The world is getting smaller every day

For the smallest of the planets, Mercury is a surprisingly active place with a complex internal structure and magnetic field, however, it’s tectonics is dominated by its size.

Smaller bodies lose heat a lot faster than bigger bodies, due to a higher ratio of surface area to volume, this means that Mercury has cooled relatively rapidly. As the planet cooled it has shrunk. Estimates for this contraction of its radius range from  ~2 – 7 km over its lifetime, whilst this is only 0.2% of its total radius, this equates to its diameter shrinking   44 km at its equator.

Mercury’s surface is a single tectonic plate, it doesn’t have the subduction and rifting zones which can accommodate strain. As the planet has cooled and contracted the crust at the surface has become compressed as the larger diameter crust is pulled in to fit into a smaller area. Rocks when under compression buckle and eventually break and form faults. In the case of Mercury, shallow angle faults known as thrust faults are created. As one part of the crust rides over another it forms cliffs (called “Rupes”) which can be 100’s of km long and several hundred meters high.


Carnegie Rupes, 2 km high wall running diagonally across the image created by a thrust fault (NASA/Johns Hopkins University Applied Physis Laboratory/Carnegie Institution of Washington)

Where multiple faults interact they can produce complex structures. The Great Valley is 1000 km long valley formed from thrust faults which make up either side and has been bent downwards in the middle.


The Great Valley (in Blue) close to Rembrandt basin on Mercury, (Nasa/JHUAPL/CIW/DLR/SI)

Smaller scale scarps have also been identified, whilst smaller (10’s of km long and 10’s of meters high) they are often found cutting across small impact craters and smooth younger planes which means that these features are less than 50 million years old and suggests that Mercury is still tectonically active now.



Smaller scarpes from recent tectonic activity, (NASA/JHUAPL/ Carnegie Institution of Washington/Smithsonian Institution


Whilst there are other features such as some structures on its surface which are linked to thicker patches of crust probably linked to mantle structures Mercury is dominated by the side effects of being a small cooling planet which is still slowly shrinking.

The Anti-Crust

Most people know that the Earth can be divided up into layers, starting with the crust – a skin of hard rock on the surface. Beneath this crust is the mantle; silicate rock which, although solid, moves very slowly over time; convecting and moving heat up to the surface. Beneath this mantle is the core. The Earth’s core is comprised of two parts. The outer core is liquid metal which is spinning away generating the magnetic field for the Earth. The innermost layer is the inner core. A solid lump of iron and nickel.


Mercury, the smallest of the rocky planets (It’s smaller than the moons Titan and Ganymede) but is very dense (denser than both Venus and Mars even though they are bigger). Whilst Mercury is not as dense as the Earth it is much smaller so it doesn’t compress the material inside as much, its high density is best explained by a much bigger iron core and a smaller silicate mantle/crust compared to the Earth.  Mercury is also the only other rocky planet other than Earth to have a strong magnetic field, which tells us about the internal composition of the core.


Comparison of Earth Mercury sizes (Earth image: Nasa, Mercury: NASA – APL)

The magnetic and gravity data from Nasa’s MESSANGER probe suggests that it has a crust of about 50 km thick, beneath which there is a mantle, and beneath that we know there is a  core at the centre, as shown by the strong magnetic field and the high density.

This core, like Earth, will likely have a solid inner part and then a liquid outer part, made of iron & nickel.  As the core cools it solidifies in the middle and iron and nickel is removed from the liquid part, causing a relative increase in the other elements such as sulphur, silica, and oxygen left in the liquid. Depending on the composition this mix of different elements may become like oil and water. A sulphur – iron mix separates out from the rest. This sulphur mix is less dense than the rest and floats up to the core-mantle boundary.

The iron-sulphur mix solidifies and forms a hard layer on the underside of the mantle – this is the anti-crust, a thick layer of iron sulphide separating the liquid outer core from the silica-rich mantle.


Gravity studies of the planet made by looking at small changes in the MESSENGER spacecraft as it orbited suggest that this layer may be present, however, there are a lot of variables to the models used to estimate this. Whilst on Earth seismic studies could be used to verify these, there are no plans to land a probe on Mercury anytime soon to undertake such tests, slight changes in the amounts of Sulpher and other elements could have prevented the formation of the anti-crust.

