Astronomy Picture of the Day
May 18, 2013

Look at me!
Look at me!

Credits: NASA/JPL/Space Science Institute - Credits for the additional process. and color.: Dr Paolo C. Fienga/Lunar Explorer Italia/IPF

This image-mosaic, created from pictures taken by the NASA - Cassini Spacecraft during its closest Fly-By of the Saturnian moon Mimas, looks straight at the moon's huge Herschel Crater and reveals new insights about the moon's scarred Surface. For instance, bright-walled Impact Craters, with Floors and surroundings about 20% (twenty-percent) darker than the steep Crater Walls, are clearly notable in this view.


In fact, Mimas' original Surface, like the Surfaces of most of the other Major Saturnian moons which do not possess an Atmosphere, is not pure Ice but contains some dark impurities. Herschel Crater (approx. 130 Km - or about 80,73 miles wide) and some of the smaller Impact Craters visible in this mosaic show relatively dark markings along the lower portion of their Walls. Cassini Scientists interpret this darkening as evidence for the gradual concentration of dark impurities from evaporating Icy Materials in areas where they slowly slide down the Crater Wall. There, bright Ice is - litterally - "baked away" by the action of the Sun, the Cosmic Rays and the Vacuum of Space (---> a.k.a. "Space Weathering"). At Herschel, the Edge where the Darker Regions contact the Crater Floor is interrupted by an extensive hummocky area. Scientists believe the hummocky texture (---> meaning a Surface with Ridges, Knobs and other kind of irregular Reliefs) came from the flow of melted Ice that was created during (---> caused by) the Impact Event. That melt, in the end, filled the bottom of the Crater and then set and solidify all around its Central Peak.


Dark Streaks are seen making their way down the sides of some other Impact Craters, often originating from pockets of dark contaminants embedded just below the Rim of their Walls. The pockets themselves likely represent small, pre-existing, dark-floored Impact Craters that were buried by the Blanket of Material that was thrown out from the newer impact that created the Rim of Herschel Crater. The Material from a newly exposed dark layer eventually moved downslope and formed a Streak (remember that the Streaks are sometimes often, if not always, seen starting from the Floors of smaller, dark-floored Impact Craters which perch along the Rims of larger Craters).


The interior of Herschel Crater is significantly less cratered than the continuous blanket of ejected material that extends radially outward from its Rim. The powerful Meteor Strike that excavated Herschel blasted pulverized debris, including massive chunks of Ice, upward. The fallback of this Ejected Material all over the Crater Rim created a thick Debris' Blanket and dotted it with Secondary Craters. Furthermore, the presence of a fluid pool of melted material on the Crater Floor, which rapidly solidified after the debris fell, probably explains the relative absence of Impact Craters on Herschel's Floor.


However, all the aforementioned Events and Processes that formed and characterized Herschel Crater are, in fact, common Events and Processes which have actually occured, as far as we knoe, on all the Celestial Bodies without Atmospheres that can be found throughout the whole Solar System; in the end, the circumstance that they might seem kind of "accentuated" on Mimas can be related to (explained by) the extremely large dimensions of Herschel when compared to the moon's' size.


This frame (which has been cropped from an Original NASA - Cassini Spacecraft b/w image-mosaic published on the NASA - Planetary Photojournal with the ID n. PIA 12568) has been additionally processed, contrast enhanced, and then colorized in Absolute Natural Colors (such as the colors that a human eye would actually perceive if someone were onboard the NASA - Cassini Spacecraft and then looked outside, towards the Saturnian moon Mimas), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team. Different colors, as well as different shades of the same color, mean, among others, the existence of different Elements present on the Surface of Mimas, each having a different Albedo (---> Reflectivity) and Chemical Composition.


Note: it is possible (but we, as IPF, have no way to be one-hundred-percent sure of such a circumstance), that the actual luminosity of the Surface of Mimas - as it is presented in this image-mosaic - would appear, to an average human eye, way lower than it has been shown (or, better yet: interpreted) here.



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