Structure of Mars

Scientists are still debating the reasons why Mars appears to not experience plate tectonic processes.

Is liquid water at the surface an essential component of plate tectonics?

Did Mars simply cool too quickly due to its higher surface area–volume ratio?

Plate tectonics and Mars

Plate tectonics governs the nature and shape of the surface of the Earth, from ocean basins to mountain ranges. It also governs the motions of the surface of Earth, providing a range of natural hazards such as earthquakes and volcanic eruptions. Plate tectonics is the main mechanism through which Earth loses its heat.

However, plate tectonics only occurs on Earth. This is puzzling. Why does plate tectonics only occur on Earth? In the absence of plate tectonics, how do other terrestrial planets lose their heat? These are major questions in Earth and planetary sciences research.

This section provides a brief introduction to the ways in which terrestrial planets lose their heat, comparing Earth and Mars, and discusses our current understanding of plate tectonics.

The structure of terrestrial planets

Earth and Mars can both be thought of as a series of approximately spherical layers, defined either chemically or mechanically. For example, starting at the centre and working outwards, Earth is chemically composed of an inner core, outer core, mantle, and crust; it is mechanically composed of an inner core, outer core, lower mantle, upper mantle, asthenosphere and lithosphere.

The lithosphere is composed of the crust and the rigid uppermost part of the mantle, and is the ‘plate’ of plate tectonics. Although it is also solid, in contrast to the rigid lithosphere, the underlying asthenosphere is plastic (i.e. it can flow on geological timescales).

We now know that the lithosphere and asthenosphere behave relatively independently, in contrast to the original idea that the motion of the tectonic plates was controlled by motion in the asthenosphere.

How do terrestrial planets lose their heat?

Terrestrial planets such as Earth and Mars are generally thought to have been initially hot, and gradually cooling, with many planetary processes (e.g. volcanism and tectonism) being driven by this cooling. The sources of heat within these planetary bodies can be categorised as either primordial (i.e. inherited from processes occurring during formation) or the result of radioactive decay.

Heat is transferred within planetary bodies and eventually lost to space through a combination of convection, conduction and radiation.

Different methods of heat loss dominate in the different layers of planetary bodies, and at the boundaries between these layers. For example, it is estimated that every year Earth loses 4.2 x 1013 W, or 42 TW, of heat: 32 TW conducted through the lithosphere, and up to 10 TW lost by, for example, hydrothermal activity at mid-ocean ridges [1].

There are three main modes of planetary cooling:

  • magma ocean
  • stagnant lid
  • plate tectonics

Regardless of the mode of planetary cooling, all planets lose heat from the surface to some degree via radiation. All terrestrial planets are thought to undergo a short-lived magma ocean stage early in their evolution.

When a body has cooled sufficiently, the surface solidifies and the common mode of heat loss is stagnant lid behaviour, where heat loss from the surface is primarily through conduction. Alternatively to stagnant lid behaviour, if the conditions are appropriate, a terrestrial planet may begin to lose heat via plate tectonics.

It is theoretically possible that a terrestrial planet may alternate between a stagnant lid regime and a plate tectonics regime; this has never been observed, but the lack of observation may simply be a reflection of the long timescales involved. Ultimately, when they have become sufficiently cool, the fate of all terrestrial bodies is to continue to cool by conduction alone; they may then be considered to be inactive or dead (i.e. lacking in any force to drive planetary processes such as volcanism and tectonism).

What are the conditions necessary for plate tectonics?

This question may be thought of as a Goldilocks problem: everything needs to be just right.

  • Firstly, the planetary body in question must have cooled sufficiently so that it is too cold to sustain a magma ocean.
  • Secondly, there needs to be sufficient heat within the interior of the body to prevent the existence of a stagnant lid, i.e. sufficient heat to maintain convection within the upper layers of the body. Thirdly, the lithosphere needs to be cool enough, dense enough, strong enough and thin enough to subduct.
  • Finally, probably the most important ingredient for successful plate tectonics is liquid water, which is readily available only on Earth, not on the other terrestrial bodies.

This too is a Goldilocks problem: the Earth may be at just the right distance from the Sun to have a surface temperature between 0 and 100°C, and therefore be a stable environment for liquid water.

So far, all of the necessary conditions for plate tectonics have only been found together on Earth.

Is there any evidence for plate tectonics on other planets?

