Tuesday, January 31, 2012

View from Vavilov

Vavilov (0.8°S, 138.8°W), the 'relatively recent' 98 km crater straddles a crossroads in the violent timeline of the Moon's history and sports some of the Moon's highest elevations on the northern and western rim. Interestingly, Vavilov formed nearly on top of a similarly sized and much older crater whose rim is still visible as a semicircle immediately northeast. Both craters carved out the same unique notch in the west wall of Hertzsprung impact basin. Image from 160 kilometer wide field of view cropped from a LROC Wide Angle Camera monochrome (566 nm) mosaic stitched from eight June 3, 2010 orbital viewing opportunities averaging 76 meters per pixel with an incidence angle of 64.5° from 55 km altitude [NASA/GSFC/Arizona State University].
WAC-derived elevation model (GLD100) Scene elevation (meters)
minimum = 2131.00, maximum = 9317.00 NASA/GSFC/Arizona State
The elevation models of the Moon, built up during the record-breaking first 10,000 orbits by the Lunar Reconnaissance Orbiter (LRO), are finally allowing us to see the lunar surface in definitive detail. Naturally, this is especially true of the farside, invisible from Earth, and the polar regions. Even the vast highlands of the farside, unseen before 1959, have either been imaged at very wide angles or at low angle in part.

The Lunar Reconnaissance Orbiter Camera (LROC) in particular has been a spectacular success at imaging nearly half the Moon's surface at very high resolution and in surveying the entire Moon under a wide range of lighting conditions. One result is a highly accurate digital terrain model that just keeps getting better.

A weak attempt to represent a 100 degree wide panorama of the Vavilov interior and southern rim from its highest elevations situated along that crater's north rim. For the first time LROC is allowing us to imagine what that view might be, but it still does such a scene little justice to squeeze it into a 580 pixel-wide image [NASA/GSFC/ILIADS/Arizona State University].
Until LRO, many, but not all, of the wide angle views of the lunar farside have been focused on low resolution wavelength analysis, albedo, mineral and some low light relief. And even millions of laser altimetry measurements haven't matched the number of points recorded by LOLA during the LRO's present mission, already in lunar orbit far longer than any previous spacecraft. The record of the location of places photographed and measured hasn't been helped by the simple fact that no one really knew the Moon's actual shape and size with a high accuracy until Japan's Kaguya (SELENE-1) mission.

The Moon's highest elevations (10,761 meters) are now believed to be more than 600 km northwest of Vavilov, on the rounded wide rim of the crater Engel'gardt, but as we round the Moon's western rim, past Oceanus Procellarum and north of Mare Orientale where the farside highlands begin, and continue to proceed westward along the Moon equator the first very highest elevations encountered are on the north and western rim of Vavilov.

Easily among the highest elevations east of Engle'gardt East is on the upper reaches of the slumping wide north wall of Vavilov. We can only guess whether those heights could have once been much higher. The chaotic terraces of the western interior of Vavilov testify to a high degree of slumping, massive landslides underway since the crater formed. LROC WAC observation M130205287C (566 nm), orbit 4321, June 3, 2003; incidence 64.67° with a resolution of 77.11 meters per pixel, from an altitude of 55.31 km [NASA/GSFC/Arizona State University].
That Vavilov is deeply notched into the west-southwestern wall of the vast Hertzsprung impact basin is not something one can easily tell from Clementine (1994) albedo imagery, for example. So many craters with extended ray systems, like Jackson, overlap over the farside highlands, already bright for their relative lack of the mare-filled basins that dominate the nearside, that getting a gauge on elevations has remained elusive until the LRO mission. With the human eye alone its nearly impossible. But there is a reason why Vavilov is different, and higher, in one half than the other.

The west-southwest of Vavilov is not as stark a contrast in elevations as its north wall but the elevations are still respectable. The highest point along that rim is 9317 meters, among the Moon's highest places, and the high ejecta blanket, outside Hertzsprung on this side of the crater, tapers off less dramatically as well. The interior on the west side of Vavilov is more dramatically terraced, and this was probably not the original rim, its original circumference having collapsed, probably many times. The view seen in high detail in LROC Narrow Angle Camera (NAC) M151440688L shows a couple of kilometers-wide strip near this high elevation, and that along with other detailed images seem to show the process of slumping is still, slowly, underway. LROC WAC  observation imaged at the same opportunity, LRO WAC observation M151440362C, orbit 7451 February 4, 2011; incidence angle 51.43° at a resolution of 81.2 meters per pixel from 58.55 km [NASA/GSFC/Arizona State University].
The Vavilov impact event was not the first to carve out a place on the wall of Hertzsprung. The crater is offset just a little to the southwest from the crater, of almost identical size, that first made the notch and first interrupted the full circle of the 590 km-wide Hertzsprung impact basin. Vavilov's progenitor came close to erasing it's sister sometime after, and all that remains of the older crater is a semicircle like a cup handle attached to Vavilov's northeast.

The LROC WAC-derived Digital Elevation Model (GLD100) brings Vavilov out of the glare, in false color. The terrain was already on the rise from the southeast before the Hertzsprung or Korolov (further westward along the farside equator) because the formation of the Moon's oldest, deepest and largest known South Pole Aitken impact basin, further southeast may have help to lift the whole wider area along its perimeter here 4 billion years ago. The uplift of the third ring of mountains around Hertzspring rose still higher, first interrupted here by the arrival of Vavilov D. The area carries the deep scars and secondary craters of what some believe to be the most recent mare-filled basin-forming impact at Orientale, to the southeast. Vavilov probably formed after that event, superimposed on all those more ancient happenings. Vavilov is about seven kilometers deep, from its floor to the heights on its north and west rims [NASA/GSFC/DLR/Arizona State University].
The high western side of Vavilov perched on the southwestern outer ring of Hertzsprung and, on closer examination, the scaring and secondary crater chains radiant from the energetic impact that formed Mare Orientale, straddling the Moon's west limb and visible on edge from Earth. The most influential morphology that lifted this area is mostly invisible from Earth, the wide and deep 4 billion year old South Pole Aitken (SPA) basin at lower left. Orthographic projection over the intersection of the Moon's equator and its 240th meridian east [NASA/GSFC/DLR/Arizona State University].
Vavilov was unfavorable placed for the Apollo mapping cameras, and not well situated, nor a priority, for the Lunar Orbiter photography before Apollo. Other than the polar regions, this area of the Moon received less attention than most other areas until Clementine, and then from a low-resolution experimental remote sensing standpoint. LRO has changed that, however, and so much else. We now know that the view, and from the standpoint of science, the excavation performed by the Vavilov progenitor warrants more attention.

Even from orbit Vavilov must be spectacular.

Courtesy of the NASA ILIADS application, the LROC 100m WAC Global Mosaic draped over the LOLA 128 px DEM (v.2), the simulated "orbital view" of Vavilov from 65 km over the center of Hertzsprung basin.

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