It is far less likely that space scientists and astronauts will ever find life in the outer solar system, home to the four 'giant' planets: Jupiter, Saturn, Uranus and Neptune, according to a study.
This is because a team of Canadian scientists found that the subsurface ocean of Titan -- the largest moon of Saturn -- is most likely a non-habitable environment, meaning any hope of finding life in the icy world is dead.
"Unfortunately, we will now need to be a little less optimistic when searching for extraterrestrial life forms within our own solar system, " said astrobiologist Catherine Neish, Earth sciences professor at the University of Western Ontario in Canada.
"The scientific community has been very excited about finding life in the icy worlds of the outer solar system, and this finding suggests that it may be less likely than we previously assumed, " Neish added.
The identification of life in the outer solar system is a significant area of interest for planetary scientists, astronomers and government space agencies like NASA, largely because many icy moons of the giant planets are thought to have large subsurface oceans of liquid water.
Titan, for example, is thought to have an ocean beneath its icy surface that is more than 12 times the volume of Earth's oceans.
In the study, published in the journal Astrobiology, Neish and her team attempted to quantify the amount of organic molecules that could be transferred from Titan's organic-rich surface to its subsurface ocean, using data from impact cratering.
Comets impacting Titan throughout its history have melted the surface of the icy moon, creating pools of liquid water that have mixed with the surface organics. The resulting melt is denser than its icy crust, so the heavier water sinks through the ice, possibly all the way to Titan's subsurface ocean.
Using the assumed rates of impacts on Titan's surface, the team determined how many comets of different sizes would strike Titan each year over its history. This allowed the researchers to predict the flow rate of water carrying organics that travel from Titan's surface to its interior.
Neish found the weight of organics transferred in this way is quite small, no more than 7, 500 kg/year of glycine -- the simplest amino acid, which makes up proteins in life.
This is approximately the same mass as a male African elephant. (All biomolecules, like glycine, use carbon -- an element -- as the backbone of their molecular structure.)
"One elephant per year of glycine into an ocean 12 times the volume of Earth's oceans is not sufficient to sustain life, " said Neish.
"In the past, people often assumed that water equals life, but they neglected the fact that life needs other elements, in particular carbon."
Other icy worlds (like Jupiter's moons Europa and Ganymede and Saturn's moon Enceladus) have almost no carbon on their surfaces, and it is unclear how much could be sourced from their interiors.
Titan is the most organic-rich icy moon in the solar system, so if its subsurface ocean is not habitable, it does not bode well for the habitability of other known icy worlds.
"This work shows that it is very hard to transfer the carbon on Titan's surface to its subsurface ocean -- basically, it's hard to have both the water and carbon needed for life in the same place, " said Neish.