Seven of the eight planets in the solar system are surrounded by atmospheres. Atmospheres also surround some of the larger moons, such as Saturn’s moon Titan, and even Pluto, a Kuiper Belt object, possesses a tenuous atmosphere when ices are vaporized from its surface.
On Earth, the weather impacts on all our lives and activities. The weather is determined by the motion and state of the atmosphere and can change from day-to-day or even hour-to-hour in many places. Weather is different from climate, which can be thought of as the average conditions in an area over a longer period of time. Climate changes happen over timescales of decades to millions of years.
Just like the Earth, each planetary atmosphere has its own, often extreme and sometimes beautiful, weather patterns and phenomena.
What drives the weather?
The energy supply that powers most weather on Venus, Earth and Mars is light (including ultraviolet light) from the Sun. Over the year, the regions of each planet near the equator are heated more than the poles. The atmosphere moves in response to this unequal heating, transporting heat from warmer to cooler regions. These motions are reflected in the high and low pressure weather systems that move across those of us who live in middle latitudes. In essence, the weather is solar-powered.
Only the distant Giant Planets, Jupiter, Saturn and Neptune, emit their own internal heat at a rate similar to the power that they receive from the Sun The amount of power radiated by the Sun is enormous, roughly 1.4 kW/m2 at the distance of the Earth. Allowing for scattering back to space by clouds and ice, and on average over the year and time of day, about 200 W/m2 of this solar radiation reaches the Earth’s surface. But near the equator this is more than 300 W/m2 and near the poles less than 100 W/m2. Only the distant Giant Planets, Jupiter, Saturn and Neptune, emit their own internal heat at a rate similar to the power that they receive from the Sun (which is very much less out at their orbits, Neptune only receives about 1/900th of the solar radiation per m2 that the Earth does, since it is about 30 times further from the Sun).
There are two important influences that modify the response of an atmosphere to this heating.
The first is gravity, which not only prevents the atmosphere from quickly escaping to space, but vertically stratifies the air so that air density decreases exponentially with increasing height and ‘buoyancy waves’ (similar to waves on the surface of the sea) can form within layers of the atmosphere. These waves can often be seen in thin sheets of cloud.
The second is the rotation of the planet, which seems to deflect winds to their right in the northern hemisphere (left in the southern hemisphere), the ‘Coriolis effect’. We might believe that we are almost stationary for most everyday purposes, but in fact the Earth is rotating very rapidly on its axis (once every 23 hours 56 min; a solar day is a little longer, 24 hours, since the Earth also moves around the Sun in its annual orbit and so it takes another 4 min to rotate completely relative to the Sun) so that at the latitude of the UK we are constantly moving eastwards at over 1000 km/h compared to the Sun and stars. We experience the weather from that rotating viewpoint: a wind curves as we watch it move across the rotating Earth’s surface.
The Coriolis effect only applies to very large-scale motions (or to very fast ones) and is not readily apparent in most other human-scale experiences. Contrary to a popular myth, water does not rotate in opposite directions when going down the plughole in different hemispheres, because other factors (such as the shape of the basin, the way the water was moving when the plug was pulled out, etc.) are vastly more important on those scales. It is possible to experience the Coriolis effect in action on smaller scales if you are rotating more rapidly. Try to throw or roll a soft ball on a rotating roundabout; if the roundabout is rotating anticlockwise, the ball will seem to curve to the right of your intended target.
Mercury is surrounded by no substantial atmosphere Mercury is the only planet in the solar system not to be surrounded by a substantial atmosphere. One reason for this is Mercury’s relatively small mass and hence gravitational field, only about a third that of the Earth, which means that it is easier for gases to escape to space. The other reason is related to Mercury’s proximity to the Sun: any gases around the planet are heated and rapidly stripped away by collisions with the flow of energetic particles known as the ‘Solar Wind’. Other planets lose atmospheric gases to space as well, but at slower rates, generally because they have a stronger gravitational field (except for Mars, which is similar to Mercury) and, crucially, they are all further from the Sun. Only a few of the lightest and most energetic gas atoms can escape from the gravity field of a Giant Planet, like Jupiter, and a smaller proportion have the high energies required in the cold upper atmosphere temperatures of the outer solar system.
