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.