Planetary Geology: Earth and Its Neighbours
Why Compare Planets?
Geology is not confined to Earth. By studying the Moon, Mars and other bodies in the Solar System, geologists apply the same principles they use on Earth — uniformitarianism — to distant worlds. If we recognise a lava flow on Mars because it looks like basalt flows we know from Earth, we can infer the volcanic history of Mars without ever visiting it. Planetary geology uses space imagery, remote sensing data and, in some cases, physical samples (from the Moon and meteorites) to reconstruct the geological histories of other bodies and, by comparison, to better understand processes on Earth itself.
Comparing Planetary Conditions
The four bodies most directly compared in GCSE Geology are Earth, the Moon, Mars, and to a lesser extent Venus. Each has a very different combination of atmosphere, surface temperature, atmospheric pressure, and surface gravity. These differences determine what geological processes can and cannot operate on each body — and therefore what the surface geology looks like.
Earth has a substantial nitrogen–oxygen atmosphere (mean surface pressure approximately 101 kPa, or 1 atmosphere), a mean surface temperature of about 15°C, and surface gravity of 9.81 m/s2. Liquid water is stable at the surface, enabling erosion, sediment transport, and the chemical weathering that removes CO2 and moderates climate. Earth also has active plate tectonics, which recycles crust and drives volcanic activity.
The Moon has no atmosphere — its gravity (1.62 m/s2, roughly one sixth of Earth’s) is too weak to retain gas over geological time. Without an atmosphere, there is no weather, no water erosion, and no chemical weathering. Surface temperatures swing from about −170°C at night to +130°C during the day. The Moon is geologically inactive today: no plate tectonics, no active volcanism. The dominant surface processes are meteorite impact and the gradual gardening of the surface by micrometeorite bombardment. As a result, ancient features — including impact craters billions of years old — are preserved in extraordinary detail.
The Moon has no atmosphere, no liquid water and no plate tectonics. Its dominant surface process is meteorite impact. Because there is no erosion, weathering or crustal recycling, ancient impact craters billions of years old are preserved on the lunar surface — giving geologists a record of the early Solar System bombardment history that Earth’s active geology has largely erased.
Mars: A Planet with a Geological History
Mars is a world of particular geological interest. Its atmosphere is very thin (mean surface pressure about 0.6 kPa, less than 1% of Earth’s) and composed mainly of carbon dioxide. Surface temperatures average about −60°C. Surface gravity is 3.72 m/s2 — about 38% of Earth’s. Despite these harsh conditions, Mars shows compelling geological evidence for past liquid water, including ancient river valleys, delta deposits at crater rims, and minerals such as clay minerals that only form in the presence of liquid water.
Mars also preserves the largest volcano in the Solar System: Olympus Mons, standing about 22 km above the surrounding plains and measuring about 600 km across at its base. On Earth, volcanic hot spots produce shield volcanoes (like those of Hawaii), but the Pacific Plate moves over the hot spot, so each new eruption creates a new volcano in a chain. Mars has no plate tectonics, so the lithosphere remained stationary above the hot spot for billions of years, allowing enormous amounts of lava to pile up in one place. Exam questions often ask you to explain this difference — make sure you link Mars’s giant volcanoes to the absence of plate tectonics, not just to the size of the magma source.
Olympus Mons on Mars is about 22 km tall — more than twice the height of Mauna Kea from its ocean base. It grew so large because Mars has no plate tectonics: the crust stayed in place over a volcanic hot spot for billions of years, allowing lava to accumulate in one location rather than being spread along a chain of smaller volcanoes as happens on Earth.
Rocks and Composition
Remote sensing and rover data confirm that Mars’s surface is dominated by basalt — the same rock type that forms Earth’s ocean floors and volcanic islands. This is consistent with a planet whose geology has been dominated by volcanic activity. The lunar highlands (pale regions of the Moon) are composed largely of anorthosite, a calcium-feldspar-rich rock that formed from a global magma ocean early in the Moon’s history. The dark lunar maria (plains) are filled with basaltic lava from later volcanic eruptions, mostly between about 3.9 and 3 billion years ago.
Earth’s interior structure — a solid inner core, liquid outer core, mantle and crust — was determined using seismology. Information about the composition of the Solar System as a whole comes from meteorites. Chondrite meteorites in particular are thought to represent the primitive material from which the inner planets formed, giving geologists a baseline composition for the early Solar System. Comparing Earth’s crust and mantle composition with chondrite data reveals how planetary differentiation — the separation of denser iron to the core — has modified Earth’s original composition.
Uniformitarianism — the idea that “the present is the key to the past” — also extends across planets. When geologists see features on Mars that look like river deltas, braided channels or alluvial fans, they apply knowledge gained from studying the same features on Earth to interpret them as the product of flowing water. Similarly, lava flow morphology, impact ejecta patterns, and dune fields on Mars can all be interpreted using processes understood from Earth. The key limit is that conditions on Mars today are very different from Earth, so geologists must also consider the different atmospheric pressure, temperature and gravity when applying analogies. A direct copy of an Earth interpretation may be wrong if it ignores those differences.
A concise data comparison for your revision:
| Body | Surface gravity (m/s2) | Mean surface temp | Atmospheric pressure (kPa) |
|---|---|---|---|
| Earth | 9.81 | ~15°C | ~101 |
| Moon | 1.62 (~1/6 Earth) | −20°C mean, large range | ~0 (effectively none) |
| Mars | 3.72 (~38% Earth) | ~−60°C | ~0.6 (<1% Earth) |
Low gravity on the Moon means it cannot retain an atmosphere — gas molecules achieve escape velocity. Mars has a thin atmosphere partly because it also lost much of its internal heat (and therefore its magnetic field and volcanic resupply of gases) early in its history.
Planetary Geology and Earth’s History
Studying other planets helps geologists understand Earth’s own early history. The Moon and Mars have preserved their ancient surfaces because they lack Earth’s erosion and crustal recycling. By reading the bombardment history recorded in lunar and Martian craters, geologists can infer that early Earth was subjected to a similarly intense meteorite bombardment during the Hadean eon (4.6–4.0 billion years ago). Because Earth’s geological record from this era is almost entirely erased by plate tectonics and erosion, the Moon effectively acts as Earth’s early geological diary.