Technical Comments

Comment on "A Hydrogen-Rich Early Earth Atmosphere"

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Science  06 Jan 2006:
Vol. 311, Issue 5757, pp. 38
DOI: 10.1126/science.1117827


Tian et al. (Reports, 13 May 2005, p. 1014) proposed a hydrogen-rich early atmosphere with slow hydrogen escape from a cold thermosphere. However, their model neglects the ultraviolet absorption of all gases other than H2. The model also neglects Earth's magnetic field, which affects the temperature and density of ions and promotes nonthermal escape of neutral hydrogen.

Tian et al. (1) argued that Earth's early atmosphere contained ∼30% H2, suggesting that the rate of hydrogen escape to space would be extremely low because the base of Earth's exosphere (or exobase) would be cold. They further reasoned that a hydrogen-rich atmosphere would synthesize organic molecules important for the origin of life, an idea that has previously been dismissed (2). Do these findings undermine research concerning meteorite or hydrothermal sources of prebiotic organics? I argue that the assumptions behind the cold exobase in the Tian et al. model (1) are too unrealistic for their exobase temperature to be relevant. Second, whether the exobase temperature affects the rate of hydrogen escape needs more careful consideration, bearing in mind that today's escape of hydrogen is predominantly nonthermal.

To properly calculate early Earth's exobase temperature requires balancing the energy budget in the early upper atmosphere, accounting for all important neutral and ionic species and higher extreme ultraviolet (EUV) fluxes from the early Sun (3). Tian et al. [supporting online material for (1)] stated that their model “contains only one component—hydrogen molecules,” an assumption that automatically invokes a cold exobase because EUV absorption by other species is neglected. Other gases in a real atmosphere would not readily escape (and carry energy away), would absorb EUV, and would get hot. A related issue omitted in (1) is that the exobase temperature depends on the proportions of molecular versus atomic species. As a result of photolysis, the latter tend to increase with higher EUV fluxes and get hotter; in addition, atomic hydrogen escapes more easily than molecular hydrogen. Although Tian et al. assumed pure H2, they applied an EUV heating efficiency of 15%, which is appropriate for atomic hydrogen but too low for H2 by a factor of ∼4. Regardless, one still needs to calculate the exobase temperature for a multigas atmosphere. In the absence of a multigas calculation, Tian et al. assert that the early exobase would be cold, quoting a primitive (anoxic) atmosphere model (4) that used the EUV flux from the present-day Sun. With an enhanced EUV flux (3) appropriate for ∼4 billion years ago (Ga), the multigas model of (4) actually produces a mean exobase temperature >1400 K, greater than today's mean value (5). At this temperature, hydrogen escapes efficiently and cannot build up as a major constituent of the atmosphere. It is unconvincing to then appeal to an early atmosphere highly enriched in CO2 to engineer a cold Venus- or Mars-like exobase by CO2 radiative cooling. CO2 was limited on early Earth by a large loss flux due to seafloor weathering (6). Data from the ancient seafloor (7) indicate that the atmospheric CO2 mixing ratio was less than ∼1% (compared with >95% on Venus and Mars), which could not have greatly affected early Earth's exobase temperature (8).

The exobase temperature is also not the sole arbiter of Earth's hydrogen escape rate. Satellite measurements prove that today's hydrogen escape is predominantly nonthermal and that the proportion of nonthermal escape increases with a cooler exobase, maintaining the same diffusion-limited escape rate (911). Nonthermal escape is linked to Earth's magnetic field: H ions accelerated by Earth's magnetic field collide with neutral H atoms, exchange their charge, and become fast-moving neutral atoms with escape velocity; hydrogen also escapes along open polar field lines. Tian et al. dismiss the importance of nonthermal hydrogen escape from early Earth by comparison with the low nonthermal escape rates on Venus. However, Venus has no magnetic field and has different escape physics (10). Terrestrial helium fluxes further demonstrate the importance of nonthermal escape. Helium continuously fluxes to Earth's atmosphere from radioactive decay in Earth's interior. Today's exobase is sufficiently cold that thermal escape of helium accounts for less than one-millionth of the total helium escape flux (11). Thus, if we only considered thermal escape, we would incorrectly conclude that Earth's present atmosphere should be relatively rich in helium. In reality, 4He escapes efficiently by processes that do not depend on the exobase temperature (12, 13).

Finally, data suggest that hydrogen actually did escape from early Earth at rates close to its upper limit. The isotopic mass fractionation of atmospheric xenon is consistent with the idea that hydrogen escaped so strongly that it dragged even xenon, the heaviest gas in the atmosphere, to space (13, 14).

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