Geological history of Mars

The geological history of Mars follows the physical evolution of Mars as we learn about its past through observations, measurements, and careful analysis. Scientists use methods developed centuries ago by Nicholas Steno, such as the law of superposition and stratigraphy, to understand Mars just like they do for Earth and the Moon. These tools help us figure out which features on Mars are older and which are younger.

By watching the surfaces of planets and moons in our Solar System, we can learn a lot about how they change over time. For instance, if we see lava filling an old crater, we know the lava came after the crater formed. And if there’s a small crater sitting on top of that lava, it must be the most recent feature of all. These ideas let scientists break down Earth’s history into big chunks like the Paleozoic, Mesozoic, and Cenozoic eras — and we can do the same for Mars.

One useful way to guess the age of Martian surfaces is by counting craters. Areas covered 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 build a timeline showing how Mars has changed since it formed, helping us understand this fascinating planet better.

Relative ages from stratigraphy

Stratigraphy helps us figure out the order of layers on Mars, telling us which layers are older or younger. By looking at differences in what makes up these layers—like solids, liquids, and gases—we can learn about the planet's history. Scientists make educated guesses about how fast these layers formed, which gives them a range of possible ages for each layer.

Absolute ages

On Earth, scientists use a method called radiometric dating to find out exactly how old rocks are. This helps them understand the timeline of Earth's history. 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, which makes them less useful for pinpointing ages. 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 understand the planet's past. They have identified 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 suggested a new way to divide Mars's history into three main parts based on the types of minerals formed. 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, lots of water helped create these minerals, and this period matches when valleys on Mars were forming.

The Theiikian, named after sulphur due to the sulphate minerals created, lasted until about 3.5 billion years ago. This era saw lots of volcanism that released gases forming sulphuric acid with water.

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