"Jupiter: A Vast Realm To Explore" - Greg Bryant

In 1994, many observers around the world became familiar with Jupiter's features when the fragments of comet Shoemaker-Levy 9 struck the planet. In the leadup to the event, astronomers were not expecting the impact scars to be obvious, and so many amateurs spent much time in studying Jupiter's cloud belts to be prepared for anything unusual. As it turned out, the comet impact sites were so obvious that anyone could easily identify them.

The comet has literally come and gone, yet Jupiter still remains. In 1996, I gave a talk at a meeting of the Astronomical Society of New South Wales on Comets and Solar System highlights for that year, and I was pleasantly surprised by the number of people who indicated that they had taken the time to observe Jupiter at some time during the last 6 months of 1995, a year after the comet crash. Clearly, the majority of amateurs don't discriminate when it comes to observing. Anything that's up in the sky is potential target material. Indeed, my best view of Jupiter came from Ilford, the Society's dark sky site in the early hours of January 20, 1991 during a period of what must have been exceptional seeing. Some members may recall the view. The following is from my article "Return To Ilford", which appeared in the February/March 1991 issue of Universe.

"It was now well after midnight, and this bright "star" in the eastern sky had been catching our attention since late evening. Jupiter was very prominent tonight, being only a week away from opposition. A check of the Galilean satellite tables revealed that Io would be eclipsed tonight. We were also wondering whether Jupiter itself would occupy our attention until the eclipse. As we looked at Jupiter just before the eclipse, our minds were literally blown away.

"I, let alone anyone else present at Ilford that night, had never seen so much detail on Jupiter. Looking closely at the two main belts, we could see that they were ragged.

"The belts, however, paled into insignificance when compared with the equatorial zone that lies between the two equatorial belts. David [Collis-Bird] described the equatorial zone as having been coloured by a child. Looking through the 20, 17 and 14 inch telescopes, there were streaks everywhere in this zone. Even through my 6 inch, the zone didn't look smooth.

"The appearance of Jupiter had an effect on everyone. It is rumoured that Steve [Mencinsky], who normally doesn't focus on anything within 10,000 light years, may indeed become a closet planetary observer [a prediction that in 1996 is becoming true]. Naturally, he denies this. The view also brought Joe Cauchi and David away from their photography to join the observers. Tony [Buckley] was still talking about Jupiter the following morning."

The memories of Jupiter in January 1991 are still strong. Looking at Jupiter through a telescope, three things will firstly come to mind :

  1. the flattened shape of the planet
  2. parallel markings across the planet
  3. the bright moons that circle the planet.

Jupiter is a Gas Giant, and being gaseous and rotating quickly (in just under 10 hours), it bulges quite noticeably at the equator. Indeed, around the time of opposition, it will be 47 arcseconds across at the equator, but 44 arc seconds from pole to pole - a difference of 7%. Consequently, if you choose to do a sketching of the planet, your diagram will be more realistic if you draw the outline of the planet as a slight oval rather than a perfect circle.

Cloud Features

Jupiter's cloud bands contrast quite well with each other, making them easy to distinguish. The two most obvious belt features are the North Equatorial Belt and the South Equatorial Belt, which are separated from each other by the Equatorial Zone.

The Equatorial Zone

The Equatorial Zone is seldom situated evenly around Jupiter's equator, and its width has been known to vary by nearly 10 degrees in latitude. The region is known for rapid changes in the appearance of its details, such that the appearance of features can change in a period as short as a week. Whilst its colour is typically whitish, during the past century it has had displayed tints of orange, dusky blue-green, blue-white, and yellow-brown. It has been suggested that the reddish colouring of the Equatorial Zone reappeared every 12 years (Jupiter's orbital period) whilst the darkness of the North and South Equatorial Belts increased at perihelion and decreased at aphelion.

