Photochemistry

Photochemistry is when molecules are hit by high energy light (usually Ultraviolet light) which breaks molecular bonds, and creates molecular fragments. These are usually highly reactive, and combine with other molecules. Mars' atmosphere is so thin, that some of these unstable fragments can last minutes or hours. Further, long lived, stable species are created (such as CO or O₂) which last for a long time in the Martian atmosphere.

There are three major gases in the Martian atmosphere Carbon dioxide (CO₂), Nitrogen (N₂), and Water (H₂O). The major photochemistry in the Martian atmosphere are based on these.

There are two areas where this occurs: the lower atmosphere (from UV light), the middle regions where the molecules are largely unionized, and the edge of space (which has UV light, and also particles of the solar wind). These are all discussed below.

Our current models of the Martian atmosphere overestimate the amount of CO, and underestimate the amount of O₃ by significant amounts. This is an area of active research.

Lower Atmosphere

Nitrogen Photo-Dissociation

Nitrogen has a powerful bond, which makes it difficult for UV light to break it apart. Thus, species from the breakup of N₂ in the Martian atmosphere are quite rare, and play only a minor part in these exotic chemistries.

The major reaction is: N₂ + hv --> N + N(²D). (With wavelengths from 80 to 100 nm.)

• The '²D' means that the nitrogen's 'D' orbital is highly excited.
• The 'hv' represents high energy light.
• The 'h' is the Plank constant.
• The 'V' is the wavelength of the light.

N(²D) + CO₂ --> NO + CO

N + O --> NO + hv.

These cause further reactions:

N + NO --> N₂ + O

N + H₂O --> NO + OH

NO + H₂O --> NO₂ + OH

Note that NO₂ weakly dissolves in water, forming nitrous acid, and will be permanently lost to the atmosphere if it is absorbed in the soil.

Nitric acid (HNO₂) and peroxynitric acid (HO₂NO₂) are also formed but tend to break up quickly.

The presence of these nitrogen species act as a catalyst, helping to react CO and O₂ to form CO₂ and atomic Oxygen.

Carbon Dioxide Photo-Disassociation

Carbon dioxide plays a large part of these exotic chemistries, due to it pre-eminent proportion of the Martian atmosphere.

CO₂ + hV --> CO + O(¹D) (Where the D orbital is highly excited.)

OH + O --> O2 + H

If the hydrogen atom reaches the edge of space it is easily lost.

CO + OH --> CO₂ + H

If the atomic hydrogen manages to find an O₂, molecule, it may react thus:

H + O₂ = HO₂.

HO₂ is called hydroperoxyl, hydrogen superoxide, peroxyl radical, hydrogen dioxide, or dioxidanyl. It quickly reacts with any Ozone it encounters.

HO₂ + O₃ --> HO + O₂ + O₂

It may also encounter atomic oxygen:

HO₂ + O --> OH + O₂

Finally, if Carbon monoxide encounters atomic oxygen:

CO + O --> CO₂. (This can also be synthesized via series of other reactions.)

Water Photo-Disassociation

The most interesting series of reactions flows from the break up of water in the Martian atmosphere.

H₂O + hV --> OH + H. (The wavelength of the light must be under 200 nm.)

This atomic hydrogen is highly reactive. Water is confined to the lower atmosphere, so this hydrogen has plenty of time to react.

H₂O + O(¹D) --> OH + OH

These reactions also occur:

HO₂ + O --> OH + O₂

CO + OH --> CO₂ + H

The atomic hydrogen and OH (hydroxide) is created in significant quantities, and are used in many of the reactions described above. However, since the water on Mars is confined to the lower atmosphere, these chemical species are common in the LOWER atmosphere. (Altho, they may be moved higher by winds.)

Sulphur & Chlorine Chemistries

On Earth, sulphur dioxide (SO₂) is the most common gas produced by volcanoes. Searching for this gas would be a strong indication of active vulcanism on Mars. However, despite careful searching, no sulphur compounds have been found in the Martian air.

