ON SCIENTIFIC GOALS OF THE SEISMIC EXPERIMENT “MISS”

T. Gudkova1, P. Lognonné 2, V.N. Zharkov1, S. Raevskiy1, V. Soloviev1 and “MISS”

team1,2,3,4.

1 Schmidt Institute of Physics of the Earth, Russia; 2 Institut de Physique du Globe de Paris, France. 3 Institute of Space Research, Russia 4 Moscow Institute of Physics and Technology, Russia

Contact: gudkova@ifz.ru

Our present knowledge on the bulk composition and interior structure of Mars is based on geophysical and geochemical information and

high-pressure experiments

Geophysical constraints The mass and the mean radius of the planet M=6.4185x1023 kg, R=3389.92 km The value for the mean moment inertia (obtained from the measurements of the precession rate of the

planet by the Pathfinder mission and the MGS mission) 0.3647-0.3663 (Konoplive et al., 2006) The inferred elastic Love number (obtained from the gravity analysis of orbiting spacecrafts)

k2=0.148±0.0.009 (Konoplive et al., 2006) k2=0.11 (Marty et al., 2008) k2=0.156±0.0.009 (Konoplive et al., 2011)

Chemical composition (based on the analyses of the composition of Martian meteorites)

►The cosmochemical DW model (Dreibus, Wänke, 1989; Wänke, Dreibus, 1994) A:B=60:40 Fe/Si=1.71 Fe#=0.25 (Fe#=Fe/(Fe+Mg)x100) ► Sanloup et al., 1999 to match the δ17O/ δ18O ratio 55% ordinary chondrite H and 45% enstatite chondrite EH ► Lodders, Fegley, 1997 85% H chondrite Lodders, 2000 11% CV chondrite 4% C1 chondrote

High-pressure experiments a model of the mantle - high-pressure experiments with an analog of the DW composition up to

core-mantle boundary pressures along a model areotherm (Bertka, Fei, 1997, 1998) for the model of the core - experimental data of high PT phases of -Fe and FeS (Kavner et al 2001)

Model estimates of the elastic Love number k2 versus core radius for the models consistent with the mean moment of inertia

k2s=0.145±0.017 , Rcore = 1520-1840 km Yoder et al., 2003

k2s=0.148±0.009 (dashed lines), Rcore = 1600-1810 km Konoplive et al., 2006

k2s=0.156±0.009 (solid lines), Rcore = 1770-1830 km Konoplive et al., 2011

Rcore = 1700 - 1800 km

The determination of the core size is a key objective for seismology

The presence of a perovskite-bearing lower mantle ?

► Love number k2 =0.148 (Konoplive et al., 2006)

k2 =0.156 (Konoplive et al., 2011)

1) lower mantle becomes impossible2) the mantle is softer than the assumed elastic solid mantlebecause of partial melt at depth (Yoder et al.,2003). Thiswould have the effect of reducing the inferred core radiusby 100-150 km.► Love number k2 =0.11 (Marty et al., 2008) allows smaller

core and,hence, the presence of perovskite bearing lower mantle

1 60 0 1 70 0 1 80 0 1 9 0 0

C o re rad iu s , k m

0 .1

0 .1 2

0 .1 4

0 .1 6

0 .1 8

k 2s

a

 

• Body waves• Surface waves• Free oscillations

Seismology is the best tool for probing planetary interiors.

How a single seismometer can be useful to get information on subsurface structure and average global structure of the planet? Nontraditional ways to probe the interiors should be used: data processing of meteoroids’ impacts,

seismic hum from meteorological forcing,

the development of new methods, that can derive interior information from a single seismometer.

Many such methods already exist: source location through P-S and back-azimuth,

receiver functions,

identification of later phases (PcP, PKP, etc),

surface wave dispersion,

normal mode analysis (from single large events, stacked events, or background noise).

What can be determined:

1) Mars’ seismicity level2) The crustal and upper mantle structure: meteoroid’ impacts the dispersion curves of surface waves receiver function method free oscillations 3) Some restrictions on the seismic velocities in the deep mantle differential measurements of arrival times of later-arriving phases (PcP, PcS, ScS) in comparison to

P

No past missions have returned seismic information on the Martian interiors.By theoretical estimates Mars is assumed to be seismically more active than the Moon but less active than the Earth.

may be expected per year (Phillips and Grimm, 1991; Solomon et al., 1991; Golombek et al., 2002, Knapmeyer et al., 2006)

more than 10 events of seismic moment greater than 1023 dyne cm, more than 250 events of magnitude greater than 1021 dyne cm, a few (2-3) should have a moment greater than 1024 dyne cm.a 1025 dyne cm quake is the upper bound of the estimate of theactivity on Mars

Seismicity map (Knapmeyer et al., 2006)

The quakes are related to the thermoelastic cooling of the lithosphere, which accumulates stresses that are then released by quakes. This type of activity is the minimum expected activity on Mars. Taking into account the fact that one can see giant faults on the surface of Mars (within Tharsis region, Tempe Terra, Valles Marineris, Olimpus region), it is not possible apriori to rule out large seismic events.

