ICARUS 16, 522 527 (1972) An Orthographic Photomap of the South Pole of Mars from Marina 7 ALAN R. GILLESPIE AND JAMES M. SOHA Space Sciences Division, Jet Propulsion Laboratory California Institute of Technology, Pasadena, California 91103 Received December 5, 1971 ; revised February 12, 1972 Television pictures of the south polar regions of Mars obtained by the Mariner 6 and 7 spacecraft in 1969 are rectified to a standard mapping projection using computer image processing techniques. Mosaicking of these pictures produces the first photomap of the entire south polar cap. INTRODUCTION We have used a digital computer to provide the first photographic view of the entire south polar cap of Mars in a standard mapping projection. This orthographic photomap, simulating a view of the polar cap from directly over the south pole, was constructed from television pictures of Mars obtained in 1969 by the Mariner 6 and 7 spacecraft.. Previously, pictures of the polar regions of Mars exhibited pro- nounced foreshortening, which distorted the apparent areographic relationships of surface features. This photomap eliminates virtually all such distortion in the polar regions. To generate the photomap, Mariner television pictures were first processed by computer to remove noise and geometric and photometric distortions produced by the television camera system and to normalize the solar zenith angle. Areodetic positions of surface features, determined using the 1969 Mariner Control Net of Mars (Davies and Berg, 1971), were then used by the mapping program to define a vector field for each picture. This vector field represents the geometric distortion at each point necessary to display the picture in an orthographic projection. Finally, indivi- dual projected pictures were mosaicked to produce the finished photomap. Two photomosaics comprise the map: a base map of the entire polar cap, made by joining nine far-encounter pictures sectioned along selected meridians, and Copyright © 1972 by Academic Press, Inc. All rights of reproduction in any form reserved. a higher resolution overlay of the five A- camera members of the Mariner 7 near- encounter polar sequence.1 Digital computer mapping techniques have several advantages over other methods. One is the relative ease with which accurate photometric and geometric rectifications may be made. Another is the ease with which quick and efficient searches may be made for changes in appearance between photomaps made from pictures taken at different times. This photomap should be a significant aid in planning future television coverage of the south polar region and will allow comparison of the polar cap condition between Mariner Mars 1969 (spring) and Mariner Mars 1971 (summer) or subsequent flights. SELECTION OF CONSTITUENT PICTURES The first step in the construction of the photomap was the selection of constituent pictures. Near-encounter Mariner 7 B- camera (narrow angle) frames of the polar region were excluded because the scale of the photomap, about 30 times that of the B-camera pictures, would result in reduc- tion in size from 702 × 948 to about 20 × 20 1 The Mariner missions to Mars in 1969 con- sisted of two spacecraft, Mariner 6 an d Mariner 7. Each carried two television cameras: art "A" camera with a focal length of about 52ram, and a higher resolution "B" camera with a focal length of about 504mm. 5~o2