What makes gas giants

Working out how planets form is very challenging. The Sun's planets formed very long ago, and much of the evidence for how this happened has been lost. Other planetary systems are forming today, but they are very far away, and it's hard to see them in detail.

Models can help fill in some of these gaps by using what we know about physical and chemical processes and seeing what sequence of events can lead to the kind of planets that we see. A lot of people are interested in where we come from, how life arose, the places where life can exist, and where else we could find it.

One step towards answering these big questions is to understand how planets form and acquire their characteristics. We live on a planet, and as far as we can tell, planets are good places to look for life elsewhere. But some planets are more hospitable than others in terms of features such as their climate, atmosphere, or the presence of oceans. Knowing how planets form can help us understand why they are so varied and what makes them habitable.

Planet formation turns out to be a very complicated process, and unfortunately there are still some fundamental issues that we don't understand. For example, lab experiments show how tiny dust grains can stick together to form objects the size of a pebble. But how these pebbles clump together to form much larger bodies, like the size of an asteroid, is still being studied.

It also appears that the orbits of young planets can change dramatically in some cases, but we still don't know how often or how much the orbits change. Given these uncertainties, we should be cautious of drawing too many conclusions from models for planet formation.

Why is modeling the existence and characteristics of gas giants challenging? There are several challenges, but probably the biggest one is modeling how a solid planet grows large enough to capture gas. We think that planets first form as solid objects composed of solid materials such as rock, metal, and ice.

If a planet becomes large enough, it can capture an atmosphere from the surrounding cloud of gas in which the planets are forming. For planets the size of Mars or Earth, this atmosphere is relatively thin and represents a small fraction of the planet's total mass.

Larger planets can gain thicker atmospheres, and above a certain mass, a planet will pull in more and more gas until something shuts off the supply. Examples of these are gas giant planets like Jupiter and Saturn. However, clouds of gas surrounding young stars don't last long—typically only a few million years.

The problem is to form a solid planet that is large enough to pull in lots of gas while the gas is still available. This means the solid planet that is the seed for a gas giant has to grow very fast. To date, models have had a hard time explaining this very rapid growth.

What challenges does the core accretion model face in trying to explain gas giant creation? The main challenge is that a solid planet has to grow substantially larger than Earth in order to pull in large amounts of gas and become a gas giant planet.

The formation of gas giants has to take place within the lifetime of the gaseous protoplanetary disk surrounding a young star in which the planet is forming. Astronomers have surveyed nearby young stars to see which ones have these clouds of gas, and the answer is that only stars younger than a few million years have them. Older stars have lost their protoplanetary disks. So, solid planets have to grow large—and rapidly—if they are to become gas giants.

In the Solar System at least, the giant planets orbit quite far from the sun. Planetary growth ought to have been slow here because the orbital speeds are slow, and planetary building blocks would have been far apart, so collisions leading to growth would have been relatively rare.

Thus, as you travel hypothetically downward through the atmosphere of each gas planet, different cloud layers represent the point at which the temperature and pressure is appropriate for condensing that particular volatile. The clouds also signify the border between volatile regimes.

Below the condensation layer for a volatile, it will be well mixed as a gas. Above the clouds, the volatile is highly depleted. The presence of different cloud layers depends on the composition and thermal characteristics of each planet. For example, the highest clouds on Uranus and Neptune are composed of crystals of methane ice.

The clouds are generated by a couple different mechanisms which have already been mentioned earlier in this article. In a similar situation to that on Earth, incoming solar radiation inputs energy into the atmospheric systems of the gas giants; this then causes air to move and clouds to form.

A quick energy balance calculation shows, however, that this energy source is not sufficient to explain observed atmospheric temperatures of the gas planets. Thus, unlike on Earth, internal energy plays a large role in the generation of atmospheric motion and clouds. This internal energy is created as gravitation potential energy is converted into kinetic energy deep within the planets.

Observations of Jupiter, Saturn and Neptune show that they are all emitting about twice as much energy as they are receiving from the Sun. This is not so on Uranus, which likely indicates weaker internal energy generation.

Regardless of the source of the energy, the added heat causes convection to initiate in the atmospheres. The convective motion then gains strong spin due to the large Coriolis Effect present on these enormous and in some cases rapidly spinning planets.

These convectively driven vorticies likely drive the large cloud bands which are observed on the gas giants Jupiter and Saturn. Rising air is present in the center of the cyclonic vorticies, which allows the air to cool as it expands in the same way that rising air cools in storm systems on Earth. As the air cools, volatiles can grow through condensation, and then eventually cause rain which would obviously evaporate before ever hitting a surface as often happens on Earth.

This means that clouds on the gas giant planets are capable of generating lightning. This was first observed directly by the Voyager space probe, which visually confirmed the presence of lightning on Jupiter and detected lightning strikes on other planets though interference in radio signals.

Even though we have the conceptual models described above to explain the known presence of clouds on the gas giants, the models do not necessarily agree with observations. The clouds on Jupiter and Saturn, in particular, are hard to explain. An analysis performed by Atreya et al. This could be explained by the locations at which the measurements were taken.

The location of the band on the electromagnetic spectrum used for remote measurements on volatile concentration was chosen because it allows us to see fairly deep into the Jovian atmosphere. Similar regions may be over represented on Saturn.

The problem arises because these regions are dry, and they by their very nature are areas with low concentrations of volatiles. The Galileo probe measurements likely have the same issue, as the instrument descended through one of these hot spot regions.

Various dynamical solutions have been proposed to explain why these regions are dry and lack volatiles. Atreya et al. The same research group also performed an analysis of the cloud features using computer modeling. The model itself is somewhat simplistic. It does not contain any microphysics and thus any clouds that form cannot precipitate. The authors state, however, that it is the best model currently available for simulating the atmospheres of the outer planets. The model overpredicts cloud concentrations as a result of these inadequacies, but it seems to do a good job representing the lifting condensation level of each volatile.

According to the model, water clouds are predicted to be both the deepest and the thickest of the different layers, with ammonium hydrosulfide and ammonia cloud layers above the water clouds. The following figures show the results of the model. Jupiter is, of course, the largest of all the planets in our solar system. It has an atmosphere that is roughly three hundred times the mass of Earth. As mentioned before, its atmosphere contains mainly hydrogen with some helium as well.

The weather present on Jupiter has low seasonal variability. In fact, Jupiter has almost no seasons at all due to its low obliquity of only three degrees. The clouds on Jupiter are the most vibrant and easily recognizable in the entire solar system. The planet has twelve or more parallel bands of cloud features ranging in color from bluish gray to various shades of red, orange and pink.

The dark bands are referred to as belts and the light regions are called zones. These areas arise from overturning circulations. The belts tend to be areas of downwelling motion while the zones are areas of upwelling air. The coloration of the clouds on any gas giant, but especially evident on Jupiter, likely comes from impurities in the clouds. The suspected materials creating these impurities include elemental sulfur and phosphorus as well as various organics that are produced by photochemical reactions that result from the interaction of solar ultraviolet and Jovian hydrocarbons.

In the zones, rising air causes ammonia to condense and form clouds over top of these darker impure cloud layers. This is why the zones appear much brighter than the belts. Zuchowski et al. Their model showed temporary reversals of the downwelling in the belts. After studying the chemical composition of 'polluted' Scientists have developed a They argue that Algol has many companion stars which have not been detected from earlier The goal of the Comet Physics Laboratory is to understand the internal structure of comets, as well as how their Print Email Share.

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