  The onset and nature of the earliest geomagnetic field is important for
understanding the evolution of the core, atmosphere and life on Earth. A record
of the early geodynamo is preserved in ancient silicate crystals containing
minute magnetic inclusions. These data indicate the presence of a geodynamo
during the Paleoarchean, between 3.4 and 3.45 billion years ago. While the
magnetic field sheltered Earth's atmosphere from erosion at this time, standoff
of the solar wind was greatly reduced, and similar to that 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 volatiles, including water. Such effects would have been more pronounced if
the field were absent or very weak prior to 3.45 billion years ago, as
suggested by some models of lower mantle evolution. The frontier is thus trying
to obtain geomagnetic field records that are >>3.45 billion-years-old, as well
as constraining solar wind pressure for these times. In this review we suggest
pathways for constraining these parameters and the attendant history of Earth's
deep interior, hydrosphere and atmosphere. In particular, we discuss new
estimates for solar wind pressure for the first 700 million years of Earth
history, the competing effects of magnetic shielding versus solar ion
collection, and bounds on the detection level of a geodynamo imposed by the
presence of external fields. We also discuss the prospects for constraining
Hadean-Paleoarchean magnetic field strength using paleointensity analyses of
zircons.
