in time, these data mark the bifurcation point for potential trajectories for the dynamo. In one scenario, the dynamo starts soon after core formation and has been relatively strong ever since. In another, the dynamo started between 4 and $\sim$3.5 billion years ago. We will next suggest ways to extend the record of the magnetopause back in time to the earliest Paleoarchean and Hadean Earth. To address the presence/absence question about the earliest field we also address the question of “recording zero" and in so-doing revisit the self-shielding potential of a planet lacking a magnetic field.\

The Challenge of Recording Magnetic Deep Time
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@Stevenson1983 suggested that field strength might approximately scale with rotation, while cautioning over the accuracy of any scaling laws of the time. Nevertheless, scaling with rotation has been suggested as a viable way of determining past field strength [@Dehant2007; @Lichtenegger2010]. While there are many unresolved issues in dynamo theory, it is generally accepted that core convection is driven by buoyancy [e.g. @Roberts2000], in which case intensity estimates based solely on rotation are illusory. Clearly, observations are needed.

Obtaining paleomagnetic constraints on the oldest geodynamo, however, is difficult because of the ubiquitous metamorphism that has affected Paleoarchean 