Your Short Guide To Space
There is water ice everywhere
Water ice was once considered a rare substance in space, but now we know we just weren't looking for it in the right places. In fact, water ice exists all over the solar system. Ice is a common component of comets and asteroids, for example. But we know that not all ice is the same. Close-up examination of Comet 67P/Churyumov–Gerasimenko by the European Space Agency's Rosetta spacecraft, for example, revealed a different kind of water ice than what is found on Earth.
That said, we've spotted water ice all over the solar system. It's in permanently shadowed craters on Mercury and the moon, although we don't know if there's enough to support colonies in those places. Mars also has ice at its poles, in frost and likely below the surface dust. Even smaller bodies in the solar system have ice – Jupiter's moon Europa, Saturn's moon Enceladus, and the dwarf planet Ceres, among others.
Spacecraft have visited every planet
We've been exploring space for more than 60 years, and have been lucky enough to get close-up pictures of dozens of celestial objects. Most notably, we've sent spacecraft to all of the planets in our solar system — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune — as well as two dwarf planets, Pluto and Ceres.
The bulk of the flybys came from NASA's twin Voyager spacecraft, which left Earth in 1977 and are still transmitting data from beyond the solar system in interstellar space. Between them, the Voyagers clocked visits to Jupiter, Saturn, Uranus and Neptune, thanks to an opportune alignment of the outer planets.
There could be life in the solar system, somewhere
So far, scientists have found no evidence that life exists elsewhere in the solar system. But as we learn more about how "extreme" microbes live in underwater volcanic vents or in frozen environments, more possibilities open up for where they could live on other planets. These aren't the aliens people once feared lived on Mars, but microbial life in the solar system is a possibility.
Microbial life is now considered so likely on Mars that scientists take special precautions to sterilize spacecraft before sending them over there. That's not the only place, though. With several icy moons scattered around the solar system, it's possible there are microbes somewhere in the oceans of Jupiter's Europa, or perhaps underneath the ice at Saturn's Enceladus, among other locations.
Mercury is still shrinking
For many years, scientists believed that Earth was the only tectonically active planet in the solar system. That changed after the Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft did the first orbital mission at Mercury, mapping the entire planet in high definition and getting a look at the features on its surface.
In 2016, data from MESSENGER (which had crashed into Mercury as planned in April 2015) revealed cliff-like landforms known as fault scarps. Because the fault scarps are relatively small, scientists are sure that they weren't created that long ago and that the planet is still contracting 4.5 billion years after the solar system was formed.
There are mountains on Pluto
Pluto is a tiny world at the edge of the solar system, so at first it was thought that the dwarf planet would have a fairly uniform environment. That changed when NASA's New Horizons spacecraft flew by there in 2015, sending back pictures that altered our view of Pluto forever.
Among the astounding discoveries were icy mountains that are 11,000 feet (3,300 meters) high, indicating that Pluto must have been geologically active as little as 100 million years ago. But geological activity requires energy, and the source of that energy inside Pluto is a mystery. The sun is too far away from Pluto to generate enough heat for geological activity, and there are no large planets nearby that could have caused such disruption with gravity.