The Great Red Spot is a gigantic high-pressure system that is likely to have raged on Jupiter continuously for the past 340 years, ever since it was first observed by man. During this period it has fluctuated in both size and colour; its famous reddish hue thought to be caused by ultraviolet irradiation of phosphorus or sulfur compounds. To put its size in perspective, this anticyclone could engulf the Earth two or three times over; not surprisingly its longevity may be attributed to the cannibalisation of other atmospheric storms. Oval BA (Red Spot Jr’) is the product of three smaller storms, and encounters its bigger brother approximately every two years. Its recent transformation from white to brown to red may be linked to an increase in wind strength.
Disturbances such as these are not exclusive to Jupiter. Unlike the rocky inner planets, gas giants do not have solid surfaces, allowing for weather systems of greater intensity and duration, unimpeded by landmass and fed by internal heat. In fact, these outer planets may emit two or three times as much energy as they receive from the Sun. Uranus, the exception, has a distinct lack of striking features, although its extreme axial tilt does result in each pole experiencing 21-year-long seasons of complete sunlight and darkness, much like parts of Antarctica.
Despite its distance from the Sun, Neptune lays claim to the fastest winds in the Solar System at 2,400 km/hr, near the Great Dark Spot observed in 1989 during a flyby of Voyager 2. Also captured was a small storm with a bright centre, named the Wizard’s Eye, and the Scooter, a white cloud to the south. However, they do not appear to be as long-lived as their Jovian cousins with the GDS dissipating by the time the Hubble Space Telescope imaged Neptune again in 1994, only to be replaced by another spot.
Saturn’s equivalent is unsurprisingly called the Great White Spot (can you spot a trend here?) but in 2004, scientists analysing the images sent back by the Cassini orbiter discovered a powerful thunderstorm which has continued to this day. This intriguing offshoot of a terrestrial phenomenon was dubbed the Dragon Storm’. When the probe was sent to investigate the southern hemisphere it discovered a massive hurricane, inexplicably remaining fixed over the planet’s pole. More recently, there has been infrared confirmation of a bizarre hexagonal pattern above Saturn’s north pole. This is bound to excite conspiracy theorists with a penchant for crop circles.
One might be forgiven for thinking Mars a mundane attraction at best, with its relatively mild climate, tenuous atmosphere, and seemingly barren exterior. But irrespective of its size, Mars punches well above its weight; it may once have supported life and is also the planet that most closely resembles our own.
Valles Marineris, the so-called Grand Canyon of Mars’, is a giant rift valley 4,000 km long, 200 km wide, and in some places up to 8 km deep, almost 180 from the Hellas impact basin, a similarly stunning crater 2,300 km in diameter. One theory proposes that the Mariner Valley was formed as the Hellas impactor sent shockwaves through the planet’s interior, resulting in a gigantic rupture on the opposite side and the Tharsis bulge, on which our next feature is located.
Olympus Mons, the largest volcano in the Solar System, rises 27 km above the Martian surface three Mount Everests piled on top of one another would still not reach the summit! With a diameter of 600 km and a gentle slope averaging 2 to 5, it is structurally similar to Mauna Loa, a shield volcano in the Hawaiian island chain. Its immense size is possible because of its planet’s low gravity (38% that of Earth) and lack of active plate tectonics. Unlike the terrestrial crust, Mars’ thick lithosphere is one immobile plate, allowing fluid lava from a hotspot to accumulate on the surface over a very long period of time. However, like the rest of the Red Planet, Olympus Mons is currently dormant.
Mars has also been known to exhibit some occasional bad behaviour’; while its dynamo was shut down billions of years ago, it is still subjected to weather of a different sort. Dust storms have been suggested as a cause of global warming on Mars which has led to shrinkage of its ice caps.
Speaking of climate change, Venus, the second innermost planet, closest in size and mass to our own, is an example of the greenhouse effect run rampant, with carbon dioxide making up about 96.5% of the atmosphere. Surface temperatures reach over 400C and atmospheric pressure at the surface is 90 times that at sea level back on Earth essentially what it would feel like with 1km of ocean above your head. The Venus Express orbiter launched in 2005 has so far revealed a multitude of fast-moving, turbulent clouds, which are thought to rain sulfuric acid, perhaps intensified by recent volcanic eruptions.
For the adventurously inclined, look no further than Io, one of the Jovian moons. A curious looking surface splashed with yellow, red, green, and brown has often attracted comparisons with a certain foodstuff you would find at your local delicatessen. Io’s close proximity to Jupiter and to a lesser extent its sister satellites generates tidal heating within its core through immense gravitational forces, leading to the extreme volcanic activity shaping the landscape today. Continuing with the theme of planetary records, the average temperature of Ioian lava is a scintillating 1,600C!
In stark contrast, Titan, Saturn’s largest satellite, is often described as an early Earth with its dense atmosphere largely consisting of nitrogen and lacking in oxygen. This, combined with its exposure to solar radiation, recreates some of the conditions which spawned the first organisms on our planet in its primordial stages. Volcanism is thought to be an important contributor to this chemical complexity. On Titan is a different sort; the cryovolcano is an interesting variant of the typical fiery ash and smoke, instead spewing methane, ammonia, and water ice. It is one extraterrestrial phenomenon which does have its peer on Earth.
Enceladus is one of the few places in the Solar System thought to hold liquid water, another essential element of our carbon-based existence. The blue tiger stripes’ are actually geysers powered by processes similar to that on Io, ejecting material hundreds of kilometres high. This discovery is particularly exciting as water vapour may exist very close to the surface of this miniscule moon. On the other hand, Europa, the smallest of the Galilean moons, may hold a vast and salty subterranean ocean under its bright layer of ice, perhaps home to a new breed of extremophilic bacteria.
Since Pluto’s demotion, Mercury has the most eccentric orbit and is the smallest of all the planets, its scarred surface characterised by two huge ring craters, the Caloris and Skinakas Basins. One theory suggests that Mercury was once much larger but had its outer layers vaporised by its close proximity to the Sun. To date, Mariner 10 is the only spacecraft to have mapped part of its surface. A new probe, MESSENGER, was launched in 2004 and will reach the planet in several years’ time.
What is the reason for all this observation and study? Scientists dedicate so much of their time to these features because extraterrestrial systems are thought to be very similar to our own, only more stable, long-lived, although at times, just as unpredictable. As counterintuitive as it seems, studying them may also give us an insight into our own planet’s workings. More to the point, humans are a curious species and will always want to explore their surroundings.
Apart from these mainstays of planetary science, there are many other wonderful spectacles not listed here; some have been discovered back on Earth, in orbit by the HST, or even observed first hand’ by our probes, launched years or decades in advance. Many are simply hypothesised to exist with no definitive evidence. It is important to note that most are found in some form or another on the Earth itself. Although we may not be the largest, the fastest, or the strongest, our world has the greatest diversity in all the Solar System and particularly endearingly, it is the only one supporting life as we know it!
