土卫八的黑白双色问题解开了
来自YAHOO的消息Mystery of Saturn's Two-Faced Moon Solved
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.
::070821_11.jpg:: ……这篇文章的文字之谜还没有解开…… 呵呵，同上。词汇太专业了 大概能看懂，不过还是我们的母语更方便更亲切啊
thx for share 好心人给翻译一下！对这个问题比较有兴趣::02:: 我发现，黑色大都集中在突起的顶部 多谢六楼！:) 看了照片，总觉得专家的解释不怎么靠谱。
专家的理由应该是大面积的黑色，甚至是半球黑色，可是照片显示的却是很小的区域。再者，按照专家的解释，黑色区域应该是“坑”，照片上却是“顶”。 文章用了比较概括的写法，提供的信息不太完整。
从NASA找到了更详细的：
http://www.jpl.nasa.gov/news/news.cfm?release=release=2007-099a
Cassini Gets Close-Up Views of Saturn's Moon Iapetus
September 11, 2007
Cassini completed its closest flyby of the odd moon Iapetus on Sept. 10, 2007. The spacecraft flew about 1,640 kilometers (1,000 miles) from Iapetus' surface and is returning amazing views of the bizarre moon.
All the data were successfully recorded on the spacecraft. Twenty-one minutes into the first post-flyby data downlink, the spacecraft went into a precautionary condition called safe mode. The cause has been determined to be a solid state power switch that was tripped due to a galactic cosmic ray hit.
While in safe mode, the spacecraft turns off all unnecessary activities and transmits only essential engineering telemetry at a low data rate, while it awaits commands from Earth.
Tuesday morning, Sept. 11, commands were sent to the spacecraft to resume high rate science and engineering data playback. The project expects all data on the spacecraft will be returned to Earth during downlinks on Tuesday and Wednesday, with no impact on the Iapetus science data return beyond a brief delay.
Due to the safing event, the sequence executing on the spacecraft was halted, and Cassini's instruments will not be turned back on for three or four days. The last time Cassini was in safe mode was over four years ago.
http://www.jpl.nasa.gov/news/news.cfm?release=2007-101
Saturn's Moon Iapetus Is the Yin-and-Yang of the Solar System
September 12, 2007
PASADENA, Calif. – Scientists on the Cassini mission to Saturn are poring through hundreds of images returned from the Sept. 10 flyby of Saturn's two-toned moon Iapetus. Pictures returned late Tuesday and early Wednesday show the moon's yin and yang--a white hemisphere resembling snow, and the other as black as tar.
Images show a surface that is heavily cratered, along with the mountain ridge that runs along the moon's equator. Many of the close-up observations focused on studying the strange 20-kilometer high (12 mile) mountain ridge that gives the moon a walnut-shaped appearance.
"The images are really stunning," said Tilmann Denk, Cassini imaging scientist at the Free University in Berlin, Germany, who was responsible for the imaging observation planning. "Every new picture contained its own charm. I was most pleased about the images showing huge mountains rising over the horizon. I knew about this scenic viewing opportunity for more than seven years, and now the real images suddenly materialized."
This flyby was nearly 100 times closer to Iapetus than Cassini's 2004 flyby, bringing the spacecraft to about 1,640 kilometers (1,000 miles) from the surface. The moon's irregular walnut shape, the mountain ridge that lies almost directly on the equator and Iapetus' brightness contrast are among the key mysteries scientists are trying to solve.
"There's never a dull moment on this mission," said Bob Mitchell, Cassini program manager, NASA's Jet Propulsion Laboratory, Pasadena, Calif. "We are very excited about the stunning images being returned. There's plenty here to keep many scientists busy for many years."
"Our flight over the surface of Iapetus was like a non-stop free fall, down the rabbit hole, directly into Wonderland! Very few places in our solar system are more bizarre than the patchwork of pitch dark and snowy bright we've seen on this moon," said Carolyn Porco, Cassini imaging team leader at the Space Science Institute, Boulder, Colo.
