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Figure 2. Known or hypothetical evolution of the 0.6
Sun, Earth, and Mars. (A) Star clusters, each Stellar spindown A
containing stars of similar age, are plotted Pleiades
against rotation rate (Rebull et al., 2016, 2017;
Meibom et al., 2009, 2011, 2015; Hartman et al., 0.4
2009). These are plotted as proxies for solar M35
evolution, so cluster age on the horizontal axis
is plotted as distance from the left edge of the (rotations per Earth day)
graph rather than the right edge. Curve show- 0.2 Praesepe
ing approximate spindown rate is from Ayres t -0.6
(1997; t = time since cluster birth). (B) Ultraviolet M37 NGC 6811 NGC 6819
and X-ray radiation fluxes inferred for the Sun Sun
over time based on measured fluxes from 0.0
nearby stars with approximate solar mass but
different ages (modified from Figure 8 of Ribas 30
et al., 2005). Current radiation flux = 1. (C) Evo- Solar radiation B
lution of solar luminosity for initial solar mass 0.1-2.0 nM (X-ray)
equal to, or slightly greater than, modern solar
mass. The rate of mass loss is shown as pro- 20 2-10 nM (soft X-ray and far ultraviolet)
portional to the rate of spindown of solar type
stars as shown in (A). Current luminosity = 1.0. Radiation flux 10-36 nM (ultraviolet)
-9
Blue area at 3.2–3.5 Ga represents calculated nM = nanometers (10 m)
luminosity (~77%–79% of modern) at the time of 10
deposition of four stromatolite and microbial 92-120 nM (near ultraviolet)
mat fossil units in northwest Australia and
South Africa. (D) Some events in Earth history modern radiation flux = 1
that reflect climate and atmospheric composi- 0
tion, including ages and names of Archean
stromatolite and microbial mat deposits older 1.0
than ca. 2.7 Ga and ages and names of glacial Solar luminosity
deposits older than ca. 2.0 Ga. Archean glacial M C
M i = 1.04M ʘi = 1.04M ʘ
deposits from Young et al. (1998; Mozaan),
Ojakangas et al. (2014; Talya), and de Wit and 0.9
Furnes (2016; Noisy) (cg—conglomerate). (E) M
M i = 1.02M ʘi = 1.02M ʘ
Some events in Mars history. Pre-Noachian Solar luminosity M
magnetization of the martian crust ended M i = 1.01M ʘi = 1.01M ʘ
before formation of the Hellas impact basin. 0.8
Noachian to early Hesperian highland valley
networks formed after the large impact basins. M i = initial solar mass
= modern solar mass
0.7 M M ʘ
M i = 1.00M ʘi = 1.00M ʘ
in its core and now has ~34%. The core
Hadean
will continue to contract, while the con- Hadean Archean Proterozoic Phanerozoic
vecting outer layer will expand due to Archean stromatolites and microbial mats D
greater energy output from the core. Isua Dresser Buck Reef Strelley Pool Moodies Chobeni Mushandike Mosher Belingwe Earth
A BRIGHT YOUNG SUN? great oxidation event (GOE)
Difficulties in identifying the causes of Noisy Complex, Onverwacht suite,
Mozaan Group, S. Africa
warm climates on young Earth and Mars Talya cg, Dharwar craton, SW India S. Africa glacial deposits >2 Ga
provoked consideration of a more massive Huronian Supergroup, Ontario (brackets show uncertainty in age)
and therefore more luminous young Sun. Makganyene diamictite, S. Africa
Eastern Transvaal basin, S. Africa
Specifically, if the Sun was 4%–5% more
massive at its birth, before blowing off reducing atmosphere oxygenation of the atmosphere
mass as solar wind and coronal mass ejec- fractionation of sulfur isotopes fractionation of sulfur isotopes
independent of mass (MIF-S)
dependent on mass
tions, it could have provided elevated
luminosity to warm the young planets to
approximately modern temperatures (Fig. Noachian Hesperian Amazonian
2C) (Whitmire et al., 1995; Sackmann pre-Noachian Mars E
and Boothroyd, 2003). This is plausible global magnetic eld (pre-Hellas)
because stellar luminosity is very sensi- Hellas
tive to stellar mass. For a roughly solar- Isidis large impact basins (ages from Fassett and Head, 2011)
Argyre
mass star, absolute luminosity scales to highland valley networks (ages from Fassett and Head, 2008a)
almost the fifth power of solar mass, 4.0 3.0 2.0 1.0 0.0
while the greater gravitational attraction Age (Ga)
of a more massive star reduces orbital
radius proportional to mass. These factors
together result in insolation at Earth and (1.01 raised to the 6.75 power ≈1.07; Minton stellar magnetic fields sweep through
Mars that scales to almost the seventh and Malhotra, 2007). stellar winds and accelerate the winds
power of solar mass such that a 1% The angular momentum of spinning circumferentially, thereby flinging the
greater solar mass would result in ~7% stars is gradually carried away by stellar winds away and transferring angular
greater insolation for orbiting planets winds. This is effective because rotating momentum from the star to the wind.

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