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Loss of angular momentum is more effec- rock units were deposited during open- al., 2018) and by sulfur isotopes in sedi-
tive when stars spin faster, because mag- water, probably sunlit conditions (Figs. 2C mentary rocks that were fractionated inde-
netic fields are generally stronger and and 2D). If Earth absorbed and retained pendent of mass by ultraviolet photolysis
sweep through stellar winds faster, and solar energy with modern effectiveness, of SO in an anoxic atmosphere (Farquhar
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because coronal mass ejections are more water at ancient Earth’s surface would and Wing, 2003; Claire et al., 2014;
common (e.g., Gallet and Bouvier, 2015). have been largely if not entirely frozen Dauphas and Schauble, 2016). CH and H
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Studies of star populations in clusters in during this time, except where heated by are potential greenhouse gases but would
which all the stars have similar age indi- magmatic and hydrothermal activity or be slowly oxidized in such an atmosphere
cate that most stellar spindown, and by beneath hundreds to thousands of meters due to various reactions involving H O and
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inference mass loss, occurs during the first of ice. Geologic evidence of ancient warm CO that are triggered by ultraviolet radia-
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few hundred million years of a star’s life conditions is outlined below, followed by tion, a process that would be more effec-
(Fig. 2A) (Skumanich, 1972; Ayres, 1997). possible explanations for effective surface tive with more intense short-wavelength
This interpretation was strengthened by warming with a faint young Sun. radiation in the Archean (Pavlov et al.,
recent studies using data from the Kepler Archean strata deposited in shallow 2001; Catling and Kasting, 2017). Methane
planet-finder satellite mission (active water with evidence of sunlit conditions could have been an important greenhouse
2009–2018). The Kepler telescope was indicate surface temperatures above gas, however, if methane-generating bacte-
designed to detect changes in stellar lumi- freezing. These rock units include calcar- ria were abundant and if bacterial growth
nosity resulting from passage of planets in eous and siliceous strata interpreted as was not inhibited by cooling due to sun-
front of stars but also identified luminosity fossil microbial mats and mound-forming light-shielding organic haze produced by
variations due to transit of starspots on algal stromatolites that harbored, or were photochemical reactions involving CH
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stellar surfaces. This allowed determina- produced by, photosynthetic bacteria and CO (Domagal-Goldman et al., 2008).
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tion of rotation periods for thousands of (e.g., Grotzinger and Knoll, 1999; Noffke, Effective warming by reducing green-
stars and accurate representation of stellar 2008; Tice et al., 2011). One of the oldest house gases is suggested by the coinci-
spindown rates (Fig. 2A) (Meibom et al., examples of aquatic microbial life is the dence of earliest Proterozoic glaciations
2011, 2015; Rebull et al., 2017). It appears 3.42 Ga Buck Reef Chert in South Africa, with the great oxidation event (“GOE”) of
from these studies that, if the Sun did have which contains carbonaceous layers, fila- Earth’s atmosphere (Holland, 2006) (Fig.
greater mass during its youth, it would ments, and grains deposited within fine- 2D). The GOE would have destroyed
have lost most of that additional mass dur- grained, wave-agitated, siliceous sedi- atmospheric CH and H , thereby causing
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ing rapid spindown before deposition of ments (Tice and Lowe, 2004) (Fig. 2D). the temperature drop (Pavlov et al., 2000;
Archean algal stromatolites on Earth and The 3.35–3.43 Ga Strelley Pool Formation Haqq-Misra et al., 2008).
Noachian river-channel incision and lake- in the Pilbara Supergroup of northwestern Three-dimensional climate models of
delta deposition on Mars (Figs. 2C–2E). Australia (Wacey et al., 2010) contains Archean Earth’s atmosphere have been
abundant evidence of microbial biofilms used to evaluate the effectiveness of sev-
SOLAR RADIATION OVER TIME that precipitated carbonate and/or trapped eral atmospheric variables that are impor-
Short-wavelength solar radiation was fine detrital grains to form laminated tant for a warm climate, including CO ,
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stronger during the Sun’s youth, which stromatolites (Allwood et al., 2009; CH , and H concentration, absolute pres-
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further complicates warming scenarios Wacey, 2010; Duda et al., 2016). Multiple sure, and abundance of cloud-condensation
because some greenhouse gases such as associated sedimentary features in the nuclei. Warm, open-water conditions not
methane are readily photolyzed (broken Strelley Pool Formation, including rare unlike modern conditions were calculated
down) by such radiation or by free radicals desiccation cracks, indicate deposition in for a 1-bar atmosphere with 1% CO and
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produced by photolysis of other gases shallow-water, tidally affected, marine 0.2% CH at 2.5 Ga (Charnay et al., 2013);
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(Catling and Kasting, 2017). Six nearby environments (e.g., Allwood et al., 2006). 1.2% CO and 0.1% CH at 3.5 Ga (Le Hir
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stars similar in mass to the Sun, but with a Fossil microbial mats and stromatolites et al., 2014); and 0.1–0.36 bar CO partial
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range of ages, were identified by Ribas et resemble much younger and modern pressure with 1 bar N at 3.8 Ga (Charnay
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al. (2005) and used to estimate radiation microbial mats, photosynthetic stromato- et al., 2017). A variety of other factors
characteristics of the Sun over time (Fig. lites, and cyanobacterial mounds influence global temperature, including
2B). Short-wavelength radiation, especially (Suosaari et al., 2016). sea-ice dynamics and the abundance
far ultraviolet and X-ray, is significantly The abundance of sedimentary and of cloud-droplet condensation nuclei.
more intense for younger stars because fossil indicators of liquid water at Earth’s Appropriate values of multiple variables
their faster rotation rate increases the surface during the Archean indicates the appear to be capable of warming Archean
accumulation of magnetic-field energy effectiveness of warming mechanisms that Earth to approximately modern tempera-
in stellar envelopes, which in turn results are still not well understood. CO and N tures, but the relative significance of these
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in greater short-wavelength radiation from were likely the dominant atmospheric factors in supporting warm conditions
stellar surfaces and coronae. gases (Kasting, 2014; Catling and Kasting, under a faint young Sun has not been
2017). The absence of significant O in clearly identified.
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EARTH Earth’s Archean atmosphere is indicated
Solar luminosity is calculated to have by unoxidized detrital sulfides and MARS
been ~77%–79% of its current value at uraninite in Archean fluvial sediments The heavily cratered Noachian high-
3.2 –3.5 Ga, when at least four different (Rasmussen and Buick, 1999; Burron et lands of equatorial and southern Mars,
6 GSA Today | December 2019