Rhea: Saturn's Second Largest Moon
Rhea is Saturn's second-largest moon after Titan, with a radius of 764 km. The moon has a heavily cratered, ancient surface that shows no signs of recent geological activity. Rhea is composed primarily of water ice with a small amount of rock, and recent observations have suggested it may have a tenuous atmosphere of oxygen and carbon dioxide, though this remains uncertain. The moon's surface is dominated by impact craters, with the largest being Tirawa, 360 km across. Rhea has a few bright, rayed craters that stand out against the darker, older terrain. Unlike some of Saturn's other moons, Rhea shows no evidence of recent resurfacing or geological activity, making it a time capsule of the early solar system. This article explores Rhea's ancient surface, potential atmosphere, composition, and its place in the Saturn system.
In Simple Terms
Think of Rhea as a giant, frozen time capsule floating around Saturn. It's the second-biggest moon around Saturn (after Titan), and it looks like it's been sitting there unchanged for billions of years. The surface is covered in dents and craters from ancient collisions, like a cosmic record of all the times things crashed into it when the solar system was young. What makes Rhea interesting is that scientists think it might have a very thin atmosphere—so thin you could barely call it an atmosphere—made of oxygen and carbon dioxide. This would be unusual because most moons don't have atmospheres at all. Rhea is like the quiet, ancient relative in Saturn's moon family—it doesn't have geysers like Enceladus or lakes like Titan, but it preserves a perfect record of what the early solar system looked like, frozen in time for us to study.
Abstract
Rhea is Saturn's second-largest moon, with a radius of 764 km and a mass of 2.31 × 10²¹ kg. The moon orbits Saturn at 527,040 km, completing an orbit in 4.52 days. Rhea has a heavily cratered, ancient surface that shows no signs of recent geological activity, preserving a record of early solar system impacts. The surface is dominated by impact craters, with the largest being Tirawa crater, 360 km across. Rhea is composed primarily of water ice with a small amount of rock, with a density of 1.24 g/cm³ indicating roughly 75% ice and 25% rock by mass. Recent observations have suggested Rhea may have a tenuous atmosphere of oxygen and carbon dioxide, created by the interaction of surface ice with Saturn's magnetosphere, though this remains uncertain and controversial. The moon's surface shows some bright, rayed craters that stand out against the darker, older terrain, indicating occasional recent impacts. Unlike Enceladus or Dione, Rhea shows no evidence of past or present geological activity, making it a frozen record of the early solar system. This article reviews Rhea's ancient surface, potential atmosphere, composition, and exploration history.
Rhea as seen by the Cassini spacecraft, showing its heavily cratered ancient surface. Credit: NASA/JPL-Caltech/Space Science Institute (Public Domain)
Introduction
Rhea, named after a Titan in Greek mythology, was discovered by Giovanni Cassini in 1672. The moon is Saturn's second-largest but has received less attention than more active moons like Enceladus or Titan. However, Rhea's ancient, unchanged surface provides valuable insights into the early solar system and the formation of Saturn's moons.
Rhea's potential tenuous atmosphere, if confirmed, would make it unique among Saturn's moons and provide insights into how atmospheres can form on icy moons through non-biological processes. Understanding Rhea is important for understanding the diversity of Saturn's moon system and the processes that shape icy moons.
Physical Characteristics
Basic Properties
Rhea is a large icy moon:
- Radius: 764 km
- Mass: 2.31 × 10²¹ kg
- Density: 1.24 g/cm³ (low, indicating mostly ice)
- Surface gravity: 0.26 m/s² (very weak)
- Escape velocity: 0.64 km/s
Rhea's density suggests it's composed of roughly 75% water ice and 25% rock by mass.
Orbit
Rhea orbits in the outer part of Saturn's moon system:
- Semi-major axis: 527,040 km
- Orbital period: 4.52 Earth days
- Rotation: Synchronous (same face always toward Saturn)
- Eccentricity: 0.001 (nearly circular)
Surface Geology
Heavily Cratered
Rhea's surface is heavily cratered:
- Crater density: Very high, indicating ancient surface
- No resurfacing: Surface appears unchanged for billions of years
- Crater sizes: Range from small to Tirawa-sized
- Distribution: Craters cover entire surface
The high crater density indicates Rhea has been geologically dead for most of its history.
Impact Features
Rhea's surface is dominated by impact craters:
Tirawa crater:
- Diameter: 360 km
- Type: Multi-ring basin
- Age: Ancient, formed early in solar system history
Other craters:
- Many craters of various sizes
- Some bright, rayed craters (recent impacts)
- Darker, older craters (ancient impacts)
Bright Rayed Craters
Some craters have bright rays:
- Formation: Ejecta from recent impacts
- Appearance: Bright material against darker terrain
- Age: Relatively young (millions of years)
- Significance: Shows occasional recent impacts
These bright craters stand out against the ancient, darker terrain.
