The Mars of several billion years ago is a very different Mars than that of today. Several billion years ago, Mars was geologically active from core to crust, possessed a much thicker atmosphere, and had an internally produced magnetic field.
Today, remnants of this former magnetic field remain, in effect frozen into the surface of Mars. A currently orbiting spacecraft, MAVEN, has obtained measurements above these fields; allowing for the large-scale characterization of these remnants to provide insight into to the historical evolution and eventual, termination, of Mars’ formerly active field. See below for a video of my current work.