[论文解读] Climate Optimum on Mars Initiated by Atmospheric Collapse

Edwin S. Kite, M. A. Mischna+2 more|arXiv (Cornell University)|Mar 1, 2017

Planetary Science and Exploration被引用 1

一句话总结

本文提出，火星诺亚纪-赫斯珀里亚气候最佳期——以长期流水活动为标志——是由大气崩溃引发的，导致地下甲烷水合物不稳定，释放甲烷并引发强烈的温室效应变暖。由此产生的10⁵–10⁶年气候最佳期解释了持续的地表径流、风化作用以及泥火山活动，当甲烷被破坏或水合物耗尽时，该气候最佳期结束。

ABSTRACT

The progressive drying-out of Mars' surface was punctuated by a dramatic transient increase in fluvial erosion around the Noachian-Hesperian boundary ($\sim$3.7 Ga). Standard explanations of this climate optimum appeal to volcano- or impact-triggered climates and imply that individual runoff episodes were brief, apparently inconsistent with evidence for persistent runoff. We examine a scenario in which the duration, intensity and uniqueness of the Noachian-Hesperian climate optimum result from degassing of CH$_4$-clathrate consequent to atmospheric collapse. Atmospheric collapse causes low-latitude surface water ice to sublimate away, depressurizing and thus destabilizing CH$_4$ clathrate in subglacial pore space. Subsequent atmospheric re-inflation leads to warming that further destabilizes CH$_4$ clathrate. CH$_4$-induced warming is efficient, permitting strong positive feedbacks, and possibly raising Mars into a climate optimum. The optimum is brought to a close by photochemical destruction of CH$_4$ or by a new atmospheric collapse, and drawdown of the CH$_4$-clathrate reservoir prevents recurrence. This scenario predicts a 10$^5$-10$^6$ yr climate optimum, transient connections between the deep hydrosphere and the surface, mud volcanism, and strong surface weathering, all of which are consistent with recent observations. Crustal hydrothermal circulation very early in Mars history could yield CH$_4$ that would be incorporated into clathrate on approach to the cold surface. The scenario explains why regional watershed integration on Mars occurred relatively late and apparently only once, and suggests that the contrasts between Noachian versus Hesperian climate-sensitive deposits on Mars correspond to a transition from a never-collapsed atmosphere to a collapse-prone climate, ultimately driven by slow loss of CO$_2$ to space.

研究动机与目标

• 解决标准模型中尽管地表径流事件短暂，火星却长期存在流水侵蚀的悖论。
• 解释诺亚纪-赫斯珀里亚边界处（约37亿年前）观测到的短暂而强烈的气候最佳期。
• 说明区域水系整合为何仅发生一次且在火星历史中相对较晚。
• 将诺亚纪向赫斯珀里亚过渡的气候敏感沉积物变化与大气崩溃及二氧化碳损失联系起来。

提出的方法

• 模拟因二氧化碳损失导致的大气崩溃，引发地表冰升华及地下孔隙冰中气压降低。
• 在低压条件下模拟甲烷水合物的不稳定化，使甲烷释放至大气中。
• 应用温室效应模型，评估甲烷的辐射强迫及其反馈效率。
• 评估大气重新充填及随后甲烷光化学破坏作为气候最佳期终止机制。
• 将地壳热液活动作为早期甲烷来源，用于水合物形成。
• 利用地质约束条件，检验地表风化、泥火山活动及瞬态水圈连接的预测。

实验结果

研究问题

• RQ1 何种机制可在地表径流事件短暂的情况下，维持火星长达10⁵–10⁶年的气候最佳期？
• RQ2 甲烷水合物如何在早期火星上促成持续的温室效应？
• RQ3 为何区域水系整合仅发生一次且在火星历史中相对较晚？
• RQ4 是什么导致了诺亚纪-赫斯珀里亚气候最佳期的终止？
• RQ5 大气崩溃如何引发一系列事件，最终导致气候变暖与地表水文活动？

主要发现

• 气候最佳期持续了10⁵–10⁶年，与地质证据中持续流水侵蚀的记录一致。
• 大气崩溃触发了低纬度冰的升华，降低气压，导致地下甲烷水合物不稳定。
• 甲烷释放引发强烈的温室效应变暖，使地表液态水存在并维持长期流水活动。
• 大气重新充填进一步 destabilized 水合物，通过正反馈机制放大变暖效应。
• 最佳期通过甲烷光化学破坏或新一轮大气崩溃而结束，导致水合物储层耗尽。
• 该情景解释了诺亚纪-赫斯珀里亚过渡的唯一性，以及此后未再发生类似事件的原因。