If sulphur dioxide does appear, it undergoes the following:

SO₂ + hV --> SO + O

SO + hV --> S + O

These rapidly react:

S + O₂ --> SO + O

SO + O₂ --> SO₂ + O

SO + OH --> SO2 + H

SO + H₂O --> OH

These reactions keep SO₂ in equilibrium, with SO₂ being the dominate sulphur species up to ~45 km. Above this, SO becomes the dominate species.

The major sink for sulphur is:

SO₂ + OH --> HSO₃ (Using some other particle as a catalyst.)

HSO₃ + O2 --> SO₃ + HO₂

SO₃ + H₂O + H₂O --> H₂SO₄ + H₂O

Sulphuric acid (H₂SO₄) can either condense at the surface, or be dissolved in water and enter the water table, being lost to the atmosphere. It has been estimated that the half life of sulphur compounds in the Martian atmosphere is ~2 years. This suggests that Martian vulcanism must be at least ~1,500 times less on Mars than on Earth.

Chlorine is likewise released by volcanoes, but perchlorates from the surface dust may also be a source in the Martian air. Chlorine is removed from the air by this reaction:

Cl + HO₂ --> HCl + O₂

Hydrochloric acid (HCl) will react with water or the surface and soon be removed. There are indications of tiny amounts of HCl in the Martian air, but it is a negligible player in the photo-chemistries described above.

The Methane Controversy

Methane (CH₄) may have been detected on Mars. (Most detections are at the error limits of the instruments finding it.) If it does exist, it will be broken up by reacting with OH or O(¹D). Its expected lifetime in the atmosphere is ~300 years, so any methane formed will have plenty of time to be well mixed in the Martian atmosphere. This makes the methane spikes found by Curiosity hard to explain, unless they took place very near the lander.

Middle atmosphere

This region gets more Extreme UltraViolet light (EUV), so molecules are more easily broken apart. At this height, few molecules are ionized.

The highly energized molecules can lose this energy with collisions with other air particles, or they can radiate the energy away as light. (This later is called, 'sky glow' or 'air glow', and gives useful information about the amounts of various molecules and their energies at different heights in the atmosphere.) (It is also called 'day glow' and 'night glow' based on time of day.)

Air glow is about 40% brighter when mars is at Perihelion simply because Mars gets more light when it is closest to the sun.

These energized particles may collide with other molecules, turning this energy into heat. The heated air causes winds, which helps move the warm day time air to the night side of the planet.

Edge of Space

In the middle atmosphere, Extreme UltraViolet (EUV) is mostly absorbed by CO₂. Extreme UV is the highest energy UV light, blending into soft X-rays. EUV can easily break molecular bonds. In the upper atmosphere, surprisingly, it is most often absorbed by atomic oxygen.

One difference is this region of Mars' atmosphere is directly heated by collisions by the solar wind. Protons, atomic hydrogen, and helium nuclei collide with the atmosphere, heating it, and for a time becoming part of the atmosphere. (Mars' gravity is so weak, that these are short lived additions to Mars' air.)

Like the air glow created by absorbing light, the air can be excited by impacts by solar particles. This is called auroral glow, and is like the auroras on Earth, except that they are planet wide, and not confined to the regions above Earth's north and south magnetic poles. The aurora on Mars is very faint, and is not visible to the naked eye. Even in areas above paleo-magnetic fields locked into the rocks (which concentrate the solar particles hitting the atmosphere), the aurora is ~15 times weaker than the sky glow.

At above 200 km, it is estimated that the amount of atomic oxygen exceeds the amount of CO₂. This atomic oxygen, is lighter than the CO₂, and rises to the edge of space. However, Mars gravity is sufficient that little atomic oxygen is lost. If it collides with an atomic hydrogen, it may well form hydroxide (OH), becoming a more permanent part of the Martian air.

Models which try to understand the upper atmosphere are hindered by lack of direct measurements of the amount of atomic oxygen at the edge of space. Ideally, future probes will measure this much more accurately.

Meteors hit the Martian atmosphere and are vaporized. This dust slowly settles towards the ground, but at around 80 to 90 km metallic ions, such as Mg+, Fe+, Na+, Si+, and their oxides such as MgO+, & MgO₂+ have been detected.

Biography

The Atmosphere and Climate of Mars, Cambridge Planetary Science, Edited by Robert M. Habgerle, et all, ISBN 978-1-107-01618-7.