Evidence of past volcanic activity has been found on most planets in our solar system and on many of their moons. Here are a few examples: our own moon has vast areas covered with ancient lava flows; the largest volcano in the solar system is Olympus Mons on Mars; and, hundreds of volcanic features have been mapped on the surface of Venus.
The examples listed above and most volcanic features discovered within our solar system formed millions of years ago - when our solar system was younger and the planets and moons had much higher internal temperatures.
Observed recent eruptions are limited to Earth and three other locations: 1) Io, a moon of Jupiter; 2) Triton, a moon of Neptune; and, 3) Enceladus, a moon of Saturn.
What is an Active Volcano?
The term "active volcano" is used mainly in reference to Earth's volcanoes. Active volcanoes are ones that are currently erupting or that have erupted at some time in human history.
This definition works well for volcanoes on Earth because we can observe them easily. However, beyond Earth our abilities to detect volcanic eruptions did not begin until the invention of powerful telescopes. Today a number of telescopes are available to detect these eruptions - if they are large enough. However small eruptions would not be noticed and there are not enough telescopes to watch all areas of the solar system where a volcanic activity might occur.
Although only a few extraterrestrial eruptions have been detected, much has been learned about them. Perhaps the most important discovery is the ones that have been observed so far are very different from volcanoes that occur on Earth. They are cryovolcanoes.
What is a Cryovolcano?
Most people define the word "volcano" as an opening in Earth's surface through which molten rock material, gases and ash escape. This definition works well for Earth, however, some bodies in our solar system have a significant amount of gas in their compsition. Planets near the sun are rocky and when they were active would most likely have produced silicate rock magmas similar to those seen on earth. However, planets beyond Mars and their moons contain significant quantities of gas in addition to silicate rocks. The volcanoes in this part of our solar system are often cryovolcanoes. Instead of erupting molten rock they erupt cold or frozen gases such as water, ammonia or methane.
Jupiter's Moon Io - The Most Active
Io is the most volcanically active body in our solar system. This surprises most people because Io's great distance from the sun and it's icy surface make it seem like a very cold place.
However, Io is a very tiny moon that is enormously influenced by the gravity of the giant planet Jupiter. The gravitational attraction of Jupiter and its other moons exert such strong "pulls" on Io that it deforms continuously from strong internal tides. These tides produce a tremendous amount of internal friction. This friction heats the moon, and enables the intense volcanic activity.
Io has hundreds of volcanic vents, some of which blast jets of frozen vapor and "volcanic snow" hundreds of miles high into its atmosphere (see animated image above). These gases could be the sole product of these eruptions or there could be some associated silicate rock or molten sulfer present. The areas around these vents show evidence that they have been "resurfaced" with a flat layer of new material. These resurfaced areas are the dominant surface feature of the moon. The very small number of impact craters compared to other bodies in the solar system is evidence of Io's continuous volcanic activity.
Triton - The First Discovered
Cryovolcanoes were first observed in 1989 when Voyager 2 made a flyby of Neptune's moon Triton. Since then evidence of additional cryovolcanoes has been found in the south polar region of this moon.
Enceladus - The Best Documented
Cryovolcanoes on Enceladus were documented by the Cassini spacecraft in 2005. The spacecraft imaged jets of icy particles venting from the south polar region. This made Enceladus the fourth body in the solar system with confirmed volcanic activity. The spacecraft actually flew through a cryovolcanic plume and documented its composition to be mainly water vapor with minor amounts of nitrogen, methane and carbon dioxide.
One theory for the mechanism behind the cryovolcanism (see image above) is that subsurface pockets of pressurized water exist a short distance (perhaps as little as a few tens of meters) beneath the moon's surface. This water is kept in the liquid state by the tidal heating of the moon's interior. Occasionally these pressurised waters vent to the surface, producing a plume of water vapor and ice particles.
Evidence for Activity
The most direct evidence that can be obtained to document volcanic activity on extraterrestrial bodies is to see or image the eruption taking place. The animated image of Io's eruption above is an example of this type of documentation.
One other type of evidence is a change in the body's surface. An eruption can produce a ground cover of debris or a resurfacing. The before and after images at right shows a region of Io where a resurfacing event has occurred. Without such direct observations it can be difficult from Earth to know if the volcanism is recent or ancient.
More Volcanism / Cryovolcanism Will Probably Be Discovered
Cryovolcanoes on Enceladus were not discovered until 2005 and an exhaustive search has not been done across the solar system for this activity. In fact, some believe that volcanic activity on our close neighbor Venus still occurs but is hidden beneath the dense cloud cover. It is also very likely, perhaps probable, that active volcanoes or cryovolcanoes will be discovered on moons of icy planets in the outer portions of our solar system such as Europa, Ganymede, Titan or Miranda.
This is an exciting time to watch space exploration!
(www.geology.com)
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