Transcript aurora_meeting - School of GeoSciences

Magnetization of the Martian crust
Kathy Whaler ([email protected]), School of GeoSciences, University of Edinburgh
Mike Purucker, Planetary Geodynamics Laboratory, 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 remanent
magnetization proportional to the then dynamo field locked into rocks
formed early in its history. Mars Global Surveyor (MGS) satellite vector
magnetic measurements show several unexpected features, notably
an order of magnitude higher strength than from terrestrial remanent
magnetization, and linear features with alternating polarity reminiscent
of those associated with sea-floor spreading on Earth (see Fig. 1).
Here, we present and interpret models of Martian crustal magnetization
deduced from MGS data, using an algorithm developed for satellite
measurements of the terrestrial lithospheric field.
Figure 2: Declination, D
(angle from North) and
inclination, I (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 200km common altitude, superimposed on shaded relief topography. The
dark grey stripes were areas without data coverage (subsequently 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 to the 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).
Fig. 2 shows a linear ‘channel’ of D  0°, I  90° in the Cimmeria region.
Possible causes are a process analogous to terrestrial sea-floor
magnetic stripe formation, dyke intrusion over a period during which the
magnetic field was steady but different from when the surrounding crust
was magnetized, or (since the locus of the boundary is a great circle
arc) a transform fault.
Figure 3: Radial magnetic field at 200km altitude, and deduced magnetization D and I,
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
Our modelling strategy places no restrictions on magnetization direction
but involves solving a data-by-data system of linear equations – an
intractable problem for large satellite magnetic data sets. 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
numerically sparse system was solved using an iterative conjugate
gradient technique. 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. Global coverage was achieved with altitudes 102-426km
using data from all mission phases. Uncertainties depend on
component (horizontal data are more affected by external fields)
mission phase, local time, and altitude.
Results
The magnetization amplitude depends on the misfit to the data, but the
pattern is robust. Thus we focus here on the magnetization direction
and relative strength.
Conclusions
MGS data and models show structural and tectonic activity, non-dipole
magnetic fields and reversals. Our chronology below aids structural and
tectonic interpretation. The strong remanent magnetization is likely due
to a combination of more iron in the Martian crust, different mineralogy,
and a more powerful dynamo operating during its short lifetime.
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 largescale 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
Proto-Apollinarsis Langlais et al.
and Patera
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
Figure 4: Inferred radial magnetization component with features discussed in Table 1
identified.
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.