There is no conclusive evidence for plate tectonics on any other planets [2]. Mars is considerably smaller than Earth, but it does have water (mostly in the form of ice). Some surface features on Mars have been interpreted as indicating the possibility of plate tectonics operating there in the past. For example, it has been suggested that magnetic patterns observed by the Mars Global Surveyor spacecraft may indicate that a process similar to plate tectonics may have operated on Mars in the past.

However, other surface features have been interpreted as indicating that plate tectonics has not operated on Mars. For example, it has been suggested that the enormous size of volcanoes such as Olympus Mons may indicate that the Martian crust has remained stationary over the magma source for a protracted period of time, whereas on Earth the movement of tectonic plates over magma sources results in linear tracks of relatively small volcanoes on the surface (e.g. the chain of Hawaiian islands).

There remains no evidence for coherent planet-wide plate tectonics at any time in the history of Mars.

Are plate tectonics, continents and life related?

There are many puzzling things about plate tectonics that we are only just beginning to address. For example, Earth is unique in that it has plate tectonics, but also in that it has continents, and in that it has life. Are these issues related?

There is no clear consensus on these issues, as we do not yet fully understand how continental crust is formed. We don’t know whether it would be possible to have a world with plate tectonics, but no continents, or conversely a world with continents but no plate tectonics.

The relation between plate tectonics and life is even more speculative, and this is currently discussed as a chicken and egg problem: do we need plate tectonics in order for there to be life on Earth, or do we need life in order for there to be plate tectonics on Earth? Of course, although our attempts to address this puzzle are more speculative, this puzzle is also very exciting!

How and when did plate tectonics start on Earth? This is the question that we are most likely to be able to answer in the near future. We plan to use data from missions like InSight to understand how Mars loses its heat, and why plate tectonics does not occur on Mars.

If we can understand why plate tectonics doesn’t occur on Mars, it will help us to figure out how plate tectonics started on Earth. Will we ever find another planet that does have plate tectonics, or is Earth not just unique within the Solar System, but also within the wider universe?

If you want to find out the answers to these kinds of questions, become a geophysicist!

Download Plate tectonics and Mars

Transition zone and mantle mineral structure

Earth and Mars have similar bulk compositions (the same as primitive meteorites in the solar system). However as they cooled from homogeneous molten spheres they underwent a process called ‘differentiation’, whereby different elements settled out at different rates, with the denser metallic components (mostly iron and nickel) forming a core and the lighter silicate components creating a mantle and crust.

Within the mantle the composition is mostly made up of minerals containing oxygen, magneswium, silicon, iron, calcium and aluminium. These elements combine to form different minerals and rock types. However the same mineral can exist as different physical structures depending on how the atoms are packed together. These different structural arrangements of the same mineral are called ‘polymorphs’ or ‘phases’.

One very well-known material that experiences polymorphism is carbon, which can exist as graphite or diamond. The minerals within the mantle can exist as several different polymorphs depending on the pressures and temperatures at which they are formed.

In the Earth, this gives rise to a ‘Transition Zone’ in the mantle between about 410 km and 1000 km deep where the mineral olivine (forming peridotite) transforms into more closely packed (and denser) minerals with a perovskite-type structure.

These phase change boundaries can be detected as changes in seismic velocity. It is expected that on Mars these phase changes in the mantle will occur much closer to the core, being able to detect these with seismic data is one of the mission objectives.

Download Transition zone and mantle mineral structure

What can seismic waves tell us about a planet’s structure?

Like all waves, seismic waves can undergo reflection and refraction. The degree to which a seismic wave is refracted, or the position at which one is reflected allows us to build up a picture of the density and structure of material at various points inside a planet. Using the Earth as an example, we can see how this is achieved.

Refraction of P-waves

P-waves can travel through the Earth’s mantle, which acts like a solid for short time periods, and also the liquid core. As pressure increases with depth in the mantle, so the velocity that P-waves travel increases leading to seismic waves being continuously refracted into curved ray paths.

At sharp discontinuities like the core–mantle boundary, where the velocity of P waves suddenly decreases in the liquid core, P-waves are strongly refracted leading to a region of the Earth’s surface which does not see directly arriving P-waves — the P-wave shadow zone running from 103º–142º distance from the event.

S-waves can only travel through solid materials like the Earth’s crust and mantle. This leads to a shadow zone at distances greater than 103º from an earthquake where recording sites do not see directly arriving S-waves. In fact, the size and composition of the Earth’s liquid core were originally discovered by analysing the location of this shadow zone.

Contact

Contact Paul Denton, MarsQuake project leader, for further information.