Free Astronomy Magazine May-June 2015

PLANETOLOGY T his illustration shows the shifts in latitude of Ganymede’s auroral ovals: left, in case of absence of an ocean, and right, in case that ocean exists and has the character- istics explained in this article. [NASA, ESA, and A. Feild (STScI)] I mages of Gany- mede’s auroras taken by Hubble in the ultraviolet, which were over- laid, in accor- dance with their actual positions, on an image in white light of the large Jovian moon taken by the Gali- leo probe. [NASA, ESA, and J. Saur (University of Co- logne, Germany)] den about 150 km below the surface and about 100 km deep, thus around ten times deeper than the deepest abysses on Earth. The researchers estimated that that ocean contains more water than that of all our oceans put together, and from the astrobio- logical point of view this makes Ganymede an extremely interesting object. In fact, it cannot be excluded a priori that in that gi- gantic water reservoir may be present some form of primitive life comparable to that found in our abysses and capable of prolif- erating by drawing energy from possible hydrothermal sources; a scenario seeming- ly shared by another of the larger Jovian moons, Europa, which too is characterized by a global underground ocean. Ganymede should therefore be added to that already large group of objects of var- ious sizes and nature on which the pres- ence of water has been ascertained. Whether in a gaseous, liquid or solid state, it would seem that it can be found any- where in our solar system. With the same technique used by Saur, but applied to the more powerful telescopes of the near future, it will be possible to dis- cover oceans also on planets and large moons in orbit around other stars; an ex- ceptionally fascinating opportunity that even the most fervent and imaginative minds could have ever imagined. program using the Hubble Space Telescope (capable of recording ultraviolet light with wavelengths up to the 200 nanometers, cor- responding to UVC rays) and to aim the in- strument towards Ganymede, possibly in coin- cidence with peaks in the flow of the solar wind, so as to maximize their chances of catching the occurrence of auroras. In so doing, Saur's team was able to follow the dynamics of Ganymede's auroras for a suffi- ciently long time to establish that their mi- gration in latitude is only 2 degrees wide. Based on the models of the large moon’s in- ternal structure, this value corroborates the existence of a global salt water ocean, hid- n