Jupiter. Notes compiled by Paul Woodward Department of Astronomy

Jupiter Notes compiled by Paul Woodward Department of Astronomy

We will spend about one week on the outer, gaseous planets, focusing first on Jupiter, then on Saturn. We will not spend time on Uranus and Neptune.

Today, we will mainly discuss Jupiter. Its orbit, and its place in our solar system, are shown on the next two slides. It is a little more than 5 times further from the sun than is the earth. Its orbit, like the earth s, lies pretty much in the ecliptic plane. Jupiter rotates in the same sense as its orbital motion, as does the earth, and its rotational axis is essentially perpendicular to the plane of its orbit. This perpendicular orientation of the rotation axis of Jupiter means that Jupiter does not have seasons, like we do on earth (because the earth s rotation axis is tilted relative to its orbital plane, the ecliptic plane).

The following tables give comparisons of some of the physical properties of the planets. The most important columns of these tables to see for the moment are those that compare the sizes and material compositions of the planets. These data show that Jupiter, Saturn, Uranus, and Neptune are fundamentally different sorts of planets from the inner ( terrestrial ) planets, Mercury, Venus, earth, and Mars. They are in fact gaseous giants.

The following table compares the four outer gaseous giant planets. From this table it is clear that 1. Jupiter and Saturn are much larger than Uranus and Neptune. 2. Uranus and Neptune contain much more heavy elements along with their primary constituents, hydrogen and helium. (The average density of Jupiter is made larger than that of Uranus because its huge gravity has compressed the hydrogen and helium.)

Jupiter is the third brightest object in the sky (other than the sun), after the Moon and Venus. Galileo discovered the 4 large satellites of Jupiter, the Galilean moons, with his telescope. This apparent planetary system in miniature, where the 4 moons obviously revolved about Jupiter and not about the earth, contributed to the demise of the Ptolemaic system of the cosmos, in which the earth was at the center. The moons of Jupiter, whose orbital radii and periods can be determined through observation from the earth, can be used to weigh Jupiter. This procedure reveals that the average density of Jupiter is much lower than that of the earth. Because of its huge size, even a small telescope can resolve the disk of the planet to measure its size. Through large telescopes we can even see the weather on Jupiter.

Jupiter with its four Galilean satellites

60-inch photograph of Jupiter Caltech & Carnegie Institute

Through a 120-inch telescope, we can see quite a bit of atmospheric detail in the disk of Jupiter.

Lick Observatory, 1927 Jupiter, observed in 6 wavelength bands: UV, v, green, yellow, red, IR

The following 3 slides show the level of detail the student Rolf Karlstad was able to obtain by compositing images taken at 1/60 sec with a video camera through his small telescope.

Considering the size of the telescope, this image of Jupiter is amazingly good. It was obtained by compositing over a thousand individual 1/60 sec exposures, after each was aligned with the others. This is manual adaptive optics.

Jupiter and more than one of its moons are both visible here.

Here we see Jupiter, 3 of its moons, and the shadow of a moon on the planet.

Through a 200-inch telescope, we can see even more atmospheric detail in the disk of Jupiter, and can clearly see shadows cast by Jupiter s moons.

Jupiter viewed through the 200-inch Palomar telescope. Ganymede and its shadow are visible, as is also the Great Red Spot.

Jupiter viewed through the 200-inch Palomar telescope. Ganymede and its shadow are visible. However, the Great Red Spot does not stand out in this red light photo.

The following image of Jupiter was also made from earth, but now through the Hubble Space Telescope, orbiting above the earth s atmosphere.

Hubble Space Telescope image of Jupiter, 2/13/95

Determination of the average density of Jupiter from the characteristics of the orbits of its moons indicates that Jupiter must be largely composed of gas rather than of rock, like the terrestrial planets. We can use Newton s law of gravitation and our knowledge of the behavior of materials under pressure to get a good idea of the interior structure of Jupiter, even though we are unable to observe the interior directly. If we know the equation of state the pressure a gas exerts at any given density and temperature then we can build a model of a planet like Jupiter as a spherical object supported against its own gravity by the pressure its gaseous components exert.

