Early Space Age
 Mars & Venus

The Space Age has given us a more precise understanding of the planets and moons in the Solar System and we know now that the planetary bodies are relatively inhospitable to life. Most of the rocky planets have only slight, insubstantial atmospheres - Venus' atmosphere, thick and bountiful, is otherwise deadly and stiffling, with rains of sulfuric acid and causing temperatures at the surface to be hot enough to melt lead. Our fantasies of Jurassic-era Venusians and red-skinned Martians are behind us
 
.As our technology became ever more sophisticated, however, it started to strip away these notions and with the advent of the space age and the dispatch of our robot probes to gather data during flybys, from orbit, and ultimately with landers, it became all too clear that these planets were downright hostile to life.
Venus turned out to be a vision of hell, with surface temperatures above the melting point of lead and with a corrosive, sulfuric acid atmosphere.
The canals of Mars turned out to be artifacts of optics and imagination and the seasonal darkenings not the organic flush of spring vegetation but global dust storms covering vast areas of the planet for months and months.

On the other hand, certain Martian landscape images appeared to show signs of floodplains and ravines scoured by waterflows. Perhaps, perhaps - though desert now, in an earlier era, possibly one or two billion years past, Mars ran with liquid water. Could life have developed at that time?
 Voyager & Pioneer visit the Galilean Moons
The Galilean Moons are quite fascinating as they seem to form a sequence of planetary processes and planetoological evolution conditioned by each moon's relative proximity to massive Jupter, whose overwhelming gravity (and association with inter-body resonances) actually flexes the two innermost moons - with the net result that this fricitional heating supplies the source for extreme volcanic activity on Io and possibly liquifying warmth in the interior of Europa.

With a liquid or semi-liquid subsurface ocean, Europa becomes the best possibility for extraterrestrial life in the solar system.
 
When the Pioneer and Voyager spacecraft sped by the moons of Jupiter, they returned images with resolutions far surpassing those that had been previously acquired with land-based telescopy. Scientists were astounded by the pictures of Jupiter's four largest satellites, the Galilean moons: Io, Europa, Ganymede, and Callisto. Each was different from the other and all hinted at planetary processes unlike any seen before in our explorations of the inner planets and moon.
Io was a volcanic world, tortured by the tremendous tidal forces exerted by the enormous, looming presence of Jupiter, with outgassing plumes reaching as far as three hundred kilometers into space - its surface formed and reformed by immense lava flows link
Callisto, the most distant moon, outside the zone of Jupiter's wrenching presence, presented a picture of the most heavily cratered object in the solar system - in stark contrast to the boiling, constantly-resurfaced Io - a world untouched by dynamical surface processes, a relic from the earliest eon of planetary accretion, billions of years old. link
Ganymede, next in toward Jupiter from Callisto, was the mystery moon, whose surface was divided between darker and lighter materials and configured by obvious but unknown tectonic processes. link
But it was Europa, standing second in line from Jupiter after Io, that created the greatest excitement among the scientists. Europa was the smoothest planetary object in the Solar System, an icy pool ball. The provocative thought came immediately: could there be an *ocean* of liquid water beneath the ice? And if this were so, what might that mean for the possibility of life there as well? link