In the late 1950s and 1960s, the unusual orbital characteristics of Phobos led to speculations that it might be hollow.
Around 1958, Russian astrophysicist
Iosif Samuilovich Shklovsky, studying the
secular acceleration of Phobos' orbital motion, suggested a "thin sheet metal" structure for Phobos, a suggestion which led to speculations that Phobos was of artificial origin.
[56] Shklovsky based his analysis on estimates of the upper Martian atmosphere's density, and deduced that for the weak braking effect to be able to account for the secular acceleration, Phobos had to be very light — one calculation yielded a hollow iron sphere 16 kilometres (9.9 mi) across but less than 6 cm thick.
[56][57] In a February 1960 letter to the journal
Astronautics,
[58] Fred Singer, then science advisor to U.S. President
Dwight D. Eisenhower, said of Shklovsky's theory:
If the satellite is indeed spiraling inward as deduced from astronomical observation, then there is little alternative to the hypothesis that it is hollow and therefore martian made. The big 'if' lies in the astronomical observations; they may well be in error. Since they are based on several independent sets of measurements taken decades apart by different observers with different instruments, systematic errors may have influenced them.
[58]
Subsequently, the systemic data errors that Singer predicted were found to exist, and the claim was called into doubt,
[59] and accurate measurements of the orbit available by 1969 showed that the discrepancy did not exist.
[60] Singer's critique was justified when earlier studies were discovered to have used an overestimated value of 5 cm/yr for the rate of altitude loss, which was later revised to 1.8 cm/yr.
[61] The secular acceleration is now attributed to tidal effects,
[59] which had not been considered in the earlier studies. The density of Phobos has now been directly measured by spacecraft to be 1.887 g/cm³.
[4] Current observations are consistent with Phobos being a
rubble pile.
[4] In addition, images obtained by the
Viking probes in the 1970s clearly showed a natural object, not an artificial one.
However, mapping by the Mars Express probe and subsequent volume calculations do suggest the presence of voids within the moon and indicate that it is not a solid chunk of rock but a porous body instead.
[62] The
porosity of Phobos was calculated to be 30% ± 5%, or a quarter to a third of the moon being hollow. This void space is mostly on small scales (millimeters to ~1-m), between individual grains and boulders.
[6]
