Wednesday, November 2, 2011

Tycho's flash-frozen inferno

Tycho in a full Sun, 'low phase' illumination, the crown jewel of a Full Moon on Earth has only its relative youth to distinguish it from many similarly-sized craters of similar origin. - LROC Wide Angle Camera (643 nm) mosaic from seven orbital passes (9061 - 9067) June 11, 2011. Arrow marks location of terraced pools of impact melt detailed below [NASA/GSFC/Arizona State University].
Joel Raupe
Lunar Pioneer, LLP

Hardly rare in scope or origin from similar craters all over the Moon, Tycho stands out like a bright star in the nearside southern Highlands because its bright rays radiate outward over the face of an entire Full Moon. Those rays make Tycho visible to the naked eye on Earth.

At a youthful 109 million years of age, relentless gardening by micrometerors has not yet smoothed the crater's reflective rougher edges, nor has a never-ending rain of charged particles, from the Sun and beyond, merged it's coloration into the background.Later impacts have not superimposed themselves or covered over the flash-frozen record of the mere minutes and hours following the explosive release of kinetic energy that created Tycho.

Tycho under 'high phase' illumination, at sunrise shows it's elevations in stark contrast, without the blinding albedo that tends to blur the immediate area of impact into the much broader area affected by that impact. LROC WAC mosaic, with longitude and latitude lines released by the LROC team last July. The impact melt detailed below, pooled just beyond the southeastern rim in this 130 kilometer-wide field of view, is much easier to see in the full-size original LROC image release, available HERE [NASA/GSFC/Arizona State University].
Among these finer details retained by Tycho are the pools of impact melt on the inner terraces and not far outside the crater's high rim, like the 'paved' pond Surveyor 7 nearly landed on in 1968, seen in the stark beauty of LROC Narrow Angle Camera observations.

The progenitor, the object that struck the lunar highlands and created Tycho threw up a lot of material in those first seconds afterward. Some of this material sped away at escape velocity, casually returning to the surface much later, if ever. Some sped away laterally as an immediate shock wave, carrying with it enough force to clip the tops of mountains on the south edge of Mare Serenitatis, knocking down the bright material of the Tortilla Flats in Taurus Littrow, sampled by Apollo 17 in 1972.

Some of the cloud scooped up by the blast hesitated above the area from where it was lifted and piled back down onto the surface outside the molten scar but most of the height where Tycho formed had to have been there before the explosion. We can tell this from the deep rutted channels carved into the highlands for hundreds of kilometers away from its center. The shape of the lunar surface around Tycho is not defined by what piled up but what remained after thousands of square kilometers of material were gouged away.

Tycho seems nested in a kind of plateau, though the evidence appears to show that this plateau was defined out of the highlands by the impact event that created Tycho. Great three and four kilometer-deep gullies appear to have been scooped out and away by the blast, better seen in this virtual 3D oblique view looking north over the outer southeastern rim of Tycho. LROC WAC (643 nm) mosaic as an overlay upon the Kaguya (SELENE-1) lunar digital elevation model in Google Earth [NASA/GSFC/USGS/JAXA/Arizona State University/Google].
The terraces on the inner walls of Tycho became a place for impact melt to pool and cool, so the terraces, by and large, were unlikely to have been formed by later slumping. We're left with a picture, immediately after the Tycho impact event, of a ragged scar, glowing hot in those first hours, from the central peak of deeper rock that rebounded in a heap (never higher than a crater's rim) to the very lip of the outer rim. It must have been a scene right out of Dante's Inferno.

LROC Wide Angle Camera (WAC) monochrome (643 nm) observation M119950214M, LRO Orbit 2810, February 4, 2010; resolution 66.12 meters per pixel, incidence angle 64.73° from 47.55 km. The yellow rectangle roughly outlines the field of view within the entire from of the LROC Narrow Angle Camera (NAC) frame from which scenes following originated [NASA/GSFC/Arizona State University].
Zooming in on these "Southeast Tycho" impact melt ponds, in the images above and following, note the fan of these ponds seem to flow downhill from a particularly ragged spot on the rim and apparently from further north channeled from an less distinct portion of the circumference of Tycho's rim. It's hard to imagine Tycho filled to the "brim" with molten rock, though the original melt was probably higher before it solidified to its present level. 

It's easier to imagine very hot material briefly pasted on the inside walls of Tycho sliding down to pool and form the ponds on the inner wall terraces. The ponds on the outside of Tycho are relatively sparse, but so is the slope acreage elevations outside the crater's interior. Before the anatomy of Tycho cooled and hardened some of the hottest melt was slung high seems to have collapsed like the opening rip of an ocean wave, which quickly froze, liquid rock that fossilized forty million years before the KT Boundary Extinction event brought an end to the Age of Dinosaurs here on Earth.

As impact melt briefly ran down the exterior side of the southeast brim of Tycho and pooled, coming to a halt in the cold vacuum of space long before 'finding its own level,' what appear as grooves formed by flowing molten material appear closer in to be shattered rock that instead merely aided molten transport. LROC NAC observation M150578086R, LRO Orbit 7324, January 25, 2011; resolution 71 centimeters per pixel, incidence angle 69.84° from 44.74 kilometers [NASA/GSFC/Arizona State University].
Full resolution view of the pond shore at the upper northeast in the image immediately above. The impact melt that ponded here briefly 109 million years ago was still hot enough for gas trapped within to heave bubbles to its surface. The flow at this juncture was arriving from all direction and the rounded surface tension elsewhere testifies to the lava-like viscosity of the pond [NASA/GSFC/Arizona State University].
A second full-resolution view from LROC NAC M150578086R shows where melt from the Tycho event briefly flowed down a huge, powerful fall at a high slope between ponds more than a thousand meters apart in elevation [NASA/GSFC/Arizona State University].
Tycho may be the same as many other craters on the Moon, but it's relative youth "in Moon years" makes it an easy choice for mapping the immediate aftermath of a powerful impact on an airless body, at least for the next half billion years or so.

Related Posts:
Tycho Peak Spectacular!
Chaotic crater floor in Tycho
Polygonal fractures on Tycho ejecta
Impact melt on Tycho floor
Ejecta on Tycho floor

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