Friday, February 8, 2013

Testing Life’s Cosmic Ubiquity

Is life common in the universe? The successful detection of a second independent origin of life within our own solar system would be absolute proof that life is common in the universe. Our solar system is just one out of billions of other planetary systems in the galaxy and if life can arise twice in a single planetary system, then the galaxy is expected to be teeming with billions of living worlds. Mars, Europa and Titan are undoubtedly the best places in the solar system to search for the existence of life. Of these three worlds, Titan may prove to be the best place to look for a second independent origin of life in the solar system and to test life’s cosmic ubiquity.

Throughout Earth’s history, hypervelocity impacts caused by asteroids and comets crashing into the surface of the Earth are known to be able to throw up rock material into space. These rocks can carry terrestrial life within them through the vacuum of space and a tiny fraction of these rocks do eventually find their way to the surfaces of other planets and moons in the solar system. Mars and Europa have environments just beneath their surface where the conditions are suitable to support terrestrial life and Earthly microbes that have hitched a ride on these rocks can indeed survive there. As a result, if any life is detected on Mars or Europa, it would be difficult to definitively proof if it has an origin that is independent from that of life on Earth unless the type of life turns out to be biochemically distinct from life on Earth.

Titan is the largest moon of Saturn and it has a thick nitrogen-rich atmosphere laden with a wealth of organic molecules. The mean surface temperature on Titan is a frigid -179 degrees Centigrade and this is so cold that any water on Titan is literally rock solid. In fact, frozen water makes up the crust of Titan, creating a geological landscape where features such as mountains, sand dunes and boulders are actually made of frozen water. Instead of liquid water, liquid methane and ethane are the working fluids in Titan’s “hydrological cycle”. The surface of Titan contains widespread fluvial features such as rivers and deltas that were created through the action of liquid methane and ethane. In the high latitudes of Titan, there are lakes and seas of liquid methane and ethane. This makes Titan the only other place in the solar system besides the Earth that has stable bodies of surface liquid.

Since all life on Earth is born of liquid water and sustained by liquid water, the absence of liquid water on Titan creates an environment that is completely inhospitable to terrestrial life. Any terrestrial life that may have hitched a ride to Titan will not survive. However, it would be naive to assume that liquid water is the only solvent suitable for biology just because all life on Earth is water-based due to water being the most common solvent on Earth. Like the Earth, Titan also has the basic requirements for life such as the presence of a fluid environment, a source of energy and abundant organic molecules. If any form of life exists on Titan, it is expected to use the abundant liquid methane and/or ethane as a biosolvent due to the absence of liquid water in such a cryogenic environment. This means that any form of life detected on Titan is expected to be so biochemically distinct from life on Earth that it would be proof for a second independent origin of life in the solar system.

The most common type of star in the universe are not G-dwarf stars like our Sun but the much less massive M-dwarf stars, also known as red dwarfs. M-dwarf stars vastly outnumber G-dwarf stars. A typical M-dwarf star is so much less luminous than the Sun that an Earth-sized habitable planet with surface liquid water can only be sustained at one-tenth the Earth-Sun distance from the star. However, such a close proximity causes the planet to be tidally locked where one hemisphere perpetually faces the M-dwarf star; creating permanent day and night sides. As a result, an Earth-sized habitable planet around an M-dwarf star is unlikely to resemble the Earth. On the contrary, a Titan-like habitable planet could exist at a distance equivalent to the Earth-Sun distance from a typical M-dwarf star. The planet would not be tidally locked and is expected to rotate freely about its axis. If life is indeed found on Titan, it means that Titan-like habitable planets are likely to be very common and may even be more common than Earth-like habitable planets, given that M-dwarf stars vastly outnumber G-dwarf stars.

When considering the exploration of Titan, the only disadvantage is the large distance between Titan and Earth. Apart from that, everything else about Titan such as its low gravity, dense atmosphere, low radiation environment and calm low altitude winds are advantages compared to the exploration of other places in the solar system. For future missions to Titan, the challenges associated with finding an exotic form of life that is biochemically distinct from life on Earth cannot be underestimated. If life is found on Titan, such life would have an origin that is independent from life on Earth and will be proof of life’s cosmic ubiquity.

1. J.I. Lunine (2009), “Saturn’s Titan: A Strict Test for Life’s Cosmic Ubiquity”, arXiv:0908.0762 [astro-ph.EP]
3. Titan Mare Explorer (TiME) (, February 2012
4. Aerial Vehicle for In-situ and Airborne Titan Reconnaissance (AVIATR) (, March 2012