Saturn's Earthlike Moon

EU probe studies Titan....

Saturn's Earthlike Moon

New data from the Huygens space probe shows Titan contains many of the building blocks that produced life on Earth


Posted Wednesday, Nov. 30, 2005

Searching for signs of life on other planets—at least in our own solar system—has always been a bit of a fool's errand. Other than Mars, which may have been teeming with biology long ago but has dried to dust since, the planets look decidedly barren. But the moons may be another story. Some of the smaller bodies that circle our sibling planets are wet, hazy, and dense with organic chemistry and could be little cosmic nurseries. Of all of them, few top Saturn's giant moon Titan for pure biological potential.

Astronomers analyzing Titan's chemically rich atmosphere had long believed the moon could be an organically astonishing place, but until the Huygens space probe parachuted down onto the moon's surface last January, nobody knew for sure. This week, a flurry of seven papers were published on the website of the journal Nature, analyzing the data the ship was able to send home in the few hours it stayed alive on the bitterly cold world. That data confirmed everything astronomers had hoped might be true about the oddly Earth-like place.

One thing investigators knew from the start was that nothing remotely like the life we know could exist on Titan. Its paralyzingly cold, -290* Fahrenheit (-179* Celsius) climate ruled that out entirely. But the orange, opaque haze that shrouds the moon—and hid its face completely from the Voyager 1 and 2 spacecraft that ventured out that way in 1980 and 1981 —suggested that the place may swirl with water and hydrocarbons such as ethane and methane, the same chemical bricks that allowed life to emerge on Earth. Scientists have thought Titan could serve as a sort of flash-frozen version of the Earth in its pre-biotic days, just the kind of place you'd want to go to study how living things did succeed in emerging here.

Huygens was designed to conduct those studies and it succeeded, spectacularly so. The descending spacecraft detected a multilayered atmosphere around Titan in which winds blow at a savage 280 mph (450 kph) at high altitudes and slow to a pleasant walking speed on the surface. Water is abundant, though the temperature is far too cold for it to exist in a liquid state. The atmosphere is nonetheless humid with methane which freezes, thaws, evaporates and rains out precisely as water does on Earth. The Titanian air is also rich in nitrogen, which is the dominant element in Earth's atmosphere too. If Titan were loosed from its orbit around Saturn and could somehow fly closer to the warmth of the Sun, it would likely—eventually—bloom into life.

Mottled markings on Titan's surface originally suggested that the moon might be home to methane or ethane lakes, but Huygens did not detect evidence of that. It did see rock-strewn highlands, drainage channels, lowlands and basins that look for all the world like earthly riverbeds. These too would have been the result of flowing ethane or methane, not water. Titan's surface—at least on the spot where the spacecraft landed—was neither frozen solid nor entirely yielding. Rather, it had the consistency of heavy slush or wet clay, leading some researchers to believe it might have landed in a spot where a methane tide had just receded.

Most tantalizing of all, the Huygens probe detected bursts of energy throughout the atmosphere that had the telltale electromagnetic signature of lightning. It's lightning, many biologists believe, that first kick-started the process on Earth that converted inert hydrocarbons to early life. On Titan too that process could be trying to get under way, though the bitter cold will likely never let it happen.

Still, if Titan is forever fated to be Earth's stillborn sister, it is its sister all the same. The two worlds came from a common chemical incubator and began to evolve in common ways. Even if Titan never becomes the loamy, living place our planet is, it can still teach us a lot about how we took that fortunate turn. (external - login to view)

European Space Agency.

Highlights of ESA’s Huygens mission

30 November 2005

ESA’s Huygens probe was released on 25 December 2004. It reached the upper layer of Titan's atmosphere on 14 January 2005 and landed on the surface after a parachute descent of 2 hours and 28 minutes.

Clear images of the surface were obtained below 40 km altitude – revealing an extraordinary world, resembling Earth in many respects, especially in meteorology, geomorphology and fluvial activity. The images show strong evidence for erosion due to liquid flows, possibly methane, on Titan.

The probe descended over the boundary between a bright icy terrain eroded by fluvial activity, and a darker area that looked like a dry river- or lakebed. Huygens landed in the dark area. Water-ice pebbles up to a few centimetres in diameter were scattered near the landing site, and the surface here was found to have the consistency of loose wet sand.

Winds were found to blow predominantly in the direction of Titan’s rotation, west to east winds, with speeds up to 450 km/h above an altitude of 120 km. The winds decreased with decreasing altitude and then and changed direction close to the surface. An unexpected layer of high wind-shear was encountered between altitudes of 100 and 60 km.

Location of landing site
Huygens also surprised the scientists by finding a second lower ionospheric layer, between 140 km and 40 km, with electrical conductivity peaking near 60 km, and its instruments may also have recorded the signature of lightning.

‘Haze’ was detected all the way down to the surface, contrary to the predictions of pre-Huygens models. It was predicted that the atmosphere would be clear of ‘haze’ in the lower stratosphere, below around 60 km. Fortunately, the haze was transparent enough for good images of the surface to be obtained below 40 km.

Huygens enabled studies of the atmosphere and surface, including the first in-situ sampling of the organic chemistry and the aerosols below 150 km. These confirmed the presence of a complex organic chemistry in both the gas and the solid phase, which reinforces the idea that Titan is a promising place to observe chemical pathways involving molecules that may have been the building blocks of life on Earth.

Argon 40 was also detected at the surface and its presence indicates that Titan has experienced in the past, and is most likely still experiencing today, internal geological activity.

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