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Where the hot stuff is: volcanism takes a variety of forms on other worlds.

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VOLCANIC ERUPTIONS are among nature's grandest spectacles, and thanks to interplanetary spacecraft, we can see signs of current and past activity on other worlds. Volcanic plumes rise hundreds of kilometers from the surface of Enceladus, lava lakes up to 200 kilometers (120 miles) across are bubbling away on Io, and Mars's Olympus Mons rises nearly three times higher than Mount Everest.

But our spacecraft catch only a glimpse in time of the geology of other worlds. Unless volcanoes are constantly erupting, we would have to be very lucky to catch one in the act. For example, only a small number of Earth's 600 active land volcanoes are constantly on the go. Stromboli in Italy may take the prize, erupting almost continuously for at least 2,000 years.

Most of our world's active volcanoes are hidden under the sea, dotting the boundaries of tectonic plates. Earth is the only known body to have plate tectonics. This is probably due to the planet's size and crustal thickness. If the crust is too thick relative to a planet's size, plate tectonics can't get started. If the crust is too thin, it might break up into so many pieces that it will form a jumbled mess. Earth appears to be at the "Goldilocks" point for plate tectonics.

Even though many of Earth's volcanoes form near plate boundaries, other worlds clearly show that volcanoes can exist without plate tectonics. Volcanoes represent a body's effort to rid itself of internal heat. That heat can either be left over from a body's formation, or in the case of Io, the result of some external force. The variety of features we see on other worlds demonstrates that volcanic activity can manifest itself in a multitude of ways.

Remarkably, many extraterrestrial volcanoes look like their terrestrial cousins, though few resemble the steep, foreboding Mount Doom of Lord of the Rings. Depending on the lava's composition, eruptions can be gentle or violently explosive. Sometimes lava pours out onto a planetary surface and just spreads out, forming flat plains such as the lunar maria. Fluid basaltic lavas often build up shield volcanoes, such as those in Hawaii and Iceland. On explosive volcanoes such as Arizona's Sunset Crater, the fragments of lava that landed close to the vent built a relatively small but steep-sided cone. The combination of lava flows and explosive eruptions over long periods builds the tallest volcanoes, such as Mount Fuji in Japan.

A volcano's shape provides clues about the type of eruption and magma. Explosive volcanoes on Earth usually erupt silicon-rich lavas (andesites) rather than sodium-rich basalts. Using remote sensing to relate a magma's composition to eruption type is important for planetary studies, because sending human field geologists to distant worlds is a daunting proposition that lies far in the future.

Volcanoes on the Moon and Mercury

Our Moon once had vast oceans of liquid lava, but today it's volcanically dead--its last eruptions occurred about a billion years ago. Analysis of rocks from various maria show that their lavas were very fluid basalts that spread relatively quickly to cover vast areas; it would have been an amazing sight! But there were no Mount Dooms on the Moon, only a few small cones and domes, probably formed in short-lived eruptions of alternating lava and ash.

Apollo astronauts returned rock samples with intriguing glass beads, formed when tiny droplets of magma erupted into cold space, froze, and then fell back to the surface. What caused some lunar eruptions to be explosive when mare activity appears to have involved outpourings of fluid lava? Most likely, small pockets of gases (particularly sulfur and carbon monoxide, traces of which are found on the glass beads) propelled the magma into lava fountains perhaps similar to those in Hawaii.

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Mercury and the Moon look superficially alike, with impact craters dotting their surfaces. But Mercury has no maria covered by vast expanses of lava. The question of Mercury volcanism remained open until recent results from NASA's Messenger spacecraft. Like the Moon, Mercury volcanism appears to have been both explosive and effusive. Scientists have identified an irregular depression surrounded by a diffuse halo of bright material northeast of Rachmaninoff basin, interpreted as an explosive volcanic vent. Messenger entered Mercury orbit in March 2011, so we can expect the story of the innermost planet's volcanic history to slowly reveal itself.

Venus: Land of Volcanoes

Volcanic materials cover at least 90% of Venus's surface. Sinuous channels thousands of kilometers long run across plains formed by lava flows. The lavas must have been extremely viscous to flow such long distances. They may be ultramafic lavas, whose high magnesium content makes them very fluid. Such lavas erupted on Earth millions to billions of years ago. When Venus had active volcanism, the planet must have been even more hellish than it is today.

How recently were Venus's volcanoes active? The surface has relatively few impact craters, so it must be young. In the late 1970s, NASA's Pioneer Venus spacecraft measured a steady decrease in sulfur dioxide (SO2) above the cloud tops. An earlier massive volcanic eruption could have pumped large amounts of SO2 into the atmosphere and by the late 1970s the SO2 was slowly breaking down.

More recently, some exciting results emerged from Europe's Venus Express orbiter. The spacecraft's Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) measures how much energy the surface radiates at a wavelength of about 1 micron. Fluctuations across the surface relate to variations in chemical composition. When scientists correlated VIRTIS data with Magellan radar images and topographic data of the surface, they found that three areas, Imdr, Themis, and Dione Regiones, are anomalously bright. Taken together, these results indicate relatively recent lava flows (S&T: July 2010, page 20). The fact that these flows are so pristine is evidence that they are considerably younger than most others on Venus, possibly 250,000 years old or younger--a blink of an eye in geologic time.

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This finding fuels the debate of whether Venus was catastrophically resurfaced between 300 million and 1 billion years ago. Proponents of the hypothesis argue that volcanoes covered most of the surface relatively quickly with molten flows that buried or destroyed any preexisting craters. Since then, volcanism has not occurred and only occasional impacts have changed the surface. The fact that VIRTIS data shows several young flows is inconsistent with this idea, but it doesn't pound a nail in the coffin either. Defenders of catastrophic resurfacing argue that it's unnecessary for all volcanism to have stopped. The debate goes on.

Towering Volcanoes on Mars

Mars features enormous shield volcanoes, vast plains of lava flows, numerous lava channels, domes, cones, and considerable evidence of explosive volcanism. Olympus Mons is the solar system's largest volcano, standing 24 km high. Its 600-km-wide base would cover Arizona. Measurements from various orbiters and landers show that most Martian volcanic deposits are similar to terrestrial basalts and andesites. NASA's rovers Spirit and Opportunity found plenty of evidence of volcanic rocks and even ash--the layers Spirit found at the Columbia Hills resemble ash fall or ash flow deposits.

Despite widespread volcanic features and multiple spacecraft studying Mars, we have not found any conclusive evidence for current volcanic activity. But there's plenty of evidence of recent volcanism. Analysis of Martian meteorites show that the most recent of these rocks crystallized out of lavas about 170 million years ago, young in planetary terms. High-resolution images from Europe's Mars Express orbiter show few impact craters over some volcanic areas, leading geologists to conclude that some of the lava flows are less than 2 million years old. It's possible that Martian volcanism is not yet dead.

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Measurements of methane (CH4) gas are also intriguing. The atmospheric methane concentration is only a few parts per billion, so ground-based measurements from Earth are particularly challenging. But the presence of methane is very significant, because chemical processes in the Martian atmosphere destroy the gas, so its lifetime is less than a few hundred years--indicating that something is replenishing it.

Recent data confirmed the presence of methane and showed that it's concentrated in particular locations (S&T: April 2009, page 20). The source could be either volcanic gas or biological activity. While many scientists view volcanic gas as more likely, the degassing implies that hot magma still lies below the surface. If there is heat, there could be liquid water, and possibly microbial life. We look forward to future missions, such as NASA's Mars Science Laboratory (also named Curiosity), a Mini-Cooper-size rover that carries instrumentation capable of detecting methane at the level of a few parts per billion. In 2016 the European-led ExoMars orbiter will study trace gases in the Martian atmosphere and how they vary with time.

Frenetic Volcanism on Io

Jupiter's large moon Io is the most volcanically active body in the solar system. Io is just 5% larger than our Moon. If it had been left alone, it would have cooled and formed a thick crust long ago. But Jupiter's gravity pulls on Io so strongly that it creates a tidal bulge. The moons Europa and Ganymede distort the bulge as they pull it toward them. This tidal friction generates heat, which keeps Io's interior molten. Remarkably, scientists predicted Io's volcanism on the basis of these orbital interactions before the 1979 Voyager 1 flyby revealed Io's frenetic volcanic activity. NASA's Galileo and New Horizons spacecraft, and ground-based observations, have since revealed many more active volcanoes; we now know of nearly 200 and undoubtedly there are many more.

Most of Io's volcanoes are caldera-like depressions. Loki, about 200 km across, is the largest caldera in the solar system. Io's volcanoes rarely build structures such as shield volcanoes or domes, but long lava flows are common. The largest active flow field in the solar system reaches some 250 km from Amirani. Io's impressive lava flows are possibly similar to continental flood basalt lavas on Earth, such as the Columbia River basalts in the northwestern U.S. These terrestrial lavas flowed tens to hundreds of millions of years ago. Studying Io's volcanism thus gives us a window into Earth's past.

Io's surface is nearly entirely covered by SO2 frost, which condenses from spectacular volcanic plumes, some of which rise several hundred kilometers high. The last spacecraft to fly by Io, New Horizons, captured remark able images of the plume over Tvashtar.

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We still don't know the composition of Io's magma. Temperatures at active hot spots provide the best clues to magma composition, since the temperature at which different types of magma melt depends on their composition. Galileo measurements of Pillan showed temperatures of about 1500[degrees]C, too high for basalt (basalts on Earth rarely exceed 1200[degrees]C). More recent calibration of these data lowered the temperature to around 1300[degrees]C, still unusually high for basalts but possible in some circumstances. Io's magmas might be ultramafic, similar to those suggested for Venus and ancient Earth, or basalts.