The return of images and other data was delayed early Tuesday due to a galactic cosmic ray hit which put the spacecraft into a precautionary state called safe mode. This occurred after the spacecraft had placed all of the flyby data on its data recorders and during the first few minutes after it began sending the data home. The data flow resumed later that day and concluded on Wednesday. The spacecraft is operating normally and its instruments are expected to return to normal operations in a few days.
"Iapetus provides us a window back in time, to the formation of the planets over four billion years ago. Since then its icy crust has been cold and stiff, preserving this ancient surface for our study," said Torrence Johnson, Cassini imaging team member at JPL.
Cassini's multiple observations of Iapetus will help to characterize the chemical composition of the surface; look for evidence of a faint atmosphere or erupting gas plumes; and map the nighttime temperature of the surface. These and other results will be analyzed in the weeks to come.
Iapetus flyby images are available at: http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini and http://ciclops.org.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the Cassini mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.
RELATED MULTIMEDIA: Video file with animation, images and sound bites will air tomorrow on NASA TV.
http://www.jpl.nasa.gov/news/news.cfm?release=2007-113
NASA scientists are on the trail of Iapetus' mysterious dark side, which seems to be home to a bizarre "runaway" process that is transporting vaporized water ice from the dark areas to the white areas of the Saturnian moon.
This "thermal segregation" model may explain many details of the moon's strange and dramatically two-toned appearance, which have been revealed exquisitely in images collected during a recent close flyby of Iapetus by NASA's Cassini spacecraft.
Infrared observations from the flyby confirm that the dark material is warm enough (approximately minus 230 degrees Fahrenheit or 127 Kelvin) for very slow release of water vapor from water ice, and this process is probably a major factor in determining the distinct brightness boundaries.
"The side of Iapetus that faces forward in its orbit around Saturn is being darkened by some mysterious process," said John Spencer, Cassini scientist with the composite infrared spectrometer team from the Southwest Research Institute, Boulder, Colo.
Using multiple instruments on Cassini, scientists are piecing together a complex story to explain the bright and dark faces of Iapetus. But yet to be fully understood is where the dark material is coming from. Is it native or from outside the moon? It has long been hypothesized that this material did not originate from within Iapetus, but instead was derived from other moons orbiting at a much greater distance from Saturn in a direction opposite to Iapetus.
Scientists are now converging on the notion that the darkening process in fact began in this manner, and that thermal effects subsequently enhanced the contrast to what we see today.
"It's interesting to ponder that a more than 30-year-old idea might still help explain the brightness difference on Iapetus," said Tilmann Denk, Cassini imaging scientist at the Free University in Berlin, Germany. "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."
Once the leading side is even slightly dark, thermal segregation can proceed rapidly. A dark surface will absorb more sunlight and warm up, explains Spencer, so the water ice on the surface evaporates. The water vapor then condenses on the nearest cold spot, which could be Iapetus's poles, and possibly bright, icy areas at lower latitudes on the side of the moon facing in the opposite direction of its orbit. So the dark stuff loses its surface ice and gets darker, and the bright stuff accumulates ice and gets brighter, in a runaway process.
Scientists say the result is that there are virtually no shades of gray on Iapetus. There is only white and very dark.
Ultraviolet data also show a non-ice component in the bright, white regions of Iapetus. Spectroscopic analysis will reveal whether the composition of the material on the dark hemisphere is the same as the dark material that is present within the bright terrain.
"The ultraviolet data tell us a lot about where the water ice is and where the non-water ice stuff is. At first glance, the two populations do not appear to be present in the pattern we expected, which is very interesting," said Amanda Hendrix, Cassini scientist on the ultraviolet imaging spectrograph team at NASA's Jet Propulsion Laboratory, Pasadena, Calif.
Because of the presence of very small craters that excavate the bright ice beneath, scientists also believe that the dark material is thin, a result consistent with previous Cassini radar results. But some local areas may be thicker. The dark material seems to lie on top of the bright region, consistent with the idea that it is a residual left behind by the sublimated water ice.
Some other mysteries are coming together. There are more data on the signature mountain ridge that gives Iapetus its "walnut" appearance. In some places it appears subdued. One big question that remains is why it does not go all the way around.
