Mars Crustal Magnetism: Through the Lens Sharply, by Dr. John E.P. Connerney, NASA Goddard Space Flight Center. Mars has no global magnetic field of internal origin (a "dynamo"), but must have had one in the past when the crust acquired intense magnetization, presumably by cooling in the presence of an Earth-like magnetic field (thermoremanent magnetization or TRM). The Mars Global Surveyor (MGS) spacecraft, in polar orbit at about 400 km altitude, produced maps of the crustal magnetic field with extraordinary signal to noise. These maps yeild valuable insight.