How would the ice melt? Is it a different kind of ice? Because I don't understand how -230 degrees would allow for melting. _______________________________________________________ Saturn's moon Iapetus has virtually no gray. Rather, its features are all stark black and white. The appearance has long puzzled astronomers. New detailed images suggest sunlight is melting ice on one side of Iapetus, leaving the moon's dark surface exposed, while the opposite half retains its reflective ice-mixed shell. Since the moon's discovery by Giovanni Domenico Cassini in 1671, Iapetus' appearance has baffled astronomers. The leading edge of Iapetus, which faces the direction of its orbit, is black as asphalt, while its trailing side appears bright as snow. Iapetus is 907 miles (1,460 kilometers) wide and circles Saturn at a distance of about 2.2 million miles (3.6 million kilometers). High-resolution images of Iapetus acquired last month by the Cassini-Huygens spacecraft during its low pass over the moon have uncovered telling details on its surface that may well yield the reason for its strange bright and dark patterns. "While there are many details yet to be worked out, we think we now understand the essence of why Iapetus looks the way it does," said Carolyn Porco, the leader of the imaging team at the Space Science Institute in Boulder, Colo. The new observations add support to a two-part explanation for Iapetus' appearance. First, as Iapetus treks around Saturn, its leading edge scoops up a thin coating of dark material, which amplifies sunlight absorption. "Dusty material spiraling in from outer moons hits Iapetus head-on and causes the forward-facing side of Iapetus to look different than the rest of the moon," said Tilmann Denk, Cassini imaging scientist at the Free University in Germany. Over time, as the black-ish surfaces warm, the rate of evaporation increases until finally all the surface ice in that region melts away. Infrared observations from the Cassini flyby confirm the dark dust material is approximately -230 degrees Fahrenheit (-146 degrees Celsius)--warm enough for the release of water vapor from the ice. The water vapor formed then condenses on the nearest cold spot, such as along polar regions and icy areas at lower latitudes on the trailing side of the moon. In that way, the dark material loses the mixed-in ice and gets even darker, while the bright material accumulates more ice and gets brighter, in what the astronomers call a runaway process that leaves no gray area
It's called sublimation, Orelander. The ice - a solid - slowly turns to vapor (a gas) without going through a liquid stage. It's actually a pretty common thing. One very common example of it is dry ice (solid carbon dioxide) that never forms a liquid.
there are many sorts of ice...Mars for example has dry ice or carbon dioxide frozen ice. However, Iopetus spectral analysis shows that it is most likely H2O ice... Please Register or Log in to view the hidden image!
Under the right conditions, notably under low atmospheric pressure, many substances will go directly from solid to gas. Water is one. That's how they freeze dry things btw.
No, in this case it IS ordinary water ice. But at very low pressures - practically no atmosphere at all - it will easily sublime. As Kevin rightly pointed out, that's exactly the process they use for freeze-drying foods. Coffee is a very good example of that. Even warm temperatures cause some of the volatile oils (that's equal to taste) to evaporate away. So it's done at low pressures and temps and the water simply turns to gas and is removed leaving all the "good stuff" behind to be packaged and sold.Please Register or Log in to view the hidden image!