The Exotic Cryovolcanoes

In the icy moons of the outer solar system, materials erupt that would be frozen solid at their normal surface temperatures. For this so-called cryovolcanism to occur, the body must have interior liquid water (probably with other constituents such as ammonia) that can rise to the surface to erupt. Spacecraft have detected possible cryovolcanic features on several icy satellites, but active cryovolcanism has only been confirmed on Saturn's medium-sized moon Enceladus.

The discovery of active volcanic plumes on Enceladus was one of the most exciting of NASA's Cassini mission, though it was not entirely unexpected. Enceladus's sur face appears smooth and young, with few impact craters; it's also highly reflective, indicating that it's covered with pristine ice dusted with fresh materials. As Cassini flew through an extended plume on July 14, 2005, its instruments detected water vapor, methane, and carbon dioxide, as well as heat over one of the long parallel rifts ("tiger stripes") in the south polar region. Images revealed spectacular plumes originating from the tiger stripes, which are about 2 km across and up to 130 km long.

Enceladus is embedded in Saturn's tenuous E ring, which is replenished by fine particles ejected by the plumes. The plume material is being shot out at about 60 meters per second (135 mph). The plumes are probably linked to subsurface reservoirs of liquid water, though their depths remain unknown. These reservoirs could be potential abodes for life because they have biology's key ingredients: heat, liquid water, and organic compounds.

Scientists are debating the existence of cryovolcanism on Saturn's largest moon, Titan. Some flow-like features appear to be rivers of methane. But recent analysis of radar data shows the topography of the Sotra Facula region. A 1-km-high mountain lies next to a deep pit and flow-like features. There are no river channels nearby that could have created these flows, the pit is not circular like an impact crater, and the mountain is isolated and not part of a chain. This combination of features is difficult to explain with fluvial or other nonvolcanic activity. Although the debate goes on, Titan could be yet another world where heat and liquid water coexist. Thanks to volcanism, the conditions for life may occur even in the frigid environments of the outer solar system.

Seeing new worlds up close helps us better understand our own. We can use other worlds as "volcano laboratories" to understand what happens if conditions are vastly different from those on our home planet. Thanks to interplanetary missions, we now know that volcanism can occur in the absence of plate tectonics, in worlds where the magma is water rather than molten rock, and where surface temperatures and pressures are much higher or lower than Earth's. As we explore new worlds such as Pluto and asteroids, we may find more volcanoes, maybe some that erupted in the distant past, maybe some that are currently active. And as we learn more about exoplanets, we may learn that some sustain volcanism due to tidal heating or other processes. Stay tuned!

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MARS

The Red Planet was at one time a volcanically active planet, with giant volcanoes that dwarf those on other planets. But as Mars's interior cooled, activity has dwindled to the point where the planet shows no signs of current volcanism. Top: This false-color perspective image of the volcanic cone Nili Patera is derived from images taken by NASA's Mars Reconnaissance Orbiter. The cone is about 5 kilometers across, and it features two areas of hydrothermal deposits (arrowed) in the foreground, evidence of recent activity that might be indicative of potential abodes for microbial life. Bottom left: NASA's Mars Global Surveyor acquired this image of the isolated volcano Apollinaris Patera. The number and sizes of impact craters on the flanks suggests that the 5-km-high volcano was probably last active about 3 billion years ago. Bottom right: This false-color image from the European Space Agency's Mars Express orbiter shows 24-km-high Olympus Mons, the largest mountain in the solar system. The colors represent different elevations on this ancient shield volcano. Rising magma from a hot spot in Mars's interior formed Olympus Mons. Because of Earth's moving plates, similar hot spots on our planet produce chains of volcanoes.

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Sunlight-Driven Volcanism

When Voyager 2 flew by Neptune's large moon Triton in 1989, its pictures showed dark streaks that originate from rising geyser-like plumes. Scientists think the plumes are produced when sunlight penetrates a transparent surface layer of frozen nitrogen. Dark, carbon-rich impurities a few meters below the surface absorb and trap the sunlight. Sunlight this far from the Sun is feeble (only >900 the amount received by Earth), and warms Triton's surface to only -235[degrees]C (-391[degrees]F). But this minimal amount of heating is enough to vaporize the icy nitrogen near the surface. The ice expands and then explodes into the near-vacuum of space, forming the plumes. If this model is correct, active plumes on Triton are a side effect of sunlight rather than an internally driven process--demonstrating yet another example of nature's creative ability to produce different styles of volcanism.

Planetary volcanism expert Rosaly Lopes is a Senior Research Scientist at NASA's Jet Propulsion Laboratory. Her fifth book, Volcanoes, was published in January as part of the Oneworld Beginnner's Guide series.
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Title Annotation:Volcanoes in the Solar System
Author:Lopes, Rosaly
Publication:Sky & Telescope
Geographic Code:1USA
Date:Jul 1, 2011
Words:2718
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