Earth will one day be much like Titan. Lakes near the poles, occasional flash-flooding nearer to the equator, but otherwise a planet girt by dunes.
Usually the analogy between Earth and Titan goes the other way – Titan is a frozen pre-biotic Earth. And, in many ways, it is. But it’s also drying out as its liquid phase is destroyed by the Sun’s rays. Methane photolyses, makes haze and heavier organicky junk, tholins. What might have been a planetary ocean – though we don’t really know yet – is now remnant lakes huddling around the poles.
And, according to Jonathan Lunine, that’s where Earth’s headed inexorably as the Sun brightens over the aeons…
Titan as an analog of Earth’s past and future
…in about a billion years the stratospheric cold-trap will fail, water will rise into the ozone layer, and UV will bust it up into oxygen and hydrogen. Once there’s enough of it, then the hydrogen will start blowing away, carrying the oceans with it.
This, apparently, happened on Venus, but with some important differences. Firstly, Venus was hotter from the beginning, and so the loss began very early in its geophysical evolution. Secondly, Venus seems to have had less water than Earth and more carbon dioxide. Thus it may never have formed the immense carbonate beds that have locked up Earth’s CO2. Instead it dried too quickly to bury that particular by-product of its volcanism and eventually it became the dry, lead-melting furnace we know today.
Earth, instead of broiling, may become increasingly parched. Dunes of wind-blown sand may replace the vast current-formed dunes on the sea-floor, but water may find refuge nearer the poles, persisting for perhaps the whole of the Sun’s Main Sequence. As we’ve discussed here earlier a desert planet can avoid a runaway greenhouse state up to 1.7 times the current insolation – even more if the atmosphere thins out and the pressure spreading of carbon dioxide’s IR spectrum is reduced. The Sun will leave the Main Sequence at about 1.85 times its present insolation… close to what will drive Earth to uninhabitability.
Percival Lowell thought Mars was a desert planet, criss-crossed by canals carrying melt-water from the poles. The real Mars is too cold, but the far future Earth won’t be. Lowell measured Mars’ surface pressure to be 85 millibars – oddly enough that’s the end figure for a thinned out Earth atmosphere resistant to greenhouse runaway. And, like Lowell’s Mars, the remaining refuge for water will be the poles. If inhabited, that desert Earth might be watered across its deserts via deep canals, with narrow strips of vegetation staying close to the water supply. After losing its oceans to space, Earth’s deserts will be heavily oxidised by the oxygen from photolysis, becoming rust-red like Mars.
Lowell had the wrong planet, some 5.5 billion years early.