10:45 AM - 12:15 PM
[PPS07-P18] The influence of accretion of cosmic dust on the evolution of atmospheric carbon isotopic composition on Mars
Keywords:Mars, Atmosphere, Isotope, Cosmic dust
Martian atmosphere is enriched in heavy carbon isotopes with the isotopic ratio δ13C 50‰ higher than that of the Earth. This characteristic isotopic composition is a tracer for the Martian atmospheric evolution since the enrichment of heavy isotopes indicates the fractionation through atmospheric escape.
Hu et al. (2015) constructed an atmospheric evolution model considering atmospheric escape to the space, outgassing and carbonate formation. This study indicated that the early surface pressure ~3.8 billion years ago was smaller than 1 bar to satisfy the present-day carbon isotopic composition of CO2 in the Martian atmosphere. They also suggested that the carbon isotopes have been mainly fractionated by the photochemical escape process. Furthermore, recent studies of photolysis-induced isotopic fractionation of CO2 imply that it enhances the degree of the isotopic fractionation through the photochemical escape (Schmidt et al., 2013; Yoshida et al., under review). It should lead to an excessive carbon isotopic fractionation inconsistent with the present-day isotopic composition.
As a solution to this problem, we suggest the accretion of cosmic dust compensates for the effective isotopic fractionation by the photochemical escape. Assuming that cosmic dust has a similar composition to carbonaceous chondrites with δ13C of -10‰ (Kerridge et al., 1985), the accretion should dilute the carbon fractionation of the Martian atmosphere. However, its effect on the evolution of the carbon isotopic composition on Mars remains poorly investigated. In this study, we model the evolution of carbon reservoir and its isotopic composition considering the accretion of cosmic dust as well as atmospheric escape, volcanic outgassing, and carbonate deposition. The present influx rate of dust with the size of 20-2000μm is set at 1.50-2.30 tons/sol (Carrillo-Sánchez et al., 2022) and its carbon concentration is ~3 wt% (Rojas et al., 2021). The past influx rate is estimated from the formulation of accretion rates (Pham et al., 2016) based on the crater distribution (Ivanov, 2001), assuming that the change in the accretion rate of cosmic dust with time is similar to that of relatively large objects.
Our results show that the most of carbon have been supplied mainly by 2.5 billion years ago by volcanic outgassing and accretion of cosmic dust. The final isotopic ratio δ13C becomes small by a few tens of ‰ relative to the case without dust supply, which is consistent with the present-day isotopic composition of the Martian atmosphere. Our result suggests that cosmic dust influx can compensate for the effective isotopic fractionation by the photochemical escape to satisfy the present-day carbon isotopic composition.
Hu et al. (2015) constructed an atmospheric evolution model considering atmospheric escape to the space, outgassing and carbonate formation. This study indicated that the early surface pressure ~3.8 billion years ago was smaller than 1 bar to satisfy the present-day carbon isotopic composition of CO2 in the Martian atmosphere. They also suggested that the carbon isotopes have been mainly fractionated by the photochemical escape process. Furthermore, recent studies of photolysis-induced isotopic fractionation of CO2 imply that it enhances the degree of the isotopic fractionation through the photochemical escape (Schmidt et al., 2013; Yoshida et al., under review). It should lead to an excessive carbon isotopic fractionation inconsistent with the present-day isotopic composition.
As a solution to this problem, we suggest the accretion of cosmic dust compensates for the effective isotopic fractionation by the photochemical escape. Assuming that cosmic dust has a similar composition to carbonaceous chondrites with δ13C of -10‰ (Kerridge et al., 1985), the accretion should dilute the carbon fractionation of the Martian atmosphere. However, its effect on the evolution of the carbon isotopic composition on Mars remains poorly investigated. In this study, we model the evolution of carbon reservoir and its isotopic composition considering the accretion of cosmic dust as well as atmospheric escape, volcanic outgassing, and carbonate deposition. The present influx rate of dust with the size of 20-2000μm is set at 1.50-2.30 tons/sol (Carrillo-Sánchez et al., 2022) and its carbon concentration is ~3 wt% (Rojas et al., 2021). The past influx rate is estimated from the formulation of accretion rates (Pham et al., 2016) based on the crater distribution (Ivanov, 2001), assuming that the change in the accretion rate of cosmic dust with time is similar to that of relatively large objects.
Our results show that the most of carbon have been supplied mainly by 2.5 billion years ago by volcanic outgassing and accretion of cosmic dust. The final isotopic ratio δ13C becomes small by a few tens of ‰ relative to the case without dust supply, which is consistent with the present-day isotopic composition of the Martian atmosphere. Our result suggests that cosmic dust influx can compensate for the effective isotopic fractionation by the photochemical escape to satisfy the present-day carbon isotopic composition.