A tenuous, collisional atmosphere on Callisto

For the past fifty years of space exploration, mass spectrometry has provided unique chemical and physical insights on the characteristics of other planetary bodies in the Solar System. A variety of mass spectrometer types, including quadrupole, magnetic sector, time-of-flight, and ion trap, have and will continue to deepen our understanding of the formation and evolution of exploration targets like the surfaces and atmospheres of planets and their moons. An important impetus for the continuing exploration of Mars, Europa, Enceladus, Titan, and Venus involves assessing the habitability of solar system bodies and, ultimately, the search for life--a monumental effort that can be advanced by mass spectrometry. Modern flight-capable mass spectrometers, in combination with various sample processing, separation, and ionization techniques also enable sensitive detection of chemical biosignatures. While our canonical knowledge of biosignatures is rooted in Terran-based examples, agnostic approaches in astrobiology can cast a wider net, to search for signs of life that may not be based on Terran-like biochemistry. Here, we delve into the search for extraterrestrial chemical and morphological biosignatures and examine several possible approaches to agnostic life-detection using mass spectrometry. We discuss how future missions can help ensure that our search strategies are inclusive of unfamiliar life forms.

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Planetary Mass Spectrometry for Agnostic Life Detection in the Solar System

Spacecraft flybys provide access to the chemical composition of the gaseous envelope of the planetary object. Typical relative encounter velocities range from km/s to tens of km/s in flybys. For speeds exceeding about 5 km/s, modern mass spectrometers analyzing the rapidly encountering gas suffer from intrinsic hypervelocity impact-induced fragmentation processes causing ambiguous results when analyzing complex molecules. In this case, instruments use an antechamber, inside which the incoming species collide many times with the chamber wall. These collisions cause the desired deceleration and thermalization of the gas molecules. However, these collisions also dissociate molecular bonds, thus fragmenting the molecules, and possibly forming new ones precluding scientists from inferring the actual chemical composition of the sampled gas. We developed a novel time-of-flight mass spectrometer that handles relative encounter velocities of up to 20 km/s omitting an antechamber and its related fragmentation. It analyzes the complete mass range of m/z 1 to 1000 at an instance. This innovation leads to unambiguous analysis of complex (organic) molecules. Applied to Enceladus, Europa or Io, it will provide reliable chemical composition datasets for exploration of the Solar System to determine its status, origin and evolution.

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Advances in Mass Spectrometers for Flyby Space Missions for the Analysis of Biosignatures and Other Complex Molecules

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Development of an electron impact ion source with high ionization efficiency for future planetary missions

This review of miniaturised instrumentation is motivated by the ongoing and forthcoming exploration of the confirmed, or candidate ocean worlds of the Solar System. It begins with a section on the evolution of instrumentation itself, ranging from the early efforts up to the current rich-heritage miniaturized mass spectrometers approved for missions to the Jovian system. The geochemistry of sulphur stable isotopes was introduced for life detection at the beginning of the present century. Miniaturized instruments allow the measurement of geochemical biosignatures with their underlying biogenic coding, which are more robust after death than cellular organic molecules. The role of known stable sulphur isotope fractionation by sulphate-reducing bacteria is discussed. Habitable ocean worlds are discussed, beginning with analogies from the first ocean world known in the Solar System that has always being available for scientific exploration, our own. Instrumentation can allow the search for biosignatures, not only on the icy Galilean moons, but also beyond. Observed sulphur fractionation on Earth suggests a testable "Sulphur Hypothesis", namely throughout the Solar System chemoautotrophy, past or present, has left, or are leaving biosignatures codified in sulphur fractionations. A preliminary feasible test is provided with a discussion of a previously formulated "Sulphur Dilemma": It was the Galileo mission that forced it upon us, when the Europan sulphur patches of non-ice surficial elements were discovered. Biogenic fractionations up to and beyond δ34S = −70‰ denote biogenic, rather than inorganic processes, which are measurable with the available high sensitivity miniaturised mass spectrometers. Finally, we comment on the long-term exploration of ocean worlds in the neighbourhood of the gas and ice giants.

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Miniaturised Instrumentation for the Detection of Biosignatures in Ocean Worlds of the Solar System

The JUpiter ICy moons Explorer (JUICE) of the European Space Agency (ESA) will investigate Jupiter and its icy moons Europa, Ganymede and Callisto, with the aim to better understand the origin and evolution of our Solar System and the emergence of life. The Neutral gas and Ion Mass spectrometer (NIM) is one of six instruments of the Particle Environment Package (PEP) on board the JUICE spacecraft. PEP will measure neutral atoms and molecules, the ion population, and the electron population over an energy range covering from meV to MeV. The NIM instrument is designed to measure the chemical and isotope composition of the exospheres of three of Jupiter's satellites, the icy moons, both, during several flybys and during its final destination in Ganymede orbit. From measurements of the exosphere, we will derive the chemical composition of the surface, which will allow us a better understanding of the icy moons formation processes, interaction processes with the magnetospheric plasma and energetic particles of Jupiter's magnetospheric system. The NIM instrument is a compact time-of-flight mass spectrometer allowing measurements of thermal neutral molecules and ionospheric ions. To minimize the background radiation on the detector and protect electronics against the harsh radiation environment around Jupiter, elaborated radiation shielding was designed. NIM consists of two major subunits, namely, the ion-optical system and the electronics. This study presents details on the technical design and the results obtained from the calibration campaigns of different subsystems of the flight instrument including a mass range of m/z 1 to 650, a mass resolution $m/\Delta m$ of at least 750 (FWHM), and an instantaneous dynamic range of almost 6 decades. These results are discussed in detail with respect to the scientific requirements. This performance in combination with its radiation tolerance allows for both a detailed analysis of the chemical composition of Jupiter's icy moons' exospheres and ionospheres, and to explore environments, where formation of life might be possible.

Hans Peter Munz

Papers

Description of the Mass Spectrometer for the Jupiter Icy Moons Explorer Mission