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), and the edge of space (which has UV light, and also particles of the solar wind). These are both 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.

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

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

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.)