Photogrammetric Brief

ARTHURD. HOWARD Stanford University Stanford, Calif. 94305 Lunar Stereopairs

High-resolution photos were obtained during the Lunar Orbiter program.

INTRODUCTION vertical stereopairs contrasts with the ex- tremely small scale of the February oblique HE PRESENT NOTE was motivated by the high-altitude stereopair cover illustration that is less than 1:2,000,000 Texcellent in the central area. The larger scale reveals of the Aristarchus region of the M~~~ which appeared on the cover of the February issue considerably more detail and makes possible refined Although the primary of this journal. Although the stereopair had actually been set up pseu~oscopica~~y(see PU'POSe of this note is to Present these ex- May issue, Erratum, page499), the writer had amples '' stereopairs! a few 'Om- ments on significant features and their possi- no difficulty viewing it properly because of his familiarity with the topography of the ble interpretation are area. THESTEREOPAIRS In a NASA report on possible lunar analogs of fluvial landscapes (Howard, Arthur, Lunar Figure 1 is a view of Schroter's Valley, the Analogs of Fluvial Landscapes: Possible Im- prominent valley that appears in the February

ABSTRACT:Lunar Orbiter vertical photographs taken in 1966 and 1967 have small areas of overlap. A number of stereopairs with scales ranging from ap- proximately 1:7,000,000 to 1:35,000 have been prepared from this photog- raphy. Four examples are presented with brief discussion. plications, NASA Report NGR-05-020-267, cover photograph. The viewer will undoubt- 95 p., Feb. 1970) a number of stereopairs edly be distracted by the apparent vertical illustrating lunar phenomena were presented. offsets of the topography between adjacent The stereopairs weie prepared largely from horizontal strips (framelets). The offsets are high-resolution Lunar Orbiter IV photo- due to the scanning, transmitting, and re- graphs which cover the entire near face of the assembly techniques employed in producing Moon and, to a lesser extent, from scattered the photographs. Schroter's Valley starts in a photographs including medium resolution crater, Cobra Head, on the northeast flank of Lunar Orbiter IV and medium- and high- the great crater Herodotus and continues resolution Lunar Orbiter V photographs. The some 80 miles (130 km) before terminating scales of these photographs varied because of abruptly in a dead end. The inner rille, how- changing altitude of the spacecraft. The ever, (Schroter's Rille) continues another 30 scaler of the high-resolution Lunar Orbiter IV to 35 miles (48 to 56 km) eventually dying photos generally ranged between 1:600,000 out on the surface of Oceanus Procellarum. and 1 :700,000 whereas scales of the other The overall length of the rille, measured photos used ranged from approximately through the middle of the meander belt, is 1:300,000 to as large as 1 :35,000. Many of thus about 110 miles (180 km). The outer the photos have variable amount of overlap. valley is interpreted as a graben whereas the Stereopairs of pertinent features of interest appearance of the inner rille suggests erosion which happened to fall within the areas of by a fluid agency. The two interruptions overlap were prepared; four examples are along the course of the rille in the lower pal t herein. The relatively large scale of these of the stereopair require explanation in the LUNAR STEREOPAIRS

fluid erosion hypothesis but these difficulties deposits of relatively high albedo, are sur- are not insurmountable. Disruption by rounded by dark mare deposits identified meteorite impact or side slope deposition are elsewhere as basaltic in composition. Prinz among possible explanations. Single large- Crater also antedates the more deposits that scale photos clearly indicate that side slope extend into its interior. debris has crowded into the meanders where A variety of rilles are displayed. Recti- these hug the valley sides. linear segments, the trends of which fall into Figure 2 illustrates the Harbinger Moun- regional sets, and the angular turns suggest tains area just east of the Aristarchus fracture control of at least parts of several of Plateau. The area is off to the left of the the rilles. The evidence for fracture control is February cover photograph. The large clear in the headward portions of most of the crater Prinz is shown in the upper left. The rilles but rectilinearity and angularity give Harbinger Mountains, consisting of terra way to sinuosity in the lower courses. A possi-

FIG.1. Schroter's Valley and Schroter's Rille in the Aristarchus Plateau. Approximate coordinates: 26ON, 50°W. Stereopair prepared from high-resolution Lunar Orbiter V photos, frames 203 and 205. Date of photography; 18 August 1967.

LUNAR STEREOPAIRS

FIG. 2. Harbinger Mountains, east of Aristarchus Plateau. Approximate coordinates: 27ON, 43OW. Stereopair prepared from medium-resolution Lunar Orbiter V photos, frames I88 and 191. Data of photography: 18 August 1967.

ble explanation may be that fluids reached the generally recognized fracture feature. Note surface by way of these fractures or graben- that in addition to the two main branches EB like depressions in the headward areas and and EJ which extend from Hyginus Crater E developed sinuous courses away from the there are other fainter fracture traces at C fracture zone. Note that the shortest of the and D. The main branches are clearly graben four rilles, below the center of the model, with strings of craters of presumably internal has no depression at its head. Numerous such origin. The craters are rimless as, for example, examples on the Moon argue against the sug- at F and G, but crater I is an exception. A gested general explanation that sinuous small crater, H, indents the rim of Hyginus rilles are eroded by internal fluids released at Crater. A remnant of the upland which impact craters. The interrupted rectilinear failed to subside along the graben appears at rille in the lower part of the model parallels J. A fault splinter and fault terrace are indi- one of the major regional fracture trends and cated at A and B, respectively. may indicate collapse along a reactivated Figure 4 is a view in the southern Apen- subsurface fault. The rille that crosses the nines. The bold, linear, facetted escarpment prominent ridge above the center of the model overlooking Mare Imbrium in the upper left presents a unique problem. Superposition, is almost certainly a fault scarp. Regional antecedence, or piracy seem inadequate. It is fractures in the hinterland are indicated not more probable that a gap existed here similar only by long lineaments (see arrows) but by to those between the hills in the lower part the edentate crests of low ridges. A good ex- of the model. The gap could, of course, have ample is shown along the east margin of the been created by faulting. The spatula-like figure at the fourth framelet from the bottom. depression across the rille below the center The most prominent fracture sets are north- of the model also parallels a regional fracture west-southeast and northeast-southwest. Val- trend and is probably a graben. The fact that leys seem to be integrated in some areas as the downdropped segment of the rille is not indicated by the exampl~sdelineated in shown on the graben floor suggests a sub- white. Many other examples, but on a much sequent fill. smaller scale, are detectable in the low hinter- Figure 3 is a view of Hyginus Rille, a land areas.

FIG.3. Hyginus Crater and Hyyitlus Rille. Approximate coordinates: 8ON, 6OW. Stereopair prepared from medium-resolution Lunar Orbiter Vphotos, frames 94 and 96. Date of photography: 14 August 1967. PHOTOGRAMMETRIC ENGINEERING, 1972