In Intelligent Life in the Universe (1966) astrophysicists Iosif Shklovsky [Shklovskii] and Carl Sagan devote a chapter to the argument that scientists and historians should seriously consider the possibility that extraterrestrial contact occurred during recorded history; however, Shklovskii and Sagan stressed that these ideas were speculative and unproven. Shklovskii and Sagan argued that sub-lightspeed interstellar travel by extraterrestrial life was a certainty when considering technologies that were established or feasible in the late 1960s; that repeated instances of extraterrestrial visitation to Earth were plausible; and that pre-scientific narratives can offer a potentially reliable means of describing contact with aliens.
Ganymede: Ganymede is both the largest moon of Jupiter, our Solar System's planetary behemoth, as well as the largest moon in our entire Solar system. Observations of Ganymede by the HST in 2015 suggested the existence of a subsurface saline ocean. This is because patterns in auroral belts and rocking of the magnetic field hinted at the presence of an ocean. It is estimated to be approximately 100 kilometers deep with a surface situated below a crust of 150 kilometers.
"The rectangular pattern of gravity anomalies was completely unexpected. Using the gradients in the gravity data to reveal the rectangular pattern of anomalies, we can now clearly and completely see structures that were only hinted at by surface observations," Dr. Jeffrey Andrews-Hanna explained in the October 1, 2014 NASA Press Release. Dr. Andrews-Hanna, a GRAIL co-investigator at the Colorado School of Mines in Golden, Colorado, is lead author of the paper.
For those craters smaller than 30 kilometers in diameter, he discovered impacts both increased and decreased porosity in the upper layer of the lunar crust.