Assuming a cold interior (cold in that we believe there are no nuclear reactions going on, as there are in the sun) allows us to build an approximate spherical model of Jupiter, because the behavior of most constituents at zero temperature is believed to be known from theory. Even if we did not have this theory available, we could use information about the density and temperature of various materials at high pressures that we can obtain on the earth. We can obtain these measurements, for example, by driving the materials to high pressures by subjecting them to shock waves from high explosives. Our knowledge of material properties is therefore gained from a combination of theoretical calculation (using very large computers) and from laboratory experiments. This knowledge tells us how much pressure a given mix of materials will exert when compressed to a particular density and maintained at a particular temperature.

For example:

At relatively low temperatures, the pressure is determined mainly by the density of the material and does not depend much at all upon the particular value of the temperature. From Newton s law of gravitation, we can determine the gravitational force at any particular distance from the center of a spherical planet, even at a distance that is inside the planet. This gravitational force must be balanced by the pressure of the material making up the planet at that radius (i.e. at that distance from the center of the planet). Balancing the gravitational and pressure forces at each distance from the planet s center, and using our knowledge of the material density we require to generate the pressure, we can build a model to tell us how the density of the material must vary with distance from the center of the planet. We do this on a computer, and it takes about one second.

For each material composition we choose for our model of the planet, and for each planet mass we might obtain from weighing the planet by observing the orbital motion of its moons, we get a run of density with radius. The density is largest in the planet s core, and we find that at a large enough distance from the core the density becomes zero. This is the surface of our model planet. Therefore: 1. given a material composition, 2. given a mass for the planet, and, of course, 3. given the assumption that the temperature is everywhere in the range of cold values for which the pressure is insensitive to temperature, we can compute a radius for the model of the planet.

The result of such model building is the set of mass-radius curves on the next slide. Points J, S, U, and N on this plot indicate the observed data for the planets Jupiter, Saturn, Uranus, and Neptune. The point J, representing Jupiter, lies very close to the mass-radius relations for cold planets constructed from pure hydrogen (the top curve) and for hydrogen-helium mixture (next curve down, labeled x=0.25) with 25% helium by mass. We may therefore conclude that Jupiter is largely hydrogen and helium, not only near its surface but in the interior as well. We also note that the point S, representing Saturn, lies right on the curve for cold planets consisting of hydrogen-helium mixtures with 25% helium by mass.

Mass-radius curves for objects of various compositions at zero temperature. Curve labeled X = 0.25 is for an approximately solar mixture of hydrogen and helium. Points J, S, U, and N represent the outer planets. Radius is in units of hundredths of a solar radius (1 R = 6.96 10 5 km) and mass is in units of solar masses (1 M = 1.99 10 11 g). Jupiter and Saturn are clearly composed predominantly of hydrogen and helium; Uranus and Neptune must have a large complement of heavier elements. (from Encyclopedia of the Solar System)

It is interesting to note that the figure indicates that if Jupiter were much more massive, it would actually be smaller! This strange result can be true because the gases that make up planets like Jupiter are compressible, just like the pillows in the illustration from your text book that appears on the next slide.

Jupiter has enormous pressures in its interior which compress hydrogen into a liquid metallic state. No one has observed liquid metallic hydrogen in the laboratory. Instead, we believe it exists under the conditions in Jupiter s interior as a result of theoretical calculations done on computers. Liquid metals should not be unfamiliar to you. Mercury is a metal that is a liquid at room temperature.

The phase diagram on the next slide divides a pressure-temperature plane into regions in which hydrogen is either liquid or solid, molecular or metallic. The interiors of models of the outer planets trace lines in this phase diagram, where progressing along one of these lines corresponds to moving in or out in radius within the planet. Uranus and Neptune do not reach the metallic region of this phase diagram, but both Jupiter and Saturn do. The shaded area, which is crossed by the line representing the Saturn interior model, indicates a region of pressure and temperature in which it is believed that any helium mixed in with the hydrogen would naturally separate out. It is believed that such separation of helium from hydrogen is happening inside Saturn, with the helium falling toward the center of the planet, and releasing gravitational potential energy in the form of heat as it does so.

