Abstract

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 [1906].

Introduction

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

The NetLander mission aims at deploying on the surface of Mars a network of 4 geophysical and meteorological landers implemented by the CNES and realized by a European and American consortium : Finland (FMI), Germany (DLR), Belgium (SSTC, 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 (mov format, avi format) 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: · deep internal structure, · global atmospheric circulation, · planetary boundary layer (PBL) phenomena, · subsurface structure at the km scale, down to water rich layers, · surface mineralogy and local geology, · alteration processes and surface/atmosphere, interaction, · atmospheric 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, IPGP, FMI.

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 (mov format, avi format)

Scientific Objectives

For body 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 [1993]. 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 [1998] 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 of attenuation.

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 ^1/2, and very likely close or less to 10^-9 m.s^-2/Hz ^1/2.  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 [1998], Suda et al [1998], these turbulences on the Earth excite indeed continuously the fundamental branch of spheroidal Earth normal modes. As shown by Kobayashi & Nishida [1998], 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 papers of space.com 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 [2000].

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/Hz^1/2 near 1 sec and few 10^-9 m.s^-2/Hz^1/2 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 /Hz^1/2. 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 [1996]. A level of 10^-9m.s^-2/Hz^1/2 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/Hz^1/2.

Instrument description

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/Hz^1/2 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)

Conclusions

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 [1906], the core of a second telluric planet might therefore be characterized.

Acknowledgments and Team composition

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