The Red Planet
The Mars that we see today is very different from the planet that formed about 4.5 billion years ago.
Mars is currently cold, dry and covered in rusty-red oxidised iron minerals and dust, which give it the nickname of the ‘Red Planet’. However, studies of Mars reveal that, early in its history, the planet was warmer and wetter, had a thicker atmosphere and had a magnetic field. It may even have hosted conditions suitable for life.
Mars is the fourth planet from the Sun. It has a similar, rocky structure to Earth, although it is only around half the size.
A Brief History of Mars
All the objects in our Solar System formed from material (initially gas and dust) clumping together to form larger building blocks.
In the case of Mars, this process, called accretion, appears to have taken place very early on in the history of the Solar System.
The surface of the newborn Mars would have been molten. As it cooled, the young planet underwent ‘differentiation’, whereby the heaviest materials, such as iron, sink to the core. The structure of Mars separated into an outer crust, mantle and core.
Early Mars was volcanically active and pummelled by impacts with asteroids, comets and embryonic planets. Large impacts are likely to be the cause of one of the strangest features of Mars: the southern hemisphere is, on average, around 5 km higher than the flat, northern hemisphere. The underlying crust in the south is also about 20 km thicker.
The most ancient rocks that survive on Mars are about 4.1-3.7 billion years old and are located in the planet’s southern hemisphere. These regions are scarred by numerous craters, telling a story of heavy bombardment by asteroids. However these terrains also show evidence that liquid water may have, at times, flowed across the martian surface and formed river valleys, deltas and lakes.
Between 3.7 and 2.9 billion years ago, volcanic activity dominated. The largest volcano and canyon in the Solar System were formed on Mars during this period. By 3 billion years ago, the water on Mars had largely dried up.
The changes on Mars over the last 2.9 billion years have been much less dramatic, with the most distinctive features sculpted by the movement of wind or ice. For the majority of this period, Mars has been cold and dry with evidence of only occasional small floods.
Every mission to Mars provides new knowledge and insights into the geological history of Mars, as well as raising new questions that inform the direction of future missions and research.
Deep Dive into the Geological History of Mars
Since the 1960s, scientists have sent robotic spacecraft to study Mars and understand more about the processes that have shaped the Red Planet. Flyby missions, orbiters, landers and rovers have sent back a huge volume of data that has shed light on the geological evolution of Mars since its formation around 4.5 billion years ago.
The geological history of Mars is divided into three main periods that are named after regions on Mars that were formed at these times:
• Noachian
• Hesperian
• Amazonian.
The age of features on the surface of Mars can be dated by establishing their relationship to other features and looking at the number of impact craters that have scarred them over time. The study of the size, number and levels of erosion of craters on a surface is an important tool for scientists to work out the age of planetary objects.
Pre-Noachian (4.5 – 4.1 billion years ago)
The first 400 million years or so after the formation of Mars is known as the Pre-Noachian period. Little is known about the first crust that formed on Mars, as no rocks from this period survive on Mars today. Evidence the accretion and differentiation of Mars happened early in the Solar System’s history has come from the analysis of martian meteorites, which have preserved and carried ancient material from Mars to Earth. The ALH 84001 meteorite is a 4.1-billion-year-old Mars rock that was formed through volcanic processes. The meteorites NWA 11220 and NWA 7034 (Black Beauty) contain fragments of rocks dating back 4.5 billion years to the formation of the first martian crust. These meteorites provide evidence that the accretion and differentiation of Mars happened early in the Solar System’s history.
Magnetic anomalies detected in localised areas of the martian crust by NASA’s Mars Global Surveyor mission suggest that the early Mars had a magnetic field. During the Pre-Noachian period, Mars is also likely to have experienced a very high rate of impacts with other objects in the Solar System. Large impacts are thought to have caused a striking difference between the northern and southern hemispheres of Mars. This ‘global dichotomy’ means that there is a surface elevation difference of approximately 5km and crustal thickness difference of approximately 20 km between the northern and southern hemispheres of Mars.
Noachian (4.1 – 3.7 billion years ago)
The Noachian period is named after the heavily-cratered Noachis region (Land of Noah) on Mars. During this period, Mars experienced heavy bombardment by asteroids and comets, which created the Hellas, Isidis, and Argyre basins – some of the largest impact structures still visible on Mars today.
Large-scale volcanic activity formed the ‘Tharsis bulge’, an elevated region in the western hemisphere of Mars that is approximately 5000 km wide and around 9 km high.
The Noachian period was also characterised by high rates of erosion and valley formation. Scientists believe that terrains dating back to the Noachian period contain the best evidence that Mars was once warmer and wetter.
This evidence includes:
• Numerous valley networks
• River deltas
• ‘Open’ basin lakes that drained into rivers or other bodies of water.
Minerals such as phyllosilicates indicate that water-related alteration of martian rocks occurred. Similarly, the existence of chlorine-rich deposits, formed through evaporation, indicate the presence of water. Mars may have even hosted oceans during the Noachian period.
View the Noachis Terra in the Mars 36 Atlas.
Hesperian (3.7 – 2.9 billion years ago)
As Mars entered the Hesperian period (named after the Hesperia Planum region of Mars), the rate of meteor impacts dropped significantly. Volcanism continued, which resulted in the resurfacing of about 30% of Mars. It is likely that Olympus Mons, the largest volcano in the Solar System at over 40 km high, started to form during the Hesperian period. Canyons began to form due to the stress that the bulging Tharsis region placed on the martian crust. The biggest of these, the Valles Marineris system of canyons, is about ten times the size of Earth’s Grand Canyon.
Huge quantities of sulphur dioxide, released by the volcanic activity, would have made any water more acidic. This led to changes in the surface geochemistry, with extensive sulphate-rich deposits forming in the Hesperian period. Evidence for water on the surface of Mars during the Hesperian period is episodic. Flood channels were carved out by massive, sudden outflows of groundwater or sub-surface ice.
View the Hesperia Planum in the Mars 36 Atlas.
Amazonian (2.9 billion years ago – present)
Although more than half the Red Planet’s history is covered by the Amazonian period (named for the smooth Amazonis Planitia region), the surface of Mars hasn’t changed much during this time.
Geological activity slowed during the Amazonian. The rates of volcanism, impacts, weathering and erosion all reduced. For the majority of the period, Mars has been cold and dry, with evidence of only occasional small floods. The dominant processes shaping the landscape are caused by wind and ice. The effects of wind can be seen by the presence of dunes. Warming and cooling caused by periodic changes in the axial tilt of Mars have created layered deposits of dusty water-ice in polar regions. Evidence of ice can also be found at high latitudes, and in glacial deposits on the flanks of volcanoes at the martian equator.
View the Amazonis Planitia in the Mars 36 Atlas.
References
https://sci.esa.int/web/mars-express/-/55481-the-ages-of-mars
In Depth | Mars – NASA Solar System Exploration
Carr, M. H. and Head, J. W. 2010. Geologic History of Mars. Earth and Planetary Science Letters, 294, pp.185-203.
https://home.csulb.edu/~rodrigue/geog441541/lectures/final/3rdhesperian.html
