An European seismic network on
Mars with NETLANDER
Banerdt2 J. Gagnepain-Beyneix1, 2, A.Mocquet7,
T. Spohn5, J.F. Karczewski10, 2, P. Schibler1,
S. Cacho1, W.T. Pike4, C. Cavoit11, A.
Desautez1, M. Favède1, J. Pinassaud12,
T.Gabsi1, M. Campillo13, A. Deschamp14,
J. Hinderer6, J.J. Lévéque6, J.P. Montagner2,
L.Rivéra6, W. Benz15, D. Breuer5,
P.Defraigne8, V. Dehant8, A.Fujimura16,
H. Mizutani16, J. Oberst9.
des Etudes Spatiales, Institut de Physique du Globe de Paris, 4 Avenue
de Neptune, 94100 Saint Maur des Fossés Cedex, France.
de Sismologie, Institut de Physique du Globe de Paris, 4 Place Jussieu,
75252 Paris Cedex 05, France
of Geophysics, ETH-Hönggerberg, CH-9093 Zürich, Switzerland
Propulsion Laboratory, California Institut of Technology, 4800 Oak
Grove Dr. Pasadena, CA 91109, USA
of Planetology, Wilhelm-Klemm Str. 10, D-48149 Münster, Germany
de Physique du Globe de Strasbourg, 5 Rue René Descartes, 67084
de Planétologie et Géodynamique, Faculté des Sciences
et des Techniques, 2, Rue de la Houssinière, BP92 208, 44322 Nantes
Observatory of Belgium, 3, avenue Circulaire, B-1180 Bruxelles, Belgique
Institute of Planetary Exploration, Rudower Chaussee 5, D-12489 Berlin,
des Observatoires, Institut de Physique du Globe de Paris, 4 Avenue de
Neptune, 94100 Saint Maur des Fosses Cedex, France.
de Recherche Géophysique, Institut National des Sciences de l'Univers,
Technique de l'Institut National des Sciences de l'Univers, 4 Avenue de
Neptune, 94100 Saint Maur des Fosses Cedex, France.
de Géophysique Interne et de Tectonophysique Observatoire de Grenoble,
IRIGM, BP53X 38041 Grenoble Cedex, France.
Géosciences Azur, 250 rue A. Einstein, 06560 Valbonne, France.
Institüt, Universitaet Bern, Sidlerstrasse 5, 3012 Bern - Switzerland
of Space and Astronautical Science, Yoshinodai 3-1-1, Sagamihara, 229-8510,
Netlander mission -
Instrument description -
The interior of Mars is today
poorly known, in contrast to the Earth interior and, to a lesser extent,
to the Moon interior, for which seismic data have been used for the determination
of the interior structure. This is one of the strongest facts motivating
the deployment on Mars of a network of very broad band seismometers, in
the framework of the Netlander mission, a set of 4 landers developed by
a European consortium.
Despite a low mass, the seismometers
will have a sensitivity comparable to the present Very Broad Band Earth
sensors, i.e. better than the past Apollo Lunar seismometers. They will
record the full range of seismic and gravity signals, from the expected
quakes induced by the thermoelastic cooling of the lithosphere, to the
possible permanent excitation of the normal modes and tidal gravity perturbations.
All these seismic signals will be able to constrain the structure of Mars'
mantle and its discontinuities, as well as the state and size of the Martian
core, possibly close in time for the centennial of the discovery of the
Earth core by Oldham .
Our present understanding of
the interior structure of Mars is mostly based on the interpretation of
gravity and rotation data, or on the chemistry of the SNC meteoroids, and
its comparison with the much better-known interior structure of the Earth
(e.g. Schubert and Spohn, 1990; Longhi et al, 1992). Mars is unfortunately
a seismically unexplored planet. Most of the past seismic experiments have
indeed failed, either due to a launch failure, as for the
seismometer [Lognonné et al, 1998] onboard the small surface stations
of Mars 96 [Linkin et al, 1998], or after a failure on Mars, as for the
seismometer onboard one of the two Viking landers. The only attempt to
address seismology on Mars was therefore made with the Viking seismic experiment
onboard the remaining Viking 2 lander [Anderson et al, 1977]. However,
it did not result in convincing marsquake detection, basically due to too
strong wind sensitivity, as well as to a too low resolution during non-windy
conditions. Seismology on Mars thus remains to be done. See Lognonne and
Mosser, 1993, for a review on Planetary seismology.
