The paradox of a weak young Sun is the observed contradiction between paleoclimatic data and astrophysical models of the evolution of the Sun.
The standard model of the evolution of stars claims that 4 billion years ago, the Sun radiated approximately 30% less energy than it does now. Under such conditions, the water on the surface of the Earth would have to completely freeze. At the same time, geological studies of Archean sedimentary rocks show that in this epoch there was a humid and warm climate on Earth. In the context of global glaciation, life might not have been able to arise.
Most scientists are inclined to explain this paradox by the global greenhouse effect in the early history of the Earth and caused by very high concentrations of volcanic gases such as carbon dioxide and methane . For the first time this model was proposed and quantitatively analyzed by Soviet scientists L. M. Mukhin and V. I. Moroz . In the role of the greenhouse gas could be a mixture of nitrogen and hydrogen , which was more in the atmosphere of the young Earth than it is now  . Alternative hypotheses explaining the heat flow that is sufficient for the generation and maintenance of life include changes in the structure of the solar system , in which the earth was born closer to the sun, and changes in the location and area of the continents of the planet  .
Mars also has traces of a warm and humid climate. Craters of different ages located in the same areas are subject to erosion processes to various degrees. For ancient craters, much higher erosion rates are determined than for younger ones. From these and some other observations, it is concluded that there is liquid water on Mars in the early stages of its existence.
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