Study Notes for NATS 1572: Searching for Life in the Outer Solar System - 8A

Introduction to Astrobiology

• Course Title: NATS 1572

• Class Focus: Searching for Life in the Outer Solar System

Habitability of Worlds

• A world can be habitable only if it has a liquid medium.

  • Primary Example: Liquid water

  • Other Possible Liquids: Ammonia, methanol, methane, or ethane

A Biological Tour of the Solar System

The Outer Solar System

Jovian Planets Overview

• Includes: Jupiter, Saturn, Uranus, Neptune

Jupiter

• Distance from the Sun: 5.20 AU

• Mass: 318 times that of Earth (318M Earth)

• Radius: 11.19 times that of Earth (11.19REarth)

• Density: 1.33 g/cm³

• Composition: Mostly hydrogen (H) and helium (He)

Saturn

• Distance from the Sun: 9.54 AU

• Mass: 95M Earth

• Radius: 9.46REarth

• Density: 0.71 g/cm³

• Composition: Mostly H and He

Uranus

• Distance from the Sun: 19.2 AU

• Mass: 14M Earth

• Radius: 3.98REarth

• Density: 1.24 g/cm³

• Composition: H compounds, rock, H and He

Neptune

• Distance from the Sun: 30.1 AU

• Mass: 17M Earth

• Radius: 3.81REarth

• Density: 1.67 g/cm³

• Composition: H compounds, rock, H and He

Jovian Planet Interiors

Structure of Jovian Planets

• Jupiter:

  • Visible Clouds: Contains gaseous hydrogen, liquid hydrogen, and metallic hydrogen

  • Core Composition: Rock, metals, and hydrogen compounds

• Saturn:

  • Atmospheric Composition: Similar to Jupiter

• Uranus:

  • Core Composition: Rock and metals; water, methane, and ammonia

• Neptune:

  • Similar composition as Uranus

Temperature of Jovian Interiors

• Upper Atmospheres: Cold due to distance from Sun

• Deep Interiors: Hot; cannot fuel geological activity due to lack of solid surfaces

• Temperature Zones: Some altitudes right for liquid water

• Energy Sources for Life: Chemical reactions powered by lightning could provide energy for life

Jupiter's Atmosphere

Environmental Conditions

• Temperature: Far below freezing at cloud tops, rises rapidly with depth

• Cloud Layers: Formed from different gas types that condense

  • Clouds at 100 km depth may contain droplets of liquid water

• Life Potential: Floating life possible?

Fatal Conditions for Life

• Wind Conditions: Strong vertical winds exceed hurricane speeds

• Particle Temperature Variations: Particles subject to wide temperature ranges which could destroy quickly forming organic molecules

• Microbial Threat: Microbes brought via meteorites faced similar outcomes

Buoyant Organisms

• Organisms could maintain altitude amidst strong winds

• Likely characteristics needed:

  • Large size

  • Gas-filled structures

• Evolutionary Challenge: Microbes find it hard to evolve into large buoyant forms; large-size organisms' survival from space travel less probable

Comparing Jupiter and Saturn

• Similarities allow conclusions regarding habitability of Jupiter to apply to Saturn as well

Uranus and Neptune

• Characteristics: Farther from the Sun than Jupiter and Saturn

• Atmospheric Conditions: Colder, less sunlight

• Cloud Layer Depth: Low altitude for clouds containing liquid water droplets; subjected to vertical winds

Interior Oceans

• Outer cores may contain water, methane, and ammonia

• Potential State: Theoretical models suggest materials might be in liquid form; high pressures complicate life potential

• Exploration Limitation: Current technology unlikely to explore these interiors soon

Jovian Moons

Variability of Moons

• Jupiter: 90+ moons

• Saturn: 140+ moons

• Uranus: 27 known moons

• Neptune: 14 known moons

• Moon Types:

  • Some moons are geologically active

  • Potential atmospheres

  • Suggest opportunities for life

Largest Moons of the Solar System

• Jupiter: Io, Europa, Ganymede, Callisto

• Saturn: Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Iapetus

• Neptune: Triton, Nereid

Smaller Moons

• Characteristics: Irregular shapes due to weak self-gravity

  • Some show ordered motion like larger moons

• Formation: Formed from circumplanetary disks, collisions, or captured asteroids/comets

Synchronous Rotation

• Concept:

  • Rotation of a moon aligns with its orbit around the planet

  • If a moon does not rotate during its orbit, it will not always face the planet

• Eternal facing: Requires completing one rotation during each orbit

Tidal Forces

Fundamentals of Tides

• Gravitational force strength decreases with distance squared

  • Moon-facing side of Earth experiences a stronger force compared to the far side

• Resulting Force: Tidal stretching forces

Effects of Tides

• Land and Ocean: Tides affect both, but ocean tides are more noticeable due to water mobility

Tidal Dynamics

• Misalignment:

  • Delayed tidal force response due to the speed of Earth’s rotation exceeding the Moon’s orbital speed

• Tidal Bulges:

  • Do not align exactly with the Moon’s position, leading to the bulge leading the Earth-Moon line

Tidal Friction Mechanics

• Earth's Rotation:

  • If Earth didn’t rotate, tidal bulges would align with Earth-Moon line

  • Friction caused by Earth's rotation pulls tidal bulges ahead of the line

  • The Moon’s gravity tries to realign the bulges, which slows Earth's rotation

  • Gravitational interactions lead to increased Moon distance over time

Tides and Synchronous Rotation Relationship

• Earth exerts tidal influences on the Moon, stronger due to mass

• The Moon likely had a faster rotation period in the past, slowed down by tidal friction

• Synchronous Matching: Rotation rate slows until it matches orbital velocity, at which point tidal friction ceases

Implications of Tidal Forces

• Tidal forces have predominantly led to moons in the solar system achieving synchronous rotation

• Geological Activity Evidence: Some larger moons show signs of geological activity, surface or subsurface waters, and atmospheric existence

Habitable Regions of Large Moons

• Potentially habitable areas within Jovian moons may include:

  • Subsurface oceans on Europa, Ganymede, and Enceladus

  • Liquid layers on Callisto and Titan

  • Surface lakes of methane and ethane on Titan

Other Small Bodies

Dwarf Planets

• Definition: Large enough to be spherical but small enough to have many planetesimals along their orbit

• Examples: Ceres, Pluto, Eris, Makemake, Haumea

  • Similarities to Jovian moons include large ice proportions, differentiated interiors

  • Possible subsurface water; however, usually lacks a heat source

Specific Cases

Ceres

• Mission Insight: Dawn mission entered orbit in 2015

• Surface Features: Heavily cratered; revealed bright spots possibly indicating mineral deposits

Pluto

• Mission Insight: New Horizons spacecraft flew by Pluto in 2015

• Surface Activity: Observational signs of geological activity; potential subsurface lakes or oceans

Life-Supporting Potential

• While subsurface liquids could earmark dwarf planets as life candidates, the lack of energy sources indicates low likelihood for life

Consideration of Small Bodies

• Includes Martian moons, smaller Jovian moons, asteroids, comets

• Habitability: Unlikely due to size; not sufficient remaining internal heat to melt ice

• Historical Life: Past life potential exists due to organics, yet no supporting evidence from meteorites found