After almost one decade of
continuous activities and proposals, such as the ESA Marsnet, NASA Mesur,
and ESA-NASA InterMarsnet [Chicarro et al, 1991, Solomon et al, 1991, Chicarro
et al, 1993, 1994, Lognonné et al, 1994, Banerdt et al, 1996], the
first network mission on Mars is expected to be launched between 2005 and
2007 under the name of NetLander. See Harri et al., 1999 for a complete
description of the Netlander mission and Lognonné et al., 2000 for
a complete description of the Seismic experiment.
The Netlander mission
NetLander mission aims at deploying on the surface of Mars a network of
4 geophysical and meteorological landers implemented by the
and realized by a European and American consortium :
PRODEX), Switzerland (PRODEX) and USA (JPL).
The NetLander Mission is to be considered in the general context of international
Mars exploration. In order to reduce the mission costs, NetLander relies
on contributions from other Mars missions: the landers will use a rocket
carrying a mission performing an orbit insertion or a Mars-fly by for the
cruise. One assumption is the Ariane V launcher and the orbiter of the
Mars Sample Return mission. Another is a Starsem launcher and the cruiser
of the Mars Asteroid ESA mission. Once on the Mars surface, they will be
operated through the Mars Express Orbiter, which will provide the telemetry
and telecommand capability, especially for returning the high data rate
associated with imaging and seismic data.
During the cruise phase a
link between the spacecraft and the NetLanders will provide the landers
with the required power and telemetry for periodical check-ups and secondary
battery refreshment during flight phase, as well as full charge at the
end of cruise phase. The NetLanders will be separated from the spacecraft
several days before arrival at Mars. Propellant provision is included to
allow for the delta-V needed to perform the orbit maneuvers in order for
the landers to land at the required sites. The entry, descent and landing
phase are performed autonomously. The Front Shield provides efficient braking
of the NetLander, allowing parachute deployment at velocity and altitude
conditions compatible with the required velocity at impact (about 25 m/s).
The impact shock is then absorbed by means of airbags. Once the airbags
are released, a Surface Module is put in its correct position, the solar
panels (located on 3 petals) are deployed, and the instruments are activated.
The entry and landing sequence is depicted in Figure 1 and a movie can
be downloaded for more details
The final selection of the
landing sites will be a compromise among the scientific requirements and
operational constraints. In particular, the altitude is limited by the
performance of the parachute system, and the availability of sufficient
solar power poses restrictions on the latitude range (e.g. ± 35°).
The network could consist of 3 of the 4 landers forming an equilateral
triangle (one station on the equator, the other two at -35° and +35°
latitude, respectively), the fourth one being diametrically opposed. The
preliminary calculations show that the total delta-V necessary to create
such a network is about 100 m/s, with the separation from the spacecraft
occurring approximately 5 days prior to the arrival at Mars.
The NetLander mission is going
to enable a leap forward in our understanding of the contemporary and past
state of Mars. To further improve the characterization of Martian atmospheric,
surface and internal phenomena - exhibiting both spatial and temporal variation
- simultaneous observations at spatially displaced sites are required;
hence the logical next step is a network of observation sites on
the surface. Several network concepts have been proposed in the past, with
meager success. The NetLander mission would hence be the first mission
to deploy a network of a moderate number of well-instrumented geophysical
and meteorological observation posts onto the Martian surface. The following
specific scientific disciplines will be addressed:
boundary layer (PBL) phenomena,
structure at the km scale, down to water rich layers,
mineralogy and local geology,
processes and surface/atmosphere, interaction,
electricity, and ionospheric structure,
The payload of the mission
was selected in April 2000 following an
Announcement of Opportunity released in November 1999. We present here the seismometer
instrument selected by this Announcement of Opportunity by the Netlander
Steering committee. More information on the Netlander mission on the Netlander
web site of CNES,
Figure 1: Entry and landing
sequence of the Netlanders. The mass of the lander after separation from
the space craft if about 65 kg, while the mass on the ground is about 25
kg. A movie can be downloaded for more details
One of the main scientific objectives
of this 4 landers network mission will be the determination of the internal
structure of the planet , which remains basically unknown (for a review
about Mars and its internal structure, see Spohn et al ). A seismometer,
a magnetometer and a geodetic experiment will therefore be deployed, allowing
a complementary approach. Geodesy and magnetic objectives are detailed
by Dehant et al  and Mocquet and Menvielle .
