Raymond Sean N, Quinn Thomas, Lunine Jonathan I
NASA Postdoctoral Program Fellow, Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, Colorado 80309-0389, USA.
Astrobiology. 2007 Feb;7(1):66-84. doi: 10.1089/ast.2006.06-0126.
The water content and habitability of terrestrial planets are determined during their final assembly, from perhaps 100 1,000-km "planetary embryos " and a swarm of billions of 1-10-km "planetesimals. " During this process, we assume that water-rich material is accreted by terrestrial planets via impacts of water-rich bodies that originate in the outer asteroid region. We present analysis of water delivery and planetary habitability in five high-resolution simulations containing about 10 times more particles than in previous simulations. These simulations formed 15 terrestrial planets from 0.4 to 2.6 Earth masses, including five planets in the habitable zone. Every planet from each simulation accreted at least the Earth's current water budget; most accreted several times that amount (assuming no impact depletion). Each planet accreted at least five water-rich embryos and planetesimals from the past 2.5 astronomical units; most accreted 10-20 water-rich bodies. We present a new model for water delivery to terrestrial planets in dynamically calm systems, with low-eccentricity or low-mass giant planets-such systems may be very common in the Galaxy. We suggest that water is accreted in comparable amounts from a few planetary embryos in a " hit or miss " way and from millions of planetesimals in a statistically robust process. Variations in water content are likely to be caused by fluctuations in the number of water-rich embryos accreted, as well as from systematic effects, such as planetary mass and location, and giant planet properties.
类地行星的含水量和宜居性在其最终聚集过程中就已确定，这一过程可能由大约100个直径1000千米的“行星胚胎”以及数十亿个直径1 - 10千米的“微行星”构成。在此过程中，我们假设富含水的物质通过起源于外小行星区域的富含水天体的撞击而被类地行星吸积。我们在五个高分辨率模拟中对水的输送和行星宜居性进行了分析，这些模拟所包含的粒子数量比之前的模拟多大约10倍。这些模拟形成了15个质量从0.4到2.6个地球质量的类地行星，其中包括五个位于宜居带的行星。每个模拟中的每颗行星至少吸积了地球目前的水储量；大多数吸积量是这个数值的几倍（假设没有撞击损耗）。每颗行星从过去2.5天文单位范围内至少吸积了五个富含水的胚胎和微行星；大多数吸积了10 - 20个富含水的天体。我们提出了一个在动态平静系统（低偏心率或低质量巨行星）中向类地行星输送水的新模型——这样的系统在星系中可能非常普遍。我们认为，水以“偶然”方式从少数几个行星胚胎中吸积的量与通过统计上可靠的过程从数百万个微行星中吸积的量相当。含水量的变化可能是由吸积的富含水胚胎数量的波动以及行星质量和位置、巨行星属性等系统效应引起的。
