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(Myr). The ensuing 500-Myr time interval, the Hadean Eon, is often referred to as the
geological dark age, since there is little preservation of this interval in the rock record.
Yet it remains a crucial stage in Earth's history because the transition to a habitable
surface environment occurred during this time.
There are few solid constraints on the Hadean Earth and a host of first-order
questions. Heat produced during accretion and core formation, together with the
higher concentrations of heat-producing radioactive elements, point to a hot, possibly
water-deficient, mantle. The consensus view is that Earth's initial atmosphere,
composed mostly of hydrogen, was lost very early, perhaps during a T-Tauri phase of
solar activity or through hydrodynamic escape to space aided by the strongly
ultraviolet-emitting young Sun (Catling, 2006). As for the early composition of the
secondary atmosphere, there is far too little in the way of direct evidence, although
the decisive events in Earth's early history point to some plausible scenarios. One
possible consequence of the Moon-forming impact is rapid evolution from a hot
silicate atmosphere to a steam-dominated greenhouse atmosphere (Zahnle et al.,
1988), and once the magma ocean solidified, liquid water could stabilize at the
surface with carbon dioxide and methane dominating the climate (Kasting and Ono,
2006). A key unknown here is the capacity of the mantle to sequester water, possibly
in the presence of early whole-mantle convection.
Clues from the Early Crust
Evidence for the earliest chapters in Earth's history comes from a variety of
sources, including the bulk composition of Earth and the Moon, the angular
momentum of the Earth-Moon system, traces of short-lived radioactive isotopes in
meteorites and terrestrial rocks, terrestrial and lunar patterns of element abundances,
and perhaps most importantly, the oldest crustal rocks and minerals. The discovery of
increasingly old crustal rocks (see Figure 2.4) provides a few tantalizing clues on the
state of Earth's surface in the late Hadean and the earliest Archean. In terms of
preservation, the most diverse suite of ancient crustal rocks is found in the Isua
terrane in Greenland, with ages as great as 3.8 Ga (Appel et al., 2001). These are
moderately metamorphosed but contain evidence to suggest that plate tectonic
processes, liquid water oceans, and perhaps life forms were present. Still older are the
Acasta gneisses from north-central Canada, dated around 4 Ga (Bowring and
Williams, 1999). The only known Earth materials that are unequivocally older are
small zircon grains that have been removed from their parent rock, transported by
fluvial systems, and deposited in sedimentary rocks of a younger age (see Box 2.2).
Advances in microanalytical techniques, especially ion microprobes, have established
ages around 4.3 Ga for the oldest of these. The overarching inference from these
oldest crustal materials is that by the late Hadean and certainly by its end, Earth's
surface environment was rather equable, perhaps not dramatically different from the
present (Wilde et al., 2001; Mojzsis et al., 2001), so that some of the conditions for
sustaining life were already in place. Other critical elements are more problematic,
however, particularly oxygen, which does not appear to have been abundant then.
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