Mars has been visited recently (1997–2001) by the
Mars Global Surveyor mission. It provided a more
accurate upper limit on its present low magnetic
moment. It also found striking evidence that Mars
had a strong magnetic field in the past, confirming a
prediction I made in 1984 (Humphreys, 1984, p. 147).
Orbiting low over the Martian surface, the spacecraft
measured alternate-polarity magnetic “stripes” of
magnetization in the crustal rocks. Figure 8 shows
these “magnetic crustal anomalies.” These linear
features are similar to those found on earth’s ocean
floors, but the Martian magnetizations are up to 20
times stronger than those on earth (Acuña et al.,
2001, pp. 23403–23417). That points to a strong field
reversing many times in the past, when the rocks
were formed. It is difficult for theorists to explain why
a Martian dynamo would be functioning robustly in
the past but not at all in the present. The water origin
theory, on the other hand has no problem with both
past and present fields, and as I said, even predicted
crustal magnetizations. The core conductivity
required is close to that of earth, so it is within the
expectations of the water origin theory.
http://www.icr.org/i/pdf/research/ICC08_Cosmic_Magn_%20Fields.pdf
Again, why did Mars’ magnetic field decay to zero while much smaller Mercury’s didn’t? By his theory Mercury’s field should have decayed to zero BEFORE Mars.