Jupiter and Saturn held on to remnants of the solar nebula that created them. Venus experienced a runaway effect due to heating of the planet as more greenhouse gases were released, further increasing the temperature. Earth’s atmosphere is likely a combination of volcanic activity and impacts from comets and wet asteroids that shaped our current atmosphere. However, our red neighbor has almost no atmosphere. Curiously the Martian surface is filled with earth-like formations. Valley networks, hydrated mineral clays, carbonates, and the remnants of deltas and rivers. These formations are the ghost of a wet planet. Mars used to have plenty of water, but now it is a frigid and barren wasteland. Where has the atmosphere gone?

Mars

What is the atmosphere?

An atmosphere around a planetary body is the collection of gases surrounding it. The gases are trapped by the gravitational field of the planetary body. When an atmosphere forms and has a specific composition it allows life to be sustainable. On planet Earth the oxygen (21% at surface level) allows us to breathe. Our atmosphere protects us from the sun’s radiation. Lastly, it keeps a steady temperature by warming during the day and cooling during the night.

The composition of the atmosphere has a large influence on the temperature of a planetary body.
Greenhouse gases are molecules that vibrate as they absorb heat. These vibrating molecules can subsequently release the energy as radiation, which is likely to be absorbed again by another greenhouse molecule. This keeps the heat in close proximity to the planetary body. The most abundant greenhouse gases in earth’s atmosphere are water vapor (H₂0), carbon dioxide (CO₂), and methane (CH₄).

We know that Mars used to have an atmosphere from analysis of the surface structure. To get an idea of why Mars lost its atmosphere we’ll have to understand how an atmosphere can be lost. There are two balancing forces that can change the density and composition of an atmosphere. There is the gravitational force as a function of the mass of the planetary object and the kinetic energy of the particles in the atmosphere. If the kinetic energy overcomes the gravitation pull of the planetary body than the particle can escape the sphere of gravitational influence. When the particle has done this is has reached escape velocity.

Attractive force F. G is the gravitational constant, m₁ and m₂ are the mass of two interacting objects, and r is the distance between the two objects.

Escape Mechanisms

Below is a quick overview of the escape mechanisms particles in the atmosphere have to escape the sphere of gravitational influence. These escape mechanisms can be divided into thermal an non-thermal influences .

Thermal Escape Mechanisms

Reaching enough kinetic energy so that the velocity of the particles is above the escape velocity has everything to do with the temperature of the gas. A higher gas temperature results in a higher average velocity for the particles of the gas. The particle velocities can be described by a Maxwell distribution. A higher temperature will cause more particles in the high tail of the distribution to reach a sufficiently high velocity. Their escape is then only dependent on the availability of a free path outward.

The kinetic energy E_kin as a function of mass m and velocity v.

Non-Thermal Escape Mechanisms

A variety of non-thermal escape mechanisms exist.

Heavy and light gases

Heavier gases tend to have a lower average velocity. If the temperature is constant than for a heavier gas there will be fewer molecules able to reach escape velocity compared to a lighter gas. This is why the giants in our solar system are able to have atmospheres with the lightest gasses hydrogen and helium, while smaller planets like ours has almost no hydrogen or helium gasses.

Sequestration

Sequestration is a second way that particles can escape from the atmosphere. Instead of having sufficient kinetic energy to be projected from the sphere of gravitational influence the particles are absorbed onto the planet. Phenomena that cause the loss of atmosphere pressure are processes such as the condensation to rain or glacial ice, the sedimentation of carbon dioxide in plant matter, or the dissolving of carbon dioxide in the sea.

Solar Winds

Solar wind are another factor in the escape of particles. Solar winds are streams of plasma and charged particles propelled from our sun at high velocities. The solar wind is highly radioactive and would cause major damage to everything that lives on planet earth, if not for Earth’s magnetic field. The magnetosphere of earth deflects the incoming solar wind so that mostly streams around the planet.

Earth Magnetosphere

Mars does not have a magnetosphere like earth, which causes it to feel the full effect of the solar wind. Highly energetic particles are impacting with Mars like a barrage. When Mars still had an atmosphere the particles of the solar wind would impact with the gas molecules in the atmosphere. This impact causes a rise in the kinetic energy of the gas molecules, which causes more particles to reach escape velocity.

Solar wind impact was always expected to be one of the main reasons why Mars lost most of it atmosphere. Except a recent thesis by Robin Ramstad showed that only the equivalent of up to 6 mbar of atmospheric pressure could have been lost due to impacts of solar wind since the estimation of when Mars has lost its water. The estimated pressure for maintaining a warmer climate on Mars is ≥ 1 bar. So solar wind would only be responsible for < 1 % of the pressure loss Mars has gone through.

Impact erosion

Another way to create enough energy for escape velocity is by the impact of a large extraterrestrial object. Meteorites that have sufficiently energetic collision can cause matter to propel away faster than escape velocities. If planetary bodies suffer many of these extraterrestrial impacts than enough atmosphere can be lost due have a significant impact. An example of such an impact would be the Chicxulub impactor hitting the earth around 66 million years ago (the one that killed the dinosaurs).

Final remarks

Now that the hypothesis of solar wind chipping away at the atmosphere of Mars being the primary influence has been disproved by Robin Ramstad the leading theory for the loss of Martian atmosphere is impact erosion. It is likely that impact of large extraterrestrial had a catastrophic impact on the state of the Martian atmosphere.

Now that plans have been set in motion to travel to Mars, will we ever get the atmosphere back?

References:
PSI - Atmosphere
PSI - Mars
Earth's Atmosphere: Composition, Climate & Weather
Earth’s Greenhouse Gases
What is solar wind?

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