The Familiar Flows and Forms of Fried and Frozen Worlds
Recent discoveries on Titan and Venus show us that nature doesn’t need water to make many of the features that remind us of home.
What do we mean when we say we are looking for other Earths out there? As we learn to locate and inspect the planets around other stars, we fantasize about finding bodies of liquid water sloshing around on the surface of a rocky world under the light of a reasonably stable star. We have convinced ourselves that our own is the best of all possible types of worlds for life. We justify this bias by invoking the unique properties of organic molecules in a water solvent. Honestly, this is hard to evaluate until we have some other examples of planetary biospheres, and explore widely enough to be sure we’re not just overlooking truly alien life.
Our terrestrial minds fetishize water worlds. Yet recent exploration has shown that two very dry worlds are emerging as our Solar System’s most Earthlike in some important ways. Both Venus and Titan are often described in terms that emphasize how forbidding and alien they are. And they are – if you look at surface conditions. But when you look at forms, patterns and processes – at what is actually going on in these places – they start to seem a lot more familiar.
Titan is far too cold for liquid water except on those rare occasions when a watery cryovolcano, thinned with ammonia antifreeze, bursts onto the surface, or when the heat from a large impact melts a temporary crater lake which quickly freezes over. Yet on Titan we see a diversity of fresh and familiar geological forms – dunes, river valleys, volcanoes and shorelines – that remind us of home, especially when combined with the dense atmosphere that rages with clouds and storms and pulses with seasonal patterns. Though Titan is devoid of surface water, its landscapes are shaped by wind, rain, flowing liquids and complex chemical and physical cycles encompassing the entire globe.
Venus, too, has been shown by recent exploration to be an unusually active world, with mysterious lightning storms, howling planetary winds, variable brightening in the clouds, active volcanoes and chemical cycles shuttling sulfur atoms between minerals in the surface and interior, gases in the atmosphere and liquids in the clouds. The surface is way too hot for water. But just as Titan surely harbors a cold ocean of water somewhere underneath its frigid surface, Venus in a sense has an ocean 50 kilometers in the sky. The clouds themselves are a realm of condensed water (drenched in strong sulfuric acid), chemical imbalances and solar energy – the kind of place that if you squint a bit, looks like an astrobiologists’ dream.
Water worlds are rare, at least around these parts. But in exotic places like the clouds of Venus or the lakes of Titan we find liquids, energy sources and interesting chemistry of unknown (as yet) complexity.
On Titan it is methane that plays the role of water on Earth. On Venus river-like forms were probably carved by exotic, carbon-rich volcanic lavas. Given a planet with active flows of matter and energy, nature seems to find a way to repeat the same kinds of forms and patterns in very different conditions, using whatever construction materials are present. Could life also be one of these phenomena that nature finds a way to make using the materials at hand, given energy and a range of construction materials? We can’t yet know. But as we look around the galaxy we may find many worlds with alien conditions but familiar forms. And, who knows, finding active worlds with complex surfaces and energetic atmospheric cycles may ultimately be more important than finding a place where we could drink the water.