Abstract :
Orbital and earth-based remote sensing infrared spectroscopy, along with in situ magnet tests, Mossbauer, and thermal infrared emission spectroscopy have determined the presence of magnetic minerals on Mars. The iron oxide minerals magnetite and grey hematite have been confirmed in situ by the Spirit and Opportunity rovers. Elemental compositions including nickel and sulfur have also been observed, which along with titanium can produce a wide range of minerals from paramagnetic through ferromagnetic species. Lalboratory measurements of the electrical and magnetic properties of magnetite and grey hematite at Mars ambient temperatures in the ground penetrating radar frequency range have produced surprisingly strong dielectric relaxations, as well as the expected magnetic properties. At the average annual Mars surface temperature of 213 K, magnetite has a strong dielectric relaxation near 10 MHz, and hematite has two strong dielectric relaxations near 15 MHz and 200 MHz. These relaxation processes are strongly temperature dependent, moving about a factor of two lower in frequency with each 10 K decrease in temperature. Thus over the temperature range of a Mars Sol, between day and night, these relaxations will move through the frequency range of the MARSIS orbital radar sounder and other proposed radar systems. Also surprising, the weakly magnetic hematite produced stronger magnetic relaxation losses than the more magnetic magnetite in the radar frequency range. The addition of titanium to this iron oxide miature can lower the Curie temperature (above which ferromagnetic minerals become paramagnetic) into the Mars day-night temperature range, which would make the magnetic signature of the minerals strong at night and make it disappear in ihe higher daytime temperatures. This is a problem for a radar orbiter since the ionosphere absorbs the radar energy in the daytime and the magnetic relaxations would be strongest at niglht, severely limiting the possibilities for dep- h of penetration. Future ground penetrating radar systems on Mars surface rovers will have to carefully choose frequencies and times of operation to maximize depth of penetration. However, the strong frequency dependence as a function of temperature also implies a broadband radar system might be able to produce a temperature versus depth profile.
