The organic Solar System.

In the first part of this two-essay series1 it was pointed out that over 100 different organic compounds have been detected in space, and that there are many locations in the Solar System where complex — and as yet uncharacterized — mixtures of organic molecules can be found. The Galilean moon Europa is a case in point. Its surface is stained with unsaturated polymers called tholins, which are formed by photochemical reactions of the simplest carbon and nitrogen species. To an astrobiologist, if you mix the likes of tholins with liquid water and energy you have the potential for life; and hence Europa, with its mantle of warm water, is an oft-cited target for future exploration (such as the Europa Clipper, a concept mission from NASA). Is life on Europa too much of a stretch? Let’s assume there’s nothing in the solar system to overly excite astrobiologists; let’s ignore the ingenuity of Mother Nature and the tenacity of life, and resign ourselves to the idea that Earth represents the only haven for life in the Solar System. Even if we do all of this and just consider the emergent phenomena that can arise in even relatively ‘straightforward’ chemical systems, there are surely startling chemical discoveries waiting to be made in many corners of the Solar System (organic molecules + energy + water + time = a heady brew). So even if conservative scientists are correct and there is nothing to satiate astrobiologists, there is still plenty going on for the average organic chemist to get worked up about. The ‘plausibility of life (POL) on other worlds’ scale2 considers a fluid medium, a source of energy, and constituents and conditions compatible with polymeric chemistry as key to the possibility of life. Based on this scale, five categories of heavenly body can be described that cover the gamut from the ideal (category I, such as the Earth) to the essentially impossible (category V, for example, the Sun). Although most Solar System bodies fall into category V or IV (conditions conducive to life may have been possible at one time but certainly aren’t now; Venus for example) there are a surprising number of category II (water + energy + organic compounds) and category III (physically extreme conditions but with a possibility of harbouring very alien life) systems. The gas giants Jupiter and Saturn are primarily composed of hydrogen and helium, but their interiors contain denser materials. It is estimated that 5% of Jupiter’s mass is made up of other elements. The Galileo probe — which descended 100 miles into the atmosphere (pressure = 23 atmospheres; temperature = 153 °C) — as well as other orbiter and ground-based measurements, have so far identified an array of compounds including methane, ethane, benzene, other organics, water, ammonia, silicon-based compounds, hydrogen sulfide, neon, oxygen, phosphine and sulfur. This richness of chemistry led Sagan and Salpeter to propose the presence of life on Jupiter3 somewhere between the metallic hydrogen core and the cold upper atmosphere — but if it is there, it would likely be devoid of compartmentalization and be really weird4. By the definitions given above, Jupiter falls into category III, but perhaps Jupiter is best classified as a category V body after all. That point ceded, even if Jupiter is only conservatively 0.1% organic, that’s nearly 2.0 × 1024 kg of organic molecules; onethird the mass of the Earth! Tossed between high pressure and heat on one side and frigid temperatures and near vacuum on the other, there must be some fascinating chemistry occurring in this turbulent giant. Less is known about Saturn, but methane, ethane, propane, acetylene, ammonia and phosphine have all been detected in the upper atmosphere, so again there are literally Earth-sized quantities of organic chemistry going on deeper down on this category III or V planet. The ice giants Uranus and Neptune are rich in energy, water and organics, and could be described as category III — or category V if you consider solid surfaces and sharp physical transitions important for supporting life. However, the composition of these two planets is visibly different; Uranus is a pale cyan colour — attributed to the presence of large amounts of methane — whereas Neptune is a vivid azure. The latter is also rich in methane, but there are evidently other compounds that lead it to absorb more in the yellow part of the spectrum than the red. Deeper down, models suggest that both planets have mantles composed of water–ammonia oceans. For reasons that are not clear, the internal heat of Uranus is very low; it is the coldest planet in the Solar System with an upper atmospheric temperature of –224 °C, some 24 °C colder than more distant Neptune. Nevertheless, in both cases the deepest sections of the mantle are so hot and under such pressure that methane is Titan (left) and a shoreline on Titan (right). N A SA /J PL /S PA C E SC IE N C E IN ST IT U TE