Geological history of Mars

The geological history of Mars follows how Mars has changed over time. We study it by looking closely at its surface and using careful analysis. Scientists use ideas from Nicholas Steno, like the law of superposition and stratigraphy, to learn about Mars just like they do for Earth and the Moon. These ideas help us tell which parts of Mars are older and which are newer.

By watching the surfaces of planets and moons in our Solar System, we can learn how they change. For example, if we see lava filling an old crater, we know the lava came after the crater formed. And if a small crater sits on top of that lava, it is the newest feature. These ideas help us divide Earth's history into big parts like the Paleozoic, Mesozoic, and Cenozoic eras — and we can use them for Mars too.

One way to guess how old Martian surfaces are is by counting craters. Areas with many large craters are usually very old, while places with few or only small craters look younger. By putting all these clues together, we can create a timeline showing how Mars has changed since it formed, helping us understand this amazing planet better.

Relative ages from stratigraphy

Stratigraphy helps us learn the order of layers on Mars. It tells us which layers are older or younger. By studying what the layers are made of—like solids, liquids, and gases—we can learn about Mars' history. Scientists make good guesses about how fast these layers formed. This helps them figure out how old each layer might be.

Absolute ages

On Earth, scientists use a method called radiometric dating to find out exactly how old rocks are. This helps them understand Earth’s timeline. We can tell which rock layers are older or younger by looking at their order, but this does not tell us the actual years. For Mars, figuring out exact ages is much harder. Scientists try to estimate ages by counting impact craters and comparing them to the Moon, but this method has many uncertainties. Martian meteorites found on Earth can be dated, but we do not know exactly where they came from on Mars. So, ages determined by crater counts on Mars should be viewed with caution.

Crater density timescale

Studies of impact crater numbers on Mars help scientists learn about the planet's past. They have found four main time periods in Mars' history, named after places on the planet.

These periods, from oldest to youngest, are:

• Pre-Noachian: From about 4.5 billion years ago to the formation of the big Hellas impact basin. Much of what happened then has been worn away.
• Noachian Period (named after Noachis Terra): From about 4.1 to 3.7 billion years ago. This time saw many big craters form and possible rivers and lakes.
• Hesperian Period (named after Hesperia Planum): From about 3.7 to 3.0 billion years ago. Huge lava plains formed, and there might have been floods of water.
• Amazonian Period (named after Amazonis Planitia): From 3.0 billion years ago to today. This period has fewer craters and includes lava flows and some ice activity.

Mineral alteration timescale

In 2006, scientists using a tool on the Mars Express spacecraft found a new way to split Mars's history into three main parts. These parts are called the Phyllocian, Theiikian, and Siderikan.

The Phyllocian, named after clay-like minerals called phyllosilicate, lasted from when Mars formed until about 4 billion years ago. During this time, water helped make these minerals, and valleys on Mars were forming.

The Theiikian, named after sulphur because of the sulphate minerals made, lasted until about 3.5 billion years ago. This time had lots of volcanism that sent out gases mixing with water to make sulphuric acid.

The Siderikan, named after iron because of the iron oxides that formed, has lasted from 3.5 billion years ago to today. As volcanoes became quiet and water vanished, iron in the rocks turned red, giving Mars its special colour.