Cassini’s Final Encounter with Saturn’s Giant Moon Titan –“Like the Early Earth”

 

During NASA’s Cassini mission’s final distant encounter with Saturn’s giant moon Titan, the spacecraft captured this view of the enigmatic moon’s north polar landscape of lakes and seas, which are filled with liquid methane and ethane containing organic molecules that likely could possibly, even under such inhospitable conditions, form structures similar to the lipid bilayers of living cells on Earth.

Punga Mare (240 miles, or 390 kilometers, across) is seen just above the center of the mosaic, with Ligeia Mare (300 miles, or 500 kilometers, wide) below center and vast Kraken Mare stretching off 730 miles (1,200 kilometers) to the left of the mosaic. Titan’s numerous smaller lakes can be seen around the seas and scattered around the right side of the mosaic. Among the ongoing mysteries about Titan is how these lakes are formed.

 

Another mystery at Titan has been the weather. With its dense atmosphere, Titan has a methane cycle much like Earth’s water cycle of evaporation, cloud formation, rainfall, surface runoff into rivers, and collection in lakes and seas. During Titan’s southern summer, Cassini observed cloud activity over the south pole .

However, typical of observations taken during northern spring and summer, the view here reveals only a few small clouds. They appear as bright features just below the center of the mosaic, including a few above Ligeia Mare.

The images in this mosaic were taken with the ISS narrow-angle camera, using a spectral filter sensitive to wavelengths of near-infrared light centered at 938 nanometers captured on Sept. 11, 2017, during Cassini’s last encounter with Titan. Four days later, Cassini was deliberately plunged into the atmosphere of Saturn.

In 2015 scientists tackled the question of whether any organic molecules likely to be on Titan could, under such inhospitable conditions, form structures similar to the lipid bilayers of living cells on Earth. Thin and flexible, the lipid bilayer is the main component of the cell membrane, which separates the inside of a cell from the outside world. This team identified acrylonitrile as the best candidate.

Those researchers proposed that acrylonitrile molecules could come together as a sheet of material similar to a cell membrane. The sheet could form a hollow, microscopic sphere that they dubbed an “azotosome.” This sphere could serve as a tiny storage and transport container, much like the spheres that lipid bilayers can form.

 

The Goddard team determined that acrylonitrile is plentiful in Titan’s atmosphere, present at concentrations up to 2.8 parts per billion. The chemical is probably most abundant in the stratosphere, at altitudes of at least 125 miles (200 kilometers). Eventually, acrylonitrile makes its way to the cold lower atmosphere, where it condenses and rains out onto the surface.

The researchers calculated how much material could be deposited in Ligeia Mare, Titan’s second-largest lake, which occupies roughly the same surface area as Earth’s Lake Huron and Lake Michigan together. Over the lifetime of Titan, the team estimated, Ligeia Mare could have accumulated enough acrylonitrile to form about 10 million azotosomes in every milliliter, or quarter-teaspoon, of liquid. That’s compared to roughly a million bacteria per milliliter of coastal ocean water on Earth.

Cassini has made hundreds of passes over Titan during its 13-year tour of the Saturn system—including 127 precisely targeted encounters—some at close range and some, like this one, more distant.

Cassini ended its 13-year tour of the Saturn system with an intentional plunge into the planet to ensure Saturn’s moons—in particular Enceladus, with its subsurface ocean and signs of hydrothermal activity—remain pristine for future exploration. The spacecraft’s fateful dive was the final beat in the mission’s Grand Finale, 22 weekly dives through the gap between Saturn and its rings. No spacecraft had ever ventured so close to the planet before.

The Daily Galaxy via Jet Propulsion Laboratory and Goddard Spaceflight Center

Image credit: NASA/JPL-Caltech/Space Science Institute

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