In planetary geology, the term Terrae (Latin for “lands”) refers to the large, continental-scale, and heavily scarred landmasses found on other terrestrial bodies. These regions represent the oldest preserved crusts on worlds like the Moon and Mars, standing in stark contrast to the smoother, lower-lying plains. Terrae are defined by their rugged topography and serve as ancient geological records, providing a window into the earliest history of the inner Solar System. This classification is primarily used to distinguish the bright, elevated surfaces from the darker, younger volcanic plains that often surround them.
Shared Defining Physical Features
The classification of a planetary surface as terrae is based on a set of universally observable physical characteristics related to morphology and age. The most prominent feature is the overwhelming density of impact craters, known as crater saturation, which demonstrates the extreme antiquity of these surfaces. These regions generally date back to the Hadean Eon, having survived the intense bombardment period of the early Solar System, including the Late Heavy Bombardment.
Terrae consistently exhibit a significantly higher elevation than the surrounding terrain, creating a substantial topographic contrast across the planetary body. This elevation difference is often accompanied by a higher albedo, meaning the surface appears lighter in color when viewed from space. This lighter hue results from the different mineral composition of the ancient crust, distinguishing it from the darker volcanic rocks of later-formed plains. These features collectively mark the terrae as the remnants of a planet’s or moon’s original, primordial crust.
Lunar Highlands
The Lunar Highlands are the archetypal example of terrae, covering approximately 83% of the Moon’s surface, including nearly all of the far side. These regions are intensely bright due to their unique composition, which is dominated by the rock anorthosite. Anorthosite is a feldspar-rich igneous rock that is significantly less dense than the basaltic rocks found elsewhere, giving the highlands their light color and high albedo.
The formation of the Lunar Highlands is linked to the crystallization of the global Lunar Magma Ocean (LMO) that existed shortly after the Moon’s formation. As the LMO cooled, lighter, calcium-rich plagioclase feldspar minerals floated to the surface. These minerals accumulated to form a buoyant crust of anorthosite through a differentiation process.
The stark contrast between the bright terrae and the dark, low-lying lunar maria is a defining feature of the Moon’s geology. The terrae crust is considerably older, forming roughly 4.4 billion years ago. The maria are younger volcanic plains that filled enormous impact basins between 4.2 and 1.2 billion years ago, providing a clear timeline for the Moon’s evolution.
Martian Highlands
On Mars, the terrae are known as the Martian Highlands, which predominantly occupy the planet’s Southern Hemisphere. This region forms one half of the dramatic Martian dichotomy, where the southern terrain is elevated and heavily cratered, standing five to six kilometers higher than the smooth Northern Lowlands. The crust beneath the southern highlands is also significantly thicker, measured to be about 58 kilometers deep compared to the Northern Lowlands’ 32 kilometers.
The southern highlands represent Mars’ oldest preserved surface, retaining the scars of the early Solar System’s heavy bombardment phase. The rocks in this ancient crust are strongly magnetized, suggesting they formed during an early era when Mars possessed a global magnetic field that has since vanished. The preservation of this ancient magnetism provides valuable insight into the planet’s thermal and internal history.
The terrain features ancient lava flows and evidence of past hydrologic activity, including traces of ancient stream valleys. The Martian Highlands are associated with some of the planet’s largest geological features, such as the Valles Marineris canyon system. Studying these features is crucial for understanding the processes that shaped Mars’ crust and the planet’s early climate, which may have included a period capable of sustaining liquid water on the surface.
Scientific Importance of Terrae
The study of terrae is important to planetary science because these ancient landforms function as time capsules of the early Solar System. Their heavily cratered surfaces provide a record of the impact flux history, which helps scientists determine the absolute ages of other planetary surfaces. By analyzing the density and size of craters, researchers can reconstruct the timeline of cosmic collisions that affected the inner terrestrial bodies.
Terrae offer evidence of crustal differentiation, the process by which a planet or moon separates into layers of different density. The anorthosite of the Lunar Highlands, for example, is a product of the Moon’s initial magma ocean cooling, revealing how planetary crusts form. Understanding this earliest stage of crustal formation is fundamental to modeling the evolution of rocky bodies throughout the Solar System.
The distinct geological and compositional differences between terrae and younger plains help scientists understand the interplay between internal planetary processes and external forces. The contrast in crustal thickness and magnetic properties in the Martian Highlands provides clues about Mars’ thermal evolution and the disappearance of its magnetic field. Terrae are invaluable archives that provide foundational data for understanding how terrestrial planets form, evolve, and differ.