7.3 Summary: The high-energy young solar system
The young, ZAMS Sun was fainter than the present-day Sun in optical light by about 30%,
and the consequent lowered irradiance of solar-system planets poses serious problems as the
Earth should have been in deep freeze if its atmosphere was not fundamentally different from
today’s; however, for both the Earth and Mars, there is evidence for a mild climate in the early
days of the Sun’s main-sequence life. But the optical and infrared radiation is not the only,
and perhaps not even the key player. Ultraviolet radiation affects the chemistry of the upper
atmospheres, and EUV and X-ray radiation is absorbed in the upper layers, ionizing and heating
the atmospheric gas. Subsequent thermal or non-thermal processes, notably also interactions
with the solar wind, led to escape of atmospheric constituents. Irradiation by UV, EUV, and
X-ray photons as well as the solar wind were tens to hundreds of times stronger in the early
solar system and should have played a key role in affecting and modifying young planetary
atmospheres. Most notably, the loss of water, for example on Venus, is an inevitable product of
photodissociation and various loss mechanisms, in particular for the light hydrogen. It may be the Earth’s
strong magnetic field that protected it sufficiently from suffering a similar fate as either Venus or
Mars.
