Olympus Mons: how Mars built a giant volcano

Olympus Mons owes its immense size to the gradual construction of a shield volcano. Its measurements, caldera and frost observed by probes reveal three distinct aspects of this Martian feature.

On Mars, Olympus Mons does not look like a mountain rising to a peak. It is a volcano so vast that its slopes stretch for hundreds of kilometres, making its summit difficult to distinguish from the ground. Its size is due to repeated lava flows and the planet’s geological history, but its altitude figure requires a caveat: it is not measured in the same way as Everest’s.

A volcano wider than it is high

Olympus Mons, Mount Olympus, is a shield volcano. This type of formation is built by the accumulation of relatively fluid lava flows. Rather than forming a narrow cone, the lava spreads out and gradually builds a broad feature with gentle slopes. Its shape therefore tells a story of a process: successive eruptions deposit layers that build up on top of one another.

The European Space Agency describes a volcano rising about 22 kilometres above the surrounding plains. Its base stretches to around 600 kilometres in diameter, according to Futura. These figures describe two different dimensions: height above the neighbouring terrain and horizontal extent. The common comparison with Everest gives an idea of the scale, but the measurement reference points need to be specified.

On Earth, Everest’s height is generally expressed above sea level. Mars has no ocean whose surface could provide a comparable zero point. Martian altitudes are based on a reference defined in relation to the planet itself. And the local height of a volcano, measured from the terrain around it, is yet another comparison. A feature can therefore be ‘the highest’ according to one measurement and less exceptional according to another.

Flows building up over time

To understand the scale of Olympus Mons, we need to consider its duration as well as its height. A single flow is not enough to build a feature of this size: it is the accumulation of volcanic material that gradually transforms the surface. The gentle slopes are consistent with construction by lava capable of spreading far from its source.

Images from Mars Express show overlapping flows and lava tubes whose roofs have sometimes collapsed. These relationships between landforms make it possible to establish a relative sequence. A flow that lies over another is more recent, even when the image alone does not allow it to be assigned a precise age.

At the summit lies a vast depression called a caldera. It is not an impact crater: it is associated with the volcano’s history, particularly the collapse of terrain at the summit. Images from the CaSSIS instrument, aboard the European ExoMars Trace Gas Orbiter, also show landforms within the depression, including terraced structures and ridges. The caldera thus offers a reading of the surface that complements the volcano’s overall scale.

Equatorial frost: a precise observation

The volcano’s size is not the only question that probes can help investigate. In 2024, teams working with data from the Mars Express and ExoMars missions reported the presence of water frost on Olympus Mons and on other volcanoes in the Tharsis region. The observation is remarkable because these sites are near the Martian equator, where frost was considered unlikely.

The image published by the European Space Agency shows frost in the caldera and near its northern rim. It does not show the entire mountain uniformly: some strongly illuminated slopes are frost-free in the area depicted. This contrast is a reminder that the presence of ice depends on local conditions, and that an observation of part of the volcano does not automatically describe the whole formation.

The colours in this image are not what the human eye would see. CaSSIS also captures near-infrared light, which is invisible to our eyes, and the processing enhances the contrast to bring out certain details. Blue here indicates frost in a false-colour image. This is a way of making a measurement legible, not a photograph in natural colours.

This observation concerns water frost on the surface. On its own, it does not demonstrate the existence of an easily accessible reserve or a resource that could be exploited for future missions. It establishes that frost can be present in certain areas near the Martian equator.

What the images allow us to distinguish

The orbiters do more than photograph a spectacular landscape. Their instruments collect information at different wavelengths, which scientists use to identify materials and structures. A false-colour image, an instrument reading and a geological interpretation are related but distinct steps. The image helps identify a phenomenon; its analysis helps determine what it may reveal.

Olympus Mons thus brings together two scales of analysis. On a planetary scale, its dimensions show how far a Martian volcano can extend. On a local scale, the landforms in the caldera and the frost observed in certain areas invite us to study the precise conditions at its surface. The giant cannot be reduced either to a figure compared with Everest or to a blue image: its geology is understood by keeping measurement, shape and interpretation separate.