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Space Topics: Planetary AnalogsStars Above, Earth BelowAstronomy and Space Exploration in America's National Parks
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Mammoth Hot Springs in winter
Volcanic hot springs battle with freezing conditions and several feet of snow near Yellowstone's northern entrance. Ultimately the melted snow will drip its way into Yellowstone's plumbing and will one day come back up as a part of the hot springs itself. Credit: Tyler Nordgren |
Cold Faithful
Yellowstone National Park -- When most people think of volcanoes, something very specific comes to mind. They think of Mount Saint Helens, or Mount Fuji. They picture a stratovolcano, an enormous cone with a black cloud coming out the top and probably molten lava pouring down the side. But the lava they picture is probably the type that erupts from Hawaiian volcanoes, which isn't at all like the viscous, gassy material that comes from stratovolcanoes; Hawaiian lava is a runny, low-viscosity fluid that spreads out into big broad shield volcanoes. So, all in all, while everyone thinks they know exactly what is meant by the word "volcano," what they actually picture is a conglomeration of many aspects of what a volcano could be.
So now consider Yellowstone National Park. In a new book called Super Volcano, author Greg Breining says virtually every aspect of the park, its geology, its geography, its biology, chemistry, and history is “all about the volcano.” But where’s the volcano? There is no giant cone. There are no rivers of lava. There is no gaping crater. The list goes on, but in reality they are all here. They’re just in forms that no one really notices.
Except, of course, for the giant steaming pools and geysers.
In the middle of winter it is obvious that something is going on when everything is covered in three feet of snow, except for the strangely warm spot of land next to the giant bubbling pool of water. At its heart, this is what volcanism is: the interior heat of a planet finding some way to escape. Big terrestrial planets with lots of radioactive elements deep inside generate a lot of internal heat that takes a long time to seep out and cool the planet off. Five billion years after the Earth formed, we still have a nice toasty interior, looking to find all sorts of creative ways to radiate that heat to space.
Old Faithful above and below
At the noontime eruption of Old Faithful, the park ranger's talk focuses on how geysers work. In her hand, Megan holds the diagram of Old Faithful's plumbing, which was determined by Susan Keiffer, who lowered a camera into the geyser vent between eruptions. Kieffer is a geologist and planetary scientist at the University of Illinois at Urbana-Champaign. Credit: Tyler Nordgren |
Eight to thirteen kilometers (five to eight miles) underneath Yellowstone, that internal heat has produced a molten plume of rock. How deep it extends and where exactly it is produced are still the subjects of some debate. Regardless of its origin, the end result is the same: the molten rock generates a lot of energy, which heats up the surrounding bedrock. Water that falls as rain or snow on this high plateau in the Rocky Mountains then percolates through the hot plumbing system of cracks in the rock caused by eons of earthquakes (the result of movement of that magma plume). Where cracks reach the surface, especially ones with some series of narrow constrictions like a bottle’s neck, relatively cool water, in contact with the air, serves as a lid on the hot water at depth, which continues to build up pressure and become superheated. Eventually the deep water begins to boil, steam bubbles rise, and the cool water cap is splashed out on the surface. Shake a bottle of soda, pop the cap, and the pressure difference between inside and out causes the gas in the liquid to escape in a frothy geyser of cola. It’s the same thing with Old Faithful. Watch an eruption and you are seeing one of the final energetic steps in a saga that started deep within the Earth. Geysers are one of the ways the planet cools.
How does this play out on other planets? A smaller body like the Moon lacks much radioactive material to begin with and has, compared to Earth, much larger surface area relative to its volume, so its internal heat has long since escaped to space. It’s a hot potato that’s long ago cooled. Look at the Moon and its lack of any geologic activity (other than sitting around and being pummeled by space debris) is obvious. Since our Moon is one of the larger ones in the solar system, its lack of internal heat and exciting geological processes should typify what you would find out there around the other planets.
Grand Canyon of the Yellowstone
Thomas Moran painting of the Grand Canyon of the Yellowstone made in 1872, when Moran was a member of the Hayden Survey. It is one of the works of art that opened the eyes of the American public to the geologic wonders of the West. Credit: Department of the Interior Museum |
Volcanic plume over Loki
Voyager 1 image of Io showing the active plume of Loki on the limb. The heart-shaped feature southeast of Loki consists of fallout deposits from the active plume Pele. Voyager 1 passed Jupiter in 1979, almost exactly 100 years after Moran visited Yellowstone. Credit: NASA / JPL |
But just as the early scientific expeditions out west to places like Yellowstone brought back reports of strange, new, geologic wonders, the same is true every time that modern, un-manned scientific expeditions travel to the planets. In the 1870s the Hayden Survey to Yellowstone sent back to the East Coast the first black-and-white photographs by William Henry Jackson and stunning color paintings by Thomas Moran. The presentation of these new images to the world was instrumental in Yellowstone becoming the world’s first national park. A hundred years later the color images of the planets and moons of the outer solar system returned by the Pioneer and Voyager spacecraft were instrumental in sharing the beauty of the cosmos with the public. These images launched me on a career in astronomy.
