Mercury: Crash Course Astronomy #13

Overview of Mercury

  • Closest planet to the Sun, as expected, it is very hot—average surface temperature can reach up to 430°C (800°F).

  • Also considered cool due to unique features and characteristics.

Naked-Eye Solar System Objects

  • Seven naked-eye solar system objects: Mercury, Venus, Mars, Jupiter, Saturn, the Sun, and the Moon.

  • Mercury is named after the Roman messenger god, known for his speed (fleet of foot).

  • Mercury visibly changes position against background stars quickly, demonstrating its swift movement.

Orbit and Visibility

  • Mercury remains relatively close to the Sun, never straying far with a maximum separation of about 28°.

  • Orbits the Sun at an average distance of 58 million kilometers (about one-third the distance Earth is from the Sun).

  • Completes orbit around the Sun every 88 Earth days, and also exhibits phases like the Moon.

  • Observing Mercury is challenging as it is often low on the horizon and affected by atmospheric turbulence.

Physical Characteristics

  • Diameter: about 4,900 kilometers, roughly one-third of Earth's width.

  • Highly elliptical orbit, ranging from 46 to nearly 70 million kilometers from the Sun.

  • Receives more than twice as much light and heat during perihelion compared to aphelion.

Day and Year Duration

  • Day length: approximately 59 Earth days.

  • Year length: about 87.97 days; thus, the ratio of day to year is approximately 2:3.

  • Tidal locking occurs in a unique manner due to its elliptical orbit, resulting in one side facing the Sun at perihelion.

Visual Phenomena

  • The Sun takes two Mercury years (176 Earth days) to complete one full cycle in the sky from a stationary viewpoint on Mercury.

  • The Sun can appear to rise, stop, and move backward in the sky due to the interplay of spin and orbital speeds.

Exploration and Surface Features

  • Difficulty observing Mercury from Earth; much is known from space probes like Mariner 10 and MESSENGER.

  • MESSENGER mapped surface features, which are characterized by craters, smooth plains, and compression folds (rupes).

  • Largest crater: Caloris Basin, spanning 1,600 kilometers across.

  • Craters named after artists and notable figures (e.g., Botticelli, Tolkien).

Internal Structure and Density

  • Mercury's interior is considered dense; possibly due to a large iron core accounting for ¾ the distance to the surface.

  • Several hypotheses for the high iron content: a significant impact or solar evaporation of lighter materials.

  • Presence of a measurable magnetic field, supported by the presence of a molten iron core.

Atmosphere and Water Ice

  • Mercury lacks a significant atmosphere but has a trace gas layer, largely resulting from solar wind interaction.

  • Observations show the presence of water ice in permanently shadowed craters near the poles, existing at temperatures below -170°C.

  • It is theorized that this water accumulated from impacts by comets and asteroids.

Conclusion

  • Mercury is an intriguing planet: airless, dense, cratered surface, with a unique day-length due to its elliptical orbit.

  • Even in extreme heat, water ice can persist in certain locations, showcasing the planet's fascinating extremes.