Astronomy Picture of the Day
November 5, 2013

On the Limb (Part I)
On the Limb (Part I)

Credits: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington and Dr Paolo C. Fienga/LXTT/IPF for the additional process. and color.

Another day of Cosmic Cruise, and another beautiful view of the Mercurian Limb (---> the far Horizon of a Celestial Body, when viewed from a great distance, or from a medium/high altitude's orbit). In this scene, which was acquired by the NASA - MESSENGER Spacecraft on October, 2, 2013, we are looking from the Shadowed portion of Mercury toward the Sunlit Side of the Planet; an ancient and Unnamed Impact Crater - whose diameter is of approx. 120 Km (such as about 74,52 miles) - stands out near the center of the frame. Emanating from this Unnamed Impact Crater you will be able to see that there are at least 4 (four) striking Chains of (according to NASA) Secondary Craters, which gouged Linear Tracks radially away from the Outer Eim of the aforementioned Crater.

However, in our humble opinion, as IPF, we do believe that those Chains of Impact Craters which have been mentioned before are all, actually, Chains of Primary Craters, which are the visible result of a "Rain" of (almost) perfectly aligned Impactors which hit the Surface of Mercury in an extremely rapid sequence. This kind of event could well occur, as you should know, and among several other plausible possibilities, in case of a Cometary Impact, when the Nucleus of the Impacting Comet (or Comets, like it appears that it has occurred in this case) got fragmented during its (or their) "traumatic" passage through the so-called "Roche Limit".

Juist out of curiosity, we have found several views of Chains of Craters - all of them being extremely similar (if not almost identical) to these four ones - on other Celestial Bodies of the Solar System and, in those cases, NASA itself explained the resulting Surface Features (such as Linear or semi-Linear Impact Crater Chains) in full accordance with the explanation that we are suggesting for this specific case. Now, in fact, you may ask us why did NASA choose, in this case, the idea of those Mercurian Surface Features being Secondary Impact Crater Chains instead of Primary Impact Crater Chains, and that would, believe us, a VERY good question! But, unfortunately, we have no idea of the reason why NASA suggested different explanations about the true nature of identical (or almost identical) Surface Features and, therefore, if you are really interested in this matter, we are afraid that you should have to ask them directly...

Date acquired: October, 2nd, 2013
Image Mission Elapsed Time (MET): 23055902
Image ID: 4933174
Instrument: Wide Angle Camera (WAC) of the Mercury Dual Imaging System (MDIS)
Center Latitude: 52,13° South
Center Longitude: 167,50° East
Solar Incidence Angle: 82,0° (meaning that the Sun, at the time that the picture was taken, was about 8,0° above the imaged Local Mercurian Horizon)
Emission Angle: 69,4°
Sun-Mercury-Messenger (or "Phase") Angle: 122,4°

This picture (which is an Original NASA - MESSENGER Spacecraft's b/w and NON-Map-Projected image published on the NASA - Planetary Photojournal with the ID n. PIA 17579) has been additionally processed, contrast enhanced, Gamma corrected, magnified to aid the visibility of the details and then 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 Limb of Mercury), 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 other things, the existence of different Elements (Minerals) present on the Surface of Mercury, each having a different Albedo (---> Reflectivity) and Chemical Composition.

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