(from Encyclopedia of the Solar System)

To get a better idea of the interior structure of giant planets like Jupiter, we can take advantage of their rapid rotation. If Jupiter were not rotating, its gravity would force it to assume a spherical shape. A spherical planet exerts a gravitational force on a satellite just as if all its mass were concentrated at a single point at its center (don t worry why this is true, but it is). The orbit of a satellite can then tell us the mass of the planet, but it cannot tell us anything about the distribution of the mass with radius within the planet. Through the centrifugal force, rapid rotation causes a fluid planet like Jupiter or Saturn to become distorted. Then the gravitational forces felt by a spacecraft orbiting the planet can reveal details of the internal distribution of mass in the planet.

Figure 11.3 (a) Gravity alone makes a planet spherical, but rapid rotation flattens out the spherical shape by flinging material near the equator outward. (b) Saturn is clearly not spherical.

To study the atmospheres of the giant planets, it is most useful to go there and look at them close up. The Voyager spacecraft returned to earth stunning views of the dynamics of Jupiter s atmosphere. Voyager-2 was able to execute a 4-planet fly-by, called by NASA a Grand Tour of the outer solar system.

Two Voyager spacecraft were launched in the 1970s to visit the outer planets. Voyager-2 took advantage of a special alignment of these planets in order to visit all four.

Voyager-1 image of Jupiter from 47 million km. Ganymede is at the right center, and Europa is at the top right.

Jupiter as observed by the Cassini space probe in true color. In the intervening 20 years, the image improved quite a bit.

The Galileo probe not only orbited Jupiter, it also descended into its atmosphere to a depth where the pressure was 20 times that of the earth s atmosphere at sea level. At that point communication with the probe was lost. As one descends through the atmosphere of Jupiter from the top of the troposphere (see graph on next slide), the temperature increases. The outermost cloud layer is composed of ammonia, which condenses only at very low temperatures. At greater depth one encounters clouds of ammonium hydrosulfide, and finally, at still greater depths, clouds of water.

Models for the temperature structure show the Jovian atmosphere getting hotter with depth, even though sunlight does not penetrate much below the clouds. Jupiter was quite hot when it formed, since the kinetic energy of motion of the bodies that collided with it to build it up to its present size was converted into heat. That kinetic energy of motion also included gravitational potential energy that had been converted into kinetic energy before the objects collided with the early Jupiter. Models predict that such a hot Jupiter would, over the course of 4.5 billion years (the approximate age of the solar system), have gradually cooled to its present temperature by emitting heat into space. Early on, as Jupiter cooled it contracted, liberating more gravitational energy and generating additional heat. But now Jupiter cools at essentially constant radius.

Thus heat is constantly rising from Jupiter s core through the interior to the surface, where it is radiated into space. This heat transport occurs by convection. Gas heated by contact with the warm interior becomes buoyant (just like a hot air balloon) and rises through the atmosphere, carrying the heat with it. Gas that has given up part of its heat by radiating it into space near the surface of the planet is cooled, is compressed by the warmer surrounding gas, becomes denser, and sinks. This convection drives powerful winds on Jupiter. Because Jupiter is rapidly rotating, the wind currents are quite complex and are organized near the surface into horizontal bands. On both Jupiter and Saturn, the atmosphere near the equator rotates faster than the planet generally.

The next slide illustrates the sort of convection in horizontal bands near the surface of Jupiter that we think is going on there. In the horizontal bands of rising air, white ammonia clouds are formed as the temperature drops due to the expansion of the gas as the pressure drops. These white ammonia clouds therefore mark bands of rising gas. When the gas, having given off some of its heat in the form of radiation into space, begins to descend again, there are no longer ammonia clouds. The horizontal bands of descending gas are therefore clear, so that we can see through them to the warmer ammonium hydrosulfide clouds below. On the next slide, an image of Jupiter in visible light, on the left, is compared to one in infrared light, on the right.

The infrared image is brightest where the gas is warmest. You can see that the bright infrared bands are the bands where there are no white ammonia clouds. These, we believe, are the bands of descending gas. Note that the Great Red Spot is dark in the infrared image.

