[P-PS06] 惑星科学

[PPS06-13] Global climate and river transport simulations of early Mars during the late Noachian and the early Hesperian

*鎌田 有紘¹、黒田 剛史¹、笠羽 康正¹、寺田 直樹¹、中川 広務¹ (1.東北大学大学院理学研究科)

キーワード：火星、ヴァレーネットワーク、全球気候モデル

Over the past decades, a number of geological observations of early Mars have suggested the existence of ancient surface river channales during the late Noachian and early Hesperian (3.85-3.6 Ga). The combination of large amounts of liquid water and stable, prolonged fluvial activity is necessary to explain these river systems, suggesting that early Mars must have experienced a long period of stable surface liquid water and a climate sufficiently warm and wet for precipitation to occur. We investigated the late Noachian and the early Hesperian climate using a 3-dimensional paleo-Mars global climate model (PMGCM), assuming a CO₂/H₂O/H₂ mixing atmosphere under the “Faint Young Sun” condition. The ancient Martian topography before the formation of the Tharsis bulge was assumed. The cases of surface pressures of 0.5, 1, 1.5 and 2 bar, with H₂ concentrations of 0%, 1%, 3% and 6%, and obliquities of 20°, 40°, and 60° were investigated. We updated the global river model of Kamada et al. (2020) to a pan-planet river model named CRIS (Catchment-based RIver Simulator), which calculates precipitation infiltration, global river discharge, and the transport of bedload and suspended load sediments for a variety of sediment particle sizes coupled with the PMGCM. We found that an early Martian atmosphere with H₂ concentration of above 3% and Ps of above 1.5 bar could have produced a hospitable surface environment that could support the stable existence of liquid water over long periods. The annual precipitation increased with surface pressure, and the distributions of the precipitation peaks under obliquities of above 40° were consistent with the locations in which the majority of the valley networks are currently observed. Moreover, CRIS predicted that surface river systems are limited at high latitudes at obliquity of 20°, but river activity is enhanced even at low and mid-latitudes as a result of increased summer precipitation at obliquity above 40°, which is close to the most probable obliquity value of 41.8° (Laskar et al., 2004). Our simulation results show that early Mars had a warm and wet environment with precipitation-fed mature river systems incising the VNs that have been recently observed.