講演情報
[PPS07-P08]Progressive Oxidation of Mars Driven by Dust Storms
*Willian Salgado1、Eito Hirai1、Yasuhito Sekine1、Shuya Tan2、Yoshio Takahashi3 (1.Earth-Life Science Institute (ELSI), Institute of Science Tokyo, Japan 、2.Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Japan、3.Department of Earth and Planetary Science, University of Tokyo, Japan)
キーワード：
Mars、oxidation、dust-storm、oxyhalogen
Halide minerals including chlorides (XCl: where X represents common cations (e.g., Na+, K+, Mg2+, Ca2+, etc.) and bromides (XBr), as well as oxidized halogen-bearing phases (oxyhalogens: chlorates (XClO3), perchlorates (XClO4) and bromates (XBrO3)), are widespread and enriched at the Martian surface, as demonstrated by ground-based observations, orbital instruments, in situ measurements, and analyses of Martian meteorites.
These minerals hold significant importance due to their ability to absorb atmospheric water vapor and deliquesce to form liquid brines. Additionally, they also act as oxidants that provide bioavailable chemical energy for putative life.
On Earth, oxyhalogens form through stratospheric photochemical oxidation of Cl- and Br-bearing species derived from marine aerosols or volcanic emissions, driven by ozone and ultraviolet radiation. These oxyhalogens accumulate at the surface in hyperarid environments, where limited precipitation prevents their removal.
On Mars, multiple mechanisms capable of producing oxyhalogens have been proposed. However, the proposed mechanisms fail to account for the chlorate and perchlorate abundances measured in the regolith, indicating that an additional formation mechanism involving material from the surface of Mars is necessary.
In this study, we propose an alternative mechanism for the formation of chlorates, perchlorates, and bromates on Mars during dust storms. In our hypothesis, Cl- and Br-bearing halide particles (XCl and XBr) are lifted into the Martian atmosphere during dust storms. In the atmosphere, halide grains may undergo heterogeneous (gas-solid) reactions with atomic oxygen formed through CO2 photolysis. This process may form chlorates, perchlorates and bromates on the surface of halide grains, which subsequently settle on the surface. Here, we perform laboratory experiments to demonstrate the reaction of O with halides and the resulting formation of oxyhalogens on halide surfaces, providing experimental support for the proposed mechanism.
In experiments, we expose NaCl and KBr to a 13.56 MHz radiofrequency (RF) oxygen plasma system (Nihon Koshuha) to investigate their oxidation. Following irradiation, we perform chemical analyses using time-of-flight secondary ion mass spectrometry (ToF-SIMS) and high-sensitivity ion chromatography (IC). ToF-SIMS results show the formation of hypochlorite, chlorate, and perchlorate on the irradiated surface of NaCl disks, indicating saturation of sample surfaces after 30 minutes of atomic O irradiation under experimental conditions. Based on the calculated atomic O flux in our experiments, the number of atomic O required for saturation is ~3.96 × 1019 O atoms. On the other hand, atomic O densities at different altitudes in the present-day Martian atmosphere have been measured using in situ observations from NASA's MAVEN NGIMS instrument and estimated through photochemical model results. These results allow us to estimate the timescales for perchlorate and bromate formation on Mars, which can be achieved in approximately 20 days, a short geological timescale. Our results suggest that XCl/XBr grains on Mars may have been oxidized to oxyhalogens during repeated dust storm events over Martian history.
Our results show that heterogeneous oxidation of XCl/XBr grains on Mars forms oxyhalogens during repeated dust storm events over Martian history. Oxyhalogens formed at the surfaces of these grains would then deliquesce producing liquid brine and sinter into regolith on current and past Mars. Such processes would explain the high concentration of perchlorate observed by robots, supply oxidants to support a putative life, and may contribute to the explanation of the formation of highly oxidized mineral phases detected in Martian rocks by rover missions. Our results suggest that dust storms contribute to driving surface–atmosphere chemical interactions on Mars and may have played a role in the long-term oxidation of Mars.
These minerals hold significant importance due to their ability to absorb atmospheric water vapor and deliquesce to form liquid brines. Additionally, they also act as oxidants that provide bioavailable chemical energy for putative life.
On Earth, oxyhalogens form through stratospheric photochemical oxidation of Cl- and Br-bearing species derived from marine aerosols or volcanic emissions, driven by ozone and ultraviolet radiation. These oxyhalogens accumulate at the surface in hyperarid environments, where limited precipitation prevents their removal.
On Mars, multiple mechanisms capable of producing oxyhalogens have been proposed. However, the proposed mechanisms fail to account for the chlorate and perchlorate abundances measured in the regolith, indicating that an additional formation mechanism involving material from the surface of Mars is necessary.
In this study, we propose an alternative mechanism for the formation of chlorates, perchlorates, and bromates on Mars during dust storms. In our hypothesis, Cl- and Br-bearing halide particles (XCl and XBr) are lifted into the Martian atmosphere during dust storms. In the atmosphere, halide grains may undergo heterogeneous (gas-solid) reactions with atomic oxygen formed through CO2 photolysis. This process may form chlorates, perchlorates and bromates on the surface of halide grains, which subsequently settle on the surface. Here, we perform laboratory experiments to demonstrate the reaction of O with halides and the resulting formation of oxyhalogens on halide surfaces, providing experimental support for the proposed mechanism.
In experiments, we expose NaCl and KBr to a 13.56 MHz radiofrequency (RF) oxygen plasma system (Nihon Koshuha) to investigate their oxidation. Following irradiation, we perform chemical analyses using time-of-flight secondary ion mass spectrometry (ToF-SIMS) and high-sensitivity ion chromatography (IC). ToF-SIMS results show the formation of hypochlorite, chlorate, and perchlorate on the irradiated surface of NaCl disks, indicating saturation of sample surfaces after 30 minutes of atomic O irradiation under experimental conditions. Based on the calculated atomic O flux in our experiments, the number of atomic O required for saturation is ~3.96 × 1019 O atoms. On the other hand, atomic O densities at different altitudes in the present-day Martian atmosphere have been measured using in situ observations from NASA's MAVEN NGIMS instrument and estimated through photochemical model results. These results allow us to estimate the timescales for perchlorate and bromate formation on Mars, which can be achieved in approximately 20 days, a short geological timescale. Our results suggest that XCl/XBr grains on Mars may have been oxidized to oxyhalogens during repeated dust storm events over Martian history.
Our results show that heterogeneous oxidation of XCl/XBr grains on Mars forms oxyhalogens during repeated dust storm events over Martian history. Oxyhalogens formed at the surfaces of these grains would then deliquesce producing liquid brine and sinter into regolith on current and past Mars. Such processes would explain the high concentration of perchlorate observed by robots, supply oxidants to support a putative life, and may contribute to the explanation of the formation of highly oxidized mineral phases detected in Martian rocks by rover missions. Our results suggest that dust storms contribute to driving surface–atmosphere chemical interactions on Mars and may have played a role in the long-term oxidation of Mars.