![]() ![]() ![]() When the Sun was accreting and losing large quantities of matter during its first 400 million years, its luminosity changed radically. It shows that between 4.3–1.3 billion years ago Earth was cool enough for liquid water to condense out from its atmosphere to form oceans. The figure shows the Sun’s luminosity history. As the Sun continues to fuse hydrogen into helium in its nuclear furnace, the increasing amounts of helium boost the Sun’s core density, which causes the Sun’s nuclear furnace to burn more efficiently. The reason why Earth possesses the quantity of surface liquid water that it does is that the Sun was fainter in the past than at the present. The planet’s atmospheric temperature and its atmospheric electric field operated to quickly desiccate Venus’s atmosphere. They are also certain that, at best, only minuscule quantities of water vapor remained in Venus’s atmosphere. Consequently, astronomers are now certain that Venus has never possessed any surface liquid water. However, this minimum is 54% greater than what is needed for liquid water to condense out from Venus’s primordial atmosphere. Venus attained a minimum surface warming of 500 W/m 2 about 4 billion years ago. The surface of Venus would need to receive no more than 325 W/m 2 of warming and Earth no more than 312.5 W/m 2. The team of astronomers also calculated the degree to which both Earth and Venus would need to cool down from their initial high accretion temperatures for liquid water to condense out from their atmospheres to form surface liquid water. These values compare with 340.5 and 675 W/m 2 of total warming, respectively, on the surfaces of Earth and Venus today. ![]() For early Earth, it is 50 watts per square meter (W/m 2). Turbet’s team calculated the net warming effect of the water clouds on surfaces of early Earth and Venus. Furthermore, water vapor is a powerful greenhouse gas. The accumulation of clouds on the nightside mitigates the thermal cooling to interplanetary space that would otherwise occur. 6 The team of six planetary astronomers led by Martin Turbet demonstrated that for both early Earth and early Venus, water clouds preferentially formed on the nightside (the side of the planet not exposed to the Sun). Consequently, the models produced highly uncertain estimates of the timing and duration of surface liquid water.įor the first time, a team of planetary astronomers has constructed and applied three-dimensional global climate models to simulate the conditions on early Earth and early Venus. ![]() 5 These models, however, could not consider the effects of atmospheric clouds and atmospheric circulation patterns. Researchers explored this question using one-dimensional numerical climate models. The question, then, was when such water existed and how long it remained on the rocky planet. In the cooling process, water vapor condenses out from the atmosphere to form oceans. Though rocky planets start off hot, due to their initial accretion energy (energy gained as a result of planetesimals merging and condensing to form the planet) 4, they cool down. After all, water (H 20/OH) is the third most abundant molecule in the universe after H 2 and H 3. These studies assumed that surface liquid water was present in the first place. Past studies sought to identify conditions that would permit rocky planets such as Earth, Mars, or Venus to possess surface liquid water in the form of oceans, seas, or lakes. Previous Research on Surface Ocean Formation 3 And, until six weeks ago, the question of whether it had ever been present on Venus’s surface remained unanswered. 2 It was briefly present as small lakes and rivers on Mars at multiple latitudes 3.6–3.8 billion years ago. 1 Liquid water has been present on Earth’s surface in the form of large oceans for the past 3.825 billion years. Without surface liquid water there is no possibility for the existence of physical life. Is Earth the only known rocky planet that shows evidence of a long-standing supply of surface water? Researchers continue to construct models to test for this planetary feature, and the results carry implications for life. ![]()
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