Tuesday, July 23, 2013

The View Inside a Tilted Crater

Oblique view of the chaotic interior of 30-km Wiener F crater. LROC Narrow Angle Camera (NAC) mosaic M1113262343LR; LRO orbit 16307, January 19, 2013, spacecraft and camera slewed 52° west from 160.39 km over 41.84°N, 140.28°E, subsampled from a scaled 2.78 meter per pixel resolution. Scene width approximately 13 kilometers from left to right, centered at 41.1°N, 150.0°E. [NASA/GSFC/Arizona State University].
Brett Denevi
LROC News System

Impact melt is commonly found in and around fresh lunar craters and can be spotted as ponds, flows, and ejecta.

This oblique view of the farside crater Wiener F highlights one of the more spectacular examples of what happens to the melt when a crater forms on a slope.

In the image above, you have a great perspective view of the chaotic crater interior, where material slumping into the crater interacted with the fluid melt, creating rough, hummocky mixtures in some regions and smoother pools of melt in others. But what is really interesting about this crater becomes clear when you zoom out to the full width of the image, below.

Thumbnail view of LROC NAC mosaic M1113262343LR, looking from west to east into Wiener F crater. For the full-resolution, zoomable view click HERE [NASA/GSFC/Arizona State University].
Wiener F formed atop a larger, older crater, so its northern rim, on the left in the picture above, ended up substantially lower than the southern rim. A profile across the crater, taken from the GLD100, shows the northern rim of the crater is over 2 km lower in elevation than the southern rim!

A profile from south to north across crater Wiener F, taken from LROC WAC-derived topography data [NASA/GSFC/Arizona State University].
So tilting the crater like this is like tilting a glass of water - it spills. In this case, the hot impact melt that would normally stay within the crater poured out, spilling over the northern rim and pooling outside the crater. Click on the image below to see this spectacular flood. You can find individual flows and places where the melt was still moving even as a crust of hard rock formed on top, resulting in cracks and wrinkles in the top layer.

View of the impact melt that escaped Wiener F, pooling outside the northern crater rim. Image subsampled from the original resolution [NASA/GSFC/Arizona State University].
Impact melt is a favorite target for LROC imaging because of its often complicated and bizarre features, and because of what it tells us about the impact process. The volume of melt can give clues as to how fast an impactor hit the surface (higher velocities mean higher shock pressures and more heat to melt rock), at what angle it impacted (melt is often thrown downrange of an impact), and how long ago the impact occurred (by observing how well preserved the melt morphology is, or by age-dating a sample of melt). Impact melt can also give insights into how portions of the crater moved and settled as the crater formed (for example, how did melt get up HERE?).

LROC Wide Angle Camera (WAC) contextual view of Wiener F crater, nested in the Farside Highlands [NASA/GSFC/Arizona State University].
Wiener F is another piece of that puzzle, showing what a dynamic environment an impact crater is shortly after formation. Click HERE for the full-resolution view of Wiener F.

Other Spectacular
Impact Melt Favorites:
Rumker E Impact Melt
Dynamics of Molten Rock
La Pérouse A Impact Melt
Rippled Pond
Breached Levee
Secondary Melt on the rim of Wiener F
Getting cracked in Wiener F
Giordano Bruno Whorl

No comments: