Living in the Universe

Living in the Outer Solar System

April 11, 2008 on 8:00 am | In Solar System, Space Program | Comments Off

 Second Week of April 2008

What would it be like to live in the outer solar system? It turns out to be not too bad and so life out there might not be as unlikely as we once thought. Past the orbits of Jupiter and Saturn, the Sun is a feeble dot in the sky. Temperatures are extremely cold, and yet under the surface of Titan, Saturn’s large moon, a vast ocean of water and ammonia may be lurking.

Astronomers have not directly observed this ocean, but recent observations with NASA’s Cassini spacecraft of Titan’s rotation and shifts in the location of surface features suggest a liquid ocean perhaps sixty miles under the surface. Titan is Saturn’s largest moon and the second biggest in the solar system, only slightly smaller than Jupiter’s moon Ganymede. It’s larger than Mercury and the recently demoted dwarf planet Pluto. Cassini has been looking at Saturn and its moons for several years and it has collected measurements using radar that penetrate Titan’s thick atmosphere, doing nineteen passes over the moon between 2005 and 2007.

Data from these early observations allowed researchers to locate fifty landmarks, including lakes, canyons and mountains on Titan’s surface. They looked at later radar data and found that prominent surface features had shifted by up to nineteen miles. That’s a lot. The spin of Titan’s crust is linked to winds that blow through its atmosphere, but this large a displacement of surface features would be hard to explain unless the crust were separated from its core by an internal ocean allowing the crust to essentially float. According to Ralph Lorenz of the Johns Hopkins University, who led the study, “It’s because Titan’s crust seemed so mobile that we infer this internal ocean.” He says the ocean is probably water, with a few percent ammonia, while the atmosphere is made up of nitrogen with other hydrocarbons that give Titan its orange color. Titan’s atmosphere consists of compounds that may have existed in the Earth’s primordial atmosphere, but Titan has more of the chemicals ethane and methane.

Titan is perhaps the most Earth-like landscape in the solar system and it probably has the most Earth-like weather. It’s much colder than the Earth, but the same processes that go on in our weather, particularly the formation of clouds and rain, happen on Titan, but in this case with liquid methane and not with water. Titan is thought to have hundreds of times more liquid hydrocarbons than all the known oil and gas reserves on the Earth. On Titan, these hydrocarbons rain from the sky and collect in vast deposits that form lakes and dunes.

Now the evidence of an underground ocean raises anew the possibility that life might exist deep under Titan’s surface. Similar underground oceans have been found on Europa, Calisto, Ganymede, and tiny Enceladus. Saturn’s tiny moon Enceladus is the subject of a second recent story. It has all the ingredients needed for life erupting in geysers beneath its surface and spewing into the atmosphere.  Instruments on the Cassini mission a few weeks ago revealed a concentration of water vapor, carbon dioxide, carbon monoxide, and organic material twenty times denser than expected, and the temperatures were higher than previously measured. Dennis Matson, the project scientist for Cassini, said, “Enceladus has got warmth, water, and organic chemicals, some of the essential building blocks needed for life.  We have quite a recipe for life on our hands.”

Saturn’s moons have long been of interest to scientists, particularly Titan with its enormous and significant atmosphere, but Enceladus’ chemical components are surprising because previously they’d only been found in comets. Cassini also measured surprisingly warm temperatures near the north pole. It doesn’t seem warm to us, but minus ninety-three degrees Celsius or minus a hundred and thirty-five Fahrenheit is tens of degrees warmer than scientists had expected. But it’s the liquid water that’s surprising, and those high temperatures near the surface make it likely that there’s liquid water not far below the surface.

There you have it. In the frigid depths of the outer Solar System, ranging from a large moon Titan to a tiny moon Enceladus, we have liquid water. We also have organic material, and we have energy: all the ingredients necessary for microbial life. Now, we just need a few billion dollars in NASA’s budget to send spacecraft out there with instruments that can make the careful measurements needed to be sure, and that’s at least a decade or more away. Astrobiology is not a subject for those in need of instant gratification.