A very interesting report. The article says "Such studies have, however, unveiled that Earth has generated a magnetic field for at least the last 3.5 billion years, and possibly as far back as 4.2 billion years, with a mean strength just over half of the present-day value. We don’t know much about how the field was behaving any earlier than that. By contrast, the Moon’s field was perhaps even stronger than Earth’s around 4 billion years ago, before precipitously declining to a weak field state by 3.2 billion years ago. At present, little is known about the structure or time-variability of these ancient fields, though."
[My observation. The giant impact model for the origin of the Moon places the Moon much closer to Earth, 4 billion years ago than the present lunar orbit is, about 60.3 earth radii distance for the average. The closer Moon and magnetic field interactions would need to be modeled with the Earth's magnetic field projected to exist some 4 billion years ago. From a source I know, "the Moon's core is too small and its rotation is too slow to make a dynamo work, even with decades of handwaving attempts by dynamo theorists."]
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