Time-of-Flight Dust Mass Spectrometry for Enceladus Plume Analysis: A Concept Study for the ESA L4 Mission

Enceladus with its abundance of water, life building blocks, and organic compounds is among the solar system’s most exciting candidates for discovering extraterrestrial life. The Cosmic Dust Analyzer (CDA) aboard Cassini already measured impact rates and mass spectra during its Enceladus plume flythroughs. A more detailed study on habitability and biomarkers of the Enceladus icy surface and subsurface oceans could provide unprecedented insights on the origin of life in the solar system.

To deepen this understanding, the ESA L4 mission aims to investigate habitability and biomarkers with more detail. This is achieved by analyzing the chemical composition of the plumes ejected from the sub-surface ocean with an orbiter and by deploying a lander to the icy surface. The plume ejecta consist mostly of salt and ice grains, whose composition can be determined by mass spectroscopy.

The measurement of the plume’s pH value, availability of important building blocks of life (CHNOPS – carbon, hydrogen, nitrogen, oxygen, phosphorus, sulfur) in the plume grains, and isotope ratios can indicate the habitability of the sub-surface ocean. Meanwhile, the occurrence and distribution of certain amino acids, aromatic hydrocarbons and deviations of species concentrations from the chemical equilibrium in non-biologic systems can be understood as biomarkers for the presence of life.

The scientific objectives of the mission yield the requirements for the proposed instrument: the required mass range (1…400 amu) results from the measurement of single-atom life building blocks (1 amu of a Hydrogen) on the one hand and aromatic hydrocarbons (< 400 amu) on the other hand.  To distinguish the expected species from their isobaric interferants, a high resolution of m/dm>1000 is required. Furthermore, the polyvinylidene fluoride (PVDF) impact detector of CDA indicated a peak impact rate of ~3000/cm²/s during a flythrough of a plume. Therefore, the dedicated plume analyzer must consider target size and processing speed to enable unambiguous analysis of individual impacts.

As CDA was designed for a more versatile purpose, it did not meet the demands for an in-depth Enceladus plume investigation. The instrument’s resolution and mass range were not sufficient to unambiguously identify the species named above. The impact rate led to superposition and the oversight of several spectra. Nevertheless, the data from CDA and the lessons learned from the instrument provide a profound knowledge base for the design of an Enceladus Dust Analyzer, specialized for the application in Enceladus plumes, which is currently developed at the University of Stuttgart.

This Enceladus Dust Analyzer is a time-of-flight dust mass spectrometer, featuring a smaller sensitive area and higher mass resolution. The instrument is able to measure ice particle compositions under extreme conditions within the Enceladus plume. It measures cation or anion mass spectra of particles with sizes between a few tens of a nanometer up to many micrometers. It will be optimized to measure at moderate encounter speeds.

This talk aims to describe the architecture of the instrument, its current development status as well as the lessons learned from preceding dust analyzers. What are the scientific and instrument requirements? What are the challenges of the environment to perform reliable measurements in the vicinity of Enceladus?