Astronomy Picture of the Day
October 11, 2012

Craters on the Mercurian Terminator
Craters on the Mercurian Terminator

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.

Today's APOD is a beautiful and dramatic image of the Surface of Planet Mercury that was taken by the NASA - MESSENGER Spacecraft just when the Sun was a mere ten degrees above the Local (imaged here) Horizon. The frame shows us four (4) Simple Unnamed Impact Craters which are all located on top of the Rim of a larger (approx. 3 Km-diameter) Simple Unnamed Crater (look from about 12 to a little after 1 o' clock of the disc drawn by the larger Crater) which, on its side, is positioned on the Outer Rim of an even larger (such as approx. 35 Km-diameter) Unnamed Complex Impact Crater which has been already caught and flooded by the shadows of the Mercurian Night.

As a matter of fact, the Rims of the four small Impact Craters show almost no signs of Erosion, or Collapse, and this circumstance marks them (always relatively speaking) as "young" Surface Features. However, while almost the entire Surface of Mercury is covered by Chains of small Craters (mostly formed by the action of repeated "waves" of Ejecta Boulders), these four small Craters DO NOT form nor constitute what is technically defined as a "Chain of Craters".

This image was acquired as a High-Resolution Targeted Observation. Targeted Observations are images of a small area on Mercury's Surface at resolutions much higher than the usual 200-meters per pixel Morphology Base Map. It is not possible to cover all of Mercury's Surface at such an High Resolution, but typically several areas of remarkable scientific interest are imaged in this mode each week.

Date acquired: May, 2nd, 2012
Image Mission Elapsed Time (MET): 217183900
Image ID: 1750578
Instrument: Narrrow Angle Camera (NAC) of the Mercury Dual Imaging System (MDIS)
WAC filter: 7 (748 nanometers)
Center Latitude: 79,27° North
Center Longitude: 232,13° East
Resolution: 15 meters/pixel
Solar Incidence Angle: 79,8° (meaning that the Sun, at the time that the picture was taken, was about 10,2° above the Local Mercurian Horizon)
Emission Angle: 11,9°
Sun-Mercury-Messenger (or "Phase") Angle: 67,9°

This frame (which is an Original NASA - MESSENGER Spacecraft image published on the NASA - Planetary Photojournal with the ID n. PIA 15857) has been additionally processed 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 Surface of the Planet 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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