Pac-Man glows on Mimas
Wed Mar 31, 2010 at 17:27 UTC
The highest-resolution-yet temperature map and images of Saturn's icy moon Mimas obtained by NASA's Cassini spacecraft reveal surprising patterns on the surface of the small moon, including unexpected hot regions that resemble "Pac-Man" eating a dot, and striking bands of light and dark in crater walls.
"Other moons usually grab the spotlight, but it turns out Mimas is more bizarre than we thought it was," said Linda Spilker, Cassini project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "It has certainly given us some new puzzles."
Cassini collected the data on Feb. 13, during its closest flyby of the moon, which is marked by an enormous scar called Herschel Crater and resembles the Death Star from "Star Wars."
Scientists working with the composite infrared spectrometer, which mapped Mimas' temperatures, expected smoothly varying temperatures peaking in the early afternoon near the equator. Instead, the warmest region was in the morning, along one edge of the moon's disk, making a sharply defined Pac-Man shape, with temperatures around 92 Kelvin (-294°F). The rest of the moon was much colder, around 77 Kelvin (-320°F). A smaller warm spot - the dot in Pac-Man's mouth - showed up around Herschel, with a temperature around 84 Kelvin (-310°F).
The warm spot around Herschel makes sense because tall crater walls (about 5 kilometers, or 3 miles, high) can trap heat inside the crater. But scientists were completely baffled by the sharp, V-shaped pattern.
"We suspect the temperatures are revealing differences in texture on the surface," said John Spencer, a Cassini composite infrared spectrometer team member based at Southwest Research Institute in Boulder, Colo. "It's maybe something like the difference between old, dense snow and freshly fallen powder."
The upper right image shows the completely different pattern that Cassini actually saw. Instead of the expected smoothly varying temperatures, this side of Mimas is divided into a warm part (on the left) and a cold part (on the right) with a sharp, v-shaped boundary between them. The warm part has typical temperatures near 92 Kelvin (-294°F), while typical temperatures on the cold part are about 77 Kelvin (-320°F). The cold part is probably colder because surface materials there have a greater thermal conductivity, so the sun's energy soaks into the subsurface instead of warming the surface itself. But why conductivity should vary so dramatically across the surface of Mimas is a mystery.
The lower two panels compare the temperature map to Mimas' appearance in ordinary visible light at the time of the observations. The map used to create this image is a mosaic of images taken by Cassini's imaging science subsystem cameras on previous flybys of Mimas. The cold side includes the giant Herschel Crater, which is a few degrees warmer than its surroundings. It's not yet known whether Herschel is responsible in some way for the larger region of cold temperatures that surrounds it. The green grid shows latitudes and longitudes on Mimas at 30-degree intervals.
Cassini took 85 minutes to make the temperature map, as the spacecraft receded from Mimas. During that time, the distance to Mimas increased from 38,000 to 67,000 kilometers (24,000 to 42,000 miles) and the longitude of the center of Mimas' disk increased from 128 degrees west to 161 degrees west, due to the moon's rotation. Because of this changing geometry, the alignment of the temperatures relative to specific features or coordinates on Mimas is shown only approximately. The temperatures were calculated from the brightness of the moon's infrared heat radiation, measured by CIRS at a wavelength of 12 to 16 microns, and are color coded according to the scale in the lower right.
Credit: NASA/JPL/GSFC/SWRI/SSI
This image also shows dark streaks trailing down the sides of some craters (marked red), often originating from pockets of dark contaminants embedded just below the rim of the crater wall. The pockets likely represent small, pre-existing, dark-floored craters that were buried by the blanket of material thrown out from the newer impact that created the crater rim. The material from a newly exposed dark layer eventually moves downslope and forms a streak. Streaks sometimes are seen originating from the floors of smaller dark-floored craters perched along rims of larger craters.
The craters seen in this mosaic also show relatively dark markings along the lower portion of their crater walls (marked in green). Cassini scientists interpret this darkening as evidence for the gradual concentration of impurities from evaporating icy materials in areas where the dark impurities slide slowly down the crater wall and the bright ice is baked away by the sun and the vacuum of space.
Credit: NASA/JPL/Space Science Institute
Denser ice quickly conducts the heat of the sun away from the surface, keeping it cold during the day. Powdery ice is more insulating and traps the sun's heat at the surface, so the surface warms up.
Even if surface texture variations are to blame, scientists are still trying to figure out why there are such sharp boundaries between the regions, Spencer said. It is possible that the impact that created Herschel Crater melted surface ice and spread water across the moon. That liquid may have flash-frozen into a hard surface. But it is hard to understand why this dense top layer would remain intact when meteorites and other space debris should have pulverized it by now, Spencer said.
Icy spray from the E ring, one of Saturn's outer rings, should also keep Mimas relatively light-colored, but the new visible-light images from the flyby paint a picture of surprising contrasts. Cassini imaging team scientists didn't expect to see dark streaks trailing down the bright crater walls or a continuous, narrow pile of concentrated dark debris tracing the foot of each wall.
The pattern may appear because of the way the surface of Mimas ages, said Paul Helfenstein, a Cassini imaging team associate based at Cornell University, Ithaca, N.Y. Over time, the moon's surface appears to accumulate a thin veil of silicate minerals or carbon-rich particles, possibly because of meteor dust falling onto the moon, or impurities already embedded in surface ice.
As the sun's warming rays and the vacuum of space evaporate the brighter ice, the darker material is concentrated and left behind. Gravity pulls the dark material down the crater walls, exposing fresh ice underneath. Although similar effects are seen on other moons of Saturn, the visibility of these contrasts on a moon continually re-paved with small particles from the E ring helps scientists estimate rates of change on other satellites.
"These processes are not unique to Mimas, but the new high-definition images are like Rosetta stones for interpreting them," Helfenstein said.
| Source: Jet Propulsion Laboratory | |
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