Reliably Analyzing the Chemical Composition of Plumes during Flybys at Velocities Exceeding 5 km/s

Spacecraft flybys of planetary bodies provide an opportunity to analyze the chemical composition of their outer atmosphere by mass spectrometry. The chemical composition of the atmosphere provides key insights into the origin and evolution of an object. Additionally, sampling a plume, if present, during the flyby will provide detailed information on the complex chemistry present below the surface. At present, composition information available for many of these objects is sparse, as on one hand, the ground-based observations lack both sensitivity and spatial resolution for a detailed investigation and on the other hand recently performed flythroughs were conducted using outdated instrumentation resulting in a moderate sensitivity within a limited mass range. In contrast to the tenuous exosphere of celestial objects, plumes feature significantly increased number density of species in a spatially confined geometry. Flybys are quick events given the relative encounter velocity are easily exceeding 5 km/s, up to about 20 km/s, resulting in major design drivers for instrumentation. The tens of seconds spent in the region of interest requires instant recording of detailed data. The high flyby velocity requires sampling that prevents species from hypervelocity impact induced bond-dissociation. Such a fragmentation complicates the reliable identification of observed signals to species especially in complex mixtures, if not making it impossible. Here we present the development of a sensitive time-of-flight mass spectrometer that can analyze both the tenuous exosphere of celestial objects and plumes, if present, during hypervelocity flybys exceeding 5 km/s enabled by its novel direct open ion source. It uniquely prevents hypervelocity impact induced bond-dissociation as it directly analyzes species without contact with a wall of, for example, an antechamber or other structures in a relative encounter velocity range of 0 to 20 km/s. Using electron ionization, it is capable of analyzing even heavy species in complex mixtures. Such a capability allows for reliably analyzing plumes in a mass range of $m/z\ 1$ to 1000 for a relative encounter velocity of 10 km/s and a mass range of $m/z\ 1$ to 800 for 20 km/s, enabled by a mass resolution exceeding $m/\Delta m\approx 1000$ (full width at half maximum) for higher masses. Thanks to its sensitivity, it is able to analyze even traces of species in both the exosphere and the plumes. This novel instrument allows for detailed and rapid mass spectrometric investigations in all mission scenarios where high flyby speeds at planetary objects are foreseen, for example, Io and Enceladus. This offers the opportunity to measure these objects' exospheres at large distances and measure the plumes when in proximity to the closest approach.

1. Introduction

The chemical composition of plumes can easily be measured with mass spectrometers during flybys of spacecraft at low altitudes. Flythroughs provide in situ measurements overcoming sensitivity limitations of remote sensing instrumentation (see references [1]–[5], for example). For example, measurements of the ion and neutral mass spectrometer (INMS) on board Cassini provided insights into the chemical composition of the plumes of Enceladus. This instrument was capable of detecting H₂O, CO₂, and other minor components. These minor components mostly represent species of heavier masses. Especially such heavier, yet undetected molecules likely provide insights into the chemistry present in the subsurface ocean, as the plumes are believed to be connected to it [6].