The next step on the path toward another Earth is to find atmospheres similar to those of Earth and Venus—high–molecular-weight (secondary) atmospheres—on rocky exoplanets. Many rocky exoplanets are born with thick (>10 kbar) H₂-dominated atmospheres but subsequently lose their H₂; this process has no known Solar System analog. We study the consequences of early loss of a thick H₂ atmosphere for subsequent occurrence of a high–molecular-weight atmosphere using a simple model of atmosphere evolution (including atmosphere loss to space, magma ocean crystallization, and volcanic outgassing). We also calculate atmosphere survival for rocky worlds that start with no H₂. Our results imply that most rocky exoplanets orbiting closer to their star than the habitable zone that were formed with thick H₂-dominated atmospheres lack high–molecular-weight atmospheres today. During early magma ocean crystallization, high–molecular-weight species usually do not form long-lived high–molecular-weight atmospheres; instead, they are lost to space alongside H₂. This early volatile depletion also makes it more difficult for later volcanic outgassing to revive the atmosphere. However, atmospheres should persist on worlds that start with abundant volatiles (for example, water worlds). Our results imply that in order to find high–molecular-weight atmospheres on warm exoplanets orbiting M-stars, we should target worlds that formed H₂-poor, that have anomalously large radii, or that orbit less active stars.

通往另一个地球的下一步是在岩石系外行星上寻找类似于地球和金星的大气层——高分子量（次级）大气层。许多岩石系外行星诞生时就具有厚（>10 kbar）H ₂主导的大气层，但随后失去了 H ₂ ；这个过程没有已知的太阳系类似物。我们使用一个简单的大气演化模型（包括大气流失到太空、岩浆海洋结晶和火山放气）来研究早期失去厚厚的 H ₂大气对随后出现高分子量大气的后果。我们还计算了从没有 H ₂开始的岩石世界的大气层存活率。我们的结果表明，大多数岩石系外行星的轨道距离其恒星比由厚厚的 H ₂主导大气层形成的宜居带更靠近，如今缺乏高分子量大气层。在早期岩浆海洋结晶过程中，高分子量物质通常不会形成长寿的高分子量大气；相反，它们与 H ₂一起消失在太空中。这种早期挥发物的消耗也使得后期火山释放气体更难以恢复大气。然而，在以丰富的挥发物开始的世界（例如水世界）上，大气应该持续存在。我们的结果意味着，为了在围绕 M 星运行的温暖系外行星上找到高分子量大气层，我们应该瞄准那些形成贫 H ₂ 、半径异常大或围绕不太活跃恒星运行的世界。