2:30 PM - 2:45 PM
[PCG20-16] Feasibility of High-Precision Measurement of Neon Isotopic Ratio in the Martian Atmosphere Using TOF-MS
Keywords:Martian Atmosphere, Neon Isotopic Ratio, Time-of-Flight Mass Spectrometer, Sensitivity Evaluation, Residual Gas Analysis
The atmospheric residence time of Ne on Mars is significantly shorter (~ 60 to 100 million years) than the Martian history (~ 4.5 billion years). This suggests that the current Ne in the Martian atmosphere is supplied from the mantle (Kurokawa et al., 2021). Therefore, measuring the Ne isotopic ratio (20Ne/22Ne) in the Martian atmosphere could reveal the Ne isotopic composition of the Martian mantle, which would help constrain the source material of Ne in the Martian mantle and provide crucial information on Martian formation history, including the environment of dust in the primordial solar nebula and accretion timescales. To determine whether the Martian mantle's Ne isotopic ratio is solar-like (~ 13) or chondritic (~ 7–11), accurate measurements of the atmospheric Ne isotopic ratio with an uncertainty of less than 10% are required. Since the abundance of Ar in the Martian atmosphere is approximately 10,000 times higher than that of Ne (Ar: 1.9%, Ne: 2.5 ppm), measurements of Ne isotopes are hindered by interference from the doubly charged 40Ar++ ion with the singly charged 20Ne+ ion in mass spectrometers. Consequently, previous missions such as Viking and Curiosity were unable to measure 20Ne in the Martian atmosphere. To address this issue, our research group has been developing a separation method using a permeable membrane to reduce the Ar-to-Ne ratio (Miura et al., 2020; Cho et al., 2024). Additionally, JAXA has previously used time-of-flight mass spectrometers (TOF-MS) in planetary exploration missions. However, while TOF-MS is planned for Ne measurements, its sensitivity for precise Ne isotopic measurements and the impact of background interference remain unverified. TOF-MS operates effectively only at pressures below 10-2 Pa. When used for static analysis, it is necessary to confirm whether outgassing from the analytical chamber is sufficiently low and whether residual Ar and Ne gases do not interfere with isotopic measurements. Furthermore, it must be determined whether the sensitivity of TOF-MS is sufficient to detect the trace amount of Ne expected to permeate through the membrane from the Martian atmosphere (estimated partial pressure: 2×10-8 Pa). Therefore, in this study, a newly developed TOF-MS was used to conduct the following experiments: (1) estimating the sensitivity of TOF-MS to demonstrate its feasibility for detecting Ne and (2) identifying the types and quantities of residual gases in the measurement line.
First, TOF-MS was placed in a large vacuum chamber, which was evacuated before introducing Ar gas. The pressure and mass spectra were measured before and after Ar introduction. Based on the data, the sensitivity of TOF-MS to 40Ar+ at a total pressure of 1.0×10-4 Pa was estimated to be 6×106 counts/min/Pa. Additionally, calculations of the Ne partial pressure permeating through the membrane from the terrestrial atmosphere indicated that Ne detection is possible with a signal-to-background ratio of approximately 4 under Earth atmospheric conditions. Under Martian conditions, where the Ne partial pressure is about three orders of magnitude lower than on Earth, improvements such as increasing the membrane area or reducing chamber size could increase the Ne partial pressure by about two orders of magnitude.
Next, TOF-MS was placed inside a small vacuum chamber designed for space measurements, and the system was evacuated. The pressure reached 2.51×10-3 Pa, which is suitable for TOF-MS operation. Mass spectrometry of residual gases revealed that water was the primary residual gas, and Ne and Ar were detected at approximately 3 counts/min at mass-to-charge ratios of 20 (20Ne+, 40Ar++) and 40 (40Ar+). These results indicate that the current background contributes significantly to Ne detection; however, baking the chamber to reduce residual gas pressure is expected to lower the background level.
In conclusion, the feasibility of high-precision measurement of the Ne isotopic ratio in the Martian atmosphere using TOF-MS has been demonstrated. Future work will focus on further validation experiments to establish the technological foundation necessary to determine whether the Ne isotopic composition of the Martian mantle is solar-like or chondritic.
First, TOF-MS was placed in a large vacuum chamber, which was evacuated before introducing Ar gas. The pressure and mass spectra were measured before and after Ar introduction. Based on the data, the sensitivity of TOF-MS to 40Ar+ at a total pressure of 1.0×10-4 Pa was estimated to be 6×106 counts/min/Pa. Additionally, calculations of the Ne partial pressure permeating through the membrane from the terrestrial atmosphere indicated that Ne detection is possible with a signal-to-background ratio of approximately 4 under Earth atmospheric conditions. Under Martian conditions, where the Ne partial pressure is about three orders of magnitude lower than on Earth, improvements such as increasing the membrane area or reducing chamber size could increase the Ne partial pressure by about two orders of magnitude.
Next, TOF-MS was placed inside a small vacuum chamber designed for space measurements, and the system was evacuated. The pressure reached 2.51×10-3 Pa, which is suitable for TOF-MS operation. Mass spectrometry of residual gases revealed that water was the primary residual gas, and Ne and Ar were detected at approximately 3 counts/min at mass-to-charge ratios of 20 (20Ne+, 40Ar++) and 40 (40Ar+). These results indicate that the current background contributes significantly to Ne detection; however, baking the chamber to reduce residual gas pressure is expected to lower the background level.
In conclusion, the feasibility of high-precision measurement of the Ne isotopic ratio in the Martian atmosphere using TOF-MS has been demonstrated. Future work will focus on further validation experiments to establish the technological foundation necessary to determine whether the Ne isotopic composition of the Martian mantle is solar-like or chondritic.