Phobos 2 Encounter

The spacecraft Phobos 1 and 2 were launched on 7 and 12 July 1988. The mission was to have three stages: investigations of the Sun and interplanetary space during the flight from Earth to Mars; studies of both Mars and Phobos during the orbit of the spacecraft around Mars; and studies of Phobos as the spacecraft approached to within 50 m of its surface. Contact with Phobos 1 was lost on 1 September 1988 and contact was also lost with Phobos 2 just before the third stage of the mission. This article introduces fourteen papers which cover the results of the second stage of the mission.

IMPORTANT information about the properties of the outermost layer of regolith on Phobos can be obtained from infrared thermal radiometry and spectrophotometry of the reflected radiation. From the analysis of Mariner 9^1,2, Viking 1,2^2–4 and ground-based experimental data it was concluded that the surface layer of Phobos consists of fine-grained material (regolith) ¹ with a composition close to that of carbonaceous chondrites^2,3. In the course of these measurements, Phobos was observed as a point-like object. Here we present results obtained with a resolution 1/20–1/40 of the mean Phobos diameter showing, for the first time, inhomogeneities in the thermal and spectral properties of the surface of Phobos.

PREVIOUS orbiter and lander pictures have given an indication of the general morphology of the surface of Mars and of some relatively small features on the surface. Here we have determined the elemental composition of martian soil from Phobos 2 using orbital γ-ray spectrometry. This technique is used to analyse the surfaces of planets which either do not have an atmosphere or which have only a low-density atmosphere (that is, there is negligible absorption of γ-rays). The γ-radiation is caused by the decay of radioactive elements excited by the interaction of solar (SCR) and galactic (GCR) cosmic rays with the surface materials. Measurements of this radiation by a -γray spectrometer enable the elemental composition of the near-surface rocks to be determined. The first successful measurements of γ-radiation were conducted from Luna 10 (ref. 1). The method was used subsequently during studies of the Moon from Apollos 15 and 16 (ref. 2), and Mars 5 (ref. 3).