The Great Dark Spots

Neptune's First Great Dark Spot
Image Credit:

Voyager Project, JPL, and NASA

The Great Dark Spot is actually a series of storms that were first discovered by Voyager 2 in 1989. They are anticyclonic (weather systems with high pressure eyes) much like the Great Red Spot on Jupiter. However, unlike the Great Red Spot, these storms are generally cloud-free and only last a few months to few years.

The winds associated with the Great Dark Spots are the fastest known in the solar system, topping out at 2400 kilometers per hour (about 1500 miles per hour). They are thought to be holes in the methane cloud deck, occuring in the troposphere at lower altitudes than the clouds. The first spot to be discovered varied in size and shape as it was viewed from Earth and there had been a plan to photograph the storm with Hubble space telescope in 1994. By the time, Hubble was in position to take an image, the storm had dissipated. However, the spots do reappear on Neptune, and a new storm appeared in the northern hemisphere of Neptune. Unlike the Great Red Spot which is a single storm, the Great Dark Spots is a series of storms with similar appearances and properties to each other.

One of the prevailing theories of what happens to the storms is that as the storms migrate towards the equator of Neptune, the storms break up and disappear. Also, clouds usually appear outside the storms so may indicate that a storm has just dissipated or may appear soon.

The storms themselves are relatively stable because they are vortexes, but again, don't last as long as the Great Red Spot.

The Hexagonal Storm on Saturn

Saturn's Hexago

Image Credit:

NASA/Cassini

North Polar Hexagon on Saturn

Image Credit:

NASA/JPL-Caltech/Space Science Institute

 

One of the strangest features on Saturn is the hexagon-shaped storm in its northern hemisphere. The storm was first discovered by Voyager in 1981-1982 and has been persistent since. It is located at 78° N latitude, but does not change longitude over time, unlike other storms on Saturn. The sides of the storm are about 13,800 km, longer than the Earth's diameter of abotu 13,500 km.

The Cassini mission also has taken images of the storm since its arrival in 2006. And if the conditions are right, i.e. the storm is in daylight, fuzzy images of the storm can be seen from Earth-based telescopes, even by amateur astronomers.

Oxford University astronomers proposed a hypothesis for the formation of the storm. In the lab, regular shpaes were created in a circular tank of liquid that had different rotation rates at the center and at the edges. Squares, hexagons, and octagons were all created, with hexagons being the most common shape. These latitudinal gradients in the rotation are one probable cause for the hexagon on Saturn.

The Great Red Spot

Jupiter's Great Red Spot just above a smaller, white storm

Image Credit:

NASA

 

Jupiter's Great Red Spot is, by far, the largest storm in the solar system. It is located in the southern hemisphere of Jupiter and has been around since it was first officially discovered in 1668. There is evidence however that it may have been discovered in 1635. At over 340 years old, it is the oldest known storm in the solar system.

Since it's original discovery, it has undergone changes in its size, being anywhere from 2 to 3 Earth diameters. It has ranged from 24,000 km to 40,000 km on its long axis (along lines of latitude) and 12,000 km to 14,000 km on its short axis. Since 1904, the Great Red Spot has decreased to half its longitudinal extant (its long axis is half as long as it was in 1904), and it is estimated if the trend continues, the Great Red Spot will become more circular by 2040. This is assuming that the storm is finally petering out, or if this is just a natural cycle of the Great Red Spot.

Comparison of Earth to Jupiter. Lower right of Jupiter is Great Red Spot

Image Credit:

NASA

The spot itself does not change latitude, but has been in the same location relative to the equator since its discovery. Unlike storms on Earth, the Great Red Spot is constrained to its latitude because of the banded atmosphere, and the sharp boundaries between the bands.

Voyager imaging also showed that the Great Red Spot is colder than the surrounding gas, which implies that the center is higher in altitude than the rest of the atmosphere. If you recall from the post on sunspots, the center of a sunspot is colder than the rest of the Sun, and sits below the surface. However, atmospheric science tells us that the cooler the gas, the higher the gas is. The Great Red Spot is 8 km above the surrounding gas.

The Great Red Spot has been observed to rotate counter-clockwise (anticlockwise) in about 14 Jovian Days (140 Earth hours). The speeds at the edge of the spot have been clocked at 432 km/hr (270 mph). On Earth, a Category 5 hurricane achieves wind speeds of 158 mph and an F6 tornado reaches 300 mph. In both cases, however, the atmosphere blown around by the hurricane or tornado is much less dense than the gases on Jupiter. Also, F6 tornadoes only are about 2 miles in diameter, which is still pretty big, and do not last as long. Granted, if you've been unlucky enough to have been through a tornado (and an F1 at that), they do seem to last forever. Imagine having to experience a Category 5 hurricane or an F6 tornado for over 300 years. If we were to equate an F6 tornado to the size of the Great Red Spot, the F6 tornado would span the entire US from coast to coast.

Scientists do not know why its red, but it is believed to be due to organic molecules, sulfur compounds, or phosphorus compounds. The shade of red has changed over time from pink to deep red and back again, so it is likely that the color is due to a combination of all three compounds.

The Great Red Spot has a little brother (sister?) called Oval BA or Red Jr., which is south of the Great Red Spot, and a little farther to the west. Like the Great Red Spot, Oval BA is red and has persisted for many years, being discovered in 2000 after three small white storms collided and merged.

The Southern Hemisphere of Jupiter.

Image Credit:

NASA and Christopher Go