The seismic data will first
consist of the recording of the natural quakes of the planet, and possibly,
the recording of seismic signals generated by meteoritic impacts. Marsquakes
may be generated through the release of thermal stresses, but the
amplitude of the Mars seismic signal is still expected to be about 4 orders
of magnitude lower than on the Earth (see Golombek et al  for estimates
from surface fault observation and by Philipps et al  from a modeling
thermo-elastic cooling of the lithosphere). It might provide about 14 quakes
of seismic moment 1015 Nm per year, with an increase/decrease
of the frequency by 5 for a decrease/increase of the seismic moment by 10.
waves, another limitation could be related to attenuation and scattering.
The high attenuation of Mars is confirmed by the high secular acceleration
of Phobos, which needs a quality factor of about 100 at the tidal period
of Phobos and was extrapolated in the seismic range to about 350 by Lognonné
& Mosser . Very likely, the Mars mantle, due to its low pressure,
is in a thermodynamically state more comparable to the highly attenuating
Earth upper mantle rather than to the lower mantle, which features lower
attenuation. Mocquet  has studied the propagation characteristics
on Mars and has shown that a severe reduction of the amplitude of S waves
at epicentral distances greater than 50° is founded as a consequence
The detection of small
magnitude quakes will however be much easier than on Earth, due to the
a-priori low noise level on the Martian surface as Mars has no ocean, responsible
for the high noise level in the 30-1s range on the Earth. We can
therefore almost expect noise level less than 10-8 m.s-2/Hz
and very likely close or less to 10-9 m.s-2/Hz
For such noise, a quake with a seismic moment of 1015 Nm will
therefore generate at epicentral distances of 50° signals with a S/N
ratio of about 30 for the S wave and 200 for P. Figures 2a and 2b
detail these perspectives for a possible Network configuration, assuming
total detection for signals with accelerations greater than 10-8
m.s-2 peak-to-peak. They show that a rate of 60% can be achieved
for the detection of quakes with seismic moment greater than 1014
Nm, i.e., corresponding to Earth magnitude greater than 3.2. This might
provide about 100 detected quakes out of the 140 quakes that are expected
during the two years of the NetLander mission lifetime to occur with at
least this seismic moment.
Figure 2a: Detection zone
of P and S waves for one of the studied configuration (MEM, LYS, TTS and
HEE are respectively Menomnia Fosae, Lycus Sulci, Tempe Terra South and
Hellas East). The white zone on to the east of TTS corresponds to the joint
shadow zone of MEM and HEE. Note that the detection efficiency is very
high in the Tharsis area, where small quakes of seismic moment of 1014
Nm might be detected.
Figure 2b: Detection zone
for a detection of a PKP wave in at least one station and of P&S waves
in the three other stations. The success rate is 16%, leading to about
22 quakes detected during the two years of operation.
However, in parallel to these
quake data, the experiment will also search for continuous seismic and
gravity signals. These signals are associated to two continuous sources:
- The first one, in the frequency
band of 0.1-10 mHz, is the atmosphere, more exactly the atmospheric turbulences.
As recently discovered by Nawa et al , Suda et al , these turbulences
on the Earth excite indeed continuously the fundamental branch of spheroidal
Earth normal modes. As shown by Kobayashi & Nishida , such excitation
process on Mars might be almost as strong as on the Earth. The inversion
of the detected free frequencies of the excited normal modes might then,
without any quake, provide information on the shear structure of the upper
Martian mantle, as well as on the state and size of the core for the gravest
modes [e.g. van Hoost et al, 2000]. More information can be found in recent
and New Scientist.
- The second one is the tide
of the Sun and of the two small Martian satellites, Phobos and Deimos.
The observation will focus on the tides produced by Phobos and the Sun,
the latter producing a displacement of a few cm associated with a tidal
forcing of the order of 10-7 m/sec2. These observations
do not only give information on the direct attraction of these bodies on
Mars, but also on the surface deformations (Love number h) and on the induced
mass redistribution (Love number k). The contribution of the core to the
sun tidal response could be of the order of half a mm. Here, the signal
used will be the amplitude of the tidal gravimetric factor, which is a
linear contribution of the Love numbers h and k. It corresponds to the
Mars transfer function to the external tidal forcing. Details on this transfer
function are given in Dehant et al .
