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).

What this dot point is asking

Eduqas wants you to interpret the surfaces of the Moon and Mars and say what they record about each body's geological history. For the Moon: the heavily cratered highlands and the smoother dark maria (ancient basalt plains), recording impact and volcanism on a body that is now dead. For Mars: the giant volcanoes, the vast canyon system, the dried-up channels and the polar ice, recording past volcanism and flowing water. You also need to use the density of impact craters to work out the relative ages of different surfaces: more craters means an older surface. This applies uniformitarianism to two specific worlds.

The answer

The Moon

The Moon is small, airless and geologically dead, so its surface preserves an ancient record. Two main terrains stand out:

• The highlands are pale, mountainous and heavily cratered. The dense cratering shows these are very old surfaces, battered by impacts over billions of years and never resurfaced.
• The maria ("seas") are smoother, darker, lower plains with far fewer craters. They are vast sheets of basalt that flooded large impact basins as lava, then solidified. Because they have fewer craters, they are younger than the highlands (the lava resurfaced and erased the older craters).

So the Moon records two processes, impact (the craters) and volcanism (the maria), on a body that long ago cooled and became inactive. The lack of atmosphere, water and active geology is why this ancient record survives.

Mars

Mars is larger than the Moon and was far more active. Its surface shows a rich history:

• Giant volcanoes (such as Olympus Mons, the largest volcano known) record past volcanism on a grand scale.
• A vast canyon system (Valles Marineris) records large-scale tectonic splitting and erosion of the crust.
• Dried-up channels that branch like river valleys record flowing water in the past, so Mars once had liquid water on its surface, which is why it is studied for past life.
• Polar ice caps show that water and carbon dioxide ice are present now, and that Mars still has a thin atmosphere and seasons.
• Dunes record that the thin atmosphere still moves dust by wind today.

Mars therefore records a body that was once volcanically active and wet, and is now cold and dry but not entirely dead.

Crater dating: the density of craters gives relative age

Because impacts happen randomly over time, the number of craters on a surface measures how long that surface has been exposed:

• a surface with many craters has been exposed for a long time, so it is old;
• a surface with few craters has been resurfaced more recently (by lava, by erosion, or by burial), so it is young.

This is relative dating (it gives the order, older or younger, not an age in years) and it is the planetary equivalent of superposition. It explains why the cratered lunar highlands are older than the smooth maria, and lets geologists order the surfaces of any cratered world.

Examples in context

Example 1. Apollo samples and crater dating. Rocks the Apollo missions brought back from the lunar maria were dated radiometrically, calibrating the crater-density method so that crater counts on other surfaces can be turned into approximate ages.

Example 2. Olympus Mons. This Martian volcano is about two and a half times the height of Everest. Its size, with relatively few craters on its flanks, shows long-lived volcanism on a surface that stayed young for a long time.

Try this

Q1. State what the lunar maria are made of. [1 mark]

• Cue. Basalt (solidified lava flows that flooded impact basins).

Q2. Explain why a heavily cratered surface is older than a smooth, sparsely cratered one. [2 marks]

• Cue. Craters accumulate over time, so many craters means long exposure (old); few craters means the surface was resurfaced recently (young).

Q3. Name one feature of Mars that records past flowing water. [1 mark]

• Cue. Dried-up, branching channels or valley networks (resembling river systems).