Exploring Volcanism in our Solar System: A complete survey of all geologically active bodies
Written by Rev. Robert A. Vinciguerra Friday, 23 November 2007 02:48
Earth has long been known to be geologically active. On Earth, such forces are responsible for continental drift, earthquakes, the rise of mountain ranges, and of course, also volcanoes.
The existence of volcanism on Earth has been known throughout all of history; even to the most primitive humans and almost certainly to human ancestors. However, it is only within the past few decades that mankind‚Äôs understanding of volcanic activity outside of Earth has been discovered.
Outside of Earth, there are four worlds where volcanism has been observed, one where it may have been detected, and more that are strongly suspected. ¬†Beginning with those that are closest to the sun and ending with objects that are far, frigid, frozen and cold, this is a journey to our solar system‚Äôs volcanoes.¬†
Ancient astronomers gazed towards Venus and saw an object of great beauty and named it after the goddess of love and splendor.
Scientists studying Venus in the early 20th century and before imagined in Venus a world that is a sister to our own. Venus is made out of the same kind of ‚Äústuff‚ÄĚ as our Earth, is very close to the same size as Earth, having 95% of the Earth‚Äôs diameter and 80% of the Earth‚Äôs mass. Thus, it was long known as ‚ÄúEarth‚Äôs Twin.‚ÄĚ
Speculation that beneath its clouded veil lies an expansive, Venus had a global tropic rain forest with exotic animals and where equally fabulous people lived. However, such conjecture quickly subsided in the mid 20th century, as remote robotic probes from Earth orbited, landed on, and mapped the planet‚Äôs hidden surface with radar.
It quickly became apparent Venus is not Earth-like at all, at least not anymore. Though it may have had large amounts of water on its surface at one time, they have since been evaporated; and though it may have once had a large moon, it has since been destroyed; and though it once may have had a strong magnetosphere, the planet now spins too slowly on its axis to generate one.
The singular kinds of features that dominate the Venusian surface are signs of volcanism. Approximately 80 percent of the planet‚Äôs surface was created by lava flows. Surveys of the planet‚Äôs surface conducted from orbit using radar have documented over 55,000 volcanoes on the planet‚Äôs surface; other sources put the number at over 100,000.
There are three main types of volcanoes that exist on Venus; shield volcanoes, pancake volcanoes, and ‚Äúticks.‚ÄĚ Shield volcanoes are similar to their Earth counterparts, the most famous of them being the Hawaiian Islands. These structures are identified by a central vent from which material spews out in a radial pattern.
The pancake volcanoes are round domes that are flattened, thus they resemble a pancake in structure. These domes are thought to consist of viscous lava. They vary in size, and many are much larger than their Earthly relatives.
‚ÄúTick‚ÄĚ volcanoes and named so because they visibly resemble a tick on Earth. Imagine volcanoes that are round like the pancakes, but have been penetrated by material on the edges, leaving the impression that they have ‚Äúlegs.‚ÄĚ These features are thought to be exclusive to Venus in our solar system, and the process that creates them is currently unknown.
The staggering amount if volcanic activity is blamed for the planets cloud cover, lack of liquid water, and its high surface temperature, a global average of 900 degrees Fahrenheit; temperatures so hot that the sulfuric acid rain that is produced in the upper-atmosphere evaporates before it can hit the ground.
Venus may have once had an Earth-like atmosphere as well, but volcanic gases have caused a run-away greenhouse effect that is responsible for the hellish surface temperature that exists today.
To date, no active volcano has ever been directly detected. This is in part due to the cloud cover; no eruption can be observed from orbit. Another reason is because of the planet‚Äôs dense atmosphere and heat; no lander could survive long enough on the surface to perform meaningful seismic experiments.
However, there is indirect evidence that may reveal recent volcanic eruptions.
The first is a study of the Venusian atmosphere. From July 1980 to August 1981 NASA‚Äôs Pioneer Venus Orbiter performed some the first detailed scans of the composition of Venus‚Äô atmosphere, including the levels of sulfur dioxide (SO2) and methane (CH4) , known to be gasses related to volcanism.
Since the Pioneer mission, subsequent robotic missions to orbit the planet, as well as ground observations, confirm that the SO2 and CH4 levels have steadily declined. A possible explanation is that a relatively recent eruption caused the presences of the gasses in the atmosphere, which is why they are dissipating in such a short timeframe.
Maat Mons, the largest volcano on the planet, rising 8 km (5 miles) above the surrounding plane, is the primary suspect for recent volcanic activity on the surface of Venus.
Mars is famous for possessing the largest known volcano or mountain in the entire solar system, Olympus Mons, which 27 km or 16.7 miles above the surrounding plane. However, it has not erupted in more than 2 million years, leaving Mars a geologically dead planet since before the time that humans evolved on Earth.
However, in 2006, a surprising discovery was made in one of Mars‚Äô coldest regions, its south polar ice cap. Mars‚Äô atmosphere consists primarily of carbon dioxide (CO2). During a Martian winter, the temperature falls to -200 degrees Fahrenheit, a point where the CO2 in the atmosphere ‚Äúfreezes out‚ÄĚ and clings to the surface of the permanent water-ice polar cap.
