Earth’s magnetic field, a familiar directional indicator over long distances, is routinely probed in applications ranging from geology to archaeology. The work was conducted in the NMR laboratory of Alexander Pines, one of the world’s foremost NMR authorities, as part of a long-standing collaboration with physicist Dmitry Budker at the University of California, Berkeley, along with colleagues at the National Institute of Standards and Technology (NIST). The exquisite sensitivity of NMR for detecting chemical composition, and the spatial resolution which it can provide in medical applications, requires large and precise superconducting magnets. But chemical characterization of objects that cannot be placed inside a magnet requires a different approach. Instead of a superconducting magnet, low-field NMR measurements may rely on Earth’s magnetic field, given a sufficiently sensitive magnetometer.
Paul Ganssle is the corresponding author of a paper in Angewandte Chemie describing ultra-low-field NMR using an optical magnetometer.
Earth’s magnetic field is indeed very weak, but optical magnetometers can serve as detectors for ultra-low-field NMR measurements in the ambient field alone without any permanent magnets.
In high-field NMR, the chemical properties of a sample are determined from their resonance spectrum, but this is not possible without either extremely high fields or extremely long-lived coherent signals (neither of which are possible with permanent magnets). A key difference between this and conventional experiments is that the relaxation and diffusion properties are resolved through optically-detected NMR, which operates sensitively even in low magnetic fields. The design was tested in the lab by measuring relaxation coefficients first for various hydrocarbons and water by themselves, then for a heterogeneous mixture, as well as in two-dimensional correlation experiments, using a magnetometer and an applied magnetic field representative of Earth’s.
This technology could help the oil industry to characterize fluids in rocks, because water relaxes at a different rate from oil.
The next step involves understanding the depth in a geological formation that could be imaged with this technology. Eventually, probes could be used to characterize entire borehole environments in this way, while current devices can only image inches deep. Other authors on the Angewandte Chemie paper include Hyun Doug Shin, Micah Ledbetter, Dmitry Budker, Svenja Knappe, John Kitching, and Alexander Pines.
For more information about the research in Alex Pines’ Nuclear Magnetic Resonance group, go here. Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe.
DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. Evidence for the existence of Earth’s magnetic field has been pushed back about 250 million years, new research suggests. Earth’s magnetic field was born by 3.45 billion years ago, a team including researchers from the University of Rochester in New York and the University of KwaZulu-Natal in South Africa report in the March 5 issue of Science.

That date falls?during life’s earliest stages of development, between the period when the Earth was pummeled by interplanetary debris and when the atmosphere filled with oxygen. The researchers measured the magnetic strength of certain rocks found in the Kaapvaal craton of South Africa, a geologic region known to date back more than 3 billion years. The Greenstone Belt in South Africa had rocks that were just right: crystals of quartz less than two millimeters long with nanometer-sized bits of iron-containing magnetite embedded in them. Tarduno and his colleagues had studied similar rocks in 2007 and found that a magnetic field half as strong as today’s was present 3.2 billion years ago.
To determine if the early magnetic field was enough to hold back the rain of radiation, the team needed to know what the sun was doing. The amount of radiation regularly reaching Earth from the sun 3.45 billion years ago would be comparable to what rains down on the planet during the most powerful solar storms today, Tarduno says.
The study “can be used to guide our searches for other life-bearing planets” as well, says astronomer Moira Jardine of the University of St. Despite the fact that no extrasolar planets with magnetic fields have ever been detected, Jardine and Tarduno remain optimistic. Now it has provided the basis for a technique which might, one day, be used to characterize the chemical composition of fluid mixtures in their native environments.
Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) conducted a proof-of-concept NMR experiment in which a mixture of hydrocarbons and water was analyzed using a high-sensitivity magnetometer and a magnetic field comparable to that of the Earth. The work will be featured on the cover of Angewandte Chemie and is published in a paper titled “Ultra-Low-Field NMR Relaxation and Diffusion Measurements Using an Optical Magnetometer.” The corresponding author is Paul Ganssle,  who was a PhD student in Pines’ lab at the time of the work.
In ex situ NMR measurements, the geometry of a typical high-field experiment is reversed such that the detector probes the sample surface, and the magnetic field is projected into the object.
This means that ex-situ measurements lose chemical sensitivity due to field strength alone.
In contrast, relaxation and diffusion measurements in low-field NMR are more than sufficient to determine bulk materials properties. Relaxation refers to the rate at which polarized spin returns to equilibrium, based on chemical and physical characteristics of the system. There were attempts to perform oil well logging starting in the 1950s using the Earth’s ambient field, but insufficient detection sensitivity led to the introduction of magnets, which are now ubiquitous in logging tools. Other applications include measuring the content of water and oil flowing in a pipeline by measuring chemical composition with time, and inspecting the quality of foods and any kind of polymer curing process such as cement curing and drying. The combination of terrestrial magnetism and versatile sensing technology again offers an elegant solution. The current publication presents some of the work for which Berkeley Lab won an R&D 100 award earlierthis year on optically-detected oil well logging by MRI.

Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes. The field may therefore be old enough to have shielded some of the planet’s earliest life from the sun’s most harmful cosmic radiation. Several earlier studies had suggested that a magnetic field is a necessary shield against deadly solar radiation that can strip away a planet’s atmosphere, evaporate water and snuff out life on its surface.
Using a specially designed magnetometer and improved lab techniques, the team detected a magnetic signal in 3.45-billion-year-old rocks that was between 50 and 70 percent the strength of the present-day field, Tarduno says. Tarduno and Eric Mamajek, an astronomer at the University of Rochester, used observations of young sunlike stars to infer how strong a solar wind the Earth was up against. This quick rotation powered a strong magnetic field, which heated the sun’s atmosphere and carried away mass and angular momentum in a strong solar wind of charged particles. The aurora borealis, caused by solar wind particles accelerating along Earth’s magnetic field, would have been visible as far south as present-day New York City.
Further, the sample of interest must be placed inside the magnet, such that the entire sample is exposed to a homogeneous magnetic field.
A main challenge with this situation is generating a homogeneous magnetic field over a sufficiently large sample area: it is not feasible to generate field strengths necessary to make conventional high-resolution NMR measurements. Iron minerals record the strength and direction of the magnetic field that was present during their formation.
But you also have to have a magnetic field, because that protects the atmosphere from erosion and the complete removal of water.” Mars may be dry today because it lost its magnetic field early on, he adds. The team calculated that the point where the Earth’s magnetic field cancels out the solar wind would be only about five Earth radii above the planet’s center, less than half of the 10.7 radii it is today.
Astronomers might want to focus more on older, less active stars or search for planets with their own magnetic fields, she says. So, add us to your ad blocker’s whitelist or pay $1 per week for an ad-free version of WIRED. But when rocks are heated in subsequent geological processes, they can lose or overwrite that record. The material on this site may not be reproduced, distributed, transmitted, cached or otherwise used, except with the prior written permission of Conde Nast.

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