Wednesday, October 3, 2012

Oblique views of Moon's highest and lowest places

East rim of 21 km Engel'gardt crater (5.69°N, 200.29°E), north of Korolov, basin on the Moon's farside, host to the Moon's highest elevation above the global mean (10,761 meters). LROC Narrow Angle Camera (NAC) M176265113LR, orbit 11111, November 18, 2011; foreground resolution 4.6 meters. A spectacular oblique observation, from spacecraft and cameras slewed -76.3° off nadir. Beyond the high point, first identified by Japan's Kaguya (SELENE-1), the east wall of Engel'gardt descends more than 4000 meters to the crater floor (not visible). View full-size and other versions HERE [NASA/GSFC/Arizona State University].
Searching through the catalog of LROC Narrow Angle Camera observations, it's difficult to find many images outside it's optimal straight-down 'push broom' design range, and fewer not already released as LROC Featured Images.

On a dare, I accepted a colleague's small wager (and his pocket change) and quickly found oblique LROC NAC observations of both the Moon's highest and lowest places. That was the easy part, because just as we discovered when we pieced together the rudimentary image in Taurus Littrow Oblique (September 29, 2012), squeezing, stretching and stitching together a huge, incredibly detailed final product of this kind is best left to the better-equipped.

Be that as it may, here is a partial result of those clumsy efforts, presented primarily because of the stark beauty of this side glance at "Engel'gardt Heights," the Moon's 10.761 km highest point. After deciphering the width and height scales the result was surprising.

Clearly, I only thought I fully appreciated all there was to see of what seemed to be a pretty bland landmark. Once again, that's the way the Moon works. You stare at a familiar feature for years, and then one simple change in perspective makes you wonder whether you ever really "saw" it all.

It became necessary to briefly review why this feature escaped our notice until the 21st century.

Dynamics of the slewed early lunar morning, oblique LROC NAC observation of Engel'gardt, during the 11,111th orbit of LRO, November 18, 2011. The spacecraft was well over 100 km away and over 40 km in altitude, and these facts help to explain the scope of the resulting field of view. X marks the spot of the Moon's highest place, in the left frame of the twin NAC observation. The correcting ratio of pixel width to height can largely be corrected. The upper half of the image reproduced up above, however, is, literally, bounded by "infinity," the dark of Space. Many objects appear higher in altitude than the Moon's "summit," because, even at a distance, LRO is looking "down" on the entire foreshortened field of view. Mountains in the distance really loom only over local terrain [Virtual Moon Atlas].
A glance through the Related Posts below will demonstrate that identifying the Moon's highest and lowest points has been discussed at length here and elsewhere, in recent years, perhaps because the Moon's true diameter, and the range of certainty as to its true center, moment of inertia, and actual shape were not narrowed to within two kilometers until very recently.

The final three Apollo "J" missions mapped large swaths of the sun-lit lunar surface with cameras operating in their patiently orbiting service modules, much of that work underway as the lunar module was away on the surface below. Unfortunately, the demands for lower Sun at their three landing sites meant the area of the Moon's farside with the highest elevations were rotating through the long lunar night. The totality of photography from all lunar missions, even the earliest, first views by anyone of the farside (just a little more than five decades ago) allowed for a low-resolution understanding of the stark differences between the Moon's near and far sides. Early on, it was already generally known where the Moon's highest elevations were, but a detailed survey would wait until lunar exploration again, albeit briefly, became the public policy of nations following the loss of Space Shuttle Columbia in 2003.

The lunar map available in Google Earth, though impressive in many places, reflects an earlier understanding of the area on the northeast frontier of the SPA basin long known to contain the Moon's highest places. The blue rectangle is the footprint of the left frame of LROC NAC observation M176265113L, used to build the lower portion of the oblique mosaic of the Moon's highest spot, on the wide eastern rim of Engel'gardt crater. The right-hand frame, bordered in part by sky, lacks official scaling instructions [NASA/JAXA/DOD/USGS/ASU/Google].
The Google Earth representation of the Moon is impressive in many places, though large areas still depend on 100 meter resolution albedo photography gathered by Clementine (1994), a component of that mission dependent on a high Sun. Without the more recent comprehensive Wide Angle Camera maps, subtle differences in relief go missing. And that's a fair illustration of at least part of the reason the magnificent high east rim of Engel'gardt crater went missing, and is still missing from the Google Moon elevation model.

Laser altimetry began with Apollo, advanced without needed "granularity" with Clementine and became nominally 'comprehensive' with the LALT laser altimeter on-board Japan's Kaguya. As the first long-duration lunar orbiter, launched together with LCROSS in 2009, advanced photo-mapping from LRO's LROC systems continues to marched our understanding at a faster pace than has yet to be achieved through its steadily growing database of LOLA laser altimetry. As a total mission, the very economical LRO mission may finally advance our understanding of the Moon to a point that matches most presumptions.

