Enceladus Plume Discovery Fuels JUICE to Jupiter Icy Moons

This evening's discussion focuses on discoveries made at the moons of Saturn and Jupiter, particularly how findings at Saturn's moons led to a mission to Jupiter to explore its icy satellites.

Jupiter and Saturn: A Comparative View

Jupiter and Saturn are vastly different in size. Jupiter is notable for its Great Red Spot, a prominent feature in its atmosphere. Saturn, on the other hand, is recognized by its visible rings, including a more diffuse E-ring further out. Close observation of the E-ring reveals plumes emanating from it and a small moon within it. A striking image captured by the Cassini spacecraft orbiting Saturn for nearly 17 years shows Earth as a tiny blue dot, highlighting the immense scale of the solar system and our planet's smallness in comparison to Saturn.

The Cassini Mission: Unveiling Saturn's Secrets

The Cassini spacecraft, a collaborative mission between NASA and the European Space Agency (ESA), was instrumental in these discoveries. It featured a large high-gain antenna for data transmission to Earth and a long boom for magnetometers, instruments designed to measure magnetic fields away from the spacecraft's own magnetic interference. The spacecraft was covered in a gold thermal blanket to maintain operational temperatures in the extreme cold of Saturn's environment (-170°C). A key component was the Huygens probe, built by ESA, which descended through Titan's atmosphere, capturing images and data.

Understanding Magnetic Fields

Magnetic fields, like Earth's, are generated by dynamo processes within a planet's interior. These fields have both direction and strength, measured by three components that combine to determine the overall strength. Earth rotates every 24 hours, Saturn every 10 hours, and Jupiter every 9 hours. As these planets rotate, their magnetic field lines rotate with them. The interaction between these rotating field lines and a moon provides clues about the moon's composition and nature.

Saturn's Moons: The Enceladus Revelation

Before Cassini, the E-ring was known to exist from Voyager and Pioneer missions, and its particles were understood to be primarily water ice. However, the source of this ice was unknown.

Enceladus, one of Saturn's moons, presented a significant surprise. Unlike its heavily cratered neighbors, Mimas and Tethys, Enceladus had very few craters, suggesting a geologically active surface that was constantly being resurfaced. Its surface, like the E-ring, was also composed mainly of water ice. This led to the hypothesis that Enceladus might be supplying the E-ring material. The surface of Enceladus also displayed deep cracks and mountains.

The Discovery of Enceladus's Plume

Cassini arrived at Saturn in 2004. Initial flybys of Enceladus in early 2005, at distances of 1,300 km and 500 km, revealed unusual magnetic field signatures. Magnetometer data showed distortions in Saturn's magnetic field around Enceladus, indicating that the moon was acting as a larger obstacle than its physical size. This suggested that the magnetic field lines could not penetrate its surface, implying the presence of an ionized atmosphere. Additionally, increased noise in the data correlated with a surge in water group ions.

These observations led scientists to propose that Enceladus possessed a large, diffuse atmosphere of water group ions interacting with Saturn's magnetic field. Based on these findings, a third, much closer flyby was planned, at just 173 km above the surface. This close approach was fortuitously directed below Enceladus's south pole.

The third flyby revealed a spectacular plume emanating from the south pole, resembling a cometary jet, composed of water group ions. This plume was not a diffuse atmosphere covering the entire moon but was concentrated at the south pole.

Evidence for Life's Ingredients on Enceladus

Further data from other instruments confirmed the significance of this discovery:

• Imaging Instrument: Revealed large cracks on the surface, dubbed "tiger stripes," at the south pole.
• Temperature Sensor: Showed a much hotter region at the south pole (-80°C) compared to the equator (-120°C). This hot spot was directly over one of the tiger stripes, indicating internal heat leaking from the moon.
• Ion Neutral Mass Spectrometer: "Tasted" the plume, finding water vapor, methane, carbon monoxide, carbon dioxide, and crucially, organic material.

These findings confirmed the presence of three key ingredients for life: liquid water (from the plume originating from a subsurface ocean), a heat source, and organic material. The fourth ingredient, long-term stability of these conditions, remains difficult to ascertain.

The Journey to Jupiter: The JUICE Mission

The discoveries at Enceladus prompted a re-evaluation of potential habitable environments in the outer solar system. Previously, the search for liquid water focused on the inner solar system, within the "snow line," where surface water would remain liquid. However, the findings at Enceladus and earlier data from NASA's Galileo spacecraft at Jupiter's moons suggested that subsurface oceans could exist far beyond this line.

Jupiter's Icy Moons

Galileo data from the 1990s indicated that three of Jupiter's large moons—Europa, Ganymede, and Callisto—likely harbor global liquid water oceans beneath their surfaces. This was inferred from magnetic field measurements, where changing magnetic fields induce electrical currents in conducting bodies (like saltwater oceans), generating a measurable magnetic field.

• Io: A volcanic moon with sulfur dioxide volcanoes.
• Europa: Evidence of a subsurface liquid water ocean.
• Ganymede: Also shows signs of a subsurface liquid water ocean.
• Callisto: Also shows signs of a subsurface liquid water ocean.