When spring arrives, the sun warms the darker sand and dust particles trapped between the water-ice and the clear CO2 ice. The Carbon dioxide then begins to return to its gas state. The pressure is so great that the ice slab actually becomes levitated.
Finally, cracks in the ice give way to massive jets of CO2 gas escaping back into the atmosphere, and throwing sand and dust hundreds of feet into the air, creating spider-like patterns in the ice.
This mechanism for volcanism ‚Äėis unlike anything that occurs on Earth,‚ÄĚ says Phil Christensen, the Arizona State University scientist who first published the discovery in the August 17, 2006 issue of the scientific journal Nature.
‚ÄúIf you were there you'd be standing on a slab of carbon-dioxide ice." Christensen says as he describes the scene from ground-based explorer‚Äôs point of view. ‚ÄúLooking down, you would see dark ground below the three-foot-thick ice layer. All around you, roaring jets of CO2 gas are throwing sand and dust a couple hundred feet into the air."
This type of ‚Äúice volcanism‚ÄĚ is certainly different than almost anything else that‚Äôs known in the solar system. The discovery of these Martian geysers may lead to the understanding of other extreme geological processes that exist in the solar system beyond the orbit of Neptune.
Io is the third largest moon of planet Jupiter, and the fourth largest natural satellite in the entire solar system at 3,642 km in diameter (1,942 miles). The little moon is composed of a rocky-silicon surface and is thought to have a molten iron-sulfide core. What makes Io truly remarkable is that it is the most geologically active body in the solar system.
Not only is it the most volcanic body, but it has the first active volcanoes ever discovered outside of Earth; and what a complete surprise it was. The first extraterrestrial volcanic eruption was observed in 1979 by the Voyager 1 space probe. A small, distant, and cold world, Io wasn‚Äôt supposed to be active, and no one paying any attention; Jupiter was the real interest.
However, in one photo, a blue cloud was discovered on the horizon of Io. The cloud turned out to not be a cloud at all; it was a volcanic plume spewing from Prometheus, (a caldera) the largest and most active volcano on Io. It was an immense discovery, one that prompted intense study of Io, and changed the way we think about deep space objects.
Despite size, temperature, and distance, Io‚Äôs interior was being heated by a process long known to exist on the Earth called tidal flexing. At home, gravitational forced between the moon and the Earth cause the tides to roll in. On Io, Jupiter‚Äôs gravity causes the moon‚Äôs surface to flex by as much as five stories. This constant flexing keeps the interior hot and molten, leaving Io a volcano-world for the foreseeable future.
Hot zones range in temperature from 1800 degrees Kelvin to 300 degrees Kelvin. The hotter areas produce basaltic lava flows, like on Earth, whereas the cooler areas actually spew molten sulfur.
An astronaut standing on Io‚Äôs surface would bear witness to a landscape that is smudged with yellow to orange sulfuric patterns. Geyser-like volcanoes would be erupting, sending their blue-hot plumes several miles into space. Then, sulfur dioxide snowflakes would gently drift back down to the surface.
Io may not be the only Jovian moon to be geologically active. It is widely speculated that Europa may also have volcanism triggered by tidal forces, though it cannot be seen. The indirect evidence, however, is profound.
In 1997, the Galileo space probe discovered evidence of ice flows and ‚Äúicebergs‚ÄĚ on the surface of Europa. Unlike Io, Europa‚Äôs surface is covered with a shell of water-ice. Large cracks in the ice appear to rub against each other in a fashion that is similar to tectonics on Earth, to create ice geysers, icebergs, and ice flows on the surface.
But where is the water coming from? From below the surface of the ice. It is theorized that the rocky core of Europa is heated by tidal flexing, and is littered with volcanic vents, such as those found beneath Earth‚Äôs oceans. These vents can supply enough warmth for a warm liquid water ocean to exist just a few kilometers below the surface ice.
Though humans have not sent an orbiter or a lander to Europa to map the subsurface ocean or explore it, the ocean is believe to exist based on the evidence presented, in addition to variations in the moon‚Äôs north magnetic pole and in its magnetosphere that suggest an electrical conductor, such as a liquid saltwater ocean, and it is believed to be up to 100 kilometers deep.
Unfortunately, a 2008 mission to Europa was cancelled due to budget overruns and President Bush‚Äôs new manned flight initiatives to the moon and Mars. Still, Europa‚Äôs salty oceans remain the best chance at discovering life outside of our own world.
On November 27th, 2005 the Cassini orbiter caught a cryovolcanic eruption in the act. This makes Enceladus one of only four bodies in the solar system where eruptions have been directly observed. (Two others, Mars and Io, have been previously discussed.)
The existence of geological activity on Enceladus was a surprise, even decades after the discoveries on Io. The mechanisms and the nature of the eruptions are a mystery as well.
