Today's APOD is a mosaic of multiple Mercury Dual Imaging System (MDIS) images and it shows the Caloris Basin Region of Mercury in its entirety. The Caloris Basin was discovered in the AD 1974 from images taken by the NASA - Mariner 10 Probe, but when the Mariner 10 flew-by Mercury, only the Eastern Half of the Basin was in full daylight.

On the other hand, during the NASA - MESSENGER Orbiter's 1st Fly-By of Mercury, the Spacecraft was able to acquire High-Resolution images of the entire Basin, revealing the full extent of this Great Impact Surface Feature for the first time.
Geologists use the term "Basin" to refer to an Impact Feature that is larger than about 300 Km (such as approx. 186,3 miles) in diameter and that often displays multiple Concentric Rings. Caloris Basin has only 1 (one) Main Topographic Ring (which is about 1550-Km in diameter); however, a few patterns of Concentric Ring-like Structures - both inside and outside the Main Ring - have been mapped and interpreted as (a possible) evidence for additional Rings (not visible from orbit).

Caloris is one of the largest Impact Basins in the whole Solar System, and the NASA - MESSENGER's orbital observations will provide much more data that will be useful to study this impressive Geologic Surface Feature, including high-resolution color as well as low-Sun images which will be used to discern its Morphology.

This mosaic was obtained when the Sun was high overhead the Caloris Basin and, as you know, such lighting conditions can really emphasize the differences in brightness existing among the Surface Materials, with also a little shadowing that was fit to provide us with a sense (---> idea, perception) of both the Texture and Topography of this whole Impact Surface Feature.

The interior of the Basin has been filled with lighter-hued Plains, which in turn have been modified by other Impact Craters. These "Inner" Craters range from small bright dots, to larger Craters with bright Rays.

Some of the larger Inner Impact Craters show the presence of a white (and so far unknown) Substance covering their Floors (look, for instance, at approx. 9, 11 and 2 o' clock of Caloris, inside the brighter Region of the Basin to see 3 - three - Impact Craters showing this specific characteristic), while other Inner Impact Craters exhibit very Dark Rims.

While nothing has been said (so far) about the white Substance covering, as we said before, the Floor of a few Inner Impact Craters, it has to be underlined that, as far as the Dark Rims are concerned, some Planetary Scientists have suggested that some kind of Dark Material (whose nature, in any case, is still not known) underlies the Interior Plains of Caloris (but only in some specific places) and that such Dark Material was exposed (---> brought to the Surface) throughout (and during) the formation of the aforementioned Inner Impact Craters.

Date Acquired: January, 14th, 2008
Instrument: Narrow Angle Camera (NAC) of the Mercury Dual Imaging System (MDIS)
Scale: Caloris Basin is roughly 1550 Km in diameter (about 962,55 miles)

This frame has been colorized in Absolute Natural Colors (such as the colors that a human eye would actually perceive if someone were onboard the NASA - MESSENGER Spacecraft and then looked down, towards the Surface of Mercury), by using an original technique created - and, in time, dramatically improved - by the Lunar Explorer Italia Team.