or time:
Viscous remanent magnetization (VRM) has certainly occured over the billions of years since the Magnetic Field died on Mars. This is a much slower process than other forms of remanent magnetization. Magnetic minerals lose the strength of the original magnetism over time. On earth, we have to understand the VRM to make sure we are reading the actual paleomagnetism.
A factor in the VRM and how long it takes to reset the magnetic signal is the size of the magnetic domains, which is in turn influenced by the size of the grains. Smaller grains suffer VRM at a much faster rate than large grains.
Temperature works the VRM as well, but here the colder it is the slower the process.