What is known is that the eruptions originate from the south polar region of the moon, which ‚Äď for reasons unknown ‚Äď is warmer than the rest of the satellite. The eruptions come from areas called ‚Äúsulci‚ÄĚ; these are also referred to ‚Äútiger stripes‚ÄĚ due to their appearance. The tiger stripes are thought to be cracks or areas where tidal flexing causes the ice sheets to rub against each other in a way that is similar to tectonics.
The eruptions are like those of geysers, they jet icy material up to 400 km (250 miles) above the moon‚Äôs surface. This process is also thought to explain the presence of Saturn‚Äôs E ring. Material from cryovolcanoes on Enceladus is constantly feeding the ring; not only creating it but also maintaining it.
Most theories regarding the mechanisms that leads to the geological activity on Enceladus include tidal flexing, but that‚Äôs where the similarities end. Some believe that there could be a subsurface lake that‚Äôs created by heating; the ejaculate into space is steam (water vapor) that breaks through the cracks in the surface.
Other scientists do not believe that tidal forces alone are enough to melt water, suggesting that there could be other unknown chemical mechanisms at work, or that the ejected particles are not water ice, but a material that has a lower melting point.
To fully understand the forces at work on Enceladus will surely required years of additional research and a return trip to the icy moon.
The king of the Saturnian moons, Titan is the second largest moon in the solar system, is has a more dense atmosphere than Mars, and it‚Äôs larger than Mercury. It is the only moon other than Earth‚Äôs to have a probe land on its surface, and it may have active volcanoes too.
On January 14th, 2005 the Huygens probe (which was released by Cassini) landed on Titan, took pictures, and sent them back to Earth. What it didn‚Äôt find was a global methane ocean. The existence of such an ocean was thought to be the reason for the Titan‚Äôs atmosphere.
Scientists immediately began looking for a new explanation. In June 2005 NASA‚Äôs Jet Propulsion Laboratories (JPL) and the European Space Agency (ESA) issued a press release stating that they may have discovered a cryovolcano on its surface.
The feature is 30 km (19 miles) in diameter and resembles a volcanic dome. Cryovolcanism could release atmospheric gasses into the Titan sky, enough to sustain the dense atmosphere. Additionally, there has been no evidence found for craters found on the surface, suggesting that active volcanoes may be responsible for ongoing resurfacing. However, methane and ethane rainfalls combined with winds may also be responsible for erasing impact craters from Titan‚Äôs surface.
The volcanic mechanism, as with Jovian moons, would likely be tidal forces between Saturn and Titan; but the only evidence for active volcanoes in circumstantial. ¬†Given the potential for science return, it is certain that Earth will send additional probes to the Titanian surface. Sooner is better than later.
The giant moon of the outer solar system, Triton is also the largest moon of Neptune. It is comparable to Pluto in size, composition and mass; these facts, combined with its retrograde orbit, leads astronomers to believe that Triton is a captured Kuiper Belt Object (KBO).
In August, 1989 the Voyager 2 space probe made its closet flyby and revealed that Triton is as interesting of a world as many of the more dynamic moons of Jupiter and Saturn. Photos of the moon revealed a tenuous atmosphere containing wispy clouds of nitrogen ice only 13 kilometers above the surface.
More shocking, however, were images of active volcanic activity that was beamed 4.3 billon kilometers (2.6 billion miles) back to Earth. These geyser-like cryovolcanic eruptions shoot up, like black smokestacks, into the upper atmosphere, where winds sheer them off, creating a smudgy ‚ÄúL‚ÄĚ shaped appearance.
The volcanoes are thought to spew methane or nitrogen gas. Heat generated by tidal flexing with Neptune could create pools of liquid methane and nitrogen close to the surface. Because the liquid gas is less dense than the ice and rocky material of Triton‚Äôs crust, it comes to the surface and vaporizes due to the low pressure of the atmosphere (15 microbars).
Voyager 2‚Äôs images of active ice geysers on Triton make it the fourth object directly observed during an active geological event (Mars, Io, and Enceladus being the other three).
There has been some thought that objects like Pluto‚Äôs companion Charon and the KBO Quaoar may also have cryovolcanoes present and active on their surfaces due to the detected presence of crystalline ice, but it is all highly speculative and none of it concrete.
While most astronomers believe that the odds of volcanism existing in such far-flung reaches of the solar system are unlikely, mankind may soon have the answer with the arrival of the New Horizons space probe in 2015, which will flyby the planet Pluto (or dwarf planet, for those who prefer to call it that) and then move on to encounter other KBOs until the year 2020.
Each new volcanic discovery made in the solar system leads to a further understanding of natural processes, not only ones that exist ‚Äúout there,‚ÄĚ but ones that exist right at home as well. Venus is a model for what happens when global warming takes over; Titan is a pristine example of a frozen embryonic Earth, and Europa just may be hiding vast arrays of well-developed marine life, just waiting to be discovered; all made possible by volcanoes throughout our solar system.
Further missions these worlds and to others will expand mankind‚Äôs understanding of the cosmos, as well as life on Earth.