Acoustic signatures of helium abundance in hydrogen for planetary science

Abstract

The aim of this work is to model and measure the acoustic signatures of varying mixtures of helium, ortho-hydrogen, and para-hydrogen. In particular, we hope to demonstrate a sensor for helium abundance and hydrogen ortho:para ratio for planetary science missions to Saturn or Uranus. Gas composition as a function of depth is important for understanding atmospheric dynamics and planetary formation of the gas giants but is not well known. In the presence of molecular relaxation, sound speed and absorption in polyatomic gas mixtures become dispersive due to frequency-dependent heat capacities. This may potentially allow the identification of gas composition through relaxational acoustic spectroscopy. We compare theoretical predictions to measurements done in a pressure vessel up to a maximum pressure of 20 bar of relevance to gas giant atmospheres. Experiments for different gas mixtures at various pressures and temperatures are conducted using wide-bandwidth ultrasound between 20 kHz and 10 MHz. A technique is proposed to determine the ratio of hydrogen to helium using speed of sound and attenuation spectra. This approach may be further utilized to determine the ortho:para ratio. Challenges include providing sufficient bandwidth and adjusting for temperature, diffraction, and reflections. [Work supported from NASA Center Innovation Fund.]