Martian paleomagnetism with the SQUID microscope

Abstract

Rocks should preserve natural remanent magnetizations with stable directional and intensity information at levels ~1000 times below that of the noise level on today?s best moment magnetometers. The superconducting quantum interference device (SQUID) Microscope is a new, high-resolution magnetometer that can now detect such weak signals. It maps the magnetic fields above samples with a spatial resolution of <100 ?and a moment sensitivity of <10-15 Am2. It therefore provides data with a resolution directly comparable with that of other common petrographic techniques. This thesis describes applications of SQUID microscopy to a variety of problems in the planetary sciences. A SQUID microscope paleomagnetic conglomerate test demonstrates that ALH84001 has been cooler than ~40?C since before its ejection from the surface of Mars at 15 Ma. Because this temperature cannot sterilize most bacteria or eukarya, these data support the hypothesis that meteorites could transfer life between planets in the solar system. These and other data on panspermia demand a re-evaluation of the long-held assumption that terrestrial life evolved in isolation on Earth. Subsequent magnetic and textural studies of the meteorite show that 4 Ga ALH84001 carbonates containing magnetite and pyrrhotite carry a stable natural remanent magnetization. 40Ar/39Ar thermochronology demonstrates that this magnetization originated at 3.9-4.1 Ga on Mars. This magnetization is the oldest known for a planetary rock, and its strong intensity suggests that Mars had generated a geodynamo at or before 4 Ga. The intensity of the field that magnetized ALH84001 was roughly within an order of magnitude of that at the surface of the present-day Earth, sufficient for magnetotaxis by the bacteria whose magnetofossils have been reported in ALH84001 and possibly for the production of the strong crustal anomalies. 40Ar/39Ar thermochronology calculations also provide an explanation for why ALH84001 contains a sample of an apparently ancient martian atmosphere. Because this gas is enriched in light isotopes of H and N relative to that on present-day Mars, this supports the hypothesis that the planet has experienced significant atmospheric loss since 4 Ga. These calculations also suggest that for the last 4 Gyr, average surface temperatures on Mars may not have been much higher than the present cold conditions.