The North Equatorial Belt

The North Equatorial Belt has been described as one of the most consistently active regions on Jupiter. It is darker and narrower than the South Equatorial Belt, and it has sometimes been seen to be divided into two or three non-continuous components. Since the mid 1980s, an area of study has been intense blue features found along the southern edge of the North Equatorial Belt, extending even well into the southern part of the Equatorial Zone. Their existence has been known since 1928, and they were photographed by the Voyager spacecraft. Nevertheless, their nature has been largely ignored. There are now at least a dozen such blue features along the Belt. Large aperture telescopes will make the colour more obvious.

The South Equatorial Belt

In contrast to the North Equatorial Belt, the South Equatorial Belt, which usually consists of two well-defined components widely separated by the non-continuous South Equatorial Belt Zone, is quite erratic in its appearance, being very quite for long periods (sometimes being invisible) and then subject to sudden outbursts of activity that follow the fading of the southern component. This activity is known as the South Equatorial Belt Disturbance and has been described as "one of the most dramatic and beautiful of all solar system phenomena" with a cyclical nature of around 3 years. In the presence of a clear South Equatorial Belt Zone and weak South Equatorial Belt (southern component), it begins with a brilliant white spot forming in the South Equatorial Belt Zone, and has sometimes been so large that it can be seen projecting from the limb. Within a few days, a small dark blob is "ejected" by the bright disturbance and is torn apart by the South Equatorial Belt Zone's winds. The result is a long dark filament. As material continues to erupt, small knots appear in both components of the South Equatorial Belt, and as other sorts erupt from the same longitude position at which the first spot surfaced, the entire South Equatorial Belt region is filled within a few weeks with a series of white ovals and numerous dark knots. Interestingly, the South Equatorial Belt disturbances originate from one of only three longitudes. If Jupiter has no solid surface, why are there specific locations?

The North Temperate Belt

The North Temperate Belt is often more bluish than any of the other belts on Jupiter, and is frequently divided into two narrow components. When the North Temperate Belt has appeared as a single broad band, it is usually followed by eruptions of dark spots. These features are very rare, however, having only been seen in 1880, 1891, 1929, 1939, and 1964. These spots usually last for only a few months, and it is worthwhile keeping an eye on the area for any further outbreaks.

The South Temperate Belt

There have been several times when the South Temperate Belt has been darker than either the North or South Equatorial Belts, and in general it is a noticeable area of Jupiter. Outbreaks of white ovals lasting for long periods, dark spots, and general chaos are common in this region.

So far, we've just discussed various areas that circle the planet. However, there are a number of individual features, some of which are transient in nature, that are well worth observing.

The Great Red Spot

The most famous feature on Jupiter is the Great Red Spot, a huge high-pressure system in the South Tropical Zone near the South Equatorial Belt that is believed to have first been seen by Robert Hooke in 1665 and Giovanni Cassini in 1666. In 1878, it became the most conspicuous marking on Jupiter and has been a key feature of Jupiter itself. Its cloudtops, while being reddish in general, have been described over time as being brick red, carrot orange, salmon pink, pale yellowish-grey, and even green-tinged white! Likewise, its dimensions have ben known to vary.

Remember of course that the Great Red Spot will not necessarily be visible when you first turn your telescope to Jupiter. As the planet rotates, there is a 50% chance that the Great Red Spot may be on the other side of the planet when you look. However, if you record the time that you observe the Great Red Spot, it is easy to calculate the approximate time of near-future observing sessions when you will sight the Great Red Spot as a multiple of Jupiter's 10hr rotation period (likewise, if you don't observe the Great Red Spot, you'll know when it won't be visible).

For the observer with a telescope 20cm and more in aperture (that's 8 inches for the old-timers), here's an interesting observational challenge: Interactions of the Great Red Spot with features on the southern edge of the South Equatorial Belt, in the South Tropical Zone, and on the northern edge of the South Temperate Belt, often result in noticeable accelerations and decelerations of the features when they encounter the Great Red Spot. Can you spot the changes?

White Ovals

Three white ovals, called "BC", "DE", and "FA", are currently visible in the South Temperate Belt. Believed to be of a similar nature to the Great Red Spot (a high-pressure anti-cyclone), they were first observed in 1939 and 1940 in the South Temperate Zone, when they extended over 90 degrees in longitude. Their appearance quickened the rotation period of the South Temperate Current by 13 seconds from its normal period of 9 hours 55 minutes 20 seconds. Since their appearance, they have contracted in size, and brightened. In 1980, they were 11 degrees in longitude length, but now occupy less than 10 degrees. There is some discussion about whether these features will gradually shrink completely, stop their contraction, or even suddenly disappear. It would be worthwhile just to occasionally keep an eye on these features to notice any change.

1998 update : two of the white ovals have now merged. Jupiter is a changing environment!

Satellites

Were it not for the glare of Jupiter, the four Galilean satellites (so-called after the discoverer Galileo) would be easily visible to the naked eye. As it is, eagle-eyed observers have been known to sight the occasional outer satellite when Jupiter is at opposition.

Io, Europa, Ganymede, and Callisto are all visible with a small pair of binoculars, and a small telescope will provide a fine view of Jupiter and its four major moons, with their configuration changing noticeably over the space of an hour.

Careful study of the satellites at opposition will reveal that they are tiny discs rather than mere points of light. Owners of telescopes of aperture 25cm and upwards viewing under circumstances allowing for magnification of 400x plus, will reveal surface details under superb seeing. The best times to look are when the satellite is transiting Jupiter, since there is a reduced glare when seen against a light background. Indeed, Ganymede's polar frosts can be detected with a telescope of only 15cm aperture because they closely match the albedo of Jupiter's zones, giving Ganymede an elliptical rather than circular appearance.

If atmospheric transparency is near the best end of the scale, you can even try to detect the colours of Jupiter's satellites. Io is yellow, Europa is off-white (making it a difficult satellite to see during transits), Ganymede is tawny grey, and Callisto is a dusky blue-grey.

Those of you who own a yearbook or read such magazines as Astronomy and Sky & Telescope will see the monthly tables that show the movements of the Galilean satellites around Jupiter. Thus, you can easily determine which moon is which.

Looking at the diagrams, you'll notice that the orbital paths regularly cross Jupiter, and here is where some of the excitement following the satellites comes into play. Because the plane of the Galilean satellite orbits lies almost in the same plane as Earth's orbit, we see each satellite (except Callisto on occasions) pass directly in front of Jupiter and the behind it on each orbit.

The shadows cast by the satellites onto the face of Jupiter as they transit the disk can be seen as small black dots in any telescope. The satellites themselves appear as bright dots against Jupiter's dark belts. However, when they are in front of the brighter zonal areas, they become harder to distinguish unless you know exactly where to look. It is often best to follow a satellite as it starts to cross the planet's face. Prior to the date of opposition, the shadow precedes the transiting satellite ie. it falls on the disc before the transit begins. After opposition, the shadows fall on Jupiter after the transit, and remain on the planet even when the satellite itself has finished its transit.

On the other side of the orbit, the satellite is eclipsed by Jupiter. In the months leading up to opposition, a satellite approaching occultation can be seen to gradually fade away as it enters Jupiter's shadow. At opposition, the shadow is directly behind Jupiter as seen by us. After opposition, the shadow is now on the other side of Jupiter, and a satellite will reappear from eclipse at some distance from the limb of the planet.

All of these events (satellite transits, shadow transits, satellite eclipses, and satellite occultations) are listed in some yearbooks and magazines, so it's worth checking these tables to prepare for the night's viewing.

Once every six years, the plane of Jupiter's equator passes through the Sun, and at these times, we can see occultations and eclipses of Jupiter by each other. 1997 was the last time that this happened.

The study of Jupiter has once again returned to prominence, with the Galileo spacecraft currently in orbit around it, and returning images until the end of 1999. It's worthwhile taking a look for yourself. Indeed, keeping an eye on Jupiter, and following the movements of its satellites, should be incorporated into any night's observing plans. Best of all, observing Jupiter is so easy, and can be done at home with the simplest of telescopes. The satellite phenomena provide for exciting events as you see the Solar System "in action" - indeed, you're observing "a miniature Solar System". Set yourself a goal to undertake a simple exploration of the Jovian realm. You'll be surprised how rewarding and fascinating it is.

(Adapted from the July and August 1996 issues of Universe)