Venus is shrouded in a dense layer of clouds which scatters most sunlight back out to space. This is why it appears bright and almost featureless to the eye in the early morning or evening sky. It has long been thought that Venus is warmer than the Earth, as it is only two-thirds of the distance from the Sun, and it was once believed that it might harbor a lush, tropical climate underneath the cloud decks. The reality is not so pleasant. Venus’s clouds are at about the same pressure and temperature as those on Earth, but are made largely from concentrated sulfuric acid. The surface lies around 60 km below the clouds, at a pressure 95 times that on Earth. A runaway greenhouse effect means that temperatures are typically over 460°C, hot enough to melt lead. If Venus ever had oceans they have long since evaporated. No evidence remains on the surface, which is covered with more recent lava flows and scarred by huge faults and fractures.
A runaway greenhouse effect means that temperatures are typically over 460°C, hot enough to melt lead One of the most interesting features of Venus is that it rotates very slowly, once every 243 Earth days, and in the opposite direction compared to the other planets (perhaps a result of large collisions very early in the planet’s history, which effectively turned the planet upside-down). This is less than one full rotation on its axis per Venus year (225 Earth days). The Coriolis effect is therefore of little importance on Venus and the atmosphere rises at the equator and sinks around the poles, like the circulation originally proposed for Earth by George Hadley in 1735, before the importance of rotation was fully understood (on Earth the ‘Hadley Cells’ only extend to roughly 30° latitude from the equator). Remarkably, the east-west winds at the height of the clouds on Venus reach over 340 km/h, meaning that the cloud decks rotate once around the planet roughly every 4 days, about 60 times faster than the solid surface beneath. This phenomenon is called ‘super-rotation’ and must be explained by complex interactions with wave-like features feeding momentum into the large super-rotating jet.
Just as life has altered the atmosphere, it is the atmosphere that has made life on Earth possible The Earth’s atmosphere stands out in the solar system for its composition of nitrogen (78%), oxygen (21%) and argon (1%) with many other gases present in trace amounts. This composition is a result of the presence of life on the planet. Mars and Venus have primarily carbon dioxide atmospheres and the Giant Planets are mainly hydrogen and helium. Earth’s atmosphere was probably very different in the early stages of the planet, most obviously the oxygen levels that we see today only came about as the result of the evolution of blue-green algae between 2 and 3 billion years ago (about half the age of the planet).
Just as life has altered the atmosphere, it is the atmosphere that has made life on Earth possible. Without a ‘natural greenhouse effect’ (from the pre-industrial atmospheric composition), of about 35°C of warming, the surface would be frozen and life impossible. In contrast, Venus has boiled under a runaway greenhouse effect of almost 500°C, and Mars has frozen with a tiny natural greenhouse of about 3°C of warming from its present, thin atmosphere. One unique feature of the Earth is that it has a coupled ocean-atmosphere system, with water in the form of ice, liquid water and vapour in the air. This active water cycle has trapped higher levels of carbon dioxide into carbonate rocks and is vital for life.
Earth’s atmosphere gives rise to many spectacular weather events, just a few of which are described in more detail here.
The dry atmosphere means that Mars’s famous dust storms can last for months Mars has a pattern of seasons very much like those of the Earth, but its surface pressure is over one hundred times lower. The thin atmosphere means that the surface weather is often an exaggerated form of a desert on Earth: a relatively warm temperature, often above 0°C, on a summer’s afternoon and a freezing night, when temperatures can plunge by up to 100°C. Just like Earth, Mars has patterns of high- and low-pressure weather systems, especially in the winter hemisphere, but the atmosphere is very dry with only a few, cirrus-like clouds of fine ice crystals. The dry atmosphere means that Mars’s famous dust storms can last for months before the fine dust particles fall out of the atmosphere. On Earth, dust storms end more quickly as water condenses on the grains.
One interesting feature of Mars is that in the winter hemisphere it can become cold enough (around -130°C, depending on pressure) for the main atmospheric constituent, carbon dioxide, to freeze. This is unique amongst the planets, and contrasts with our own snow and ice formed of water, a relatively tiny proportion of the atmosphere (less than 0.005% by mass). In martian winter a thick hood of carbon dioxide clouds forms over the pole and snows onto the surface, where carbon dioxide ice accumulates. About one third of the total atmospheric mass freezes onto the winter pole and sublimes again spring, to move to the opposite pole.