Potential Atmosphere
Discovery
Observations have suggested a tenuous atmosphere:
- Composition: Possibly oxygen and carbon dioxide
- Density: Extremely low (surface pressure ~10⁻¹² bar)
- Source: Interaction of surface ice with Saturn's magnetosphere
- Status: Controversial, not confirmed
Formation Mechanism
If the atmosphere exists, it may form through:
- Radiolysis: Radiation breaks down water ice
- Sputtering: Particles from magnetosphere knock atoms off surface
- Result: Creates oxygen and other molecules
This would be similar to processes on Europa and Ganymede.
The Debate
Arguments for atmosphere:
- Some observations suggest its presence
- Similar processes occur on other moons
- Would explain some observations
Arguments against:
- Very difficult to detect
- Alternative explanations exist
- Not confirmed by all observations
The debate continues, and future missions will help resolve it.
Composition
Ice and Rock
Rhea's composition:
- Water ice: Primary component (~75% by mass)
- Rock: Silicate material (~25%)
- Structure: Possibly differentiated (ice shell over rocky core)
The density indicates significant rock content.
Surface Composition
Rhea's surface is composed of:
- Water ice: Primary component
- Dark material: Some darker regions, possibly from impacts
- Bright material: Exposed ice in some areas
- Albedo: Moderate (reflects 60% of light)
The surface is primarily water ice with some contamination.
Exploration History
Early Observations
- 1672: Discovered by Giovanni Cassini
- 1980-1981: Voyager missions provided first detailed images
Cassini Mission (2004-2017)
Cassini provided detailed observations:
- High-resolution imaging
- Composition studies
- Gravity measurements
- Atmosphere detection attempts
- Detailed mapping
Scientific Importance
Preserving Early History
Rhea's ancient surface preserves:
- Impact history: Record of early solar system impacts
- Formation conditions: Clues about how moons formed
- Early environment: Conditions in early solar system
Understanding Moon Formation
Rhea provides insights into:
- Accretion: How large moons form
- Differentiation: How moons separate into layers
- Evolution: How moons evolve over time
Atmospheric Formation
If Rhea has an atmosphere, it shows:
- Non-biological processes: How atmospheres form without life
- Radiolysis: How radiation creates atmospheres
- Diversity: Different types of moon atmospheres
Comparison to Other Moons
Rhea is similar to Callisto:
- Both large, icy moons
- Both heavily cratered
- Both ancient surfaces
- Both show no recent activity
Differences:
- Rhea is smaller
- Rhea may have tenuous atmosphere
- Rhea is closer to Saturn
Open Questions
Many mysteries remain about Rhea:
- Atmosphere: Does it exist, and if so, what is its composition?
- Interior structure: What is the exact structure?
- Formation: How did Rhea form?
- Evolution: Why has it remained inactive?
- Bright craters: What causes the bright rayed craters?
- Dark material: What is the composition and origin?
Future missions will address these questions.
Conclusion
Rhea may be less famous than Enceladus or Titan, but it's an important part of Saturn's moon system. Its ancient, heavily cratered surface preserves a record of the early solar system, while its potential tenuous atmosphere demonstrates how atmospheres can form on icy moons through non-biological processes. Understanding Rhea is essential for understanding the diversity of Saturn's moon system, the formation of large icy moons, and the processes that shape worlds throughout the solar system. As we continue to study Rhea and other Saturnian moons, we gain new insights into the formation and evolution of moon systems.
For related topics:
- Callisto - Jupiter's moon with a similar ancient surface
- Titan - Saturn's largest moon with lakes and seas
- Enceladus - Saturn's active moon with geysers
- Saturn - The ringed planet and Rhea's parent world
- Planetary Science & Space - Overview of planetary science topics
^[NASA Solar System Exploration - Rhea] NASA. (2024). Rhea: In Depth. NASA Solar System Exploration. https://solarsystem.nasa.gov/moons/saturn-moons/rhea/in-depth/
^[Rhea Atmosphere] Teolis, B. D., et al. (2010). Cassini finds an oxygen–carbon dioxide atmosphere at Rhea. Science, 330(6012), 1813-1815.
^[Rhea Surface] Wagner, R. J., et al. (2008). Geology of Saturn's satellite Rhea on the basis of the high-resolution images from the targeted flyby 049. Icarus, 193(2), 323-344.
^[Cassini Rhea] Porco, C. C., et al. (2005). Cassini imaging science: Initial results on Saturn's rings and small satellites. Science, 307(5713), 1226-1236.
