily of microtechnology sensors, with
nine sensor subsystems weighing 180 grams. A
meteorological package will record wind speed and
direction, and atmospheric pressure and temperature;
a life environment subsystem will measure the local
radiation environment, the surface UV flux, and
measure the presence of oxidants such as hydrogen
peroxide at the ppb level. Additional sensors will
record atmospheric conductivity, dust impact rates,
and the upper atmosphere density profile (determined
by the acceleration encountered during probe entry
and descent).
Introduction: The primary goal of the
environmental sensors, given the limited resources
available, is to support the rest of the mission by
providing measurements of short (minutes, days) and
long term (seasonal) variations in the local
environment.
Specifically
this
involves
measurements in three particular areas:
• Meteorology, including transient processes (such
as 'dust devils')
• Local radiation environment and likely
biological influences.
• Atmospheric properties during Beagle 2
entry/descent
In addition to this, three additional priority
measurements of particular scientific interest were
identified. Resources on the Beagle 2 lander are
particularly scarce, leading to an overall mass budget
for the nine chosen sensors of 180gm in total. This
has forced a move to commercial microtechnology
based solutions, several of which have not been used
on space missions before.
Wind,
Atmosphere
conductivity
Temperature
Dust, Oxide,
UV,
Radiation,
Pressure
Accelerometers
0.6m
Meteorology sub-system
Wind sensor This sensor is mounted on the top of
a 0.6m mast, and uses an innovative commercial
design[2], that utilises the ultrasonic resonance
between parallel disks. This allows the sensor to be
relatively insensitive to atmospheric pressure, unlike
conventional anemometers. Range is 0-40m/s,
direction is measured to within to 3O. In addition, this
approach also gives a direct reading of the
atmosphere temperature as a 'by-product' of the
technique.
Temperature sensor Air temperature will be
monitored using a copper-constantan thermocouple
arranged similarly to the Mars Pathfinder design[3].
It is located on the edge of one of the solar panel
sheets, to minimise interference from the probe body.
Expected absolute accuracy is 0.1K, with a resolution
of 0.05K. In combination with the wind sensor, this
will give simultaneous measurements of temperature
at two different heights.
Pressure Sensor This sensor is based on a
capacitive diaphragm design. It has a range of
0-25mBar, with an absolute accuracy of 200µBar and
a resolution of 50µBar[4].
Life environment sub-system
Radiation sensor This sensor is a silicon diode,
and will provide count rate and magnitude
information on the flux of high energy cosmic rays
and solar protons at the Martian surface.
UV sensor Short wavelength UV, such as UVB
and C are harmful to life, and can directly damage
DNA. The UV environment on Mars is known to be
harsh, and it is unlikely that life can survive on the
surface, but subsurface life may still be possible. The
sensor is a simple array of 5 photodiodes with
Lunar and Planetary Science XXXI
1028.pdf
THE BEAGLE 2 ENVIRONMENTAL SENSORS: M.C.Towner et al
appropriate bandpass filters, giving a 5 point
spectrum from 200-400nm.
Wavelength
Comment
(nm)
210
Main TiO2 dust absorption band
230
Biologically damaging and
rapidly time varying regime
250
Secondary TiO2 band
300
Mid UVB
350
Mid UVA
Oxidant sensor One controversial issue arising
from the results from the Viking landers is the
postulated presence of hydrogen peroxide (H2O2) or
other oxidising compounds in the soil, used in several
cases to explain the results of the experiments
designed to detect Martian life. This sensor uses an
acoustic approach to detect levels of oxidising gas,
with a minimum detectable level of 10ppb of H2O2 or
Ozone.
Additional sensors
Relaxation sensor A measurement of atmospheric
electrical conductivity provides useful science for
several reasons: estimates can be made of the work
function of the surrounding material, the
photoelectron flux density, and the magnitude of dust
charging effects[6].
The measurement technique uses a simple
electrode that is biased and then isolated, allowing
the charge to decay via leakage through the
atmosphere. By cycling the sensor during both the
day and night, photoelectric effects can be
distinguished.
Dust impact sensor Impacts from dust in the
atmosphere of Mars will help to indicate how
material is moved over the planet's surface. The
detector is a simple 50x50mm Al sheet, 0.25mm
thick, with a piezoelectric film on the rear face.
Planned sensitivity is 1x10-10 kgms-1 (equivalent to a
0.2mm glass bead from 10mm height on Earth)
Accelerometer Measurements of deceleration of
the probe during the atmospheric entry and landing
sequence can be used (in combination with the drag
coefficient for the heat shield) to derive the upper
atmosphere density and pressure. See for example
Schofield et al[7] for analysis of the Mars Pathfinder
data.
A three axis sensor will be used, with ranges of
±25g on the probe axis, and ±5g perpendicular to the
axis. During the early entry phases pressure and
density can be derived in upper atmosphere with an
initial vertical resolution of 150m. Horizontal wind
speeds will be monitored during later descent after
chute deployment, and the probe tilt once at rest will
also be recorded.
Measurement Strategy Throughout the surface
mission lifetime, each sensor will be sampled at a
low rate, typically taking one reading from each
sensor every 30 minutes. In addition to this, to study
quickly changing conditions such as the dust devils
seen by Viking[8] and Mars Pathfinder[7], the wind,
temperature, pressure, and dust sensors will have a
high sampling rate mode (1 per second), whereby
data from the previous 5 minutes is buffered and only
returned to earth (along with a further 5 minutes of
data) should any transient effects be detected. Of
particular interest are ‘dust devils’, which may be the
primary source of dust movement on Mars, and
responsible for the homogeneity of the dust measured
at the Viking and Pathfinder sites.
Acknowledgements: David Catling and Aaron
Zent, NASA Ames are thanked for fruitful discussion
concerning the UV and Oxidant sensors. To date
work has been carried out on this project has been
funded by the Particle Physics And Research
Council. Future work will be directly funded through
the Beagle 2 project, of which Colin Pillinger at the
Open University is Lead Scientist.
References: [1] R Schmidt, JD Credland, A
Chicarro, P Moulinier, ESA Bulletin-EUROPEAN
SPACE AGENCY, 1999, No.98, pp.56-66[2] FT
Technologies Ltd, Church Lane, Teddington,
Middlesex, UK. http://www.fttech.co.uk [3] A Seiff,
JE Tillman, JR Murphy, JT Schofield, D Crisp, JR
Barnes, C LaBaw, C Mahoney, JD Mihalov, GR
Wilson, R Haberle, JGR 102, E2, 4045-4056, 1997
[4] Kavlico Corp, CA [5] HP Klein, NH Horowitz, K
Biemann, p1221-1233, Mars, eds Kieffer, Jakosky,
Snyder and Matthews, Univ of Arizona Press, 1992
[6] R Grard, Icarus 114, 130-138, 1995 [7] JT
Schofield, JR Barnes, D Crisp, RM Haberle, S
Larsen, JA Magalhaes, JR Murphy, A Seiff, G
Wilson, Science 278, 1752-1758, 1997 [8] For
example, PP Thomas and P Gierasch, Science 230,
175-177, 1985.