Fig. 11.7 Air Flow on Jupiter

The motions of the gas in the atmosphere of Jupiter are also strongly constrained by the rapid rotation of the planet, which gives rise to the Coriolis effect. The Coriolis effect, together with unequal rates of heating by sunlight at different latitudes, causes a phenomenon called meridional circulation. This circulation tends to even out the heat deposited by the sun by setting up a convection flow. On a slowly rotating planet like Venus, the meridional circulation is very simple, on a more rapidly rotating planet like the earth it is more complex, and on a very rapidly rotating planet like Jupiter we get the many horizontal bands that are the most striking aspect of the planet. Because Jupiter, unlike Earth or Venus, has heat coming up from its interior via convection, whether the sun is heating the outer atmosphere or not, the cause of the meridional circulation bands is more complex. Nevertheless, it is the Coriolis force, which expresses the principle of the conservation of angular momentum, that constrains the gas to circulate in horizontal bands.

You can observe the coriolis effect on a merry-go-round. The ball rolled inward veers to the right so that its angular momentum is conserved. The ball rolled outward from the center also veers to the right to conserve its angular momentum.

Notice the low pressure zone centered in NW Minnesota creating counterclockwise surface winds. This is from conservation of angular momentum as air moves toward the low pressure spot.

On a rapidly rotating planet like the earth, the Coriolis effect causes each of the 2 large circulation cells of a slowly rotating planet to split into 3 cells.

Here the resulting surface wind patterns are shown. These motions combine with the circular flows around low pressure regions shown earlier to give the overall winds.

Fig. 11.7 Air Flow on Jupiter

Figure 11.8: This photograph shows Jupiter s Great Red Spot, a huge highpressure storm that is large enough to swallow two or three Earths. The smaller photo (right) of the Great Red Spot is overlaid with a weather map of the region.

This Web site has lots of nice movies.

A number of images follow which show some of the great wealth of weather phenomena on Jupiter. The short animated sequences produced by NASA from the Voyager data give life to these scenes.

Cassini image of Jupiter with Io in the foreground.

Galileo image of Jupiter with Great Red Spot.

Cassini image of Jupiter, 12/31/00, in true color on the left and in false color on the right. Color on the right denotes the height of the clouds. Red regions are deep water clouds. Bright blue regions are high haze (as for the region of the Great Red Spot). White spots are energetic lightning storms high in the atmosphere. The darkest blue regions are very deep hot spots from which thermal emission escapes freely.

Figure 11.8: This photograph shows Jupiter s Great Red Spot, a huge high-pressure storm that is large enough to swallow two or three Earths. The smaller photo (right) of the Great Red Spot is overlaid with a weather map of the region.

Galileo image of Jupiter s Great Red Spot.

More recently, the Galileo spacecraft produced very high resolution images of atmospheric phenomena on Jupiter, including, of course, the Great Red Spot. The image on the next slide was made by combining images taken through 3 infrared filters: 1. 886 nm, where methane strongly absorbs, is shown as red. Due to this absorption, only high clouds can reflect light in this wavelength. 2. Reflected light at 732 nm, where methane absorbs less strongly, is shown in green. 3. Reflected light at 757 nm, where there are essentially no absorbers in the Jovian atmosphere, is shown in blue. This light is reflected from the deepest clouds. Thus blue or black areas of the next slide are deep clouds; pink areas are high, thin hazes; white areas are high, thick clouds. This Galileo mosaic was taken on June 26, 1996, and it has been map-projected onto a uniform longitude-latitude grid.

A false color mosaic of Jupiter s Great Red Spot from the Galileo spacecraft, made from images through 3 infrared filters.

A true color mosaic of Jupiter s Great Red Spot from the Galileo spacecraft, made from images through near infrared & violet filters.

Nov 28, 2014: The ruddy color of Jupiter's Great Red Spot is likely a product of simple chemicals being broken apart by sunlight in the planet's upper atmosphere, according to a new analysis of data from NASA's Cassini mission. The results contradict the other leading theory for the origin of the spot's striking color -- that the reddish chemicals come from beneath Jupiter's clouds.

Images taken by the Hubble Space Telescope over a span of 20 years show that the Great Red Spot is shrinking.