decorative picture for the mainstream pages

Planetary Atmospheres

IN SHORT - Atmospheres at a rocky planet are a natural part of the process of formation of those. As the first atmosphere around a planet is made of those elements found in the protoplanetary disk, outgasing are at the origin of a second atmosphere. Gas giants, on the other hand, strictly are mostly an atmosphere only, with their gas layers surrounding a small rocky core. Rocky planets may loose their atmosphere as it's eroded

by the solar wind -like at Mars, or have it changed in composition by life process -like at Earth. The role of an atmosphere mostly is to shield the planet from the Sun's, or the extra-solar, harmful radiations as the atmosphere is moderating the heat budget too. In some cases, an atmosphere even may be the place where fundamental, life compounds are manufactured

Atmospheres are part of any planet formation process. At the very first any planet or planetary body -like the moons of the gas giants- have an atmosphere of hydrogen and helium, that is made of these elements which are the fundamentals of the protoplanetary disc. Then, planets get secondary atmospheres, that is atmosphere which are the product of outgassing. Once in the process of differentiation, the planet interior is heating. At a moment, this heat is evacuated. As part of the evacuation, an atmosphere forms. Volcanoes e.g. are rejecting important masses of gas into the planet's immediate surrounding. Strictly the outer gas giants of the solar system are small solid cores with very large atmospheres. These atmospheres however are not due to any differentiation/outgassing process. They are simply an accretion of hydrogen, helium, and other gases when the planets formed. Such gases were those found in the protoplanetary disc. Some large moons of the outer gas giants got an atmosphere due to the differentiation process

Further, the atmosphere endures evolution. A planet may loose its atmosphere. It's mainly seen now like such atmospheres being eroded by the solar wind. It was the case at Mars, atmosphere of which ceased to be protected when the magnetic field there disappeared. Or an atmosphere may change in composition. It was the case at Earth where first organisms using photosynthesis turned a mostly nitrogen atmosphere into a mostly oxygen one

The role of the atmosphere is mostly to shield the planet from Sun's harmful radiations like the UV or the X-rays. The shield is acting too against other high-energy events like the cosmic rays. Any planetary atmosphere is interacting with the solar radiation though. An atmosphere is moderating the energy balance at a planet. First, it's moderating the infall of solar energy to the surface. The planet's surface is moderately heated only. On the other hand the atmosphere is forbidding the diurnal heat to radiate back into space at night. The planet is retaining a part of the heat of the day. The difference between the diurnal, and nocturnal, temperatures is feeble, like at Earth. On bodies like Mercury or Moon, which have no atmosphere, the temperature gradient is much more high. On the other hand, from what is presumed at Venus, and from what is about certain at Titan, the main Saturn's moon, it seems that some interaction between Sun radiation and atmosphere are manufacturing there either primordial organisms, or prebiotic compounds which, falling to the surface, form the basis to a further possible apparition of life

see a table of data about planets' atmospheres