Low temperature superconductivity SQUID microscopy is by far the most sensitive high-resolution SQUID technique available for quantitatively mapping sample magnetization. I have been using the Ultra High Resolution Scanning SQUID Microscope (UHRSSM) in collaboration with Franz Baudenbacher (Living State Physics Group, Vanderbilt University) to make two-dimensional images of the magnetic fields of rocks at room temperature. The UHRSSM rasters over a sample, producing a grid of magnetic field measurements that can be inverted for magnetization. Click here to see our recent scan of a 30-micron thin section of martian meteorite Los Angeles. Did you ever realize that a dollar bill is magnetic?

Its extraordinary sensitivity of 10^-15 Am2 (1000 times that of the best available superconducting moment magnetometers), spatial resolution of down to 80 microns (~200 times that of moment magnetometers), and speed (several thousand independent measurements per hour) allows us to measure directly the magnetic properties of each grain and structure in a rock thin section, and to monitor the changes in magnetization on a grain-by-grain basis during both demagnetization and rock magnetic experiments. It can therefore provide data with a resolution comparable with that of other common petrographic techniques such as optical and electron microscopy. In collaboration with F. Baudenbacher, we are currently constructing a newer version of the UHRSSM at Caltech over this year to be optimized for paleomagnetic research.

For an introduction, check out our recent article in Eos or our recent paper on Martian meteorite ALH84001 in Science. More details about how the instrument works can be found in Franz's article in Review of Scientific Instruments. The latest, high-resolution bare SQUID sensors are described in a recent article in Applied Physics Letters.

Martian Paleomagnetism

I have been studying the magnetism and magnetic properties of Martian meteorite ALH84001 and other SNC meteorites.

We have shown that ALH84001 was transferred from Mars to Earth without its interior being shock-heated above ambient surface temperatures, indicating that it was not thermally sterilized. This supports the panspermia hypothesis that meteorites can transfer life between planets in the solar system. See the articles by Reuters, New York Times,first and second from space.com, Caltech, the BBC. Why not also practice your German, Portuguese, or Polish while you're at it? See also some of the press materials.

This and related work is published in Science, Palaeontologia Electronica, EPSL, and The Planetary Report. Click here to download pdf files of these papers.

We have now also shown that ALH84001 acquired its magnetization 4 billion years ago (Weiss et al. 2002a). Th files of these papers.

We have now also shown that ALH84001 acquired its magnetization 4 billion years ago (Weiss et al. 2002a). This is the oldest magnetization known in any planetary rock, and shows that Mars had generated a dynamo and global magnetic field within the first 50 million years of its existence. The subsequent decay of the magnetic field may have been responsible for drastic climate change on Mars. Our thermochronology calculations (Weiss et al. 2002b) also strongly support the hypothesis that ALH84001 contains a sample of 4 billion year old Martian atmosphere. The composition of this gas is consistent with the notion that significant atmospheric loss has occurred on Mars since 4 billion years ago. This thermochronology work also confirms the results of our Science paper that this meteorite was not shock-heated during ejection from Mars. See articles in New Scientist and Pasadena Star News describing these results.

In collaboration with Sam Kim (JPL) we also recently developed a suite of rock magnetic techniques--ferromagnetic resonance, low-temperature magnetization--which can be used to rapidly identify biogenic magnetites (Weiss et al. 2004a, 2004b). These techniques can then be used to search for magnetofossils in ancient sediments.

We are currently using the SQUID Microscope to read the four billion year history of the lunar magnetic field recorded in impact melt-produced glass spherules. These ~0.2 mm diameter balls should record any magnetic fields in which they cooled, and range in age from 4 billion years old to zero-age. Our goal is to measure the paleointensity of the paleofield from several hundred Apollo spherules.

Our purpose is to determine whether the magnetization in lunar samples originated from a lunar dynamo, from impact-generated magnetic fields or some entirely different process.

We recently detected the moments of our first two spherules with signal-to-noise ratios of 100 and 4000, respectively. The weaker of these moments would be below the sensitivity limit of 2G Enterprises Superconducting Rock Magnetometers.

I am engaged in a variety of investigations into the nature of magnetism on the Early Earth and the evolution of life. We are focusing in particular on outcrop samples, drill core from recent deep drilling projects, and mineral separates from the Yilgarn and Pilbara cratons in Australia.