The first goal of the experiment is determining Mars’ seismicity level

Meteoroid impacts

an additional and very important seismic sources for a planet with a weak atmosphere for constraining the crustal and upper mantle structure the number of impacts are expected to be 2-4 times larger then for the Moon their impact time and location can be known with orbital imaging (high-resolution cameras is on orbiting Mars spacecraft)

Both P and S arrival time can be used on a seismometer. If the time is not known, the P-S differential travel-times can be used.

The main characteristics of the seismic source generated by an impact are its amplitude and cutoff frequency. These parameters allow us to constrain the mass and velocity of the impactor.

The larger an impact is, the lower is its cutoff frequency.

Free oscillations, if they are excited, are particularly attractive to probe beneath the surface of an extraterrestrial body into its deep interior. Interpretation of data on free oscillations does not require knowledge of the time or location of the source; thus, data from a single station are sufficient.

Since the planet has finite dimensions and is bounded by a free surface, the study of the free oscillations is based on the theory of vibration of an elastic sphere. The planet reacts to a quake (or an impact) by vibrating as a whole, vibrations being the sum of an infinite number of modes that correspond to a set of frequencies.

Free oscillations

Can the free oscillation method be used to study Martian interiors?

Current broad band seismometers can measure accelerations (Lognonné et al., 1996)

a N,E= - 2 uN,E ≈ 10-8 cm/s2,

a N,E - the ground acceleration, uN,E - the ground displacement in the North and East direction.

Functions 0Ul proportional to the displacements for spheroidal oscillations for the fundamental mode, l=2 to 10 vesus relative radius r/R. 0Ul is normalized to unity at the surface.

The fundamental modes sound to those depth in the interiors where its displacement 0.3

The horizontal line drawn at level =0.3 enables one to judge graphically which modes give information about one or another zone of the planet.

Torsional oscillations:

M0=1025 dyn cm: l3 (up to 1600 km)

M0=1024dyn cm: l6 (up to 1100 km)

M0=1023dyn cm: l12 (up to 700 km)

Spheroidal oscillations:

M0=1025 dyn cm: l17 (up to 700-800 km)

M0=1026dyn cm: l6 (up to 2000 km)

The depth to which surface waves are sensitive depends on frequency, with low frequency waves feeling to greater depth and therfore propagating with higher speeds. Low frequency waves are arriving earlier than higher frequencies. They are extremely sensitive to subsurface structure (to the crustal thickness). The velocity can be calculated from arrival time and estimate of distance from the source, which can be obtained from R1-R2 difference, where R1 is the direct Rayleigh wave arrival, R2 is the arrival of the wave propagating around the planet in the opposite direction.

The dispersion curves of surface waves can be used to solve problem of determining the structure of the crust and the upper mantle

Profiles of density in the different models of the Martian crust (MK1M, MK2M, MK2H and MK2L) are on the left (the data are from (Babeiko A. et al., 1993)) and group velocities oUn for a fundamental mode of Rayleigh waves as function of the period of oscillation for these models: 1, MK2L; 2, MK2M; 2, MK2H; 4, MK1M.

0 50 100 150 D epth, km

3.2

3.3

3.4

3.5

Dens

ity

, g/cm

3 M K2L

M K2M

M K2HM K1M

0 50 100 150 200 250 Period T , s

3.4

3.6

3.8

4.0

4.2

Grou

p velo

city o

U n, k

m/s

1234

The data on Rayleigh waves enable one to distinguish between not only the crusts with different composition (MK2M and MK1M), but also between the models based on different temperature distribution in the crust (MK2M, MK2H and MK2L).

BODY WAVES

Differential measurements of arrival times of later-arriving phases (PcP, PcS, ScS) in comparison to P could put some restrictions on the seismic velocities in the deep mantle. Synthetic seismogram analysis for interoir structure models can lead to its identification.

0 40 80 120 160Epicentral d istance (km )

0

100

200

300

400

Trav

el-tim

e diffe

rence

s T(

s)

SScS

PPcP

PK PSKS

0 0.2 0.4 0.6 0.8 1

re lative radius r/R

0

2

4

6

8

density

, g/cm

3

The difference are up to 40 s for P and PcP , and up to 100 s for S and ScS arrivels. PcP and ScS, phases reflected from the core, could provide a strong constraint on the core’s radius. For diagnostic purposes, the core phases PKP and SKS are the most promising phases in Martian seismology. The difference between models are about 300-350 s.

Travel times P, PKP, PcP, S, SKS, ScS waves difference between a trial model M7_3 (Zharkov et al., 2009) (Rc=1766 km; the density of 50-km thick crust is 3000 kg/m3) and the model A (Rc=1468 km; the density of 110-km thick crust is 2810 kg/m3) of (Sohl, Spohn, 1997): solid line - the source is on the surface dashed line - the source is at the depth of 300 km.

We have showed the mission possibility to get seismic information on Martian interiors from only one seismic instrument using non-traditional sources of seismic waves and new seismic techniques.

Very Broad Band seismometer will record the full range of seismic signals, from the expected quakes induced by the thermoelastic cooling of the lithosphere, to the possible permanent excitation of the normal modes. All these seismic signals will be able to constrain the structure of Mars’ interiors.