Astronomy Picture of the Day
March 1, 2013

Engelier Crater and the Trailing Hemisphere of Japetus
Engelier Crater and the Trailing Hemisphere of Japetus

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

The NASA - Cassini Spacecraft captured, in the month of September of the AD 2007, the first High-Resolution glimpse of the bright Trailing Hemisphere of Saturn's moon Japetus. This Absolute Natural Color image-mosaic shows the entire Hemisphere of Japetus that was visible from the NASA - Cassini Spacecraft on the outbound leg of its encounter with the two-toned moon. Also shown here is the complicated Transition Region of Japetus (meaning the Region which is located in-between its Dark Leading Hemisphere and the Bright Trailing one). This Region, visible along the right side of the image, was observed in many of the images acquired by Cassini near the closest approach of the encounter.

Revealed here for the first time in detail are the Geologic Structures that mark the Trailing Hemisphere of Japetus. The Region appears to be Heavily Cratered, particularly in the North and South Polar Regions of the moon. Near the top of the mosaic, numerous Impact Features that hade been already caught in several frames obtained by the NASA - Voyager 2 Spacecraft during the Fly-By of Japetus that took place in the AD 1981, are well visible, including the Impact Craters Ogier and Charlemagne. However, the most prominent Topographic Feature in this view, in the bottom half of the mosaic, is the approx. 450-Km (such as about 280-miles) wide Engelier Impact Basin: one of at least 9 (nine) huge Impact Basins found on Japetus. In fact, the Engelier Impact Basin overlaps an older, similar-sized Impact Basin located to its South/East.

In many places of Japetus, the Dark Material (that is thought to be composed of Nitrogen-bearing Organic Compounds - called "Cyanides" -, as well as by Hydrated Minerals and other Carbonaceous Minerals) seems to "coat" the Equator-facing Slopes as well as the Floor of the Impact Basin. The distribution of this Dark Material as well as the variations in the color of the Bright Material that is spread all across the Trailing Hemisphere, will be crucial clues to solve (or, at least, ro clarify) if we really want to get a better understanding of the origin of Japetus' extremely peculiar bright-dark dual personality.

The view was acquired with the NASA - Cassini Spacecraft Narrow-Angle Camera on September 10, 2007, at a distance of about 73.000 Km (such as approx. 45.333 miles) from Japetus. This mosaic consists of 60 images covering 15 footprints across the Surface of Japetus. The view is an Orthographic Projection of the Saturnian moon that has been centered at 10,8° South Latitude, 246,5° West Longitude and which has a resolution of 426 meters (0,26 miles) per pixel. An Orthographic Projection (like this one) is something like the view that a distant observer would get by looking at a given Celestial Object through a telescope.

This frame (which is an Original NASA - Cassini Spacecraft false-color image mosaic published on the NASA - Planetary Photojournal with the ID n. PIA 08384) has been additionally processed, contrast enhanced, and then re-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 bright Trailing Hemisphere of the Saturnian moon Japetus), 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 Japetus, each having a different Albedo (---> Reflectivity) and Chemical Composition.

Note: in the EDM (upper Right Side of the image-mosaic), a view of the Terrain located in the Transition Zone of Japetus. The Bright Material visible on the frozen Surface of Japetus is, most likely, Water Ice, while the Dark Material is, probably, a Carbonaceous Mineral.

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