At Jupiter, Voyager discovered volcanoes. While Moran’s painting of the Grand Canyon of the Yellowstone is an iconic representation of the riot of color at play in the walls of the canyon due to the spectacular volcanic geology, Voyager 1’s color images of the equally colorful Io with volcanic plumes rising over its riotous limb, are iconic images of NASA’s exploration of the solar system.
Granted, the internal heat for moons like Io and Europa around Jupiter is different than for planets like the Earth. For these moons, about the size of our own dead Moon, the internal heat is generated by tidal heating due to nearby Jupiter’s huge mass. (For more information see the post in which I describe tidal heating as I wander in Acadia National Park). But while the source is different, the effects are broadly the same. On Io, where all the water has long ago boiled away to space, basaltic lava flows and lakes are a dark black amid a golden wasteland of sulfur compounds. With every change of winds here in the Upper Geyser Basin surrounding Old Faithful, I wonder if the spacesuit an astronaut might someday wear on Io would completely block the smells I’m experiencing here on Earth. For me it’s the smell of volcanism.
For comparisons, Yellowstone’s caldera, the collapsed surface above the most recently erupted magma chamber, measures 55 by 80 kilometers (35 by 50 miles). Tupan Patera on Io (a patera looks like a caldera but it is unknown if the formation mechanism is the same) measures about 50 kilometers (30 miles) in diameter, while Io’s Loki Patera is the largest active volcano in the solar system with a diameter of about 250 kilometers (155 miles).
Fountains of Enceladus
Icy plumes tower above the south polar region of Saturn’s moon Enceladus. The plumes are backlit by sunlight, which makes them visible. Credit: NASA / JPL / Space Science Institute |
But standing in Yellowstone it is the geysers that most people are familiar with. The caldera is so filled with lava flows and forests hardly anyone would notice it if it wasn’t drawn on the park maps. In 2005 NASA’s Cassini spacecraft at Saturn photographed in beautiful black and white (a la W. H. Jackson perhaps) backlit geysers erupting off the south pole of Saturn’s moon Enceladus. Like Io, Enceladus is believed to be tidally heated. On this frozen moon, however, it is hypothesized that the internal heat seeking to escape has melted regions of the ice at the South Pole producing pockets of liquid water beneath the surface. Cracks in the ice above yield an outlet for the water. The difference in pressure between the water under the surface and the vacuum of space above the surface results in geyser plumes that extend about 400 kilometers (250 miles) above the moon. This is 10,000 times higher than Old Faithful. But, to be fair, Enceladus is smaller than the Earth, so if we scale the sizes of the Earth and Enceladus to be the same, imagine standing on the boardwalk beside Old Faithful and seeing its plume rise nearly 6,000 kilometers (3,500 miles) into space. One would not have to fight the crowds of tourists for a photograph of that.
The geysers on Enceladus quickly got dubbed Cold Faithful. At a workshop in Boulder last August I talked with Terry Hurford (of NASA’s Goddard Space Flight Center) who published a paper in Nature last year about the tidal forces at work on Enceladus. In his model, every time Enceladus reaches its closest passage to Saturn in its elliptical orbit, the tidal forces should open the southern polar cracks and the geysers should erupt. As Enceladus passes on, the tidal forces begin to orient differently relative to the cracks, and the geysers should shut off. The cycle repeats itself every orbit. The geysers of Cold Faithful, just like Old Faithful before me, really should be periodic and therefore predictable.
Old Faithful by starlight
Old Faithful erupts by starlight in winter. The bright star within the plume is the planet Saturn around which the E ring is replenished by ice from the geysers of Enceladus. Credit: Tyler Nordgren |
The weather has turned quite cold; it has snowed every day I have been here. In order to try and actually catch Old Faithful by starlight (just so I can get in my Enceladus mood completely), I get up three hours before dawn, the only time of night that ever seems to be clear. I’m wearing a parka loaned to me by an Arctic researcher (it has a neon orange “Polar Bear Alert” tag on the front), and it comes in handy as I snowshoe my way over to the quite empty Old Faithful viewing area. Starlight (and my head lamp) illuminates the snow-covered path as I attempt to avoid any bison in the area. I am told their eyes glow green in the night but suspect if I get close enough to tell, I’m already too close. By the time I make it to the geyser the clear patch of sky has grown small, but I happily note that, of the few stars still visible, the brightest is the planet Saturn. At 4:40 am, just as I arrive under a Moonless sky, I hear Old Faithful erupt. I plant my camera, snap my photos, and feel something brush against my face. The superheated water erupting from the ground is freezing in the air above me and raining down as gentle snow. I look up at Saturn. On Enceladus, where the force of gravity is so much lower, most of this flash-frozen water doesn’t fall back to ground, but rather continues upward to eventually become the E ring of Saturn. One of the most beautiful and otherworldly aspects of the distant planet Saturn is the result of the very earthly phenomenon before me.
With that I pack up my equipment and go home to bed.
Enceladus and the E ring
With the Sun almost directly behind the view, the tiny moon Enceladus (slightly over 500 kilometers in diameter) is embedded in the E ring, which is created from the material spewed from Enceladus' south polar plumes. Credit: NASA / JPL / SSI |
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