Life in the Solar System - Let's be reasonable - II


The Sirens of Titan from giantfreakinrobot.com

Titan

Since Cassini/Huygens' arrival at Titan (2004) we have seen that it fills the need for at least some of the requirements of life: These observations alone were sufficient to fuel speculation by authors such as McKay and Smith, 2005 about methanogens, possible life forms living in a methane/ethane medium.


Artists impression of Titan's surface from Universe Today
On the con side: Pre-biotic mix: Overall, Titan looks like a very cold version of the early Earth, prior to the rise of life. Oceans filled with dissolved organic molecules waiting to be assembled into living things. Unlike Earth's oceans, Titan's may have been kept in a deep-freeze for the history of the Solar System, impeding the rise of life.

Hydrogen/Acetylene surprise: Does Titan have a chemical pathway that could conceivably power living systems? Maybe. Like Viking, Huygens has presented some tantalizing if inconclusive data. Huygens was equipped with a GCMS instrument. As is hung on its parachute, its GCMS sniffed the air, compiling profiles of the atmospheric densities of gasses at different elevations ( Niemann et al., 2005). This profile was surprising in two major ways:

This has fueled speculation by Strobel, 2010 that these chemicals are being consumed by methanogenic life forms. Indeed, at this moment, the methanogen hypothesis looks like the strongest one on the table.

Earth Biosphere Analogy:

On Earth, most life is powered by the oxidation of the sugar glucose by oxygen in this reaction:

C6H12O6 + 6O2 --> 6H2O + 6CO2

This is misleading because at Earth temperatures this reaction doesn't take place without a complex set of catalysts. In contrast, if we put hydrogen and acetylene together, we get a vigorous reaction:

3H2 + C2H2 --> 2CH4

Like the glucose oxidation, the oxidation of acetylene by hydrogen releases considerable energy. Arguably, the depletion of these chemicals near Titan's surface means that they are being consumed by some surface chemical process. But the cool thing: At Titan temperatures, this reaction can only proceed in the presence of complex catalysts.

This doesn't "prove" or "confirm" anything about the presence of life in Titan's liquids, but it is highly suggestive. Either some sort of catalyzed metabolic or protometabolic process is using up the H2 and C2H2 or some completely unknown inorganic reaction is doing the same. Either way, Titan just got lots more interesting.

On a more speculative note, a water ocean is thought to exist deep beneath Titan's icy crust. Could it, too harbor life? There is no evidence at all to support this, but it is interesting to imagine that Titan might support two totally separate biospheres.


Artists impression of Europa's surface from NASA-JPL

Europa

Now, suppose we could identify an environment identical to the hydrothermal vent environments of the ancient Earth in which terrestrial life arose? The prospects for life in such an environment ought to be good. That place is Europa.


Europa's ocean from Portal to the universe
Recall: Thus, we have an environment that provides the same: that the first terrestrial microbes enjoyed. For this reason:

Europa is generally regarded as the most likely place in the Solar System to harbor life.


Hyopthetical Europan multicellular life form from Stephen Hawking's Aliens
Oxygen: Adding color to this assessment is the assertion by Greenberg, 2010 that exposure of surface ice to cosmic rays would convert it to oxidizers. If the ice is thin enough to allow these to be transported into the liquid mantle/ocean, it may result in its widespread oxygenation. In such an event, life would be able to proliferate throughout the mantle/ocean, and it could support large, complex, energy intensive life forms. (An imaginary example appears at right). Indeed, Europa seems like the only alien world in the Solar System with any chance of supporting more complex life than simple microbes.


Rising water lens and chaos regions from University of Texas - Jackson School of Geosciences
Europan lakes?: Schmidt et al., 2011 propose that Europa's chaos regions result from rising lenses of liquid water interacting with the surface. If true, it strengthens the idea that the transfer of material between the deep ocean and surface is possible.

Before you get too excited: Currently, no speculation on Europan life is supported by any data. We have only conjecture based on the perceived conduciveness of an environment that may or may not exist. Potential pitfalls:

But the real tribute to Europa's promise is that it has become the focus of the next generation of high-profile "flagship" mission concepts by NASA and ESA. Major priorities of these Europa missions include:

Roth et al., 2013 report Hubble Space Telescope evidence that Europa's subsurface material has erupted into space at least once. How often this happens is anyone's guess.

If Europa continues to look interesting, the next step might include deployment of a lander to a region of thin ice to search directly for chemical or physical signs of life.


Geysers of Enceladus from Armaugh Planetarium News
A simpler option? But wait! Many other icy worlds are thought to harbor sub-surface water. The problem is getting to it. On these worlds (E.g. Ganymede), the water, if it exists, is at very great depth.

But on Enceladus, we know about the water because we see it erupting. Clearly, it has enough energy at depth to power its cryovolcanoes. Could life have appeared in this deep environment? If it has, we might be able to sample it from the surface, because Enceladus is one place where it might actually be snowing microbes. In fact, mission concepts to:

Have been proposed by NASA and DLR (the German Center for Air and Space Travel.) So all we need is some cash.

Key concepts and vocabulary:
Additional reading: