Predicted and observed magnetic signatures of martian (de)magnetized impact craters

Research areas:
The current morphology of the martian lithospheric
magnetic field results from magnetization and
demagnetization processes, both of which shaped the
planet. The largest martian impact craters, Hellas,
Argyre, Isidis and Utopia, are not associated with
intense magnetic fields at spacecraft altitude. This
is usually interpreted as locally non- or
de-magnetized areas, as large impactors may have
reset the magnetization of the pre-impact
material. We study the effects of impacts on the
magnetic field. First, a careful analysis is
performed to compute the impact demagnetization
effects. We assume that the pre-impact lithosphere
acquired its magnetization while cooling in the
presence of a global, centered and mainly dipolar
magnetic field, and that the subsequent
demagnetization is restricted to the excavation area
created by large craters, between 50- and 500-km
diameter. Depth-to-diameter ratio of the transient
craters is set to 0.1, consistent with observed
telluric bodies. Associated magnetic field is
computed between 100- and 500-km altitude. For a
single-impact event, the maximum magnetic field
anomaly associated with a crater located over the
magnetic pole is maximum above the crater. A 200-km
diameter crater presents a close-to-1-nT magnetic
field anomaly at 400-km altitude, while a 100-km
diameter crater has a similar signature at 200-km
altitude. Second, we statistically study the 400-km
altitude Mars Global Surveyor magnetic measurements
modelled locally over the visible impact
craters. This approach offers a local estimate of
the confidence to which the magnetic field can be
computed from real measurements. We conclude that
currently craters down to a diameter of 200 km can
be characterized. There is a slight anti-correlation
of 0.23 between magnetic field intensity and impact
crater diameters, although we show that this result
may be fortuitous. A complete low-altitude magnetic
field mapping is needed. New data will allow
predicted weak anomalies above craters to be better
characterized, and will bring new constraints on the
timing of the martian dynamo and on Mars'