9.2 Lunar Surface
NASA's LCROSS Mission
In 2009, NASA conducted a mission involving the Lunar Crater Observation and Sensing Satellite (LCROSS).
LCROSS was crashed into the crater Cabeus near the Moon’s south pole.
The impact occurred at a speed of 9,000 kilometers per hour, releasing energy equivalent to 2 tons of dynamite.
The impact created a plume of water vapor and other chemicals, which rose high above the lunar surface.
This plume was observable by telescopes orbiting the Moon and was measured by the LCROSS spacecraft as it passed through.
Lunar Reconnaissance Orbiter (LRO)
A NASA spacecraft named the Lunar Reconnaissance Orbiter (LRO) conducted further measurements:
It recorded very low temperatures inside several lunar craters.
The LRO's sensitive cameras successfully imaged the interiors of craters using starlight.
Discoveries on the Moon
The estimated quantity of water ice in the Moon’s polar craters is hundreds of billions of tons.
Although in liquid form it would only fill a lake 100 miles across, this volume is significant compared to the dry lunar crust.
The polar water is likely accumulated from comets and asteroids impacting the Moon.
A small fraction of this water froze in extremely cold regions (known as cold traps) where sunlight never reaches, such as the bottoms of deep polar craters.
Importance of Lunar Water Ice
The discovery of water ice raises important considerations for future lunar exploration:
It paves the way for potential human habitation near lunar poles.
It suggests the possibility of establishing a lunar base as a way-station for missions to Mars and beyond.
Mining the ice could provide water and oxygen for human support.
Water could be broken down into hydrogen and oxygen, serving as potent rocket fuel.
The Lunar Surface
Learning Objectives:
By the end of this section, you should be able to:
Differentiate between major surface features of the Moon.
Describe the history of the lunar surface.
Explain the properties of lunar “soil.”
General Appearance
Observing the Moon through a telescope reveals it is covered by a multitude of impact craters.
The most noticeable features visible to the naked eye are darker lava flows, referred to as “maria” (Latin for seas).
Early observers thought the Moon had continents and oceans, naming the darker regions “seas,” examples include:
Mare Nubium (Sea of Clouds)
Mare Tranquillitatis (Sea of Tranquility)
In contrast, the lighter land areas between the seas are not named; however, thousands of craters have been named, typically honoring great scientists and philosophers (e.g., Plato, Copernicus, Tycho, Kepler).
The lunar features, while superficially resembling terrestrial ones, have different origins due to:
The Moon's relative lack of internal activity.
The absence of air and water, resulting in a geological history distinct from Earth’s.
History of the Moon
Dating Lunar Rocks
To understand the Moon's history, estimating the ages of individual rocks is crucial.
Apollo astronauts collected lunar samples, enabling the application of radioactive dating techniques used for Earth, revealing solidification ages between 3.3 to 4.4 billion years.
These ages are significantly older than most terrestrial rocks (which are younger).
Both Earth and the Moon originated around 4.5 to 4.6 billion years ago.
Composition of the Moon's Crust
83% of the Moon's crust comprises silicate rocks known as anorthosites, identified as the lunar highlands.
Anorthosites are low-density rocks that solidified on the cooling Moon, resembling slag in a smelter.
The highlands are heavily cratered due to billions of years of impact from interplanetary debris (Figure 9.7).
Physical Characteristics of Lunar Highlands
The lunar highlands possess neither sharp folds nor significant elevation variations like Earth's mountains.
Their profiles are low and rounded, resembling the most eroded mountains on Earth.
Without atmosphere or water, there are no wind or water-driven erosive processes shaping these features.
Maria and Their Formation
About 17% of the Moon's surface comprises the maria, which are flat plains of basaltic lava and are much less cratered than the highlands.
These maria cover primarily the side facing Earth and result from volcanic eruptions billions of years ago.
The basalt and lava flows in maria filled the depressions created by early impact collisions, often similar in composition to terrestrial oceanic crust and volcanic rocks.
The Lunar Basalt and Impact Basins
The youngest lunar impact basin, Mare Orientale, is about 3.8 billion years old, retaining a “bull’s-eye” structure (Figure 9.11).
The Moon experienced early volcanism, with major lava releases happening until about 3.3 billion years ago, followed by a cooling interior, leading to limited volcanic activity.
The Lunar Surface and Soil
Neil Armstrong's description of the lunar surface post-landing:
“The surface is fine and powdery.
I can pick it up loosely with my toe. But I can see the footprints of my boots and the treads in the fine sandy particles.”
The lunar surface is covered with fine-grained soil made of tiny shattered rock fragments;
Each astronaut's footstep created dark basaltic dust, contributing to the mix on their suits and equipment.
The surface layers consist of porous dust that allowed astronauts' boots to sink several centimeters (Figure 9.12).
The dusty soil results from impact cratering over billions of years, breaking down larger rocks into sand-like particles.
Temperature Extremes on the Moon
The absence of an atmosphere leads to extreme temperature variations:
Near local noon, temperatures exceed the boiling point of water.
During the long lunar night (lasting about 14 Earth days) temperatures drop to about 100 K (–173 °C).
This rapid cooling behavior is due to the porous nature of the lunar soil, which cools faster than solid rock due to the lack of air and insulating atmosphere.
Learning Resources
Supplement your understanding with the NASA Lunar Reconnaissance Orbiter (LRO) team's video on the Moon's craters and maria formation:
This video traces the Moon's evolution from its origin approximately 4.5 billion years ago to the current state observable today.
Note: The lunar cycle includes about 14 days for the phase transition from a full moon to a new moon.
-- More detailed discussions in Chapter 4: Earth, Moon, and Sky.