PIMS
Europa Clipper's Plasma Instrument for Magnetic Sounding
One question has long intrigued humanity
Does life exist beyond Earth?
In its search for that still elusive answer, NASA has been probing other worlds in our solar system for signs of habitability. Mission observations have led the space agency to home in on the Jovian moon Europa. Enclosed in its icy shell is an apparent abundance of one of life’s essential ingredients: water. It is estimated there is more liquid water in Europa’s subsurface than in all of Earth’s oceans combined.
Launching in 2024, NASA’s Europa Clipper mission will perform the first detailed reconnaissance of the ice-covered moon. Johns Hopkins APL is building and managing the Plasma Instrument for Magnetic Sounding, or PIMS, instrument to support some of the mission’s most pressing science goals, including determining the thickness of Europa’s icy shell and the depth and estimated salinity of the subsurface water, and better understanding of how the moon interacts with Jupiter’s powerful magnetic field.
Generated by the planet’s outer core, consisting of liquid metallic hydrogen, Jupiter’s magnetosphere is massive — if it were visible to the human eye, from Earth it would appear larger in the sky than the Moon. This magnetosphere will also serve as an important tool for accomplishing the mission’s science goals. While Europa has no appreciable internal magnetic field of its own, Jupiter’s propagates through the Jovian moon. By using the spacecraft’s magnetometer to measure the strength of these induced magnetic fields, scientists will be able to interpret the information they carry to unlock the aforementioned mysteries of Europa’s icy shell and subsurface ocean.
Yet the Jovian system is a busy energy highway. Neighboring moon Io contains a thin atmosphere produced by massive volcanic eruptions yielding ionized oxygen and sulfur gas, or plasma, that is swept up by Jupiter’s magnetosphere before propagating through Europa. The currents from this rapidly flowing plasma produces a secondary magnetic field on the ocean moon. This poses a challenge for the mission, since it masks the magnetic field induced by Jupiter’s magnetosphere. As a solution, PIMS directly measures these plasmas to account for and isolate its influence on the overall magnetic field.
PIMS uses four Faraday Cup sensors — conductive metal cups designed to catch charged particles — to measure the characteristics of the plasmas near Europa. The sensors are impervious to radiation damage and are not receptive to other types of radiation, making them ideal for targeting only plasma currents. The measured plasma properties along with detailed computational models will establish their contribution to the total magnetic field on the ice-covered moon.
Science Goals
The PIMS investigation is targeting three scientific objectives:
1. Estimate ice shell thickness, and the inner ocean's salinity and depth.
PIMS will measure the temperature, density and other characteristics of Jovian plasma — ionized gases sourced by the moon Io’s volcanic activity — in order to account for its influence on Europa’s induced magnetic field. Europa Clipper’s magnetometer will retrieve the induction signal caused by Jupiter’s magnetosphere and, by incorporating PIMS data to separate the plasma’s effects, will estimate the moon’s ice shell thickness, and the inner ocean's depth and salinity.
Those measurements are critical to determining whether Europa is capable of supporting life. The thickness of the ice shell dictates how much energy transfer from the surface to the ocean can be expected and also how deep a spacecraft would need to drill to reach liquid water. By estimating salinity, comparisons can be made to Earth’s ocean, since similar levels of salinity would bolster the chances that life also arose on the Jovian moon. Too salty an ocean could result in uninhabitable living conditions. Ocean depth would tell us how far down we may have to probe for possible lifeforms, as the pressure buildup at the ocean’s bottom generates higher temperatures, making it a favorable if not difficult location for future exploration.
2. Understanding how Europa influences its local space environment and Jupiter’s magnetosphere, including Europa’s purported water vapor plumes and its own field of ionized gas.
Jupiter’s ultrafast rotation rate combined with its liquid metallic hydrogen outer core creates a powerful magnetosphere that measures on average three million miles from end to end. The gargantuan magnetic field continuously washes over the planet’s constellation of moons, including Io and Europa, where it ionizes the gas particles that originate from Io’s volcanic gases and Europa’s oxygen atmosphere. Swept up in the magnetosphere’s momentum, these now high-velocity, piping-hot ion particles circulate around Jupiter as plasma tori.
The region between Europa and Ganymede in particular operates like a racetrack for plasma, as electric fields and waves accelerates them into higher-energy particles traveling near the speed of light. PIMS will measure the properties and spatial variation of this circulating plasma, altogether called the Jovian torus, to shed light on their source location, rates, and variability. Specifically, the instrument will also be able to home in on how much Europa’s purported water vapor plumes contribute to Jupiter’s magnetosphere.
3. Understanding the mechanisms for weathering and releasing material from Europa's surface into the atmosphere and ionosphere.
Mapping the distribution of ices, salts, organics, and the warmest hotspots on Europa’s surface will help to unlock its geologic history and determine its habitability. For its part, PIMS will determine the rate at which particles from Jupiter’s magnetosphere are hitting the surface to better understand the role that radiation damage plays in the moon’s geologic and chemical evolution and composition.
As those Jovian particles smash into Europa, they eject the oxygen and water vapor that comprise most of the moon’s atmosphere and also the ionized carbon dioxide, methane, nitric oxide and ammonia that make up its ionosphere. PIMS will characterize plume and other activity in Europa’s ionosphere and atmosphere to better understand its surface features and the processes that create this “sputtered” atmosphere.
Instrument
The Europa Clipper mission will use magnetic sounding to probe Europa’s subsurface ocean. Plasmas produced by both Europa and Jupiter’s magnetosphere produce a detectable secondary magnetic field that must be characterized to reveal the primary induced magnetic field, which carries information about the properties of Europa's subsurface ocean, such as its depth and conductivity.
The Plasma for Magnetic Sounding (PIMS) is composed of four Faraday Cups, each with a 90-degree field of view and tasked to measure the 1.5-dimensional velocity distribution function of ions and electrons. Through sensors consisting of inert metal plates, the Faraday Cups measure the current produced by charged particles that have a sufficient energy per charge to pass through a grid placed at high-voltage variable alternating currents. In any one measurement, a high-voltage waveform consisting of a direct current level voltage plus a sine wave with amplitude change in voltage that sets the energy resolution is applied to the modulator grid. Particles with E/q › V+ΔV, where E/q represents energy per charge and V stands for voltage, always make it through the modulator, producing a constant current. Particles with E/q ‹ V-ΔV are reflected out of the sensor. Particles with V-ΔV ‹ E/q ‹ V+ΔV produce an alternating current. Electronics within the instrument amplify and digitize the current waveform from each collector plate and then perform a synchronous detection to lock in on only the alternating current component of the current at the high-voltage modulation frequency. This lock-in process makes the Faraday Cups insensitive to noise sources such as ionizing radiation or ultraviolet light.
The sum of the currents from all collector plate tridents gives the particle flux, and the ratios of the currents give the precise flow angle of the plasma. Since the currents are measured with a very high signal to noise, the instrument is capable of measuring the flow angle with better than 5-degree resolution. A full Faraday Cup measurement cycle consists of stepping velocity on the modulator from negative values to modulate electrons up to positive values to modulate ions. The energy per charge range scanned by the instrument is set by the range of voltage applied. The energy resolution of the instrument is determined by selecting the value of the voltage; simply changing the amplitude of the alternating current component of the waveform changes the Faraday Cup energy resolution on the fly.
Each Faraday Cup is built from machined cylindrical housings, each containing a set of metal grids and insulating spacers. Grids are either connected to the chassis (ground) or driven to a voltage by a power supply. Each cup also contains two independent retarding grids; one grid is driven by a modulated high voltage, and the other by a modulated low voltage. When either retarding grid is being driven, the other will be held at ground; this dual modulation approach extends the sensor’s energy range, and allows the PIMS Faraday Cups to observe plasma in both Jupiter’s magnetosphere and Europa’s ionosphere. A suppressor grid is placed directly above the collector plates to ensure that the measured current is due only to incoming particles and not secondary electrons being ejected from the collector plates.
In the Jovian magnetospheric plasma PIMS measures the density and flow velocity of ions with energies below 6 kiloelectron volts (keV), and the density and energy of electrons with energies below 2 keV. In Europa's ionosphere, PIMS measures the density and temperature of ions and electrons.
The Europa Clipper PIMS instrument, showing two of its specially designed Faraday cups to measure plasma around Jupiter's moon Europa.
| Parameter | Performance |
|---|---|
| In Magnetospheric Mode | Electron energy: 50 eV 2 keV Ion energy: 50 eV 6 keV |
| In Ionospheric Mode | Electron energy: 1 - 70 eV Ion energy: 1 - 70 eV |
| Energy resolution | 10% ΔE/E (3% ΔE/E intrinsic) |
| Field of view | 4 × 90° cones |
| Temporal resolution | 1 second for a full ion and electron sweep in Ionospheric Mode 4 seconds for a full ion and electron sweep in Magnetospheric Mode 5 seconds for a full ion and electron sweep in Transition Mode |
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