Lunar surface traces of engine jets of Soviet sample return probes: The enigma of the Luna-23 and Luna-24 landing sites

We use a photometric method called phase-ratio imaging to study the landing sites of the Soviet Luna-16, Luna-20, Luna-23 and Luna-24 probes using the survey data of the lunar surface, which was carried out with the Narrow-Angle Cameras (NACs) of the Lunar Reconnaissance Orbiter (LRO) spacecraft. The phase-ratio images clearly show diffuse features associated with structure perturbations of the lunar regolith. We suggest that these features are caused by the impact of the gas jets from the rocket engines. The photometric anomalies around the landing sites suggest that the impacts smooth out the surface, destroying the primordial fairy castle structure that effectively produces the shadow-hiding effect. The same characteristic features have been found previously for the Apollo spacecraft landings, but over larger spatial scales. The only exception is the landing site of the Luna-24 probe, for which the feature of the possible impact of the gas jets is shifted to the northwest by approximately 150 m. As the Luna-24 descent module worked in the regular mode and could not allow such a shift as the probe was descending vertically, a possible explanation is that the sites of Luna-23 (an unsuccessful sample return mission) and Luna-24 are misidentified. The distance between the sites is about 2 km, which is within the inaccuracy of their coordinate determination. We suggest that because of faulty processing of the radar system for distance/speed control, the incorrectly operated engine and/or thrusters of Luna-23 produced the 150 m lateral drift before final deactivation and hard descent. To better understand the geologic situation, we produce brightness and phase-ratio anaglyphs for the vicinity of the landings. Published by Elsevier Ltd.