Early sun

Cosmochemistry is the study of the chemical composition of the universe and the processes that produced those compositions. This is a tall order, to be sure. Understandably, cosmochemistry focuses primarily on the objects in our own solar system, because that is where we have direct access to the most chemical information. That part of cosmochemistry encompasses the compositions of the Sun, its retinue of planets and their satellites, the almost innumerable asteroids and comets, and the smaller samples (meteorites, interplanetary dust particles or IDPs, returned lunar samples) derived from them. From their chemistry, determined by laboratory measurements of samples or by various remote-sensing techniques, cosmochemists try to unravel the processes that formed or affected them and to fix the chronology of these events. Meteorites offer a unique window on the solar nebula - the disk-shaped cocoon of gas and dust that enveloped the early Sun some 4.57 billion years ago, and from which planetesimals and planets accreted (Fig. 1.1). 

Hydrogen isotopic compositions, expressed as molar D/H ratios, of solar system bodies. The relatively low D/H values in the atmospheres of Jupiter and Saturn are similar to those in the early Sun, whereas D/H ratios for Uranus and Neptune are intermediate between the Jupiter-Saturn values and those of comets and chondrites. The Earth s oceans have D/H shown by the horizontal line. Mars values are from SNC meteorites. Modified from Righter et al. (2006) and Lunine (2004). 

Ackermann, 1971) and for two additional values 300 times the present value and for the wavelength-dependent ultraviolet flux emitted by early sun-like stars (T-Tauri) as observed with the International Ultraviolet Explorer (lUE) satellite (Canuto et al., 1982, and Canuto et al., 1983). The model extends from the surface to 53.5 km with 1 kilometer spatial resolution between the surface and 10 km, and 1.5 kilometers spatial resolution between 10 and 53.5 km. Further details about the model are given in Levine et al. (1981) and Levine et al. (1982). 

Figure 6 The X-wind model for meteoritical processing involves subjecting solids to high temperatures and fluxes of energetic particles from the early Sun during and after being lifted from the midplane of the solar nebula in a bipolar wind, and afterward falling back onto the nebula at greater distances (Shu et al., 2001) (reproduced by permission of University of Chicago Press and American Astronomical Society from Astrophys. J., 2001, 548, 1029-1050).

It is possible that the atmosphere was blown off by a major impact like the Moon-forming giant impact, but this is far from clear at this stage. Another mechanism that often is considered is the effect of strong ultraviolet wavelength radiation from the early Sun (Zahnle and Walker, 1982). This might affect Xe preferentially because of the lower ionization potential. It is of course possible that the Earth simply acquired an atmosphere, with xenon, like today s (Marty, 1989 Cafifee et al, 1999). However, then it is not clear how to explain the strong isotopic fractionation relative to solar and meteorite compositions. 

Sagan C. and Chyba C. (1997) The early Sun paradox organic shielding of ultraviolet-labile greenhouse gases. Science 276, 1217-1221. 

Patients with skin types IV-VI have a higher risk of PIH, even with low concentrations of superficial peeling agents. Abnormal pigmentation may develop very soon after the peel or months later, and it is often the result of poor technique or early sun exposure after the peel. Other causes of PIH include estrogens, photosensitizing drugs or... 

Because the early Sun was less luminous, the temperatures could have been lower on Earth. A global frozen ocean (several 100 m thick) could have provided an ideal shield against UV radiation. 

Before discussing the occurrence of water on surfaces of the terrestrial planets at present and in the past we shortly describe the early Sun evolution since this had important influence on the water history of the planets. 

However, the early sun was quite different from the sun we know. Its luminosity was only about 70% of its present value. It rotated much faster. SteUar rotation is an important parameter to trigger stellar activity. The present day sun shows an activity cycle with a period of about 11 years. The number of sunspots varies but also the number of flare occurrence (these are energetic outbursts caused by a reconnection... 

The early sun was less luminous but the energetic outbursts were of higher amplitude and occurred more frequent than presently. The UV emission was about 10, the extreme UV 100 and the X-ray emission 1000 times the present values. Interactions of high-energy radiation and the solar wind with upper planetary atmospheres may have led to the escape of important amounts of atmospheric constituents (GUdel, 2007 [148]). 

The lower luminosity of the early Sun would imply lower surface temperatures on the terrestrial planets and e.g. the Earth would have been frozen totally. From geology we know that Earth was never frozen totally. This paradox is called the... 

A study of the spectral signature of water described above showed that 66 percent of the C-class asteroids in the sample investigated have hydrated silicate surfaces. Differences in hydration among different types of asteroids may be understood by the fact that the early Sun had a very strong wind which heated the protoasteroids. 

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