For many of us, the motivation for reading science fiction is the opportunity to explore worlds beyond our own.  Only in fantasy can we fly to faraway planets and see the unusual sights they afford us.  But, as I try to convey in this column, science can also reveal places every bit as interesting as the those that are the fruits of imagination. 

For instance, there are eight planets besides the Earth whirling around the sun, each of them a wildly different orb from ours and each other.  Moreover, while we are still on the eve of a new era of observation, utilizing space probes like the recently failed Venera and the ambiguously targeted Pioneer 5, yet the progress of technology has revolutionized even ground-based observation.  Our conception of the planets has evolved significantly in the last half-century (to say nothing of a full century ago).  It boggles the mind to imagine what we might know in another fifty years.

Let me show you these worlds, as we know them today, and as we used to know them.  I've written about Venus, and I've written about Pluto.  Today is Mercury's turn.

Mercury was known to the oldest civilizations.  It was named after the swiftest of the Roman gods because, being the closest planet to the sun, it completes its trip around the star in the shortest time.  A hundred years ago, we knew very little about this little world, in large part because it is usually lost in the sun's glare; from our vantage, Mercury never strays far from its parent star.  We knew its period (year): 88 days.  We had a rotation (day): slightly longer than that of the Earth.  The latter was a guess – it seemed that some vague features could be resolved on Mercury's tiny disk, and since they did not move much from day to day, it was thought that Mercury's day must be similar to ours. 

We knew that Mercury has no moon.  This actually makes it harder to determine the size and mass of the planet; luckily, Mercury is occasionally visited by Encke's comet, on which it exerts a measurable pull.  From that and optical observations, it was guessed that Mercury was just over 3000 miles across, about fifteen times less voluminous than the Earth.  This made it by far the smallest planet in our solar system.  We knew nothing of the planet's tilt, and there was speculation that, if the seasons were severe enough, that life might survive at one of Mercury's poles.  The relative dimness of the planet, even taking into account its size, suggested that it didn't have much of an atmosphere – at least, not a reflective one. 

And that's it!  Not a big scientific haul for a planet that was closer to Earth, on average, than Mars.  Even the early science fictioneers had little to say about the planet: Edgar Rice Burroughs' Martians knew that Mercury, which they called "Rasoom," was inhabited by an advanced race, but nothing more.

Now we move to the present day…and we still don't know a whole lot about Mercury!  We do now know that Mercury must be airless or nearly so.  It would be hard for a planet so small to hold onto the energetic gases that make up an atmosphere, particularly a superheated one.  Additionally, whenever Mercury has crossed the disc of the sun, in an event known as a transit, observers have spotted no telltale halo that would betray the existence of air.  The romantic notion that life could exist on the planet seems forever excluded even from the realm of science fiction, though it should be noted that some mid-century polarimetry observations (measuring how sunlight scatters off of things) suggest that there is some Mercurian atmosphere. 

We still don't know much about the surface of the planet, but it is assumed that it mimics that of Earth's moon.  It has a similar color, and the difference in the density of light reflecting depends on Mercury's phase (both of the planets closer to the sun than the Earth exhibit phases, of course – from new, to crescent, gibbous, then full, and back again); this suggests that the planet's surface is rough.  Imagine giant Mercurian craters, jagged mountains, deep canyons, all more outsized than we generally conceive thanks to Mercury's light gravity.

And that 24-hour Mercurian day?  Well, there is another rotation scheme that fits the evidence even better.  If Mercury doesn't rotate at all, presenting one face to the sun at all times, as the moon does to the Earth, this also is consistent with its unvarying surface features over the span of several days.  In fact, given Mercury's proximity to the great gravitational pull of the sun, it is likely that Mercury is "tidally locked". 

Thus, one side of the planet is forever being broiled with terrific intensity, hot enough to melt lead!  Then you have the back side that never sees the sun.  It may well be the coldest place in the solar system – even more frigid than faraway Pluto.  Imagine an eternally dark landscape so cold that there could be lakes of hydrogen.  The dimmest of shadows would be cast by the rugged Mercurian mountainscape in the meager Venus-light.  Talk about bleakly exotic!

And at Mercury's ring of unchanging twilight, perhaps there is a temperate zone where life could yet flourish, especially if there is, though evidence be against it, a measurable atmosphere on the smallest of our solar system's worlds.  I suppose there may yet be stories to write about the first planet from the sun…