All these possibilities are
however closely related to the performance of the instruments and their
installation. Concerning the installation, the first priority will be to
have a direct contact with the ground together with a mechanical decoupling
of the sensor from the lander, as opposed to the Viking design. In this
configuration, it will still be necessary to shield the seismometer from
the direct effect of the wind on its structure. Tests were performed with
a mock-up of a seismometer platform covered by a simple windshield. They
have shown that vertical noise spectral amplitudes of a few 10-8
m.s-2/Hz1/2 near 1 sec and few 10-9 m.s-2/Hz1/2
close to 20 sec can be realistically achieved with seismometer deployed
at the Earth surface [Lognonné et al, 1996]. On horizontal components
the noise has a spectral density of a few 10-8 m.s-2 /Hz1/2.
In both cases, a further improvement of the signal to noise ratio can probably
be achieved by recording the variations of the meteorological parameters
(temperature, pressure) in the seismic frequency band and by performing
a real-time decorrelation, as demonstrated by Beauduin et al . A
level of 10-9m.s-2/Hz1/2 resulting from
the conjunction of a windshield, a good thermal protection, and the a priori
low ground noise is a reasonable assumption. It is therefore necessary
to have an instrument with a smaller noise level, in the seismic band,
as close as possible to 10-10 m.s-2/Hz1/2.
The Very Broad Band (VBB) seismometer
was developed through a Research and Technology Program of CNES, in preparation
of the InterMarsnet program, using some heritage of the OPTIMISM seismometer,
which was on board the Mars 96 Small Surface Stations [Lognonné
et al, 1998]. This instrument is composed of two tilted axes (see Figure
3a) providing, after recombination, data of one horizontal axis and one
vertical axis. The limitation to two axes is related to the size of the
lander, much smaller than the InterMarsnet lander. However the use of a
micro-seismometer (see Figure 3b) for the second "missing" horizontal component
allows the recovery of all three axes , even though the instrument noise
on this second component is expected to be about two orders of magnitude
greater [Figure 4].
Figure 3a, Left: Breadboard
of one VBB axis. All the structure of the seismometer is realised in titanium,
in order to reduce the thermo-elastic deformations.
Figure 3b, Right: Silicium
wafers of the 3-axis BRB seismometer proposed by the Jet Propulsion Laboratory.
The assembly consists in the stack of the three wafers. (Note: the French
coin, on the left, and the US coins, on the right have about the same size,
2.4 cm in diameter)
The VBB instrument has been
designed to study the very low accelerations associated with the small
magnitude of Marsquakes, in a thermal environment featuring very strong
temperature variations [Cacho et al, 1999]. Therefore, this sensor is very
sensitive, has a very low self noise and has a design relatively insensitive
to environmental parameters ( as low as 80 nm/K for the inertial mass sensitivity
to temperature). The seismic mass is therefore in an evacuated sphere and
less sensitive to pressure changes. This vacuum as well as a carefull design
allow also a severe reduction of the brownian noise: despite the small
inertial mass, a Q-factor close to 400 for a frequency close from 1.5 Hz
is achieved. Its has an internal temperature compensation and high thermal
dewar-like protection, limiting the seismic mass displacement to less than
5 microns over the daily cycle. And finally, the design is compatible with
an operation both with Earth and Mars's gravity.
Two independent transducers
do the displacement measurement:
- An Oscillating Cavity Sensor
(OCS), producing a digital tidal output (f < 10-3 Hz), with
a dynamic range of 32 bits;
- A Differential Capacitive
Sensor (DCS), providing an analogue long period seismic output (10-3-10
Hz), with a dynamic range of 140 dB (with a spectral noise close to 1 pm/Hz1/2
at 1 Hz).
Pressure and temperature will
be recorded with high resolution (µb and µK) in order to remove
their influence on the seismic signal. This will be done by the CDMS for
the seismic signals with sampling rate higher than 1 sps, and on the Earth
for the tidal outputs. The total mass of the complete instrument (VBB,
SP, environment sensors with installing, levelling devices and data logger)
will be about 1935 g without margins. The average power requirement will
be about 600 mW (900 mW peak during the installation).
