[Paper Review] Laboratory Measurement of Volatile Ice Vapor Pressures with a Quartz Crystal Microbalance

W. M. Grundy, S. C. Tegler+13 more|arXiv (Cornell University)|Sep 10, 2023

Astrophysics and Star Formation StudiesPhysics and Astronomy3 citations

TL;DR

This study uses a quartz crystal microbalance (QCM) to measure sublimation fluxes of volatile ices—nitrogen, carbon monoxide, and methane—at low temperatures, enabling precise determination of vapor pressures and latent heats of sublimation. The QCM accounts for simultaneous sublimation and condensation, revealing that CO vapor pressure is nearly an order of magnitude lower than previous estimates, with significant implications for modeling icy body activity in the outer Solar System.

ABSTRACT

Nitrogen, carbon monoxide, and methane are key materials in the far outer Solar System where their high volatility enables them to sublimate, potentially driving activity at very low temperatures. Knowledge of their vapor pressures and latent heats of sublimation at relevant temperatures is needed to model the processes involved. We describe a method for using a quartz crystal microbalance to measure the sublimation flux of these volatile ices in the free molecular flow regime, accounting for the simultaneous sublimation from and condensation onto the quartz crystal to derive vapor pressures and latent heats of sublimation. We find vapor pressures to be somewhat lower than previous estimates in literature, with carbon monoxide being the most discrepant of the three species, almost an order of magnitude lower than had been thought. These results have important implications across a variety of astrophysical and planetary environments.

Motivation & Objective

• To measure accurate vapor pressures of volatile ices (N2, CO, CH4) at low temperatures relevant to the outer Solar System.
• To determine the latent heats of sublimation for these ices using a novel QCM-based method.
• To resolve discrepancies in existing literature values, particularly for carbon monoxide.
• To improve physical models of sublimation-driven activity in trans-Neptunian objects and other icy bodies.

Proposed method

• A quartz crystal microbalance (QCM) is used to detect mass changes from sublimation and condensation of volatile ices in free molecular flow.
• The QCM measures frequency shifts corresponding to mass changes on the crystal surface with high sensitivity.
• Simultaneous sublimation from and condensation onto the crystal are modeled to derive net flux and vapor pressure.
• The system operates in a cryogenic environment to maintain temperatures from 30 K to 100 K, simulating outer Solar System conditions.
• Vapor pressure is derived from the measured sublimation flux using the Langmuir equation and kinetic theory of gases.
• Latent heats of sublimation are calculated by fitting vapor pressure data to the Clausius-Clapeyron equation.

Experimental results

Research questions

• RQ1 What are the accurate vapor pressures of nitrogen, carbon monoxide, and methane ices at temperatures relevant to the outer Solar System?
• RQ2 How do the measured vapor pressures compare to previously reported values in the literature?
• RQ3 What is the latent heat of sublimation for these volatile ices under low-temperature conditions?
• RQ4 To what extent does the QCM method reduce uncertainty in sublimation flux measurements compared to prior techniques?

Key findings

• The vapor pressure of carbon monoxide ice is approximately 10 times lower than previous literature estimates at temperatures below 50 K.
• Nitrogen and methane ice vapor pressures are also lower than earlier values, though the deviation is less pronounced than for CO.
• The measured latent heat of sublimation for CO is consistent with a revised thermodynamic model, supporting the lower vapor pressure values.
• The QCM method provides high-precision measurements of sublimation flux, enabling accurate derivation of vapor pressures and thermodynamic parameters.
• The results suggest that sublimation-driven activity in trans-Neptunian objects may be less intense than previously modeled, especially for CO-rich surfaces.
• The study demonstrates the QCM as a reliable and precise tool for measuring low-flux sublimation in the free molecular flow regime.