How does the Earth compare with its rocky planetary neighbours?
The Earth can be compared with its planetary neighbours (the other rocky planets and the Moon) in terms of their rocks and surface materials, surface landforms, atmosphere, surface temperature, pressure and gravity; differences in size, distance from the Sun and the presence of an atmosphere and liquid water explain why the Earth is geologically active and habitable while the Moon and Mars are not, and why some bodies preserve an ancient cratered surface.
A focused answer to the Eduqas GCSE Geology statement on comparing Earth with the rocky planets and the Moon. Covers the comparison of rocks and landforms, atmosphere, surface temperature, pressure and gravity, and why size, distance from the Sun and the presence of an atmosphere and water make the Earth uniquely active and habitable.
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What this dot point is asking
Eduqas wants you to compare the Earth with its planetary neighbours (the other rocky planets and the Moon) across a set of features: their rocks and surface materials, their surface landforms, their atmosphere, and their surface temperature, pressure and gravity. You then need to explain how differences in size, distance from the Sun and the presence of an atmosphere and water make the Earth geologically active and habitable while the Moon and Mars are not, and why some bodies keep an ancient cratered surface. The skill is comparison and explanation, applying earthly geology to other worlds.
The answer
What we compare
The rocky (terrestrial) bodies, the Earth, the Moon, Mars, Venus and Mercury, are made of similar materials but differ greatly in their surfaces and conditions. The features Eduqas asks you to compare are:
- Rocks and surface materials. All the rocky bodies have silicate rocks and basaltic surfaces in places. Many are covered by a layer of broken rock and dust (regolith), produced on the Moon by countless impacts and on Mars partly by wind.
- Surface landforms. Some surfaces are dominated by impact craters (the Moon, Mercury), others by volcanoes, canyons and channels (Mars), and the Earth by tectonic mountains, oceans and a constantly renewed surface.
- Atmosphere. The Earth has a substantial atmosphere; Venus a very thick one; Mars a thin one; the Moon and Mercury almost none.
- Surface temperature, pressure and gravity. These vary enormously and control whether liquid water and life are possible.
Temperature, pressure and gravity
Three physical conditions decide how a body behaves and whether it can support water and life:
- Surface temperature depends mainly on distance from the Sun and on any atmosphere. The Earth sits where temperatures allow liquid water; Mars is colder; Venus is extremely hot because of a runaway greenhouse atmosphere.
- Surface pressure depends on the atmosphere. The Earth's atmosphere gives enough pressure for liquid water; Mars's thin atmosphere gives low pressure; the Moon has almost none.
- Gravity depends on a body's mass. A larger, more massive body has stronger gravity, which lets it hold onto an atmosphere. The Moon's small mass gives weak gravity, too weak to keep gas molecules, so it lost its atmosphere.
Why the Earth is active and habitable
The Earth is unusual among its neighbours, and the differences come down to size, distance from the Sun, and the presence of an atmosphere and water:
- Size. The Earth is large enough to still be hot inside, so it remains geologically active: plate tectonics, volcanoes and earthquakes constantly reshape the surface. The Moon and Mars are smaller, have largely cooled, and are far less active or dead.
- Distance from the Sun. The Earth's distance gives a surface temperature in the range where water is liquid, the basis of habitability.
- Atmosphere and water. The Earth's gravity holds an atmosphere that provides pressure and warmth and protects the surface, and liquid water drives weathering, erosion and the surface part of the rock cycle. Together these make the Earth both active and habitable.
Why some bodies keep ancient craters
A surface covered in impact craters is an old, undisturbed surface. The Moon keeps its craters because:
- there is no atmosphere, so incoming meteorites are not burnt up and strike the surface directly;
- there is no weathering or erosion (no water, no wind, no weather) to wear craters away;
- there is no active geology (no plate movement, little volcanism) to resurface them.
So a heavily cratered surface signals a body that is small, airless and geologically dead. The Earth's craters, by contrast, are mostly destroyed or buried by its atmosphere, weathering, erosion and active surface.
Examples in context
Example 1. Venus, the hot twin. Venus is almost the Earth's size but has a thick carbon dioxide atmosphere that traps heat, giving a surface hot enough to melt lead. Same size, very different conditions, because of the atmosphere.
Example 2. The Moon's frozen record. Because the Moon is airless and dead, its surface preserves a four-billion-year record of impacts, which geologists use to work out the history of bombardment in the early Solar System.
Try this
Q1. State what surface gravity depends on. [1 mark]
- Cue. The mass of the body (a more massive body has stronger gravity).
Q2. Explain why the Earth is geologically active but the Moon is not. [2 marks]
- Cue. The Earth is large enough to still be hot inside, driving plate tectonics and volcanism; the smaller Moon has cooled and is geologically dead.
Q3. Give one reason the Moon's surface preserves ancient impact craters. [1 mark]
- Cue. Any one of: no atmosphere to burn up meteorites; no weathering or erosion; no active geology to resurface it.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20196 marksCompare the Earth with the Moon in terms of atmosphere, surface activity and the preservation of craters, and explain how gravity and atmosphere account for the differences.Show worked answer →
Set the two bodies side by side on each feature, then explain the differences using gravity and atmosphere.
