Astrobiology Top 10: Water on Enceladus?
|Recent Cassini images of Enceladus show fountain-like sources of a fine spray of material that towers above the south polar region. The image was taken looking more or less broadside at the ‘tiger stripe’ fractures observed in earlier Enceladus images and shows discrete and small-scale plumes above the limb of the moon. Credit: CICLOPS|
NASA’s Cassini spacecraft may have found evidence of liquid water reservoirs that erupt in Yellowstone-like geysers on Saturn’s moon Enceladus. The rare occurrence of liquid water so near the surface raises many new questions about the mysterious moon.
This finding and others are reported by the Cassini Imaging Science Team in the journal Science.
"We realize that this is a radical conclusion — that we may have evidence for liquid water within a body so small and so cold," said Carolyn Porco, Cassini imaging team leader at the Space Science Institute in Boulder, Colorado, and the lead author of the Science report. "However, if we are right, we have significantly broadened the diversity of solar system environments where we might possibly have conditions suitable for living organisms."
High-resolution Cassini images show icy jets and towering plumes ejecting large quantities of particles at high speed. Scientists examined several models to explain the process, and they determined there are too many particles being released from the south pole for the source to be merely frozen mist condensing out of a plume of water vapor. They don’t think the particles are being blown off by jets of water vapor that arise from warm ice, either. Instead, scientists have found evidence for a much more exciting possibility. The jets might be erupting from near-surface pockets of liquid water above 0 degrees Celsius (32 degrees Fahrenheit), like cold versions of the Old Faithful geyser in Yellowstone.
"We previously knew of three places where active volcanism exists: Jupiter’s moon Io, Earth, and possibly Neptune’s moon Triton. Cassini changed all that, making Enceladus the latest member of this very exclusive club, and one of the most exciting places in the solar system," said John Spencer, Cassini scientist from the Southwest Research Institute in Boulder.
|The color-coded image of Enceladus was processed to enhance faint signals, making contours in the plume of material even more apparent. The greatly enhanced and colorized image shows the enormous extent of the fainter, larger-scale component of the plume. Credit: CICLOPS|
"Other moons in the solar system have liquid-water oceans covered by kilometers of icy crust," said Andrew Ingersoll, imaging team member and atmospheric scientist at the California Institute of Technology in Pasadena. "What’s different here is that pockets of liquid water may be no more than tens of meters below the surface."
In the near-vacuum conditions at the moon’s surface, liquid water would boil away into space, erupting forcefully into the void and carrying particles of ice and liquid water along with the vapor. Analysis of the jets and plumes indicate that most of the particles eventually fall back to the moon’s surface, giving the south pole its extremely bright veneer. The particles that escape the moon’s gravity go into orbit around Saturn, forming the E ring.
Cassini images have revealed the geology of Enceladus in startling detail, including relaxed craters and extensive surface cracks and folds. Imaging scientists report that the moon has undergone geologic activity over the last four and half billion years up to the present, with the active south pole being the only place where liquid water may currently exist near the surface. Telltale geologic features throughout the southern hemisphere of Enceladus also point to a change in the body’s shape with time. Scientists believe these to be related to an episode of intense heating in the moon’s past that may, in turn, explain the anomalous warmth and current activity in the south polar region.
|This image was taken during Cassini’s close approach to Enceladus on July 14, 2005, with the narrow angle camera from a distance of approximately 103,230 kilometers (64,140 miles) at a Sun-Enceladus-spacecraft, or phase, angle of 50 degrees. |
The sources of this warmth are a major puzzle. Some combination of tidal flexing and heating of the interior by naturally radioactive material may provide the heat to power the geysers, which almost certainly erupt from the narrow, warm fractures, called ‘tiger stripes,’ seen crossing Enceladus’ south polar region. However, obtaining enough energy to reproduce the observed heat emanating from the south pole is still a problem.
Dr. Torrence Johnson, a co-author of the Science paper and satellite expert at NASA’s Jet Propulsion Laboratory in Pasadena, notes: “Active water geysers on little Enceladus are a major surprise. We’re still puzzled about the details and energy sources, but what’s exciting is that Enceladus obviously figured out how to do it. Now it’s up to us to crack the mystery.”
In the spring of 2008, scientists will get another chance to look at Enceladus when Cassini flies within 350 kilometers (approximately 220 miles), but much work remains after the spacecraft’s four-year prime mission is over.
"There’s no question, along with the moon Titan, Enceladus should be a very high priority for us. Saturn has given us two exciting worlds to explore," said Jonathan Lunine, Cassini interdisciplinary scientist from the University of Arizona in Tucson.
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