Of the inner planets, Mercury, Venus, Earth and Mars, Mercury is too hot and small to support an appreciable atmosphere. The other three each have oxygen-containing constituents in their atmospheres which have led to the development of their respective ozone layers.
The outer planets Jupiter, Saturn, Uranus and Neptune have atmospheres which are composed mostly of hydrogen-containing gases (H2, CH4) and helium with no known oxygen. Without oxygen-containing compounds, Ozone (O3) cannot occur there either.
The Atmosphere of Earth
The Earth's ozone layer for the current atmosphere is well documented. It is likely that living organisms caused the production of the ozone layer into what we see today. The Origin of Life may have been in the oceans, which were themselves likely supplied from comets and asteroids.
The water would have formed an effective shield against the ultraviolet (UV) from the Sun. Oxygen (O2) was then released by the primitive life forms over millenia, and it accumulated in the atmosphere. Once O2 was present, an ozone layer could form by
O2 + hv ---> 2 O ........................................................R1
O + O2 + M --> O3 + M .............................................R2
where hv is an ultraviolet photon and M is any 3rd body needed to conserve energy and momentum of the collision between O and O2. The ozone layer then formed a barrier to UV which enabled life to emerge from the oceans and adapt to the use of oxygen for respiration.
The Atmosphere of Venus
The atmosphere of Venus is very dense, about 90 Earth atmospheres, consisting mostly of carbon dioxide and nitrogen with trace amounts of sulfur dioxide. Measurements from the Venus Express satellite, reported in the November 2011 issue of the journal Icarus, now show, for the first time, the presence of a thin ozone layer in the atmosphere of Venus.
Ozone arises from the breakdown of CO2:
CO2 + hv --> CO + O .................................................R3
The oxygen atoms can then combine in several steps to form first O2 and then O3 via reaction R2. The measurements show that the layer occurs at about 100 km altitude, and is very thin, only about 0.01% of the depth of Earth's ozone layer. It is possible that more ozone exists lower down in the atmosphere, but the instruments were unable to penetrate below altitudes of about 60 km due to the thick clouds.
The Atmosphere of Mars
Ozone was first in the atmosphere of Mars in the 1970s (Barth and Hord, 1971) Similar reactions occur in the atmosphere of Mars as on Venus. However, the atmosphere is much less dense, and the UV from the sun much less intense than on Venus.
Ozone loss is catalyzed by reactions with OH and HO2, so that O3 is limited to about 1% of the depth of the Earth's ozone layer.
The Possibility of Life
Examination of the atmospheres of Venus and Mars have provided insights into the presence of life either in the solar system or elsewhere. If that life is carbon-based, like on Earth, a significant quantity of water is apparently needed. This follows from the requirement of protection of primitive life from the effects of solar UV.
For life to become land-based, primitive life needs to release sufficient oxygen to build up the planetary ozone layer. Selsis et al. (2002) have speculated that the ozone layer needs to be at least 20% of that of the depth of the Earth's ozone layer before life can exist. "We can use these new observations to test and refine the scenarios for the detection of life on other worlds," says Dr Montmessin, reported in ESA news online following the anouncement of the ozone detection in the atmosphere of Venus.
Neither Venus or Mars are currently close to the 20% criterion. Production of ozone indirectly by life can be a positive feedback process: the more oxygen produced by plants, for example, the more ozone is produced to protect the living tissue further. This leads to a build up in the depth of the ozone layer until the layer is limited by other processes. As noted in the study of Montmessin et al., if Venus (and by inference Mars) previously had such a positive feedback process, it has now disappeared.
References
Barth, C.A., Hord, C.W., Mariner ultraviolet spectrometer. Science 173, 795–796, 1971.
Selsis, F., Depois, D., Parisot, J.-P., 2002. Signature of life on exoplanets: Can Darwin produce false positive detections? Astron. Astrophys. 388, 985–1003.
Montmessin et al., A layer of ozone detected in the nightside upper atmosphere of Venus, Icarus, 216, 82-85, November 2011.
NASA, Lunar and Planetary Science, online publication.
