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arXiv 1702.08618astro-ph.EP
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一个火山氢宜居带

A Volcanic Hydrogen Habitable Zone

Ramses Ramirez, Lisa Kaltenegger

摘要:

经典的宜居带是恒星周围的圆形区域,其中液态水可能存在于岩石行星的表面。传统的N2-CO2-H2O宜居带(HZ)的外缘在我们的太阳系中延伸到近1.7 AU,超过这个范围,CO2的凝结和散射超过了它的温室容量。在这里,我们表明,在靠近外缘的行星上,大气H2的火山放气可以将宜居带扩展到我们太阳系的~2.4 AU。只要火山释放的氢气抵消了从大气层顶部逃逸的氢气,这种更广泛的火山氢气宜居带(N2-CO2-H2O-H2)就可以持续下去。我们使用一个单柱辐射对流气候模式来计算这个火山氢宜居带的HZ限制的氢浓度在1%和50%之间,假设扩散有限的大气逃逸。当氢浓度为50%时,M到A型恒星维持外缘所需的有效恒星通量会减少约35%到60%。相应的轨道距离增加约30%至60%。相对于经典的HZ,这个HZ的内边缘只向外移动了约0.1%到4%,因为H2变暖在稠密的H2O大气中减少了。这种火山H2大气层的大气尺度高度在HZ外缘附近也增加,便于远程探测大气特征。

英文摘要:

The classical habitable zone is the circular region around a star in which liquid water could exist on the surface of a rocky planet. The outer edge of the traditional N2-CO2-H2O habitable zone (HZ) extends out to nearly 1.7 AU in our Solar System, beyond which condensation and scattering by CO2 outstrips its greenhouse capacity. Here, we show that volcanic outgassing of atmospheric H2 on a planet near the outer edge can extend the habitable zone out to ~2.4 AU in our solar system. This wider volcanic hydrogen habitable zone (N2-CO2-H2O-H2) can be sustained as long as volcanic H2 output offsets its escape from the top of the atmosphere. We use a single-column radiative-convective climate model to compute the HZ limits of this volcanic hydrogen habitable zone for hydrogen concentrations between 1% and 50%, assuming diffusion-limited atmospheric escape. At a hydrogen concentration of 50%, the effective stellar flux required to support the outer edge decreases by ~35% to 60% for M to A stars. The corresponding orbital distances increase by ~30% to 60%. The inner edge of this HZ only moves out by ~0.1 to 4% relative to the classical HZ because H2 warming is reduced in dense H2O atmospheres. The atmospheric scale heights of such volcanic H2 atmospheres near the outer edge of the HZ also increase, facilitating remote detection of atmospheric signatures.

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