The onset and nature of the geomagnetic field is important for understanding the evolution of the core, atmosphere and life on Earth. The geomagnetic field is generated in the liquid outer core, and hence is a probe of core conditions. The field also protects Earth from energetic particles streaming from the Sun (the "solar wind"); without this protective shield Earth might have developed into a dry and barren planet. A record of the early core geodynamo that generated the field is preserved in ancient silicate crystals from igneous rocks that contain minute magnetic grain inclusions. Our data indicate the presence of a geodynamo between 3.4 and 3.45 billion years ago, near the limit for the start of growth of the solid inner-core. While the magnetic field sheltered Earth's atmosphere from erosion at this time, the standoff of the solar wind was greatly reduced, and similar to that seen during modern extreme solar storms. These conditions suggest that intense radiation from the young Sun may have modified the atmosphere of the young Earth by promoting loss of light elements and water. Such effects would have been more pronounced if the field were absent prior to 3.45 billion years ago, as suggested in some hypotheses, or if an older geodynamo prior to inner core growth produced a weak field. In general, these considerations suggest the young Earth was more water-rich than today. The new frontier to learn more about these issues is obtaining geomagnetic field records that are more than 3.45 billion-years-old.

We are investigating the first billion years of geodynamo history and its implications for Earth evolution through the study of well-dated rock units from 6 ancient cores of continents (cratons). We are using a combination of existing methods of single silicate crystal magnetic measurements from in situ igneous host rocks and new approaches involving magnetic analyses of grains from sedimentary units. These measurements are achievable using highly sensitive magnetometers at the University of Rochester. Determination of the presence and strength of the geomagnetic field during the first billion years of Earth history is of broad interest to a range of scientists who study early Earth environments (atmosphere and biosphere) and the core. The investigation involves international collaboration with geologists from several countries, and multidisciplinary collaborations spanning astrophysics and space physics. Our program also integrates research and educational efforts. The study is contributing to graduate theses and undergraduates are receiving training in the field and the laboratory.

Project Report

Earth's magnetic field protects our atmosphere from erosion by particles streaming from the Sun. In the distant past, 4.4 to 3.5 billion years ago, this stream of particles-- known as the solar wind-- was tens to hundreds of times more intense than today. But many models of Earth evolution question whether the geomagnetic field, generated from convection in Earth's liquid iron core, existed at these times. Our project has been devoted toward addressing this paradox through direct measurement of natural minerals containing minute magnetic inclusions that can preserve a record of the earliest magnetic field. This endeavor has been challenging for several reasons. First, one must find samples whose magnetizations have survived, and been unaltered, by geologic history (e.g. deep burial, mountain building) since their formation. This has involved extensive field studies around the world where key ancient rocks are exposed. Second, the minerals that can preserve these magnetizations are extremely small -- less than one-half of a millimeter. The low intrinsic magnetizations are below the measuring limit of even highly sensitive laboratory magnetometers. To meet this challenge we employed an ultra-sensitive magnetometer, designed with sensors much closer to the sample than comparable instruments. We have coupled this field and laboratory work with modeling to better constrain early solar winds.These estimates are in turn based on analogies with stars like our Sun, but of different age. Our modeling involved collaborations with astronomers and astrophysicists. We have found that solar winds were indeed intense for the early Sun, but they were not quite as intense as might have been expected by simple extrapolation of published values for younger times. It seems very young stars have a different surface structure that limits stellar winds. Our laboratory effort has resulted in the detection of magnetic fields, and a core dynamo, much older than previous reports. This implies that heat loss was efficient through the crust and mantle of the early Earth, allowing the heat transport from the core required to drive a dynamo. The early onset of Earth's magnetic field was probably a key factor in the development of Earth as a habitable planet. All of our studies have been integrated into undergraduate and graduate studies. Students have gained experience in field and laboratory settings, and in modeling.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Stephen S. Harlan
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University of Rochester
United States
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