Lesson Overview
This lesson explores Mercury, the smallest planet and the one closest to the Sun. Learners will examine its orbit, surface features, temperature extremes, and lack of atmosphere, and understand why Mercury is one of the most extreme worlds in the Solar System.
Lesson Content
Mercury’s Position and Orbit
Mercury is the closest planet to the Sun and has the shortest year of all the planets, completing one orbit in about 88 Earth days. Its orbit is highly eccentric, meaning its distance from the Sun varies significantly during each orbit.
Despite being closest to the Sun, Mercury does not have the highest average temperature because it lacks a thick atmosphere to retain heat.
Size and Structure
Mercury is the smallest planet in the Solar System, only slightly larger than Earth’s Moon. It has:
A large metallic core
A thin rocky mantle
A heavily cratered crust
The planet’s large core makes up a significant portion of its total size, influencing its gravity and magnetic field.
Surface Features
Mercury’s surface is covered in impact craters, indicating a long history of collisions. One of its most notable features is the Caloris Basin, a massive impact crater formed early in the planet’s history.
The surface shows signs of contraction, where Mercury cooled and shrank over time, creating cliffs known as lobate scarps.
Temperature Extremes
Mercury experiences the most extreme temperature differences of any planet:
Daytime temperatures can exceed 430°C
Night-time temperatures can drop below –180°C
These extremes occur because Mercury has almost no atmosphere, preventing heat from being evenly distributed or retained.
Atmosphere and Exosphere
Mercury does not have a true atmosphere. Instead, it has a very thin exosphere composed of atoms blasted off the surface by solar wind and micrometeoroid impacts.
Because this exosphere is so thin, it cannot trap heat or protect the surface from radiation.
Magnetic Field
Despite its small size, Mercury has a global magnetic field, though it is much weaker than Earth’s. This discovery surprised scientists and suggests that Mercury’s core is at least partially molten.
Exploration of Mercury
Mercury has been studied by spacecraft missions, which revealed its surface composition, magnetic field, and geological history. These missions have significantly expanded our understanding of rocky planet formation.
Why Mercury Matters
Studying Mercury helps scientists:
Understand how rocky planets form near stars
Learn how extreme solar environments affect planets
Compare Mercury with Earth’s Moon and other terrestrial worlds
Mercury also serves as a reference point for rocky exoplanets found close to their stars.
Key Terms Introduced
Eccentric orbit
Exosphere
Caloris Basin
Lobate scarp
Magnetic field