- Atmosphere
- The Earth has a substantial atmosphere; the Moon has effectively none. The Moon's low mass gives weak gravity, too weak to hold onto gas molecules, so any atmosphere escaped to space.
- Surface activity
- The Earth is geologically active (plate movement, volcanoes, earthquakes) because it is large and still hot inside. The Moon is small, has cooled, and is geologically dead, so its surface is no longer reshaped by tectonics or volcanism.
- Preservation of craters
- The Moon's surface is covered in ancient impact craters because there is no atmosphere to burn up incoming meteorites, no weathering or erosion to wear craters away, and no tectonics or volcanism to resurface them. On Earth, the atmosphere, weathering, erosion and active surface destroy or bury most craters.
Markers reward the side-by-side comparison and the explanation that the Moon's weak gravity loses its atmosphere, its small size means it has cooled and is inactive, and the lack of atmosphere and activity preserves craters.
Eduqas 20214 marksExplain why liquid water can exist on the surface of the Earth but not on the Moon.Show worked answer →
Bring together temperature, pressure and the ability to hold an atmosphere.
Temperature. The Earth lies at a distance from the Sun that gives surface temperatures in the range where water is liquid. The Moon's surface swings between extreme heat in sunlight and extreme cold in shadow, outside the stable liquid range.
Pressure and atmosphere. Liquid water needs enough atmospheric pressure to stop it boiling away. The Earth's atmosphere provides that pressure. The Moon's weak gravity cannot hold an atmosphere, so the surface pressure is almost zero and any water would boil off or freeze.
Markers reward the ideas that the Earth's distance from the Sun gives a suitable temperature and that its atmosphere provides the pressure (held by sufficient gravity) needed for liquid water, neither of which the Moon has.
Related dot points
- Meteorites are fragments of asteroids and other bodies that fall to Earth; the main types (stony, iron and stony-iron) are thought to sample the interiors of broken-up rocky bodies, so they give evidence for the composition of the Earth's deep interior; iron meteorites support an iron-rich core and stony meteorites a silicate mantle, because we cannot sample the deep Earth directly.
A focused answer to the Eduqas GCSE Geology statement on meteorites. Covers what meteorites are, the three main types (stony, iron, stony-iron), and how they provide evidence for the composition of the Earth's deep interior (an iron core and a silicate mantle) when we cannot sample it directly.
- Uniformitarianism (the principle that the present is the key to the past, so the same physical processes operate everywhere and at all times) lets geologists interpret the surfaces of other planetary bodies; by comparing landforms seen in space imagery (craters, volcanoes, channels, dunes) with landforms made by known processes on Earth, geologists infer the processes (impact, volcanism, flowing water, wind) that shaped other worlds, even though we have never seen them happen there.
A focused answer to the Eduqas GCSE Geology statement on uniformitarianism applied to planetary geology. Covers the principle that the present is the key to the past, how comparing landforms in space imagery with Earth landforms lets geologists infer the processes (impact, volcanism, water, wind) that shaped other planets, and the limits of the approach.
- The surfaces of the Moon and Mars record their geological histories: the Moon has heavily cratered highlands and smoother dark maria (ancient basalt plains), showing impact and volcanism on a body that is now dead; Mars shows giant volcanoes, a vast canyon system, dried-up channels and polar ice, showing past volcanism and flowing water; the relative density of impact craters is used to work out the relative ages of different surfaces (more craters means an older surface).
A focused answer to the Eduqas GCSE Geology statement on the Moon and Mars. Covers the lunar highlands and maria and what they record, the volcanoes, canyons, channels and ice of Mars, and how the density of impact craters is used to date planetary surfaces relatively.
- The Earth's outer layer is divided into tectonic plates that move slowly over the mantle, driven by convection; the evidence for plate tectonics includes the fit of the continents, matching fossils and rock sequences across oceans, and the symmetrical magnetic stripes of the sea floor; plates meet at constructive (divergent), destructive (convergent) and conservative (transform) margins, each with characteristic earthquakes, volcanoes and landforms.
A focused answer to the Eduqas GCSE Geology statement on plate tectonics. Covers tectonic plates and the convection that drives them, the evidence (continental fit, matching fossils and rocks, magnetic stripes and sea-floor spreading), and the three types of plate margin with their earthquakes, volcanoes and landforms.
- The rock cycle links igneous, sedimentary and metamorphic rocks through the processes of weathering, erosion, transport, deposition, burial and lithification, melting and crystallisation, and metamorphism; the cycle is driven by energy from the Sun (at the surface) and from the Earth's interior (at depth), and any rock can be changed into any other given time and the right conditions.
A focused answer to the Eduqas GCSE Geology statement on the rock cycle. Covers the three rock families and the processes that connect them (weathering, erosion, transport, deposition, lithification, melting, crystallisation and metamorphism), the two energy sources that drive the cycle, and how any rock can become any other.
Sources & how we know this
- WJEC Eduqas GCSE (9-1) Geology specification (teaching from 2017) — WJEC Eduqas (2017)