Potentially Habitable Moons of Our Solar System
Potentially Habitable Moons of Our Solar System
“Somewhere, something incredible is waiting to be known”
~Carl Sagan
Since the dawn of time, humanity has been looking for life among the heavens. From Venus, to Mars, to exoplanets, we’re desperate to find even a single Microbe as it would fundamentally change our place in the cosmos. While looking for life among other planets, we often neglect an important celestial body, i.e Moons. Several Moons in our Solar System promise habitability far greater than Mars can. One of such Moons is estimated to have an ocean that may contain twice as much water as all of Earth's oceans combined. Bizarre isn’t it? In this article we’re going to dive deep into this bizarreness.
Before discussing the habitability of these moons separately, I would like to discuss some general considerations. First of all, the general mechanism that led to the origin of life on Earth from inanimate matter(Inorganic molecules) is Abiogenesis. Abiogenesis from an astrobiological point of view is important because life will take a general route anywhere in the universe irrespective of the environment, it’s constrained to form molecules from atoms, then molecules of higher complexity from the already existing molecules and finally a cell. Though Abiogenesis is a huge topic in itself, for the report we’ll discuss it in brief. For a cell to be able to form, we need have these requirements:
A continuous supply of reactive carbon for synthesizing new organic compounds.
A supply of free energy to power metabolic biochemistry- the formation of proteins, DNA and so forth;
Chemical catalysts to speed up and channel these metabolic reactions
Excretion of waste to satisfy the second law of thermodynamics.
Hereditary Material – RNA, DNA or an equivalent to specify the detailed form and function of the cell.
For all these requirements, alkaline hydrothermal vents are a good start, they’re a good source of energy, heat and membrane structures and they can also provide a basic medium to facilitate the proton gradient. Proton gradient is an essential requirement as it helps in ATP(Adenosine Triphosphate) synthesis. Earth has a lot of hydrothermal vents in its oceans so it's natural to assume that's where life arose. A few moons in the solar system also are hypothesized to have subsurface oceans; they might as well have hydrothermal vents.
A Hydrothermal Vent
Now that we have given a rough description of how life most likely has originated on Earth, comes the question: what are we looking for in other celestial bodies which makes them habitable or eligible for initiating life?
One of the very basic things we may be looking for in planets or moons is SPONCH. Carbon, hydrogen, nitrogen, oxygen, Phosphorus, and Sulphur—make up 99 per cent of living material by mass. Detecting the SPONCH elements in chemical forms is a very basic goal for planetary space probes searching for Earth-like life. Water specifically is very important for the origin of life. Tidally locked bodies may be very favorable because they provide heat required to sustain liquid water.
Now that we have discussed various parameters to look for, let's discuss the habitability of the moons individually.
Europa
Europa is a Moon of Jupiter. By far it shows the greatest potential for habitability then any planet (Except Earth) or Moon in our Sol System. Europa is speculated to have a large Subsurface ocean. The evidence for Europa’s subsurface ocean comes from measurements by Galileo spacecraft. Europa’s interior doesn’t have its own magnetic field. But in passing through Jupiter’s large magnetic field, electrical currents are induced inside Europa. In turn, these currents generate a weak and varying induced magnetic field. The strength of this field has an essential requirement that an electrically conducting fluid must exist within 200 km of Europa’s surface. The most likely explanation for this is a salty ocean up to twice the volume of Earth’s ocean.
The largest craters suggest that the ice cover above the ocean is at least 25 km thick. Shallow lens-shaped lakes might also exist. The possibility of life depends on energy sources, interfaces, and the availability of SPONCH elements. Exothermic reactions of water and a rock seafloor could supply energy as well as nutrients. Heat produced by the decay of radioactive isotopes within rocks (such as potassium, uranium, and thorium), might produce seafloor vents that supply carbon dioxide and hydrogen for chemoautotrophs. Oxygen is also the primary gas in atmosphere of Europa.
2) Enceladus
Jets of Enceladus
Enceladus’s geology is active. It experiences tidal heating. The heating is concentrated under Enceladus’s south pole, the reason behind why this happens is not understood yet. At the south pole, icy particles and gas spray out of parallel fractures also known as tiger stripes, which are about 130 km long, 2 km wide. These jets contain traces of methane, ammonia, and organic compounds, along with salt. Enceladus has a rocky core and an underground sea beneath the area of the jets. The combination of organic molecules, energy, and liquid water indicates that life might exist inside Enceladus. Such life could be chemoautotrophic, meaning it would consume hydrogen produced by water–rock reactions or hydrogen and oxygen from water split by radioactivity. If life does exist there, methane or organic matter in the jets could be biological.
3) Titan
Titan’s atmosphere contains a smoggy haze of hydrocarbons. At high altitude, ultraviolet sunlight breaks up methane molecules (CH4 ) and subsequent reactions build up hydrocarbons including ethane (C2H6 ), acetylene (C2H2 ), propane (C3H8 ), benzene (C6H6 ), and reddish-brown particles containing polyaromatic hydrocarbons. About 20 per cent of Titan’s surface is covered in tropical dunes that are made of sand-sized particles, which are coated with organics. At the poles, over 400 lakes are mixtures of liquid propane, ethane, and methane, with beaches made of particles of benzene and acetylene. Methane on Titan forms clouds, rain, and rivers. There is evidence that Titan has a subsurface ocean. According to a report by ESA, Titan’s surface organics surpass oil reserves on Earth.
Network of liquid channels at the surface of Titan
On this Note, I’d like to end this article, by quoting Sir Carl Sagan again.
“The universe is a pretty big place. If it's just us, seems like an awful waste of space.”
Citations
The Vital Question By Nicklane
Astrobiology: A Very Short Introduction
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