How Mars Lost Its Atmosphere
Though it doesn’t look like a nice place to live now, Mars may have had an atmosphere more like ours on Earth! But how did it lose it? One way a planet can lose its atmosphere is through a process called ‘sputtering.’ In this process, atoms are knocked away from the atmosphere due to impacts from energetic particles. Learn more in this video!
Mars is the fourth planet from the Sun and the second smallest planet in the Solar System, it comes after Mercury, Venus and Earth. It used to be as pretty as the Earth until it underwent sputtering (sputtering is a form of space weathering, a process that changes the physical and chemical properties of airless bodies, such as asteroids and the moon. This happens as atoms are practically knocked away from the atmosphere as a result of impact from energetic particles) and then changed and became as red as it is now hence making it un-inhabitable.
Several scientists believe that there was a time long ago that the Martian atmosphere was able to support liquid water on the surface because it was denser then. This suggests that Mars was wetter, warmer and had a thicker atmosphere than today. Before 4.4 billion years ago, strong hydrodynamic escape, heavy bombardment of meteorites and outgassing have been used to describe Mars atmospheric evolution (Brain and Jakosky, 1994). The heavy bombardment have lasted until 3.7 billion years ago ending about the same time that the magnetic field of Mars turned off (Acuña et al., 1999, 1998; Carr, 1999).
It is known that impact bombardment and early hydrodynamic escape is estimated to have caused a loss of about 90% of the primitive Martian volatiles. However, impact surface features show a strong degradation due to erosional processes (Jakosky and Jones, 1997). This suggests that water was still very much present even after the end of the bombardment period. Also after the period of the heavy bombardment, a 1-0.5 bar thick atmosphere was still present. It should be noted that one of the main factors that could explain the present much thinner 5mb atmosphere is the sputtering of the atmosphere by pick-up ions although photochemical escape and carbonate recycling could also contribute.
Work by some Scientists have al so shown that 85-90% of Argon, Carbon, Hydrogen and Nitrogen may have been lost from the atmosphere by sputtering. On the dayside, it is observed that solar extreme ultra violet and photo-electrons ionize a fraction of the neutral atmosphere. This newly ionized particles are picked-up by the partially deflected field field frozen in the solar wind this occurs because of the absence of a large permanent magnetic field. It is also known than Mars is slightly smaller than the Earth and therefore it has a weaker gravitational field; gravitational field holds an atmosphere in place. When there is weak gravitational field, an atmosphere will gradually leak away into outer space and this the case with the planet Mars. Also, excess kinetic energy from solar winds has such energy that when impacted on atmospheric particles allow them to reach escape velocity hence atmospheric escape occurs.
With all these facts, it can be deduced that the following may be some of the most likely reasons why Mars lost its atmospheric temperature. Mars lost its atmosphere due to atmospheric escape which is a loss of atmospheric gases to space since they have a weaker gravitational field than Earth.
Let us take some of these thoughts in detail. Although the processes that led to the loss of the Martian atmosphere is still disputed just like the Evolution theory, we can still put out the most likely causes of the loss of the Martian atmosphere.
- Solar-wind-induced sputtering
- Impact erosion
- Sequestration into the regolith and polar caps
- Photodissociative reactions.
It has been proven that not a single process has been shown to be capable of removing the entire Martian atmosphere, therefore, several processes worked together hand-in-hand to bring about the present state of the Martian atmosphere. Considering the present Mars it can be observed that the display of valley networks suggest that liquid water was once present and it can also be observed that erosion rates 3.5-4 billion years ago are much higher than they are today.
Presently the current atmospheric reservoir of Carbon (IV) Oxide gas is about 7 mbar, but 1-5 bar of the gas are needed in order to raise the surface temperature to the melting point of water (Haberle, 1998).
The indication that the Martian climate changed abruptly was the sudden decline in erosion rates 3.5-4 billion years ago. The main processes that are known to be involved in the removal of the atmosphere is the ejection of atmospheric gas by impacts, loss to space due to pick-up-ion sputtering, and sequestration into regolith or polar caps. This is not all, nitrogen is eliminated from the atmosphere by photochemical reactions.
Since impact erosion efficiency depends on planet mass this explains why impact erosion is inefficient on Earth and Venus but efficient on Mars. A small impactor is able to remove a mass of the atmosphere that is roughly equal to the mass intercepted by the impactor as it travels through the atmosphere. On the other hand, a large impactor can do greater damage and thus eject more than just the mass of the atmosphere it intercepts. It is capable of removing part of the atmosphere by creating an outwardly expanding plume.
Practically all planets are hit by solar winds(these are “bubbles” of protons and helium ions expelled from the sun). Earth has a magnetic field that deflects the solar wind around and past the planet. Mars no longer have a magnetic field although it once had one. This absence of magnetic field is as a result of the planet’s lack of plate tectonics — Plate tectonics is the concept that the Earth’s crust is composed of rigid plates that move over a less rigid interior. In this case, it can also be termed as the circulation of molten iron within a planet’s core, that give rise to a magnetic field.
As expected, Mars grew more prone to sputtering as its magnetic field disappeared. Sputtering is a process that occurs in the presence of light. In this process, the solar wind ionizes the neutral atoms in the upper atmosphere and are then accelerated by the magnetic field of the solar wind. The upper atmosphere atoms are ejected into space after they collide elastically with the high velocity ions.
Everyone knows about the effect of weathering or at least have heard of it. Weathering can deplete Carbon (IV) Oxide gas by converting it into carbonate rock. This is also the reason why through an abiotic process, the Martian Carbon (IV) Oxide gas could also have been easily converted into carbonates.
Though most of atmospheric gases in Mars are removed through the processes or mechanisms outlined above, however, Nitrogen is an exception because it is subject to photochemical reactions. These reactions could include photodissociation, photodissociative ionization, ion-molecule reactions, electron impact dissociative ionization and dissociative recombination. Photodissociation is the dissociation of a chemical compound by the action of light (in this case Nitrogen gas in Mars).
In Conclusion, Mars lost its atmosphere not just by one process but by the hand-in-hand workings of other processes. Research work is currently going on for this topic and we are yet to get new discoveries from NASA.