Day–night cloud asymmetry prevents early oceans on Venus but not on Earth
Day–night cloud asymmetry prevents early oceans on Venus but not on Earth
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昼夜云不对称阻止了金星上的早期海洋，但地球上却没有
作者：
M. Turbet;É. Bolmont;G. Chaverot;D. Ehrenreich;J. Leconte;E. Marcq
地球拥有海洋已经有近40亿年了，火星在35亿至38亿年前就有湖泊和河流。然而，水是否曾经在金星表面凝结仍然是未知的3，4，因为这颗行星现在完全干燥5，经历了全球性的表面重塑事件，掩盖了它的大部分历史6，7。水最初在太阳系类地行星表面凝结所需的条件是高度不确定的，因为到目前为止，它们只使用一维数值气候模型3进行研究，无法解释大气环流和云的影响，而这些是关键的气候稳定因素。在这里，我们使用三维全球气候模型模拟早期金星和地球的水云，它优先形成在夜面，由于强烈的太阳下水蒸气吸收，有一个强大的净变暖效应，抑制地表水冷凝，即使在温和的日照（下降到325瓦每平方米，即0.95倍地球太阳常数）。这表明水从未凝结，因此，金星表面从未形成海洋。此外，这表明地球海洋的形成需要比今天低得多的日照，这是由于年轻的太阳微弱。这也意味着现今地球存在另一种稳定状态：“蒸汽地球”，所有来自海洋的水都蒸发到大气中。全球气候模型对早期金星和地球的模拟显示，云层状况的差异阻止了金星上海洋的形成，但地球上没有。
Earth has had oceans for nearly four billion years1 and Mars had lakes and rivers 3.5–3.8 billion years ago2. However, it is still unknown whether water has ever condensed on the surface of Venus3,4 because the planet—now completely dry5—has undergone global resurfacing events that obscure most of its history6,7. The conditions required for water to have initially condensed on the surface of Solar System terrestrial planets are highly uncertain, as they have so far only been studied with one-dimensional numerical climate models3 that cannot account for the effects of atmospheric circulation and clouds, which are key climate stabilizers. Here we show using three-dimensional global climate model simulations of early Venus and Earth that water clouds—which preferentially form on the nightside, owing to the strong subsolar water vapour absorption—have a strong net warming effect that inhibits surface water condensation even at modest insolations (down to 325 watts per square metre, that is, 0.95 times the Earth solar constant). This shows that water never condensed and that, consequently, oceans never formed on the surface of Venus. Furthermore, this shows that the formation of Earth’s oceans required much lower insolation than today, which was made possible by the faint young Sun. This also implies the existence of another stability state for present-day Earth: the ‘steam Earth’, with all the water from the oceans evaporated into the atmosphere. Global climate model simulations of early Venus and Earth show that differences in the cloud regimes prevented ocean formation on Venus but not on Earth.