Whilst the smallest planet in our solar system Mercury has many similarities to that of Earth, the anti-crust is an extra layer and an interesting artefact from cooling worlds.



Into the Exosphere!

Last time I talked about the different ways rocks breakdown in the space environment. This time we will look at what happens to some of these products of the erosion and weathering of the materials go.

Whilst many of the smaller bodies in the solar system don’t have an atmosphere (which would stop most space weathering), there is still atoms/ions/dust particles floating around above the surface. Whilst this material is too spread out to interact with each other in the way that a gas does in an atmosphere, these materials are still trapped, bound by the gravity of the body – this is called the exosphere.

Unlike other bodies, these exospheres are predominantly comprised of material generated from space-weathering; sputtering releasing atoms, and melting which releases volatile molecules. Some are released from surfaces by heating at sunrise. As the solar wind of charged particles moves over a body, its surface can become charged, dust particles gain a similar charge and so are repelled floating above the surface. The amount of dust is linked to the number of micrometeorite impacts, the intensity of the solar wind, and any magnetic fields. This dust can refract light; giving the appearance of a diffuse sunrise which would otherwise not be possible on an airless world.


Lunar Horizon Glow on the Moon, generated by dust in its exosphere, taken by Nasa’s Clementine mission (NASA)

For rocker bodies, such as Mercury and the Moon atoms of  Ca, Mg, & K have been detected. For some of the other bodies in the solar system, beyond the frost line – water ice covers many of the bodies like Europa and Enceladus, when the surfaces are broken down by radiation, the hydrogen escapes, leaving oxygen and hydroxide around them.


The Calcium and Magnesium tails of Mercury as taken by MESSANGER (NASA/Johns Hopkins/ Carnegie institution

These materials don’t stay in the exosphere forever, s0me can be reabsorbed by the surface of the planet, often on the night side of a planet, and the dust and atoms can settle down. Atoms can become charged by sunlight (photo-ionisation), or interaction with charged particles in the solar wind. These ions are caught by solar winds or by magnetic fields and expelled out into interplanetary space or fired at high speed into the ground close to the poles. The force of sunlight hitting atoms or the rare collisions between atoms can slowly add momentum until some material eventually gain enough velocity and can escape the exosphere.

Weathering without any weather

I’ve been reading up on weathering on airless bodies (those without atmospheres) at the moment and so thought I would write a brief overview of the processes which break down rocks in space.

On Earth when we think about weathering as the breakdown or rocks/minerals/soils in situ by physical actions (such as wind, temperature changes, wave action, and biological action) or by chemical action (dissolution, or oxidation like rust).

For many of those physical and chemical actions an atmosphere is needed (life and liquids can’t survive long without an atmosphere) without one weathering occurs by different processes – collectively known as space weathering.

Space weathering is caused by the flow of energetic particles, rays, and fragments of rocks and comets bombarding the surfaces or objects as well as extreme temperature changes. Normal atmospheres do a lot to reduce the impact these influences, but without air to slow them down they impact the surfaces of these bodies and break them down.


Airless world Mercury is a large body subject to space weathering (NASA/JHUAPL/CIW)

One of the key weathering mechanisms is micrometeorite impacts, these are small (<1 cm) fragments of asteroids and comets, which collide with surfaces and smash them up, forming a layer of broken rock fragments, called regolith, which varies in thickness and size based on the underlying rock types, the length of time a surface is exposed, and the size of a body. The bigger a body is the stronger the gravitational force and the greater the acceleration of particles hitting them and so the higher energy the impact. Impacts also mix-up the regolith bringing material at depth to the surface where it is more exposed to radiation, this mixing progress is called gardening.

These high speed impacts generate a lot of heat which can melt some of the rocks and cause loss of volatile components as well as cause the formation of glasses. The impactors make up a significant amount of material in the regolith, with between 5-20% of the regolith on Mercury estimated to be made from macrometeorites which have impacted the planet.