Jupiter rotates extremely rapidly despite its great size, with a day less than 10 hours long, and is home to a vast array of jets and vortices. The atmospheric circulation of Jupiter is dominated by alternating, east-west jets which have created the banded appearance of the planet. Wind speeds can reach more than 600 km/h. Jupiter’s swirling clouds are what gives the planet its beautiful appearance. The high, white clouds consist of ammonia crystals, while the darker ones are deeper, water ice clouds. The clouds are coloured by a variety of chemicals, brought up from the deeper atmosphere, which react with sunlight.
The Great Red Spot is a high-pressure anticyclone, larger than the Earth, which overturns completely every six days A variety of white and coloured ‘spots’ appear within this jet pattern. These are long-lived storms that can persist for many years. The banded structure breaks down near the poles, where a beautiful pattern of vortices has recently been revealed by the NASA Juno spacecraft.
Jupiter’s ‘Great Red Spot’ is the largest and most famous example of a long-lived spot. It has been continuously observed since at least the nineteenth century, and it was possibly the feature seen either by Robert Hooke in 1664 or Giovanni Cassini in 1665 using early telescopes. The Great Red Spot is a high-pressure anticyclone, larger than the Earth, which overturns completely every six days and is perhaps similar to a ‘blocking high’ weather system on Earth, such as might bring long periods of stable, colder weather to Europe in winter. Despite the rapid, swirling motions within and around the vortex, the Great Red Spot has persisted for centuries rather than days or weeks.
Sometimes bright storms appear on Saturn, like high thunderstorm clouds on Earth Saturn’s atmosphere has a banded structure, like that of Jupiter, but with even faster winds; the equatorial jet can reach speeds of 1,800 km/h. There are fewer obvious vortices than seen on Jupiter, but the North Pole shows a regular hexagonal structure in a polar jet stream. Sometimes bright storms appear on Saturn, like high thunderstorm clouds on Earth. The image shows the so-called ‘Dragon Storm’ seen by the Cassini spacecraft in September 2004 (a false colour scale has been used to emphasize atmospheric structures). The Dragon Storm was linked to radio signals probably caused by huge lightning discharges in the atmosphere. One interesting feature of this storm was that it was seen in the same place over several months, flaring up at different times. It is thus likely to be linked to events taking place deeper in the atmosphere.
Uranus has clouds of methane ice in its cold atmosphere Unlike the ammonia clouds of Jupiter and Saturn, Uranus and Neptune have clouds of methane ice in their cold atmospheres. It is very difficult to see any features in the hazy atmosphere of Uranus in visible light, but the images above were made from the Keck telescope in the infrared part of the spectrum. These show a banded structure with jets and a few distinct atmospheric features. One fascinating aspect of Uranus is that it lies almost on its side, rotating about an axis tilted at 98° to the plane of its orbit. This results in an unusual pattern of seasons over its long year (about 84 Earth years). At some times of year, one pole points towards the Sun and the other is in an extended period of darkness, while over the course of a full orbit the poles receive more sunlight on average than the equator does. In the image shown here, Uranus is almost side-on to the Sun. Only Voyager 2 has flown past Uranus, and it may be decades before another mission visits the system and observes it in more detail.
Neptune exhibits the highest wind speeds recorded in the solar system, reaching 2,200 km/h When Voyager 2 flew past Neptune it saw a large anticyclonic storm, south of the equator, which was soon called the ‘Great Dark Spot’ because of its apparent similarities to Jupiter’s Great Red Spot. A smaller spot lay further south. The Great Dark Spot had vanished when the Hubble Space Telescope looked for it again five years later, although another feature appeared in the northern hemisphere. Despite its large distance from the Sun and relatively small solar energy input, Neptune exhibits the highest wind speeds recorded in the solar system, reaching 2,200 km/h.
Neptune has an interior heat source comparable with the small amount of energy it receives from the Sun, as do Jupiter and Saturn. Intriguingly, Uranus has no similar excess of heat. The ultimate source of this heat is gravitational energy; heat is generated either as the Giant Planets slowly contract or as heavier elements fall towards their centres. What role, if any, the interior heat source plays in driving weather systems remains mysterious and the subject of research.