11:00 〜 13:00
[MIS17-P03] 岩石穴状構造の地球アナログおよび実験試料に基づく特徴づけ：火星ジェゼロクレータ湖の水環境への示唆
キーワード：火星、タフォニ、化学風化
NASA’s Perseverance rover has landed on near delta deposits of Jezero Crater, Mars, where large lakes would have existed in ancient time. Visual images acquired by Perseverance on Jezero Crater showed enigmatic cavernous structures, with circular pits with diameter of centimeters, on surface rocks. Given the existence of carbonate rocks on Jezero Crater and possible occurrence of acidic alterations in Hesperian, the cavernous structures may have been formed by acidic alterations of carbonate rocks. Alternatively, these cavernous structures could be generated via salt weathering, similar to Earth’s tafoni, and via erosion by streamflow, similar to potholes on Earth, if saline lakes existed on Jezero Crater. However, the formation mechanisms of the cavernous structures on Jezero Crater are unconstrained based on comparison in morphology.
Here, we perform field surveys and laboratory experiments aiming to characterize morphology of the cavernous structures of tafoni, potholes, and cavities formed by acidic alterations . In field surveys, we observed coastal cavernous structures in the Shimokita and Boso peninsulas, Japan. In laboratory experiments, we simulate acidic alterations of carbonate rocks by slow melting of H2SO4 frost on limestone at low temperatures. For both field and laboratory samples, we analyzed the morphology and number density of cavities via a common methodology by constructing three-dimensional structures.
Our results of field surveys show that tafoni and potholes can be distinguished based on morphology, with being higher number density with smaller cavities for tafoni than pothole. Our experimental results show that acidic alterations could generate cavernous structures on limestone via dissolution, bubbling, and deposition of sulfate minerals. Based on the Principal Component analysis, we suggest that these three types of cavities can be distinguished mainly by the number density and diameter of cavities. By applying our approach to cavernous structures on Jezero rocks, we find that they are categorized in terms of the number density and diameter as in between tafoni and cavities formed by acidic alterations. The size and number density of some cavernous structures on Jezero rocks seen to agree with those of carbonate rocks that repeatedly altered by H2SO4 fluids, although the shape of merged cavities of altered carbonate is inconsistent with that of Jezero rocks. On the other hand, if the cavernous structures of Jezero rocks, this suggests that early Jezero lake/oceanic water was saline. The size of cavernous structures of Jezero rocks is lower than that of tafoni; whereas, their number density tends to be higher than tafoni. This suggests that Jezero’s cavernous structures might be tafoni formed in shorter periods than Earth’s, implying short-lived lakes on Jezero Crater.
Here, we perform field surveys and laboratory experiments aiming to characterize morphology of the cavernous structures of tafoni, potholes, and cavities formed by acidic alterations . In field surveys, we observed coastal cavernous structures in the Shimokita and Boso peninsulas, Japan. In laboratory experiments, we simulate acidic alterations of carbonate rocks by slow melting of H2SO4 frost on limestone at low temperatures. For both field and laboratory samples, we analyzed the morphology and number density of cavities via a common methodology by constructing three-dimensional structures.
Our results of field surveys show that tafoni and potholes can be distinguished based on morphology, with being higher number density with smaller cavities for tafoni than pothole. Our experimental results show that acidic alterations could generate cavernous structures on limestone via dissolution, bubbling, and deposition of sulfate minerals. Based on the Principal Component analysis, we suggest that these three types of cavities can be distinguished mainly by the number density and diameter of cavities. By applying our approach to cavernous structures on Jezero rocks, we find that they are categorized in terms of the number density and diameter as in between tafoni and cavities formed by acidic alterations. The size and number density of some cavernous structures on Jezero rocks seen to agree with those of carbonate rocks that repeatedly altered by H2SO4 fluids, although the shape of merged cavities of altered carbonate is inconsistent with that of Jezero rocks. On the other hand, if the cavernous structures of Jezero rocks, this suggests that early Jezero lake/oceanic water was saline. The size of cavernous structures of Jezero rocks is lower than that of tafoni; whereas, their number density tends to be higher than tafoni. This suggests that Jezero’s cavernous structures might be tafoni formed in shorter periods than Earth’s, implying short-lived lakes on Jezero Crater.