Transcript planetary_sciences_meeting

Magnetization of the Martian crust
Kathy Whaler ([email protected]), School of GeoSciences, University of Edinburgh, and
Mike Purucker, Geodynamics Branch, NASA/Goddard Space Flight Center, Maryland, USA
Introduction
The Martian dynamo operated for only the first ~0.5 billion years of the
planet’s history. Thus to-day’s magnetic field reflects magnetization locked into
rocks formed early in its history. For the Earth, the largest contribution to
magnetic field measurements comes from the main magnetic field (i.e.
resulting from geodynamo action), with small contributions from the magnetic
field induced in the lithosphere by the main field (proportional to the current
main field), and from remanent magnetization (proportional to the main field at
the time of formation). Mars has only remanent magnetization. However, the
Martian magnetic field, measured by the Mars Global Surveyor satellite
(MGS), shows several unexpected features, notably at least an order of
magnitude higher strength than that from terrestrial remanent magnetization,
and linear features with alternating polarity. Here, we present and interpret
models of Martian crustal magnetization deduced from MGS data, using a
strategy formulated for satellite measurements of the terrestrial lithospheric
field. This is just one example of applying geophysical methods to other solar
system bodies, as the acquisition of large, accurate satellite data extends the
‘territory’ of geophysicists.
Figure 2: Declination
(angle from North) and
inclination (angle from
vertical), where
magnetization strength
is sufficiently high for
the angles to be well
determined. T marks
Tyhrrhena Patera,
shown in more detail in
Figure 3. The solid line
is the dichotomy.
Figure 1: An early (Purucker et al., 2000) compilation of MGS radial component data,
reduced to a common altitude of 200km, superimposed on the topography (shaded relief).
The dark grey stripes were areas without data coverage (now filled). Note the much lower
field strength in the relatively flat, low-lying area north of the dichotomy, and the cratered,
higher, much more magnetic area south. V is a truncated magnetic feature at Valles
Marineris, G an offset feature at Ganges Chasm. A and C indicate magnetic features in
young terrain west of Olympus Mons (A) and eastern Chryse Planitia (C).
An interesting feature of fig. 2 is the linear ‘channel’ of approximately 0°
declination and 90° inclination magnetization in the Cimmeria region. It is
consistent with generation by a process analogous to the formation of terrestrial
seafloor magnetic stripes, or dyke intrusion over a period during which the
magnetic field was steady, and different from when the surrounding crust was
magnetized, or (since the locus of the boundary is a great circle arc), the pattern
associated with a terrestrial transform fault.
Figure 3: The radial magnetic field at 200km, and deduced declination and inclination of
magnetization, over Tyhrrhena Patera. The sudden reversal of inclination from steeply down
to steeply up along the ‘arms’ of a triple is the pattern expected over a triple junction formed
in a reversing magnetic field.
Method
Alternating polarity magnetic stripes
Most previous modelling methods assume purely induced magnetization.
Such methods are inapplicable on Mars which no longer has a main field. Our
new modelling strategy, which places no restrictions on magnetization
direction, involves solving a data-by-data system of linear equations. Since
satellite magnetic data sets typically comprise several hundreds of thousands,
this gives an intractable computational problem. However, each datum
depends on magnetization only in a small disc of crust directly beneath the
satellite (i.e. the satellite footprint is small), so the system of equations relating
model to data is numerically sparse. The code to calculate the sparse matrix
elements parallelizes efficiently, and we use an iterative conjugate gradient
technique to find the solution. Results shown here were obtained using 8
processors on Edinburgh Parallel Computing Centre’s sunfire system.
Data
Cain et al. (2003) assembled a 3-component data set at 111274 positions. By
using data from all phases of the mission, global coverage was achieved, with
altitudes 102-426km. Uncertainties depend on component (horizontal data are
more affected by external fields) mission phase, local time, and altitude.
Conclusions
The data and models presented here, and other evidence, suggests structural and
tectonic activity, and magnetic reversals, on Mars. The chronology below may aid
structural and tectonic interpretation. The strong magnetic field observed to-day is
likely due to a combination of: more iron in the Martian crust, different mineralogy,
and a more powerful dynamo during its short lifetime.
Results
The magnetization amplitude we deduce depends on the misfit to the data, but
the magnetization pattern is robust. Thus we focus here on the direction and
relative strength of magnetization.
Figure 4: Inferred
radial magnetization
component with
features discussed in
Table 1 identified.
Table 1: A chronology of events with a magnetic signature
Code
Event
Location
Reference
Initiation of Martian dynamo
Magnetic field creation events
1a
Cooling of primordial magma ocean(s) to yield
large-scale magnetic features
Planet-wide
Various authors
1b
Development of lineated magnetic features
associated with crustal recycling
Terra Sirenum
and Cimmeria
Connerney et al.,
Acuña et al.
1c
Development of magnetic features associated
with volcanism and plutonism
ProtoApollinarsis and
Patera
Langlais et al.
1d
Development of magnetic features associated
with volcanism and tectonism
Tyrrhena Patera
Whaler and Purucker
1e
Impact at eastern end of lineated magnetic
feature (1b above) and development of TRM
during cooling
Terra Sirenum
This study
Martian dynamo disappears
Magnetic field destruction events
2a
Internal heating and impact?
Elysium Mons
Frey et al., this study
2b
Internal heating and impact?
Ascræus Mons
Frey et al., this study
2c
Impact
Isidis
Various authors
2d
Impact
Hellas
Acuña et al.
2e
Impact
Argyre
Acuña et al.
Later tectonic events, neither constructive nor destructive
3a
Graben formation
Valles Marineris
Purucker et al.
3b
Tectonism
Ganges Chasma Purucker et al.
Code
Event
Location
Reference
Initiation of Martian dynamo
Magnetic field creation events
1a
1b
1c
1d
1e
Cooling of primordial magma ocean(s) to yield large- Planet-wide
scale magnetic features
Development of lineated magnetic features
Terra Sirenum and Cimmeria
associated with crustal recycling
Development of magnetic features associated with
Proto-Apollinarsis and Patera
volcanism and plutonism
Development of magnetic features associated with
Tyrrhena Patera
volcanism and tectonism
Impact at eastern end of lineated magnetic feature
Terra Sirenum
(1b above) and development of TRM during cooling
Martian dynamo disappears
Various authors
Connerney et al., Acuña et al.
Langlais et al.
Whaler and Purucker
This study
Magnetic field destruction events
2a
Internal heating and impact?
Elysium Mons
Frey et al., this study
2b
Internal heating and impact?
Ascræus Mons
Frey et al., this study
2c
Impact
Isidis
Various authors
2d
Impact
Hellas
Acuña et al.
2e
Impact
Argyre
Acuña et al.
Later tectonic events, neither constructive nor destructive
3a
Graben formation
Valles Marineris
Purucker et al.
3b
Tectonism
Ganges Chasma
Purucker et al.