Astronomer Petrus Martens of Georgia State University (USA) believes that the Sun was heavier in ancient times than today. This allowed the young star to shine as brightly as it does today and provide livable conditions on Earth and Mars. By now, the luminary has become lighter. Research, available from the arXiv.org electronic preprint library, is addressing the faint young sun paradox. We will talk about the history of the life of the luminary below.
The young Sun appeared about 4.5 billion years ago as a main sequence object. According to the standard theory of stellar evolution in ancient times, the Sun was about 30 percent fainter than it is today. It remains a mystery how, with such a weak star, the young Earth was warm enough to provide its surface with liquid water. This contradiction is called the weak young Sun paradox.
The paradox is also relevant for Mars, on which seas and oceans of liquid water existed for hundreds of millions of years, although the Red Planet receives about half the amount of sunlight that the Earth does.
Geological data indicate that water appeared early on Earth and Mars. The past of the Sun can be learned by observing other stars of the main sequence. Simulations indicate that stars of spectral types G, to which the luminary closest to Earth belongs, as well as objects of classes K and M, do not develop too quickly, and the habitability zone around such stars is gradually shifting outward.
The paradox of the weak young Sun has been proposed to be resolved in several ways. The reason for the heating of the planet's atmosphere was a strong greenhouse effect from carbon dioxide or methane, geothermal energy from the initially warmer than today, the Earth's core, the Earth's albedo smaller in antiquity, life developing in a cold environment under a 200-meter-thick ice sheet, even a variant with variable gravitational constant.
Mars in antiquity (as imagined by the artist)
Martens believes that most of these explanations have serious flaws. For example, it is unclear when the greenhouse effect should stop, so that what happened on Venus, whose atmosphere is so hot that life is practically impossible in it, does not happen. In addition, no sufficient traces of excess carbon dioxide have yet been found in ancient geological samples.
Martens believes that many explanations of the paradox of the young sun take into account only the processes occurring on Earth, and not on Mars, and do not suggest an explanation of this contradiction for other planetary systems. In this regard, the American astronomer decided to recall the old, but unpopular today hypothesis, according to which the ancient Sun was more massive than at present.
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A luminary belonging to the same spectral class emits more energy, the heavier it is. This means that if in ancient times the Sun at its present size was 30 percent weaker, it is possible to calculate how much the star closest to the Earth was heavier to shine as it does today.
About three billion years ago, according to the scientist's estimates, the luminary lost about 0.0000000000075 of its mass every year (about three percent of the initial mass for three billion years of existence); at present, this value is two orders of magnitude lower and is insignificant for taking into account the change in the brightness of the star. The scientist came to such conclusions, having drawn attention to the fact that over time the Sun and most of these stars slow down their rotation.
According to the author, this is due to the loss of their mass by the Sun and similar stars (when the law of conservation of angular momentum is fulfilled). For example, the large companion of the binary 70 Ophiuchus is about 1.1 times lighter than the Sun, is 0.8 billion years old, and becomes lighter at a rate of 0.000000000003 solar masses per year. In order for local planets to have conditions suitable for the existence of liquid water, such a regime of mass loss must be maintained for about 2.4 billion years.
The ancient full glaciers of the Earth, which are replaced by melting water, Martens explains in a rather prosaic way - volcanic activity, along with which greenhouse gases enter the atmosphere, as well as positive feedback.
The sun
The loss of their masses by the Sun and similar luminaries in ancient times should have been accompanied by the emergence of stable and strong solar (stellar) winds. The modern Sun does not produce such emissions of matter. It may seem that the star had no reason to do this before, so the hypothesis of an ancient massive Sun is unpopular. Martens believes that this is not the case: the current rate of mass loss by the Sun is not enough to slow down from the initial four to five days to the current 26 days.
Martens' point of view does not explain how life should be preserved on a planet irradiated by strong stellar winds. Meanwhile, explanations of the paradox of the young Sun, based on the greenhouse effect, do not lose their relevance, moreover, over time, these theories are supplemented.
For example, not only volcanoes, but also asteroids can take part in filling the Earth's atmosphere with carbon dioxide and methane. So, scientists have created a new model of gas release on Earth, which demonstrated the sufficient strength of the greenhouse effect for the existence of liquid oceans already at the early stages of the planet's development, in low light conditions. Unlike previous studies, which also offer a possible explanation for the presence of liquid water on the ancient Earth using volcanic degassing (the release of greenhouse gases into the atmosphere during volcanic eruptions), the new work takes into account the active bombardment of the planet by asteroids.
Reaching a hundred kilometers in diameter, these celestial bodies, when falling to the Earth, cause the melting of large volumes of rocks, creating huge lava lakes. As they cool, they release enough carbon dioxide and thus heat up the atmosphere. The bombardment of the planet, according to scientists, led to the release of sulfur from its bowels, which is necessary for the formation of organic life.