NASA’s InSight lander, which arrived on the Red Planet in November 2018, is making measurements of Mars’s magnetic field from the surface, which may help us find water deep underground. While this might trigger thoughts of dowsing there are some scientific reasoning behind the idea.
Mars doesnt have a magnetic field but it probably had one at sme stage n the past. This led to magnetised rocks in its crust. Electric currents created by charged particles from the sun can generate small magnetic fields in those rocks, which InSight’s magnetometer can measure.
InSight scientist Catherine Johnson says
We can use it to probe how much water is locked up in those rocks, how strongly magnetised they are is proportional to what the mineral is, how much of it is there and how strong the ancient magnetic field was.
It may be able to see how much water there is inside minerals under the surface and how strong the primordial magnetic field was. The loss of the field is probably why Mars shed its atmosphere and became cold and dry.
Magnetic fields play an important part in the formation and maintenance of life. So much so that some have suggested that for long term habitation of Mars that we should create an artificial magnetic field.