Repeated over-flights of LRO and its LROC Wide Angle Camera through an unprecedented time in lunar orbit, together with precise calculations of solar angles and distances, allowed for the rapid identification of elevations, complimenting the simultaneous build up of laser data points by LRO's LOLA. Under a mid-day Sun, the east rim of Engel'gardt seemed flat, but above, on December 9, 2009, with the Sun 21° over the east-southeast, LRO passed 59 km overhead as a bulbous shadow betrays the presence of a great height, in profile. Confirmation of Japan's discovery, with an only slightly adjusted elevation in meters, was only one of many achievements of the on-going Lunar Reconnaissance Orbiter mission. LROC WAC monochrome (604nm) data [NASA/DLR/GSFC/Arizona State University].
In truth, though it hasn't made the news, in our time the scientific world is adding more to our knowledge of the Moon each year than was learned throughout the 20th century. And that pace is likely to continue long after LRO is added to the list of artifacts of human activity on the Moon that it was, in part, sent to survey.

And, in truth, the reason the Moon's highest and lowest places were not known until more recently is more simply put. The highest of the Moon's high places (like Everest, on Earth) is a high place among high places, and the Moon's lowest place is a low place among many low places. Our knowledge of the Moon wasn't so much lacking as it was lacking granularity.

Point of Highest Elevation
By now, however, the LROC team at Arizona State has made the "east rim of Engel'gardt crater" not merely part of its WAC-derived global elevation models, but also a part of it growing list of NAC-derived localities, places with elevations mapped in fractions of a meter. Not satisfied with mapping more than half the lunar surface at high-resolution, a mind-boggling 3D model is taking shape.

"The Point of Highest Elevation" can be explored at 50 centimeter resolution, at the link on the left, or explore nearly 100 other areas of interest, at the LROC NAC DTM Viewer, HERE.

According the LROC Planetary Data System interface, unlike the still- significant part of the lunar surface not yet photographed at high-resolution through the LROC NAC cameras, the east rim Engel'gardt crater has been imaged, in either the NAC  left, NAC right, or both frames simultaneously 23 times, beginning June 4, 2010 (or after nearly a year after the spacecraft's arrival in lunar orbit. That's not to imply any neglect of other targets. The resource can't really be measured that way. But it is reasonable to conclude there is more than just a passing scientific interest in "the Roof of the Moon."

Until the release of this latest oblique view of the eastern side of Engel'gardt crater and points beyond, I confess to little more than a passing interest in this target. It seemed to lack something as intangible as a certain aesthetic quality. It seemed rather dull. But in the almost unnatural tight shot, captured from 48.16 km above a point on the lunar surface more than 100 km away, more than 6 degrees of longitude east, we have found our aesthetic.

Because it was captured from an altitude, and over an airless body, the area in the field of view seems typical of what's seen through a telephoto lens, with objects in the foreground gathered together and as much in focus as the distant high places well beyond and invisible from anywhere near the actual crater. As already mentioned, the scaling for the right frame of the NAC montage is essentially infinite, so beyond an imaginary line running north-south transecting the target crater, "objects may be further away than they appear."
This unnamed, largest crater on the floor of 178 km-wide Antoniadi (69.2°S, 186.94°E) is the location of the Moon's lowest point, 9094 meters below the Moon's mean elevation.The crater within a crater is, itself, well inside the Moon's largest (2600 km), deepest and oldest known impact, the more than 4 billion year-old South Pole-Aitken basin. Mosaic of both left and right frames from LROC NAC observation M191636857, orbit 13277, May 14, 2012; slew angle 63.8° [NASA/GSFC/Arizona State University].
Bonus image, also in the field of view from the Antoniadi NAC mosaic, the nearly flooded and comparatively diminutive central peak of Antoniadi, which was heavily inundated by melt long after it formed, demonstrating the marked differences between Antoniadi and similarly-sized craters elsewhere on the Moon. Some theorize Antoniadi may have been flooded from underground, following the energetic and relatively recent Orientalis basin-forming impact [NASA/GSFC/Arizona State University].
Then there's Antoniadi, about which much has already been written, and the subject of another LROC NAC oblique observation. So far, we've been unable to satisfactorily scale the entire image, but we do discuss two interesting fields of view from those frames immediately above.

Related Posts:
DLR: Flying over the three-dimensional Moon (December 1, 2011)
LROC's new Global Lunar Topography (November 16, 2011)
LOLA's deep Antoniadi (April 16, 2011)
LRO's unprecedented topography of the Moon (December 17, 2010)
Highest point on the Moon (October 26, 2010)
The deepest spot on the Moon nearly wasn't (September 17, 2010)
Lunar superlatives from LROC WAC (September 6, 2010)
The Moon's lowest of the  low (November 24, 2009)
Accurate topographic map of the Moon (June 13, 2009)
Spectacular refinements to Kaguya laser altimetry (May 28, 2009)
Best lunar topography derived from Kaguya (February 12, 2009)

The Moon's highest and lowest places, in relation to one another, the whole Moon's average elevation (the lunar equivalent to Sea Level here on Earth) in this very much resampled whole hemisphere view of the Moon's farside. Near center, beyond the rim of South Pole Aitken basin, is Engel'gardt crater, and the Moon's highest elevation, and near bottom, well-within SPA, is the largest crater within 178 km crater Antoniadi, site of the Moon's lowest elevation. The sites, representative of two very different places on the Moon, are only 2300 km apart, less distance than the major axis of slightly oblong SPA. LROC WAC DTM [NASA/GSFC/Arizona State University].

1 comment:

Humaun Kabir said...

Thank you for this valuable information, I hope it is okay that I bookmarked your website for further references.

Square and Stationary Earth Map