These findings strengthened the case for exploring the outer solar system for habitability. The European Space Agency approved the Jupiter Icy Moons Explorer (JUICE) mission, focusing on these three moons.

The JUICE Spacecraft

JUICE is primarily solar-powered, featuring 76 square meters of solar panels. It also includes a high-gain antenna and a 10.6-meter magnetometer boom to ensure accurate magnetic field measurements. The development of JUICE instruments faced challenges, including construction during the COVID-19 lockdown, which engineers overcame by adapting their work methods.

The JUICE magnetometer suite includes three sensors: two vector magnetometers (built at Imperial College and Technical University of Braunschweig) and a scalar magnetometer (built by the University of Graz) at the end of the boom, which measures only the field's magnitude and aids in calibration around Ganymede. These instruments underwent rigorous testing, including vibration and shock tests, to ensure their survival during launch and the long journey to Jupiter.

JUICE was launched on April 14, 2023, from French Guiana, a location chosen for its proximity to the equator to minimize fuel requirements. The launch was initially delayed by an electrical storm but proceeded successfully the following day. After launch, the solar panels and magnetometer boom were successfully deployed.

JUICE's Scientific Objectives at Jupiter

JUICE is expected to reach Jupiter in July 2031. The long transit time allows for instrument calibration and noise source identification. To reach Jupiter, JUICE will perform three Earth flybys to gain energy, with the second scheduled for September 2024 and the third for 2029.

The mission will involve orbiting Jupiter, conducting flybys of its moons, and ultimately entering orbit around Ganymede.

• Ganymede: This is a primary target because it is the only moon in the solar system with an internal dynamo-generated magnetic field, similar to Earth's. JUICE aims to understand why Ganymede has this unique feature and to confirm the presence and characteristics of its deep, global subsurface ocean, which is thought to be sandwiched between layers of ice. Studying Ganymede can also provide insights into exoplanets with similar water-world structures.
• Callisto: This moon is believed to have an undifferentiated interior, unlike Ganymede's layered structure, despite both moons forming at the same time. JUICE will investigate this difference and study Callisto's ancient, largely unchanged surface to understand the early history of the solar system.
• Europa: JUICE will perform two flybys of Europa. Europa is known to have a liquid water ocean beneath its surface, potentially in contact with silicates, which could leak onto the surface. Due to Europa's high-radiation environment, extensive study will also be conducted by NASA's Europa Clipper mission, which will perform about 50 flybys. The combined data from both missions will allow for comprehensive spatial and temporal measurements.

A key challenge for JUICE at Ganymede is to precisely measure tiny induction signals from the subsurface ocean, separating them from Ganymede's internal dynamo field and Jupiter's magnetic field. Success in this endeavor will allow scientists to determine the ocean's depth, salt content, and global extent, as well as the thickness of the ice crust. This information is crucial for future missions, such as potential landers, to identify suitable landing sites.

The Evolution of Icy Moon Exploration

The understanding of icy moons has evolved significantly since Galileo Galilei first observed Jupiter's moons in 1610. Key milestones include:

• 1974: Insight mission provided evidence of past liquid water on Mars.
• 1985: Submersibles discovered bacteria in Earth's deep ocean trenches, demonstrating life's resilience in extreme environments.
• Galileo and Cassini-Huygens missions: Provided crucial data on Jupiter's and Saturn's moons.
• 2012: Discovery of the first extrasolar water world.
• JUICE and Europa Clipper: Current missions focused on characterizing these environments for future exploration.

The Grand Finale of Cassini

Cassini's mission concluded after 13 years orbiting Saturn. To prevent potential contamination of moons like Enceladus or Titan, where life might exist, the spacecraft was deliberately plunged into Saturn's atmosphere. This "grand finale" also aimed to solve a long-standing mystery about Saturn's magnetic field. Planetary dynamo theory suggests a tilt between a planet's rotation axis and its magnetic axis is necessary for dynamo generation. However, Cassini consistently measured no tilt at Saturn.

The final year of Cassini's mission involved:

1. Ring-grazing orbits: Using Titan for gravity assists, Cassini performed orbits just beyond the edge of Saturn's rings.
2. Grand Finale orbits: Twenty-three close orbits between Saturn and its rings, with the closest approach just above Saturn's atmosphere.

These close orbits revealed that Saturn's rings are not solid but composed of individual particles. The data collected during these final orbits showed that while an internal dynamo was indeed present, a second dynamo in the upper atmosphere was masking its tilt, explaining the previous observations.

The first dive between Saturn and its rings was a high-risk maneuver, as the environment was unknown. Cassini used its high-gain antenna as a shield. Despite initial concerns, the spacecraft survived, indicating the gap was largely empty. Cassini continued its dives, getting progressively closer to Saturn.

On September 15, 2017, Cassini made its final plunge into Saturn's atmosphere, transmitting data until its signal was lost. The mission's end marked a poignant moment for the scientists and engineers who had dedicated decades to the project. The successful execution of the grand finale provided invaluable data and a fitting conclusion to a groundbreaking mission.