Was it partially destroyed after it formed, or did it never extend all the way around the moon? Scientists have ruled out that it is a youthful feature because it is pitted with craters, indicating it is old. And the ridge looks too solid and competent to be the result of an equatorial ring around the moon collapsing onto its surface. The ring theory cannot explain features that look like tectonic structures in the new high resolution images.
Over the next few months, scientists hope to learn more about Iapetus' mysteries.
New Iapetus images, temperature maps and other visuals on Iapetus are available at: http://saturn.jpl.nasa.gov and http://www.nasa.gov/cassini .
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL, a division of the California Institute of Technology in Pasadena, manages the Cassini mission for NASA's Science Mission Directorate, Washington, D.C.
http://www.jpl.nasa.gov/images/cassini/2007-09-11/iapetus-91840-200.jpg
http://www.jpl.nasa.gov/images/cassini/2007-09-12/N00092001-200.jpg
http://www.jpl.nasa.gov/images/cassini/2007-10-06/pia08384-200.jpg
http://www.jpl.nasa.gov/images/cassini/2007-10-06/pia10012-200.jpg
http://www.jpl.nasa.gov/images/cassini/pia08373-200.jpg
http://www.jpl.nasa.gov/images/cassini/pia08372-200.jpg 多谢六楼, 真是热心人.
NASA竟然用阴阳来形容土卫八 10楼的照片很好，看来黑色与迎风面确实有关。只是那些黑色物质好像能够减缓它下面的白色物融化，而不是加速融化。 其中那张看似雪堆上有黑色物质的照片,其实是一种视觉错觉,调整一下感觉你会突然发现原来是高的地方反而是个坑,那些黑色物质又出现在坑里了.在看google卫星照片时也会出现这种视觉现象
土卫八太小不会有大气,因此不会有风.(尽管有些稀薄气体) 原帖由 天关 于 2007-10-11 18:42 发表 http://www.astronomy.com.cn/bbs/images/common/back.gif
其中那张看似雪堆上有黑色物质的照片,其实是一种视觉错觉,调整一下感觉你会突然发现原来是高的地方反而是个坑,那些黑色物质又出现在坑里了.在看google卫星照片时也会出现这种视觉现象
你真的能够把这张图里的黑色看成在坑里？
我是真的不能看出来。
黑色物质是什么
？？ 二维图看成三维是需要大脑参与理解的过程,凸凹两种结果的阴影效果都是正确的.经过练习是能转换自如的.利用人为的阴影可以帮助你更容易的看出来. 要是雪坑的话看上去也一样呀,边缘暗而且发现有一些阴影是环状的,还有些阴影面积比黑色面积大(不过这个可以用是个山峰解释)
这样用影子解释就不行了 原帖由 天关 于 2007-10-11 21:54 发表 http://www.astronomy.com.cn/bbs/images/common/back.gif
二维图看成三维是需要大脑参与理解的过程,凸凹两种结果的阴影效果都是正确的.经过练习是能转换自如的.利用人为的阴影可以帮助你更容易的看出来.
经过你的人工引导，我仍然得不到坑的感觉。我也想努力转换，就是换不过来。 以下的图来源于nasa,表示在火星表面发现7个坑(http://mars.jpl.nasa.gov/odyssey/gallery/martianterrain/20070921_Cave_02.html)
图中的圆形较为容易看成山包,光线从右側照亮山的右側,背面是阴暗的,然而却没有山的影子.
如果光线是从左向右,山包就变成了坑,且不存在影子的问题.
这个图应该比上面的容易看出来. 原帖由 天关 于 2007-10-12 15:20 发表 http://www.astronomy.com.cn/bbs/images/common/back.gif
以下的图来源于nasa,表示在火星表面发现7个坑(http://mars.jpl.nasa.gov/odyssey/gallery/martianterrain/20070921_Cave_02.html)
图中的圆形较为容易看成山包,光线从右側照亮山的右側,背面是阴暗的,然而却没有山的 ...
很好，谢谢。
先把那些东西认定成圆形，就比较好转变了。
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