Ozone
Ozone (O3) is a molecule containing 3 Oxygen atoms. It is a pale blue gas with a distinctive smell (the 'electric' smell after lightning). It is less stable than the normal diatomic oxygen (O2) molecule. Ozone is formed by ultraviolet (UV) light breaking an oxygen atom in half, which then combine with two other O2 molecules. Thus an ozone layer forms high in the atmosphere, which protects the Earth from UV light. Breathing ozone is damaging to the lungs when it is above 0.1 ppm (parts per million). At ground level ozone is considered a pollutant.
Industrial Uses
Ozone is a powerful oxidant (more so than regular O2), and has many uses related to this.
It is used in the preparation of pharmaceuticals, synthetic lubricants, and in many other organic compounds where it is used to sever carbon-carbon bonds.
It is used as a bleaching agent.
It can be used as an antiseptic and can be bubbled thru water to sterilize it.
It can be used to remove iron, magnesium, arsenic, hydrogen sulphide, nitrates, and other 'color' from water .
It is used to kill insects in stored grain.
It can scrub yeast and mold spores from the air in food processing plants.
It can increase the flavour of banana, pineapple, and tomatoes by increasing the flavonoids and phenol contents.
It can destroy cyanide wastes (e.g. chemicals left over from silver and gold mining), by oxidizing cyanide to cyanate and eventually to carbon dioxide and nitrogen.
Ozone & Planetary Science
Earth's ozone layer has formed what is called the 'cold trap' to protect Earth's water. UV light will break up water into hydrogen and hydroxide. The hydrogen is light enough to float up thru the atmosphere and be lost to space. However, the water freezes into ice crystals below the ozone layer, and the photo-disassociation of water is greatly slowed, preserving Earth's oceans. It is thought that both Mars and Venus has lost much water because they don't have a cold trap.
Mars has a larger than expected amount of ozone, formed from its trace amounts of oxygen and from disassociation of Carbon monoxide (CO). However, this tiny amount of ozone is insufficient to stop UV rays from reaching the surface and destroying organic molecules there. See Photochemistry for more information.
It is a weak Greenhouse gas. (Because it is unstable, people wouldn't create it, but if naturally formed it would help warm the planet.)
If we were to begin to Terraform Mars, creating a significant ozone layer would be a high priority to protect bacteria on the surface.
