Mars, Pluto… and beyond

The story of our solar system is developing into one of the most absorbing – and puzzling – epics of contemporary science. At the heart of it lies one of the greatest questions of all – just how special is our own planet, which teems with life and (this is the difficult bit) which has teemed with life continuously through most of its 4.5 billion year lifetime? Not all of the answers are to be found here on Earth. Our world must be understood in context, by comparison with other planets, near and far. New information has recently come in from one of our nearest neighbors, Mars, and from our most distant one, that heavenly body Pluto, that used to be known as a planet. The information is puzzling and astonishing in equal measure.

On Mars, the long-running story of whether there might be water there or not (and hence whether there might be life there, or not), took another twist. Liquid water has been discovered – or at least strongly suspected – at the surface of this frigid, and seeming freeze-dried planet. It is the latest development in a question that has swung backwards and forwards ever since the ‘discovery’ of what seemed to be straight, artificial waterways on that planet – the notorious ‘canals’ – at least to the straining eyes and imaginations of Earth-bound astronomers, peering through their early telescopes.

The first spacecraft that approached Mars dashed the vision of a watery, fertile, inhabited planet, revealing (once a planet-wide dust-storm had abated) a dry, lifeless panorama of desert terrain. But among the ruins of former landscapes were ancient natural waterways, and perhaps shorelines too. More than three billion years ago, water in large amounts did flow on Mars: this is clear from delta-like structures and abandoned river meanders, captured in images sent back by the orbiting spacecraft, and reinforced by the more detailed evidence of ancient lakes such as ‘Yellowknife Lake’, the sediments of which were discovered in Gale Crater by the Curiosity rover. Might any remnants of that precious liquid still flow on Mars? A couple of years ago, a few droplets of what looked like water were snapped on the legs of the Phoenix Mars lander in 2009. It had seemingly condensed from the thin, almost dry atmosphere, even at temperatures well below zero degrees C. The critical factor here were perchlorate salts that allowed tiny amounts of very strong brine to form even at those low temperatures and pressures.

Now a similar mechanism has been invoked to explain dark streaks – which change and evolve from photograph to photograph – that form on the steep slopes of some Martian craters. Sophisticated chemical analysis from orbiting satellite data has revealed the presence of these same salts as thin crusts on the streaks. Hence, the inference is that these streaks are left from dribbles of brine intermittently forming and running down the slopes. It’s a step closer to developing an understanding of this distant landscape.

Does it mean life? Not necessarily. These are occasional traces of dampness, of extremely concentrated brine, on a surface that is in any case bombarded by radiation. The search for any remaining traces of life on the red planet continues.

Far, far away, it is the face of Pluto, quickly snapped from the New Horizons spacecraft as it sped by, that exhausted the superlatives of the happy, astonished, and deeply bemused NASA space scientists. Pluto, so distant from the Sun, and so small as to have been ignominiously demoted from full planetary status some years ago, should, by all sensible prediction, have been a rather dull rock and ice ball battered by billions of years’ worth of asteroids to produce a surface showing little more than a mass of overlapping craters.

But far from it. Even though Pluto is smaller than our own Moon, and has a surface temperature lower then -200 degrees Centigrade, it shows a face that is as varied and phantasmagoric as any in the solar system. There are bright smooth plains crossed by enormous polygonal fractures, rugged mountains, and the extraordinary ‘dragon scale’ regions with complex and swirl-like ridged patterns. How have these been produced – and where has the energy come to drive the tectonics of this clearly active planet, much of which shows remarkably few craters (always a good indication of youth)? No-one yet knows. It really is a case of going back to the drawing board – or perhaps designing a whole new drawing board from scratch, to enable the geology of this extraordinary ‘dwarf planet’ to be teased out (as much as possible) by a combination of close observation and the application of first principles.

Given these clear signs of vigorous geological activity, it has even been speculated that Pluto has, deep below its icy crust, some kind of ocean. If so, that would take the total number of ocean-bearing planets and moons in our solar system to ten – not counting the trickling brine on Mars, or the long-vanished oceans of Venus. Which of course implies that many of the almost limitless exoplanets of deep space will most likely also have oceans of liquid water. If so, the chances of finding alien life are surely increased. The universe is, before our eyes, becoming an ever more complex and fascinating place – and, just perhaps, a less lonely one than we might once have supposed.

Featured image credit: Pluto’s blue sky by NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute. Public domain via NASA.