A detailed description of the weather systems of the Giant Planet are beyond the scope of this book. The outstanding book The Giant Planet Jupiter by John Rogers (Jupiter Section Director of the British Astronomical Association) is the definitive observers guide and acquiring a copy is a must for the keen planetary observer. Unlike any other planet in the solar system, Jupiter always has a wealth of changing detail that is visible in modest telescopes. The detail on Saturn is very subtle (except when a large spot comes along) and Martian weather is usually a subtle business, too, except for major dust storms and the shrinking and growing of the polar caps.
The easiest Jovian feature to observe (and easily the most famous) is the Great Red Spot or GRS. This rotating storm, bigger than the Earth, was probably first spotted in 1665 by Cassini in Italy although there is some speculation as to whether Robert Hooke of England observed it a year earlier. However, both of those features may not have been the GRS at all: it is just impossible to be sure. The earliest undisputed observation of this remarkable feature was in 1831 by S.H. Schwabe. The Great Red Spot is in the SEB or South Equatorial Belt and thus has a longitude in System II. However, the 9 hours, 55 minutes, and 41 seconds is only a mean figure for all the nonequatorial features and so the longitude of the GRS does drift with time. At the time of writing (2005) it is close to 100° but slowly increasing. Indeed, over the last 130 years it has moved backwards and forwards in System II, going around the globe five times! From about 1870 to the late 1930s it drifted backwards in System II almost five times. Then, in the last 60 years it has crept forwards, but at a much slower pace. From 1878 to 1882 the Great Red Spot became very prominent and brick red in color. At one point it became 40,000 kilometers wide, literally double its current width.
Monitoring the drift rate and evolution of Jupiter's Great Red Spots and the numerous other smaller spots is a major feature of Jupiter observing. The method that used to be employed to determine the longitude of a spot was to time it crossing the Jovian meridian. However, with webcam images being of such a high quality, it is just as accurate to pinpoint a feature's position with a cursor and measure it straight off, whether or not it is transiting the meridian. Perhaps the most advanced system for determining system longitudes on Jupiter is the JUPOS software developed by Hans Joerg Mettig for the British Astronomical Association.
To understand the weather systems of Jupiter it is important to start by learning that the planet can be divided up into a series of dark belts and light zones. The complete novice may only be able to identify the dark SEB (South Equatorial Belt) and NEB (North Equatorial Belt) and the dark polar regions (SPR and NPR). In addition the light Equatorial Zone (EZ) and the lighter regions between the polar regions and the NEB and SEB can be spotted even through a small telescope. Needless to say, things are a lot more complicated than this and the planet can be divided into 20 belts and zones when high-resolution images are analyzed. (see Figure 13.1)
But this is not an end to matters. The advanced observer will want to understand not only how to recognize the belts and zones but also how to expect features to drift within them. When looking at a satellite image of Earth you can spot the weather fronts but the prevailing air and weather currents are not immediately obvious. Jupiter has a system of slow currents associated with the most obvious features but it also has numerous prograding (with the planet's rotation) and retrograding (against the planet's rotation) jet streams at the boundaries of these obvious features. When looking at reports of Jupiter observers observations you may be confused by two issues. Firstly, planetary images are nearly always shown with south at the top (because astronomical telescopes produce an upside-down image in the eyepiece). Secondly, the terms p and f are frequently used. These stand for preceding and following. In other words, one object precedes another if it rotates into the observer's view first, as the planet rotates, and vice versa.
When we look at Jupiter's creamy zones, brownish belts, and other features, what, precisely, are we looking at? The Jovian atmosphere consists mainly of hydrogen (90%) and helium makes up almost all of the rest. So where do the colors we see come from? This is not a question that has a simple answer, indeed, in John Rogers' definitive book The Giant Planet Jupiter, 20 pages are devoted to discussing the vertical structure of the atmosphere, the colors seen by the observer and the "cloud" features. Even after the Galileo spacecraft mission, it is not possible to easily summarize what you are looking at when you see the Jovian disc on your PC screen, or through the eyepiece. Perhaps surprisingly, spectroscopy does not automatically provide a solution to the question of what the eyes see. The situation is highly complicated. However, some simplification is possible. The colors seen on Jupiter arise from the miniscule amounts of elements that are not hydrogen or helium.
The white clouds that make up the bright zones on the giant planet seem to consist of ammonia ice crystals floating in gaseous hydrogen. (Gases heated by the considerable internal heat of the giant planet, rise into the upper hydrogen atmosphere and cool.) These bright zones appear to be higher and colder than the dark belts.
The red/brown, gray/brown, or simply brown colors that characterize the SEB and NEB may well be made from ammonium hydrosulphide polymers.
The Great Red Spot is often the most obvious red or orange feature on the Jovian surface and the color may be caused by the condensation of phosphorus. Amateur webcam images are now of sufficiently high resolution that they routinely show the rotation of the spot itself. The GRS vortex, featuring wind speeds up to 360 kilometers per hour, rotates counter clockwise. On the outer edge one rotation is completed every 12 days, and on the inner edge one rotation is completed every 9 days. This vortex appears to be a high-pressure area that has risen by 8 kilometers above the surrounding clouds, because of convection from below.
As well as the major Jovian belts and zones, Jupiter has smaller, more subtle features whose terminology can be confusing to the beginner. The five most likely confusing or ambiguous terms encountered are festoons, barges, ovals, spots, and portholes.
A festoon is a dark, usually bluish, feature that projects into the white equatorial zone from the NEB south edge, and trails backwards against the direction of the planet's rotation. Variations on this theme exist, such as arched festoons, plumes, or simply projections.
A barge is a term first used by the Jupiter observer Captain Ainslie in 1917/18. It refers to dark oblong features with a distinct "chocolate red" or "clotted blood" coloration. To Ainslie, they looked like canal barges stranded by a falling tide on the NEB north edge. White ovals is a term generally used to describe long-lived features on the planet, whereas the term white spot generally applies to smaller, short-lived features. After the long-lived Great Red Spot, the three white ovals called BC, DE, and FA were probably the longest-lived features (but much smaller than the GRS) that have been studied since the invention of the telescope and the era of serious Jovian study. These white ovals dominated the activity in the South Temperate Region of Jupiter from 1940 until the end of the 20th century. The term portholes is often used to describe anticyclonic white ovals in a dark belt such as the NEB.
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