Explanation:

North of Mare Imbrium, towards the lunar north pole, is a region considerably different than the southern pole. Instead of towering mountains and deep craters, the topography of the region poleward of 60 degrees N latitude is battered and muted with only an occasional younger crater to attract attention. What happened? This image of the half crater known as Gartner provides a clue. At 102 km wide, Gartner probably looked like Copernicus about 4 billion years ago. But then at 3.85 billion years ago a gigantic impact excavated the Imbrium basin, spewing ejecta in all directions. Much of the north polar region was splattered with mountains and boulders and flour size ejecta. The hilly rubble in Gartner is probably part of the ejecta and the smoother material surrounding the hills may also be from Imbrium. Apparently the area of Mare Frigoris subsided, carrying down the southern rim of Gartner, which was covered by later Frigoris lavas. The rille in Gartner is hard to explain. It is not radial or concentric to a basin (perhaps it could be considered radial to Serenitatis), and if it formed in ejecta rather than lava, it can not be a lava tube. Perhaps the smooth plains material is older, non-mare composition lava. In fact, with full Moon lighting the areas around the small craters near the rille's north end appear dark, like a pyroclastic deposit. This area is not covered by good Lunar Orbiter imagery and has not been investigated fully.

— Chuck Wood

Technical Details:
Sept 3, 2004, 2:31 UT. Takahashi Mewlon 300 + 3x barlow + anti-IR filter + SBIG ST7 XME CCD. Median of 10 1/10 s exposures of 181 obtained. Processed with Astroart 3.0 and Photoshop.

Related Links:
Lunar Orbiter IV View
Rukl Atlas of the Moon, Sheet 6
Bussey & Spudis Clementine Atlas of the Moon Sheet 13

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