FIGURE 1.9 A speculative history of temperature, water, and CO2 during the Hadean. The Hadean begins with the Moon-forming impact (at time = zero in this figure). For 1,000 years Earth is enveloped in hot rock vapor. After the silicate vapor rains out, the atmosphere consists mostly of CO2. Water is gradually lost from the magma ocean and added to the atmosphere. The greenhouse effect and tidal heating maintain the magma ocean for 2 million years. When the magma surface freezes over, surface temperature drops quickly and the steam atmosphere rains out to leave a warm (~500 K) water ocean under ~100 bars of CO2. This warm, wet Earth lasts as long as the CO2 stays in the atmosphere. This illustration shows CO2 being removed on timescales of 20 million years (green solid curves) or 100 million years (green dotted curves). When the CO2 partial pressure drops below about 1 bar, the oceans freeze over (blue region of graph). After the late heavy bombardment, CO2 is shown returning to an arbitrary level of ~1 bar, which allows the surface to be clement as required by geological data. SOURCE: Zahnle (2006). Reprinted with permission.

And most of those rocks are metamorphosed, some at very high temperature and pressure, obscuring their original form. Thus the period of time for which there is virtually no rock record on Earth extends from the time of the putative Moon-forming impact ca. 4,530 Ma to the age of the oldest rocks on Earth ca. 4,000 to 3,800 Ma. This period, about which we can discern very little from Earth itself, is called the Hadean Eon.

The name of this eon is unusually graphic, for good reason. Earth during the earliest Hadean was probably much less hospitable than even the grimmest representations of Hell. Yet somehow this inferno evolved into a place not only suitable for life but welcoming—with abundant oceans as well as dry land, an atmosphere dominated by nitrogen, and mostly comfortable temperatures. We have almost no idea how fast the surface environment evolved, how the transition took place, or when conditions became hospitable enough to support life. However, clues from Earth’s oldest minerals, zircons, as well as from our Moon and other planets are allowing a clearer picture of that early fiery (and perhaps sometimes frozen) Earth to gradually emerge.

How Did the Transition to Earth’s Current Environment Occur?

Current models suggest that much of Earth’s rocky mantle was melted by the Moon-forming impact and that part of it was vaporized (Stevenson, 1987; Canup, 2004a). If this was the case, liquid mantle would have been present at Earth’s surface and the atmosphere would likely have been mostly rock vapor, topped by hot silicate clouds with temperatures up to 2500 K (Zahnle, 2006).2 As Earth’s surface cooled, the silicate clouds

2

K represents the Kelvin temperature scale, commonly used in geology. The Kelvin scale is set so that zero degrees K is absolute zero, the temperature at which a substance has no remaining thermal energy. Zero K equals −273.15°C and the two scales are otherwise the same, with one degree C having the same magnitude as a one-degree increment in Kelvin.



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