Increasingly we're finding evidence pointing to some pretty interesting environments in our own solar system that might be worth visiting in person in the coming decades.
Closest to home and best known, of course, is Mars. We've been landing robots on the red planet for almost 40 years now, and there've been books and movies about colonizing Mars for much longer. Now it seems efforts are finally under way in earnest to visit our most similar neighbor. While the light that Curiosity spotted there recently may not have come from a Martian subterranean condo, perhaps we could start construction on a real one sometime in the next few decades.
The small Huygens probe landed on Titan in 2005 and sent back a few hours' worth of data, successfully pulling off the most distant landing of any sort in human history. Titan is almost too tempting a destination to resist for a future manned journey, but any such adventurous astronauts should be sure to give up smoking completely before landing anywhere near those flammable lakes.
A few of the moons circling our gas giant neighbors are highly suspected of harboring buried treasure -- specifically liquid oceans buried under layers of rock and ice. Among these, the most promising in terms of potential for life is Jupiter's satellite Europa.
The surface of Europa is made up of super-smooth solid water ice with a potentially massive liquid-water ocean beneath, kept from freezing up by tidal heating. It also has a thin atmosphere made up of oxygen, which unfortunately would probably be far too thin for humans to actually breathe.
Nonetheless, if there are extraterrestrial super-hard-core polar bears (or just some nifty alien microbes) hanging out anywhere within a light year of here, Europa might be the mostly likely spot to find them.
Just this month new data appears to have confirmed the existence of a watery ocean beneath the surface of Saturnian moon Enceladus. This is not entirely surprising, given that the Cassini spacecraft has been spotting plumes of water vapor and ice spurting through the moon's icy shell for years now. Even more intriguing, NASA has detected the presence of salts and organic molecules in those plumes.
There's currently talk in astrobiology circles of mounting a mission to collect samples from these plumes. If the results are as promising as many hope, Enceladus could become a top candidate for the next place humans visit after Mars.
NASA, ESA, and G. Bacon (STScI)
Pluto and the Kuiper Belt
While Pluto is no longer a planet, it's still a good potential destination for human exploration in the distant future. NASA's unmanned New Horizons mission to Pluto (and the Kuiper Belt of other planetoids it hangs out with) will finally begin to prepare for its close encounter with Pluto starting about a year from now in April 2015.
This far-flung region could be worth a visit by our descendants, not just because it should be rich with frozen water ice and organic molecules, but it's also where the big space rock that wiped out the dinosaurs originated from. It's always good to know your enemy, y'all.
Biden and the distant dwarves
If we ever get the means, it would also be worth traveling even farther than Pluto, to the very edges of the solar system where distant dwarf planets have been spotted in recent years, including the one lovingly nicknamed for Vice President Biden.
What's most interesting about these lonely outposts is that they could be held in the sway of a larger super-Earth-type planet still waiting to be discovered in the farthest dark reaches of our sun's gravitational influence.
NASA/University of Arizona
Like many spots along Earth's volcanically active "ring of fire," Jupiter's moon Io might not be the most hospitable locale, but it definitely seems worth a visit.
Io is the most volcanically active body in the solar system, ejecting plumes up to 190 miles above its surface at half of escape velocity. Imagine the volcanoes of Hawaii erupting with such force that they nearly send plumes of lava and ash into orbit.
Scary and maybe not the safest place to visit, but can you imagine how many great shots you could get for your Instagram feed on that trip?
Ceres and the asteroid belt
So you entrepreneurial space nerd types want to talk about mining asteroids? Well, then, let's talk about the asteroid belt and its largest resident, Ceres.
Lying just beyond Mars and certainly not the only asteroid belt in the solar system, this one is the granddaddy. Ceres is believed to make up as much as a third of the total mass of the objects in the asteroid belt. If anyone is really serious about exploiting these space rocks, this could be the place to set up shop.
How could we leave the solar system's largest moon off this bucket list? Jupiter's moon Ganymede is another of those distant satellites with a thin oxygen atmosphere and icy surface, but it's also home to some of the most complex geology observed so far beyond our own planet. So much so that it led researchers to create a geologic map of the entire sphere.
Unmanned missions to Ganymede are already planned for the 2030s, so definitely add this one to the list of places our grandchildren might be interested in checking out.
Another frozen distant moon, another possible hidden ocean. Just like its pal Ganymede, Callisto is huge -- the third-largest moon in the solar system.
While it's the most battered and cratered object in the solar system and thought to be long-dead, don't write Callisto off. As with Enceladus and Europa, there's reason to believe Callisto is also hiding a liquid and possibly even salty ocean beneath its forebidding exterior. In other words, this dead planet is the Lord Vader of our solar system -- get past the mask and there's still a potentially beating heart in there somewhere. Once we master the Force, or at least space travel, perhaps we'll be able to see it for ourselves.