The Moon, Earth’s only natural satellite, appears as a static, unchanging celestial body to the unaided eye, but its surface is a detailed geological record. This surface is a landscape of extremes, preserving billions of years of bombardment and volcanic activity due to the lack of an atmosphere or liquid water to cause erosion. The various features visible on the lunar surface offer scientists a timeline of the Solar System’s history. These distinct surface characteristics are categorized into four major types, each telling a different part of the Moon’s deep history.
Impact Craters
The most abundant and defining features on the Moon are the impact craters, formed by the hypervelocity collisions of meteoroids, asteroids, and comets with the lunar surface. The immense kinetic energy of the impactor is instantly converted into a massive explosion, which excavates material, compresses rock, and sends shockwaves through the crust. This process creates a circular depression surrounded by a blanket of ejected material, known as the ejecta blanket.
Craters are generally classified into two main types based on their size and resulting structure. Simple craters, which are typically less than 15 kilometers in diameter, are characterized by a smooth, bowl-shaped interior and sharp rim, like the Molke or Linné craters. The transition to a more complex structure occurs when the crater diameter exceeds this size threshold, though the exact size depends on the target rock’s strength.
Complex craters, which are larger than about 15 kilometers, feature a shallow, flat floor, terraces along the inner wall, and a raised central peak. The central peak forms as the highly compressed rock beneath the impact point rebounds upward after the initial excavation, while the inner wall terraces result from the collapse of the steep crater rim. These complex features demonstrate the intense modification stage that follows the initial impact event.
Lunar Maria
The dark, relatively smooth plains visible from Earth are called the lunar maria, which is Latin for “seas” and was named by early astronomers who mistook them for bodies of water. These low-elevation regions cover approximately 16% of the Moon’s surface and are predominantly located on the side facing Earth. The maria are composed of dark, iron-rich basalt rock, which is volcanic in origin and gives them their low reflectivity compared to other lunar regions.
The formation of the maria began after the Moon’s period of heavy bombardment, when massive impacts created large, deep basins. Subsequently, between about 3.3 and 3.8 billion years ago, upwelling magma from the Moon’s interior flowed out through fissures in the crust and filled these impact basins. This ancient volcanic activity involved highly fluid lava, which spread out to create the vast, flat plains of solidified basalt.
Lunar Highlands
In stark contrast to the dark maria are the lunar highlands, also known as the terrae, which appear as the brighter, heavily cratered, and mountainous regions. The highlands constitute the majority of the lunar crust, making up about 83% of the surface area. These regions are significantly older than the maria, with rocks dating back between 4.1 and 4.4 billion years, representing the Moon’s original crust.
The dominant rock type in the highlands is anorthosite, a relatively low-density igneous rock rich in the mineral plagioclase feldspar. This composition is a result of the early cooling of a global magma ocean, where less dense anorthositic materials floated to the surface to form the initial crust. The high concentration of impact craters in the highlands is a direct consequence of their great age, as they have been exposed to the bombardment of interplanetary debris for billions of years longer than the surfaces of the maria.
Rilles and Other Linear Features
Rilles are long, narrow depressions on the lunar surface that resemble channels, valleys, or trenches, and are important indicators of the Moon’s volcanic and tectonic history. These features can extend for hundreds of kilometers and are classified into three distinct types based on their morphology and origin. Sinuous rilles, like the famous Hadley Rille, meander in a curved path similar to a terrestrial river and are thought to be the remnants of collapsed lava tubes or surface lava channels.
Arcuate rilles are curved trenches that are typically found along the edges of the lunar maria, forming concentric shapes parallel to the basin rims. These features likely formed when the massive weight of the mare basalt caused the floor of the impact basin to subside, creating extensional faults along the edges.
The final major type is the straight rille, which follows a long, linear path and is believed to be a graben, a block of the lunar crust that dropped down between two parallel faults due to tectonic forces pulling the crust apart.