Melt overlap at Anaxagoras

Two lobes of impact melt marking the boundary of the floor of Anaxagoras crater overlapped before solidifying against the crater wall. LROC Narrow Angle Camera (NAC) observation M185964003L, LRO orbit 12484, March 9, 2012; image field of view near 1.8 km, angle of incidence at this high latitude 73.06° with a resolution of 1.76 meters from 179.53 km [NASA/GSFC/Arizona State University].

Lillian Ostrach
LROC News System

Although impact crater formation is a nearly instantaneous event, impact melt cooling is not. For large craters such as Tycho, Copernicus, and Anaxagoras, so much impact melt was created during the impact process that the crater floors were, in effect, flooded by molten seas of melt. The melt pooled in topographic lows, flowed around the central peaks, and became mixed with loose ejecta blocks to create a hummocky texture on the crater floor.

Cooling cracks and collapse pits are prevalent; subparallel groupings of cracks are common near melt boundaries and in regions with entrained blocks.

LROC WAC monochrome mosaic of Anaxagoras crater (73.458°N, 349.934°E, 52 km diameter). Location of the field of view shown at high resolution in the LROC Featured Image released January 16, 2013 noted by arrow [NASA/GSFC/Arizona State University].

Like volcanic lava, impact melt may remain molten for an appreciable amount of time - days, weeks, and years in some cases. The floor melt pond in Anaxagoras was molten for a while, and mobile enough to splash onto the crater walls to form "bathtub rings" (upper right corner in the opening image, also beautifully visible in more detail in the full NAC image). The splashing probably occurred as large blocks of wall material slumped into the ponded melt causing a tsunami of melt!

Today's Featured Image focuses on the boundary of the floor melt pond with the northern crater wall where two lobes of melt overlapped (73.830°N, 350.368°E). Looking to the left of the image, you can trace the melt contact with the crater wall and follow it toward the right side of the image. A moat-like boundary at the edge of the flow of this top layer of melt distinguishes it from the layer beneath. The stratigraphically lower layer of melt is first visible in the center of the image and extends toward the image right.

What may have happened was that this section of the melt splashed up the crater wall to form the first bathtub ring (barely visible in the opening image top right) and flowed back down the wall. Then the melt flowed up the wall again where some melt stuck to the wall because it had sufficiently cooled and a crust had formed, and the melt stuck up on the wall (lobe on the right side of the opening image). Perhaps the melt near the center of this sub-pond remained quite warm and mobile and the melt flowed again toward the wall. The melt then onlapped and superposed the cooled, frozen section of melt near the wall but the melt boundary with the wall cooled sufficiently to stick and totally freeze, thus preserving multiple splashes and slurps of melt on the crater wall. But that is only one possible story - topographic data, melt cooling models, and observations at other craters would be helpful in discovering the history of melt cooling at Anaxagoras.

Check it out! Take a look at the full LROC NAC image HERE - how many impact melt "bathtub rings" and overlapping melt lobes can you find?

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Impact melt in Anaxagoras crater
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UPDATE: There are LROC NAC observations of the interior melt overlap at Anaxagoras at higher resolution, more than there are of equatorial targets on average, for example, LRO revolves around the Moon in a polar orbit, and, like lines of longitude, the vehicle’s orbital path traced out on the surface below converge at the poles. Targets like Anaxagora, at relatively high latitude, have, thus far, received overlapping attention.

The Copernican age crater get more attention because its relatively new, as well, less beaten down and gardened by space weather and micrometeorites. It takes up a considerable volume of the interior of the larger, and considerably older Goldschmidt crater, where the Cassini spacecraft, on its way to Saturn, appears to have detected water or hydroxyl molecules in broad daylight.

One particularly close observation in addition to the one at the top of this post is highlighted below, showing three distinctive “bathtub rings,” a steady surf that came to a halt before the energy that created it played out. In fact, that same energy is still present. Can these observations allow scientists to measure the time between the melt formation and its frozen state? 

A 2460 meter-wide field of view overlapping the same area shown in the LROC Featured Image, released January 16, 2013, with the three areas shown at full resolution in the images that follow outlined in rectangles. LROC NAC observation M124628200R, spacecraft orbit 3500, March 30, 2010; angle of incidence 72.08° at an original resolution of 49 cm per pixel from 43.61 km [NASA/GSFC/Arizona State University].

The highest, northern most ‘slosh’ in the field of view appears to have been energetic or directed enough, or both, to have moved a considerable amount of debris [NASA/GSFC/Arizona State University].

This slosh appears to have been retreating, even as the a wave further south was gaining and over lapping it, leaving debris like sea shells in a scene witnessed in motion by anyone who has walked along a terrestrial beach [NASA/GSFC/Arizona State University].

Was this later wave really of a greater magnitude than the others? Did the surface lose its viscosity suddenly? Did material heap up behind or below? Was this process really slow or sudden? [NASA/GSFC/Arizona State University].