Figure 4 Performances
of VBB and JPL seismometers.
Feedback, necessary for an
instrument with such sensitivity and high Q will be performed with both
sensors, in order to reduce the mechanical recentering and to provide the
ideally continuous record necessary for tidal analysis. The OCS contributes
to the digital part, to prevent long period drifts, and the DCS contributes
to the analogue part, permitting an adjustment of the gain, of the stability
of the instrument and of the reduction of the resonance peak. The performances
expected by the VBB sensor are shown on the Figure 4, for the mock-up presently
being tested as well as for the future Flight Models. A comparison between
the record of a STS2 and of a mock-up is given in Figure 5. The performance
will depend on the final optimisation of the mechanical (free period, Q)
and electrical parameters (feedback and displacement transducer gain) of
the sensors, in order to minimise the sum of all the noise sources.
A commercial Earth version of this Mars seismometer is expected in a near
future by the
SODERN compagny (Corlay,
personnal communication), who realized both the
OPTIMISM experiment and R&T VBB seismometer.
Each VBB axis is equipped
with a device allowing a complete and precise levelling of the pendulum.
The system used for the seismometer deployment, even if not comparable
with a full robotic or human installation, is therefore expected to provide
a good installation. The influence of the lander on the seismometer will
be minimised due to the decoupling and coupling with ground at high frequency
(f > 10 Hz) will be enhanced. Finally, the lander will act as a thermal
and windshield for the instrument. This system will be designed to work
even if the lander is tilted (up to about 30°). Except during installation
when a special acquisition sub-routine will be used, the standard process
involved for data acquisition will be based on a high-rate sampling of
all parameters, and its transfer to the large-capacity on-board memory.
These data will be stored in a 4 Gbits turning buffer, which will therefore
complete a full turn in little less than 20 days. Between 5 to 7 Mbits
of data will be transmitted back to Earth every day. These data will
correspond to several continuous channels ( LPZ, VLPX, VLPY, VLPZ,
with 1 Hz for LP and 1/10 Hz for VLP), as well as post-detected event data
on the VBB and SP channels ( 20 Hz for VBB, 100 Hz for SP)
Figure 5: Earthquake recorded
by an STS2 and by a mock-up of the VBB seismometer, on May 30, 1998. The
epicentre was in Afghanistan. The signal was recorded in a seismic vault,
in Saint Maur, France. The quake has a magnitude of 6.9, with a focal depth
of 33 km. The STS2 outputs were used to compute the oblique output compared
to the mock-up signal. Note however that the breadboard was operating without
feedback and at a slightly different place than STS2, which may explain
the small differences between the two signals.
The NetLander mission is expected
to provide the first network on Mars. The expected performances of the
seismometer planned in the payload will allow monitoring the Martian seismic
activity, especially in the Tharsis area. An attempt to monitor the continuous
excitation of normal modes, as well as the gravity tides of the Sun and
Phobos will also be performed.
The NetLander mission, presently
in study, is expected to start the realization phase after the confirmation
of the payload in April 2000. Delivery of the instrument is expected 1
to 2 years before launch. The seismic network will operate from 2008 to
early 2010, i.e. during thus Martian year, after a launch in 2007
With these new seismic data,
much of the unknown internal structure of Mars will be discovered. Possibly
little more than one century after the discovery of the Earth core by Oldham
, the core of a second telluric planet might therefore be characterized.
and Team composition
The VBB seismometer was developed
by IPGP-DT/INSU and SODERN inc. under a CNES R&T program, and with
the academic technical staff supported by CNRS-INSU. The VBB sensor and
experiment integration is under the responsibility of the Institut de Physique
du Globe de Paris, France (PI: P. Lognonné), with support of CNES.
C. Germon, L. Simoulin, D. Desmet, Y. Bouchet, N. Striebig have contributed
to the recent VBB developpment. The electronics is under the responsibility
of Institute of Geophysics, ETHZ, Switzerland (co-PI: D. Giardini), with
support by ESA/PRODEX. The SP sensor is under the responsibility of the
Jet Propulsion Laboratory, USA (co-PI, B. Banerdt), with a possible support
of NASA. The seismometer team thanks Olivier Marsal and the CNES Team,
Ari-Matti Harri and the finish FMI team, and the IfP/DLR german team for
their contribution to the developpment opf the Netlander system and mission.
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