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6 Further Back in Time: The Pre-Main Sequence Sun 61
6.1 Where was the cradle of the Sun? . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
6.2 New features in the pre-main sequence Sun . . . . . . . . . . . . . . . . . . . . . . 61
6.2.1 Evolutionary stages: Overview . . . . . . . . . . . . . . . . . . . . . . . . . 61
6.2.2 New features: Accretion, disks, and jets . . . . . . . . . . . . . . . . . . . . 62
6.2.3 New emission properties: Solar-like or not? . . . . . . . . . . . . . . . . . . 62
6.3 The T Tauri Sun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
6.3.1 The magnetic field of the T Tauri Sun . . . . . . . . . . . . . . . . . . . . . 63
6.3.2 The ultraviolet T Tauri Sun . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
6.3.3 The X-ray T Tauri Sun in time . . . . . . . . . . . . . . . . . . . . . . . . . 66
6.3.4 Coronal excesses and deficits induced by activity? . . . . . . . . . . . . . . 68
6.3.5 X-ray flaring of the T Tauri Sun . . . . . . . . . . . . . . . . . . . . . . . . 70
6.3.6 The radio T Tauri Sun in time . . . . . . . . . . . . . . . . . . . . . . . . . 70
6.3.7 The composition of the T Tauri Sun’s corona . . . . . . . . . . . . . . . . . 71
6.4 The protostellar Sun . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
6.4.1 Magnetic activity in the protostellar Sun . . . . . . . . . . . . . . . . . . . 72
6.4.2 Magnetic flaring of the protostellar Sun . . . . . . . . . . . . . . . . . . . . 73
6.4.3 Radio emission from the protostellar Sun . . . . . . . . . . . . . . . . . . . 73
6.5 The pre-main sequence Sun’s environment in time . . . . . . . . . . . . . . . . . . 73
6.5.1 Circumstellar disk ionization . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6.5.2 Circumstellar disk heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
6.5.3 Observational evidence of disk irradiation . . . . . . . . . . . . . . . . . . . 76
6.6 The T Tauri Sun’s activity and meteoritics . . . . . . . . . . . . . . . . . . . . . . 77
6.7 Summary: The violent pre-main sequence Sun . . . . . . . . . . . . . . . . . . . . . 79
7 The Solar System in Time: Solar Activity and Planetary Atmospheres 80
7.1 The Faint Young Sun Paradox: Greenhouse or deep freeze? . . . . . . . . . . . . . 80
7.1.1 The relevance of greenhouse gases . . . . . . . . . . . . . . . . . . . . . . . 81
7.1.2 A bright young Sun? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
7.1.3 Cosmic rays and a stronger solar wind . . . . . . . . . . . . . . . . . . . . . 84
7.2 The Sun’s activity in the young solar system . . . . . . . . . . . . . . . . . . . . . 84
7.2.1 Planetary atmospheric chemistry induced by high-energy radiation . . . . . 84
7.2.2 High-energy radiation and the planetary biospheres in habitable zones . . . 86
7.2.3 Planetary atmospheric loss and high-energy radiation and particles . . . . . 87
7.2.4 Mercury: Erosion of atmosphere and mantle? . . . . . . . . . . . . . . . . . 89
7.2.5 Venus: Where has the water gone? . . . . . . . . . . . . . . . . . . . . . . . 89
7.2.6 Earth: A CO2 atmosphere and magnetic protection . . . . . . . . . . . . . 90
7.2.7 Mars: Once warmer and wetter? . . . . . . . . . . . . . . . . . . . . . . . . 91
7.2.8 Venus, Earth, Mars: Similar start, different results? . . . . . . . . . . . . . 92
7.2.9 Titan: Early atmospheric blow-off? . . . . . . . . . . . . . . . . . . . . . . . 93
7.2.10 Hot Jupiters: Mass evolution by evaporation? . . . . . . . . . . . . . . . . . 94
7.3 Summary: The high-energy young solar system . . . . . . . . . . . . . . . . . . . . 95
8 Summary and Conclusions 96
9 Acknowledgements 99
References 100