David Rothery is a geologist and planetary scientist at the Open University. He chairs the Open University’s level 2 module on planetary science and is a leading member of the science team for BepiColombo, Europe’s forthcoming orbital mission to Mercury. His books include Planets: A Very Short Introduction, Geology: the Key Ideas, and Teach Yourself Volcanoes, Earthquakes and Tsunamis. Cross posted with permission from OUPblog.

I’ve seen proud posts on the internet from people who saw five planets with the naked eye this Spring. Venus and Jupiter could hardly be missed in the west after sunset, though Mercury was more elusive as it never strays very far from the Sun and is smaller and fainter. Later in the evening Mars and then Saturn have been rising high in the east.

That’s a “full house”, comprising all five of the planets recognised by the ancients. Being a geologist, I usually insist on claiming that a sixth planet is easily visible too. Just look below your feet and there it is – the Earth. If I’m feeling really awkward, I will also point out the Moon. Astronomically-defined, it is not a planet because it orbits the Earth rather than the Sun – but geologically it shares all the important attributes common to the “terrestrial” planets: Mercury, Venus, Earth, and Mars. These are an iron-rich core surrounded by rocky material that has been deformed and melted by numerous geological processes. The giant planets Jupiter and Saturn plus their more distant (and too faint to find without optical aid) brethren Uranus and Neptune are somewhat different, being mantled in deep hydrogen-rich atmospheres topped by clouds of methane or ammonia.

Each of them is rewarding to spot, if you just pause to consider what you are looking at. Through a telescope you can lose yourself among the amazing landscapes of the Moon. You won’t see anything like the same detail on any of the planets, which appear small and fuzzy by comparison. But hang on! What you are seeing is a world tens or hundreds of millions of miles away, with your own eyes. For me, the wonder never pales.

However both professionally and personally, what I crave is close-up images (and other measurements) from spacecraft orbiting or even roaming around on the surfaces of these worlds. Geologists who try to make sense of the Earth without taking advantage of the insights that can be gained from examining comparable bodies are as blinkered as people who study the oak but never examine the ash or the elm, and yet think they understand deciduous trees. I could make the same point about the inappropriateness and out-of-datedness of geology courses that omit to include appropriate examples from the other planets.

Fortunately, there are incredible views available for all to see via NASA. You can find parts of Mars at 50 cm resolution from orbit, or even closer-up studies by rovers revealing details that you’d need a magnifying glass to see even if you were there in person. The story that comes back from Mars is that it’s “like Earth, but different”. Major flood events belong to the distant past, but there is growing evidence to suggest liquid water seeping onto the surface even today. For instance this NASA video, Possible Water Flows on Mars, presents evidence of present-day water seepage onto Mars’s surface.

18 km wide area on Mercury marked by numerous flat-floored pits, many of which coincide with bright blue material

Currently, however, it is Mercury rather than Mars that is keeping me busy professionally. I am part of a very large team preparing the European Space Agency’s BepiColombo mission for launch towards Mercury in 2015. What we are learning from MESSENGER, the NASA probe that began to orbit Mercury in March 2011, baffles and excites in equal measure. Mercury has a strong magnetic field like the Earth, but it is asymmetrically displaced towards the north by about one-fifth of the planet’s radius. Has it always been like that, or are we seeing Mercury in the act of flipping its magnetic field over, as has happened to the Earth many times in the past? Another surprise comes from Mercury’s so-called “bright crater-floor deposits”. The detailed views seen by MESSENGER from orbit show that they contain numerous pits with steep walls and flat floors that are clearly not impact craters – they aren’t round enough, and lack raised rims. Instead, the surface appears to have been etched away by some unknown mechanism (possibly turning directly to vapour – a process known as sublimation), and the bright patches that surround many of these pits might be showing traces of the volatile material that is being removed. What is this stuff? MESSENGER has shown that Mercury is rich in sulfur (2-4%), and that would fit the bill except that the bright material is also the bluest stuff so far found on Mercury, which doesn’t seem to tally with sulfur at all! When BepiColombo gets there we will find out for sure, because it has an X-ray spectrometer that will be able to determine the abundances of sulfur and many other elements with sufficient resolution to distinguish these bright, pitted areas from their surroundings.

The general “how does a planet function?” question is not all that drives planetary research. The more we learn more about microbial life on Earth, the more we have come to realise that there are bodies elsewhere in our Solar System were similar life could survive. Mars with its present-day water seeps and much wetter past is the most obvious, though not necessarily the best, candidate. Some of the icy moons of Jupiter and Saturn contain oceans of liquid water, where life could survive by feeding off chemical energy (all the time) or from sunlight (during brief episodes when cracks open). For example Enceladus, a 500 km diameter moon of Saturn is criss-crossed by fractures, some of which have been seen to vent ice-crystals to space providing evidence of warm, liquid water in its interior.

If we do discover life on Mars or one of the outer Solar System’s icy moons, then we will be close to answering one of the most fundamental questions there is: “How common is life in the Universe?” If the bugs on Mars or Enceladus turn out to be related to terrestrial life, then all this will show is that life has hitched a ride around the Solar System on chunks of debris thrown up by major impacts, and need have originated only once. However, if we can show that life on two bodies in our Solar System is unrelated then it must have begun independently in two places. If that is the case, then the chances of it happening only in our own Solar System are minute, and we can expect life to be widespread throughout the cosmos.