Tethys is one of Saturn's mid-sized moons, notable for its massive Odysseus Crater and the giant canyon Ithaca Chasma that stretches three-quarters of the way around the moon. The crater, 400 km across, is nearly half the moon's diameter and was created by an impact that likely came close to shattering Tethys. Ithaca Chasma, a canyon system 100 km wide and 3-5 km deep, may have formed as a result of the Odysseus Crater impact or from other geological processes. Tethys is composed almost entirely of water ice, with a density suggesting it's 90% ice and 10% rock. The moon has two small co-orbital moons, Telesto and Calypso, which share its orbit at the stable Lagrangian Points. Tethys's surface is heavily cratered and shows no signs of recent geological activity, making it a time capsule of the early solar system. This article explores Tethys's dramatic impact features, co-orbital moons, composition, and its place in the Saturn system.

In Simple Terms

Imagine a giant ball of ice floating around Saturn, almost like a frozen snowball in space. That's Tethys—a moon made mostly of ice that's so light it would actually float if you put it in water. What makes Tethys special is that it has a huge dent called Odysseus Crater that's almost as big as the moon itself, created by a massive collision that almost broke the moon apart. There's also a giant crack called Ithaca Chasma that wraps around most of the moon, like a deep valley carved into the ice. Tethys also has two tiny companion moons, Telesto and Calypso, that travel along the same path around Saturn, staying in special stable positions. Scientists study Tethys to understand how moons form, what happens when big objects crash into each other, and how ice behaves in the cold depths of space. Even though Tethys looks quiet and frozen today, its surface tells the story of dramatic events from billions of years ago, when the solar system was still forming and collisions were much more common.

Introduction

Tethys, named after a Titan in Greek mythology, was discovered by Giovanni Cassini in 1684. The moon gained attention when Voyager images revealed its massive Odysseus Crater and the giant canyon Ithaca Chasma. These features tell a story of catastrophic impacts and possible geological activity early in Tethys's history.

Tethys's co-orbital moons, Telesto and Calypso, are unique in the solar system—they share Tethys's orbit at the stable Lagrangian Points, demonstrating the dynamics of three-body orbital mechanics. Understanding Tethys is important for understanding the formation of Saturn's moons, the effects of large impacts, and the diversity of geological features on icy moons.

Physical Characteristics

Basic Properties

Tethys is a mid-sized icy moon:

• Radius: 531 km
• Mass: 6.17 × 10²⁰ kg
• Density: 0.98 g/cm³ (very low, indicating mostly ice)
• Surface gravity: 0.145 m/s² (very weak)
• Escape velocity: 0.39 km/s

Tethys's density is so low it would float in water, indicating it's composed almost entirely of water ice.

Orbit

Tethys orbits in the middle of Saturn's moon system:

• Semi-major axis: 294,660 km
• Orbital period: 1.89 Earth days
• Rotation: Synchronous (same face always toward Saturn)
• Eccentricity: 0.0001 (nearly circular)

The Odysseus Crater

Massive Impact

Odysseus Crater is one of the largest impact features in the solar system:

• Diameter: 400 km (nearly half of Tethys's 1,062 km diameter)
• Depth: ~5 km
• Rim: Raised above surrounding terrain
• Central peak: Absent or very small

The impact that created Odysseus was catastrophic:

• Energy: Nearly enough to shatter Tethys
• Effects: Likely created global fractures
• Formation: Early in solar system history
• Survival: Tethys barely survived the impact

The fact that Tethys survived such a massive impact suggests it may have been more solid or had a different structure at the time.

Relationship to Ithaca Chasma

Ithaca Chasma may be related to the Odysseus Crater impact:

• Timing: Possibly formed at the same time
• Mechanism: Impact may have created global fractures
• Alternative: Chasma may have formed independently

The relationship between the two features remains uncertain.

Ithaca Chasma

Giant Canyon

Ithaca Chasma is one of the largest canyons in the solar system:

• Length: 2,000 km (three-quarters of Tethys's circumference)
• Width: 100 km
• Depth: 3-5 km
• Structure: Graben (down-dropped block between faults)

The canyon is so large it's visible from a distance as a dark line across Tethys's surface.

Formation

Ithaca Chasma's formation is debated:

• Impact-related: May have formed from the Odysseus impact
• Tectonic: May have formed from internal processes
• Freezing: May have formed as Tethys's interior froze and expanded

The exact mechanism remains uncertain.

Co-Orbital Moons

Telesto and Calypso

Tethys has two small co-orbital moons:

• Telesto: 24 km radius, leads Tethys by 60 degrees
• Calypso: 21 km radius, trails Tethys by 60 degrees
• Orbit: Share Tethys's orbit at stable Lagrangian Points

These are the only known co-orbital moons in the solar system.

Lagrangian Points

The co-orbital moons occupy stable positions:

• L4: 60 degrees ahead (Telesto)
• L5: 60 degrees behind (Calypso)
• Stability: These are stable equilibrium points
• Formation: Likely captured asteroids or debris from impacts

The Lagrangian Points are where gravitational forces balance, creating stable positions.

Surface Geology

Heavily Cratered

Tethys's surface is heavily cratered:

• Crater density: High, indicating old surface
• No resurfacing: Surface appears unchanged for billions of years
• Crater sizes: Range from small to Odysseus-sized
• Distribution: Craters cover entire surface

The high crater density indicates Tethys has been geologically dead for most of its history.

Surface Composition

Tethys'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: High (reflects 80% of light), typical of icy moons

The surface is essentially pure water ice with little contamination.

Composition and Interior

Mostly Ice

Tethys's composition:

• Water ice: ~90% by mass
• Rock: ~10% by mass
• Structure: Possibly differentiated (ice shell over rocky core)

The very low density indicates mostly ice, but the exact structure is uncertain.

Interior

Tethys's interior is likely:

• Ice shell: Thick outer layer of ice
• Rocky core: Small silicate core
• No activity: No recent geological activity
• Frozen: Interior may be completely frozen

The lack of activity suggests the interior is cold and solid.

Exploration History

Early Observations

• 1684: 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
• Detailed mapping of Odysseus and Ithaca Chasma

Scientific Importance

Impact Studies

Tethys provides insights into:

• Large impacts: Effects on small bodies
• Shattering: How close to destruction can a body come?
• Recovery: How do bodies recover from massive impacts?

Orbital Dynamics

Tethys demonstrates:

• Co-orbital systems: How Lagrangian points work
• Stable orbits: Dynamics of three-body systems
• Moon formation: How small moons can share orbits

Icy Moon Geology

Tethys shows:

• Icy composition: Structure of ice-rich moons
• Impact history: Record of early solar system
• Geological diversity: Different types of features

Open Questions

Many mysteries remain about Tethys:

1. Ithaca Chasma: How did it form?
2. Odysseus impact: How did Tethys survive?
3. Co-orbital moons: How did they form?
4. Interior structure: What is the exact structure?
5. Formation: How did Tethys form?
6. Evolution: Why has it remained inactive?

Future missions will address these questions.