In-situ Investigations of the Europa Clipper Mission

1. Introduction

With a launch readiness date of late 2024, NASA’s Europa Clipper will set out on a journey to explore the habitability of Jupiter’s moon Europa. Among others, observations by Europa Clipper’s in-situ instruments will provide an understanding of the satellite’s interior structure and compositional makeup. The variability of Jupiter’s magnetic field at Europa induces electric currents within the moon’s conducting ocean layer, the magnitude of which depends on the ocean’s location, extent, and conductivity. Europa is also embedded in a flow of corotating plasma, which continuously impacts and sputters the surface to produce the moon’s atmosphere. In addition, micrometeorite impacts eject particles of the surface to wrap Europa in a cloud of dust. The neutral atmosphere is readily ionized by energetic particles to produce an ionosphere, which gives rise to current systems electromagnetically connecting Europa to Jupiter. The in-situ instruments comprise the Europa Clipper Magnetometer (ECM), the Plasma Instrument for Magnetic Sounding (PIMS), the SUrface Dust Analyzer (SUDA), and the MAss Spectrometer for Planetary Exploration (MASPEX). We present the highlights of these instruments’ development on their path to delivery to assembly, test, and launch operations (ATLO), which have begun in March 2022.

2. Relevant Investigations

2.1. Europa Clipper Magnetometer

The Europa Clipper Magnetometer (ECM) will measure magnetic fields generated by currents induced in Europa’s subsurface ocean and the electromagnetic coupling of the moon to Jupiter. Jupiter’s tilted dipole magnetic field and Europa’s eccentric orbit expose the moon to time-varying magnetic fields inducing eddy currents in the ocean. By measuring the induced field response at multiple frequencies, the ice shell thickness and the ocean layer thickness and conductivity can be uniquely determined. The ECM consists of three fluxgate sensors mounted on an 8.5-m-long boom and a control electronics hosted in a vault shielding it from radiation damage. The use of three sensors allows for dynamic removal of higher-order spacecraft-generated magnetic fields. The flight-model (FM) sensors (Figure 1) have been delivered to Instrument Integration and Testing, and the instrument is awaiting calibration in the fall of 2022.

Figure 1. ECM FM fluxgate sensor.

2.2. Plasma Instrument for Magnetic Sounding

The Plasma Instrument for Magnetic Sounding (PIMS) will measure ions and electrons in Europa’s ionosphere to quantify magnetic fields associated with electric currents driven by dynamic pressure gradients, which are produced by the interaction of corotating Jovian plasma with Europa and its neutral atmosphere. PIMS will further measure the particle population precipitating onto Europa to model surface sputtering rates and constrain the effects of space weathering. In addition, PIMS will characterize the distribution of electrons carrying field-aligned currents coupling Europa to Jupiter’s ionosphere. The PIMS instrument consists of two sensors each hosting two Faraday cups with a 90° field of view. The instrument measures electrons and ions with energies of up to 2 keV and 7 keV/q, respectively, with an energy resolution of <15% and a temporal resolution of 1−4 s. The instrument, including sensor assemblies (Figure 2), is scheduled for delivery to ATLO in June 2022.

Figure 2. PIMS FM Fully Integrated Sensor Assemblies

2.3. Mass Spectrometer for Planetary Exploration

The MAss Spectrometer for Planetary Exploration (MASPEX) measures trace neutral species to determine the composition of Europa’s atmosphere. The atmosphere is produced by particles sputtering off the surface, is augmented by possible contributions from plumes, and is altered by radiolysis. MASPEX will determine the distribution of major volatiles and key organic compounds with respect to latitude and longitude, altitude, solar local time, and will distinguish endogenic and exogenic sources. MASPEX is a multi-bounce time-of-flight mass spectrometer for species with atomic mass ranging from 2 to 500 u. The instrument’s mass resolution m/Dm of 4275 at mass 50 enables measurement of individual fragment ions generated by electron impact ionization of parent molecule. The FM instrument is presently undergoing testing (Figure 3) and is scheduled for delivery to ATLO in August 2022.

Figure 3. Testing of FM MASPEX instrument.

2.4. Surface Dust Analyzer

The SUrface Dust Analyzer (SUDA) will map the chemical composition of particles ejected from Europa’s surface by hypervelocity impacts of micrometeoroids. The dust particles have a typical size of 0.5−1 μm and exhibit a near isotropic distribution and only weak temporal variations. Small fluctuations are expected from variability of exogenous sources, resulting, e.g., from the orbital motion of sources on Io and the stochastic nature of volcanic activity. SUDA is a time-of-flight mass spectrometer for species with atomic mass ranging of up to 200 u and is capable of detecting up to 40 ejecta per second. Owing to the deterministic nature of the ballistic trajectories of the ejected particles, the composition measurements at spacecraft altitude can be correlated with geologic features on the surface. It has been demonstrated that trace amounts of complex organic species embedded in ice grains with abundances <0.1 ppm can be detected. In addition to micrometeorite ejecta, SUDA will measure the makeup of potential plumes by directly sampling microscopic particles. The FM sensor assembly (Figure 4) will be delivered to ATLO in August 2022 as part of the instrument.

Figure 4. SUDA FM sensor assembly.

3. Mission Concept

The current mission design consists of 49 flybys of Europa executed over a ~3.5 year period while the spacecraft is in orbit about Jupiter. The altitudes of closest approach typically range from 25 km to 100 km. The tour is divided into two principal campaigns, visiting first the anti-Jovian hemisphere followed by observations of the sub-Jovian hemisphere. The flybys occur over a wide range of latitudes and longitudes (Figure 5), and they are widely distributed in true anomaly and solar local time. The tour is thus well suited for characterization of the ice shell and ocean and of the atmosphere and ionosphere created through Europa’s interaction with Jupiter’s magnetosphere.

Figure 5. Planned Europa Clipper close approach ground tracks below 1000 km altitude (black), 400 km (green), 100 km yellow, and 50 km (red).