Visual Abstract

The textures and accretion ages of chondrites have been used to argue that their parent asteroids never differentiated. Without a core, undifferentiated planetesimals could not have generated magnetic fields through dynamo activity, so chondrites are not expected to have experienced such fields. However, the magnetic remanence carried by the CV chondrites is consistent with dynamo‐generated fields, hinting that partially differentiated asteroids consisting of an unmelted crust atop a differentiated interior may exist. Here, we test this hypothesis by applying synchrotron X‐ray microscopy to metallic veins in the slowly cooled H6 chondrite Portales Valley. The magnetic remanence carried by nanostructures in these veins indicates that this meteorite recorded a magnetic field over a period of tens to hundreds of years at ∼100 Myr after solar system formation. These properties are inconsistent with external field sources such as the nebula, solar wind, or impacts, but are consistent with dynamo‐generated fields, indicating that the H chondrite parent body contained an advecting metallic core and was therefore partially differentiated. We calculate the thermal evolution of the chondritic portions of partially differentiated asteroids that form through incremental accretion across 105 to 106 years, finding this can agree with the measured ages and cooling rates of multiple H chondrites. We also predict that the cores of these bodies could have been partially liquid and feasibly generating a dynamo at 100 Myr after solar system formation. These observations contribute to a growing body of evidence supporting a spectrum of internal differentiation within some asteroids with primitive surfaces.

Metallic asteroids, the exposed cores of disrupted planetesimals, are expected to have been exposed while still molten. Some would have cooled from the outside in, crystallizing a surface crust which would then grow inward. Because the growing crust is expected to be more dense than the underlying melt, this melt will tend to migrate toward the surface whenever it is able. Compressional stresses produced in the crust while it cools will be relieved locally by thrust faulting, which will also provide potential conduits for melt to reach the surface. We predict iron volcanism to have occurred on metallic asteroids as they cooled and discuss the implications of this process for both the evolution and the modern appearance of these bodies.

15Magnetoreception, the perception of the geomagnetic field, is a sensory modality well-16 established across all major groups of vertebrates and some invertebrates, but its presence in 17 humans has been tested rarely, yielding inconclusive results. We report here a strong, specific 18 human brain response to ecologically-relevant rotations of Earth-strength magnetic fields. 19Following geomagnetic stimulation, a drop in amplitude of EEG alpha oscillations (8-13 Hz) 20 occurred in a repeatable manner. Termed alpha event-related desynchronization (alpha-ERD), 21 such a response is associated with sensory and cognitive processing of external stimuli. 22Biophysical tests showed that the neural response was sensitive to the dynamic components and 23 axial alignment of the field but also to the static components and polarity of the field. This 24 pattern of results implicates ferromagnetism as the biophysical basis for the sensory transduction 25 and provides a basis to start the behavioral exploration of human magnetoreception. 26 27 109 room ~20 m away. Additional details are available in the Extended Materials and Methods 110 section, and Fig. 5 below. This diagram was modified from the figure "Center of attraction", by 111 C. Bickel (Hand, 2016), with permission.112 113 114 5 A ~1 hour EEG session consisted of multiple ~7 minute experimental runs. In each run 115 of 100+ trials, magnetic field direction rotated repeatedly between two preset orientations with 116 field intensity held nearly constant at the ambient lab value (~35 µT). In SWEEP trials, the 117 magnetic field started in one orientation then rotated smoothly over 100 milliseconds to the other 118 orientation. As a control condition, FIXED trials with no magnetic field rotation were 119 interspersed amongst SWEEP trials according to pseudorandom sequences generated by 120 software. Trials were separated in time by 2-3 seconds. The experimental chamber was dark, 121 quiet and isolated from the control room during runs. Participants were blind to Active vs. Sham 122 mode, trial sequence and trial timing. During sessions, auditory tones signaled the beginning and 123 end of experiment runs, and experimenters only communicated with participants once or twice 124 per session between active runs to update the participant on the number of runs remaining.125