講演情報
[PPS05-10]火星着陸探査機におけるその場K-Ar年代測定に向けた着陸地点検討
*三浦 弥生1、長 勇一郎2、諸田 智克3、宮本 英昭4、臼井 寛裕5、亀田 真吾2、杉田 精司6、岡崎 隆司7 (1.東京大学地震研究所、2.立教大学、3.名古屋大学、4.東京大学博物館、5.東京工業大学、6.東京大学大学院理学系研究科、7.九州大学)
キーワード：
火星、火星探査、K-Ar年代、年代学、ヘスペリアン、Syrtis Major
Mars shows a variety of surface features affected by geologic processes. Though the crater-based dating has estimated surface ages, ranging from nearly 4.5 Ga age to recent, the absolute ages have not been determined for Martian samples except a mudstone at Gale crater by the Curiosity rover mission (4.21 ± 0.35 b.y.) and meteorites that come from somewhere on Mars. The authors have been developing an in-situ K-Ar dating system for future Japan's landing mission on Mars (e.g., Cho et al., 2014, 2015). In this paper, aims of the chronologic investigation using the system onboard a Mars rover/lander and appropriate landing sites are discussed.
Studies of impact crater densities present three representative eras for geologic history of Mars; Noachian, Hesperian and Amazonian (e.g., Tanaka, 1986). Abundant water should have existed early in Martian history (likely the Noachian and a part of the Hesperian), but most of them disappeared. In order to understand habitable environment, climate changes and atmosphere evolution of Mars it is important to determine the absolute ages of geologically-well-defined Noachian/Hesperian samples. Considering crater chronology, mineralogy, geological setting and engineering requirements (altitude, latitude and thermal inertia), we propose three regions that are covered by Hesperian volcanic rocks as candidates of chronologic investigation; Syrtis Major Planum, north-east side of Tharsis and peripheral area of Amazonis Planitia. Crater counting based on CTX and HRSC images applied to five specific areas in Syrtis Major provides ages ranging in 3.0 - 3.6 Ga (where the model by Hartmann and Neukum, 2001 is adopted). Among which, two areas reveal resurfacing evidences; the crater frequency gives older ages of 3.7 - 3.8 Ga for the sizes > 1 km in diameter and the thickness of the younger lava (for the sizes < 1 km) is estimated to be ∼40 m. Syrtis Major, having gentle slope and less abundant dust, is a highly recommended region for the landing site. Further information such as local morphology and shock and alteration phenomena should also be considered.
Studies of impact crater densities present three representative eras for geologic history of Mars; Noachian, Hesperian and Amazonian (e.g., Tanaka, 1986). Abundant water should have existed early in Martian history (likely the Noachian and a part of the Hesperian), but most of them disappeared. In order to understand habitable environment, climate changes and atmosphere evolution of Mars it is important to determine the absolute ages of geologically-well-defined Noachian/Hesperian samples. Considering crater chronology, mineralogy, geological setting and engineering requirements (altitude, latitude and thermal inertia), we propose three regions that are covered by Hesperian volcanic rocks as candidates of chronologic investigation; Syrtis Major Planum, north-east side of Tharsis and peripheral area of Amazonis Planitia. Crater counting based on CTX and HRSC images applied to five specific areas in Syrtis Major provides ages ranging in 3.0 - 3.6 Ga (where the model by Hartmann and Neukum, 2001 is adopted). Among which, two areas reveal resurfacing evidences; the crater frequency gives older ages of 3.7 - 3.8 Ga for the sizes > 1 km in diameter and the thickness of the younger lava (for the sizes < 1 km) is estimated to be ∼40 m. Syrtis Major, having gentle slope and less abundant dust, is a highly recommended region for the landing site. Further information such as local morphology and shock and alteration phenomena should also be considered.