The other main source of space weathering is radiation, cosmic rays, solar particles, and sunlight, bombarding the unprotected surfaces. Ions caught in solar winds can be implanted into crystal latices or the impact can be of such high energy that it knocks out lighter elements from rocks (a process known as sputtering). Differential heating caused by the day and night cycles can cause fracturing and lighter elements can be lost through heating, in a process called thermal desorption, a key process and one of the main mechanisms behind comments getting their tail.


Space weathering and erosion in action on a comet (K. Jobse / P. Jenniskens /NASA Ames

These process add up together to cause the reduction of size, and the loss of volatile elements, chemically this leads to reduction which causes the formation of microscopic iron fragments in the regolith and on the edges of glass fragments.

All these minor alterations mean that the surfaces of bodies can be of a different composition to the underlying rocks and this must be taken into account when analysing satellite data to work out what a body is comprised off. Next time we’ll have a look at where some of the eroded products end-up: The Exosphere, but for now if you are interested in this, more detailed overview can be found here.


Tis’ the reason for the season

Happy Solstice everyone! Today in the northern hemisphere is the shortest day (longest in the southern hemisphere).


If you imagine a stick through the Earth around which the planet spins on a daily basis this is the Earth’s axis. If this axis was vertical, then the lengths of day would not change, throughout the year, however it actually lies out at an angle of  23 degrees from the vertical. At different points in its orbit the north will be pointing either in the direction of the sun or further away. Today the northern hemisphere is pointing directly away from the sun. This means that in the northern hemisphere the days are much shorter and the nights longer due to spending more time facing away from the Sun than towards it. In addition due to the curvature of the Earth the beams of light hitting the surface is more spread out towards the poles than the equator and so the amount of incoming energy spreads out, these two mean that the climate gets colder during the winter. What about other planets, are there seasons and how are they manifested?

Nether Jupiter or Mercury have seasons due to there low angles of axial tilt, Mercury has the smallest axial tilt of any planet, an upshot of this is that there are many craters on the poles which are in constant shade, this allows for permanent water ice on the surfaces of the closest planet to the Sun.


The north pole of Mercury, some of the craters are permanently in shadow (NASA/ John Hopkins/Carnegie Institution)

There are huge temperature variations on Mercury related to the eccentricity of its orbit (how elliptical it is rather than circular) linked with a 3:2 ratio of years to days but these do not cause temperature changes in latitude.

Venus has a tilt of 177 degrees, what this means is that it is completely flipped over when you look at its rotation (it spins clockwise whilst the other planets spin anticlockwise)

That being said it means that the axis is only about 3 degrees off the vertical and with the very efficient heat transport in the dense atmosphere the temperature is fairly constant over the whole globe and that Venus doesn’t have a strong seasonal changes.

With a similar axial tilt to Earth (25 degrees) Mars also has seasons, which are about twice as long as on Earth (due to the longer year). This leads to growing and shrinking of carbon dioxide ice caps and temperature changes just as on Earth. Intriguingly images from the Mars reconnaissance orbiter have shown linear features called recurring slope lineae forming on crater edges, these features grow during the warmest months then disappear during the coolest month. They are thought to be  formed from brines (very salty water) which melt and run down the slope, they do not appear in the winters due to it being too cold for these brines to melt, although the source of the water is not currently certain.


Water flows at Newton Crater, Mars (NAS/JPL/Univ. Arizona)

One other seasonal feature of mars is dust storms , which are known to cover the whole planet at times which most commonly occur during the spring and summer.


Martian dust storm (Nasa/JPL-Caltech/MSSS)


Saturn has seasons which last around 7 Earth years, changes in cloud composition and occurs during this transition and there are increased storms during spring

Uranus is lying on it side, meaning that the axial tilt is just of the equator which means that the poles experience 42 years of day light followed by 42 of darkness, the change in temperature between the side facing the sun and the side facing away the sun probably has an effect on its climate however as it has only been briefly visited by the Voyager 2 probe little is known about the long term seasonality ice giant.

Finally Neptune has a similar axial tilt to Earth of 28 degrees, at the moment a lack of observational evidence makes it difficult to say if it has any strong seasonal effects although an increase in cloud cover has been noticed by Hubble as it transitions into a 40 year long summer.

Happy Holidays!