Chapter 20: The Moon and Mercury: Comparing Airless Worlds

tidal coupling

The locking of the rotation of a body to its revolution around another body.

terminator

The dividing line between daylight and darkness on a planet or moon.

maria

One of the lunar lowlands filled by successive flows of dark lava; from the Latin word for “sea.”

sinuous rilles

A narrow, winding valley on the Moon caused by ancient lava flows along narrow channels.

ejecta

Pulverized rock scattered by meteorite impacts on a planetary surface.

rays

Ejecta from a meteorite impact, forming white streamers radiating from some lunar craters.

secondary craters

A crater formed by the impact of debris ejected from a larger crater.

micrometeorites

Meteorite of microscopic size.

multiringed basins

Very large impact basin in which there are concentric rings of mountains.

relative ages

The age of a geological feature as referred to other features. For example, relative ages reveal that the lunar maria are younger than the highlands.

absolute age

An age determined in years, as from radioactive dating

vesicular

A porous basalt rock formed by solidified lava with trapped bubbles.

anorthosite

Rock of aluminum and calcium silicates found in the lunar highlands.

breccias

A rock composed of fragments of earlier rocks bonded together.

regolith

A soil made up of crushed rock fragments.

late heavy bombardment

The surge in cratering impacts in the Solar System that occurred about 3.8 billion years ago.

jumbled terrain

Disturbed regions of the Moon’s surface opposite the locations of the Imbrium Basin and Mare Orientale, possibly focusing of seismic waves from the large impacts that formed those basins.

fission hypothesis

proposed that the Moon broke from a rapidly spinning young Earth

condensation hypothesis

suggested that Earth and the Moon condensed together from the same cloud of matter in the solar nebula

capture hypothesis

suggested that the Moon formed elsewhere in the solar nebula and was later captured by Earth

large impact hypothesis

The hypothesis that the Moon formed from debris ejected during a collision between Earth and a large planetesimal.

resonance

The coincidental agreement between two periodic phenomena; commonly applied to agreements between orbital periods, which can make orbits more stable or less stable.

lobate scarps

A curved cliff such as those found on Mercury.

intercrater plains

The relatively smooth terrain on Mercury.

smooth plains

Apparently young plain on Mercury formed by lava flows at or soon after the formation of the Caloris Basin.

Why would you include the Moon in a comparison of the Terrestrial planets?

The Moon is located in the inner Solar System and The Moon is made of rock and iron.

What may be the origin of the Moon?

The debris of the collision of Theta with Earth form over millions of years what we know now as our Moon

What is the densest planet in the Solar system?

Mercury

What do Mercury and our Moon have in common?

• They are solid

• They have no atmosphere

• They have many craters on their surface

• They have about the same size