Images from the DISR Side-Looking Imager and from the Medium Resolution Imager, acquired after landing. The horizon’s position implies a pitch of the DISR, nose-upward, by 1-2° with no measurable roll. ‘Stones’ in the foreground are 10-15 cm in size, presumably made of water ice, and these lie on a darker, finer-grained substrate. The scene evokes the possibility of a dry lakebed. Credits: ESA/NASA/University of Arizona
When the Huygens probe touched down on Titan, it landed on a relatively soft patch of material similar to lightly packed snow, researchers announced today.
But to get to that soft patch, Huygens had to descend through a treacherous atmosphere where winds raged up to 270 mph, temperatures dropped as low as -333 degrees Fahrenheit, and lightning was likely.
Nearly a year after the probe landed on Saturn's largest moon, scientists are still poring over loads of data. Today, researchers released their newest findings in a series of seven reports published on line by the journal Nature.
The findings provide a better picture of some of the most basic characteristics of Titan, a smog-shrouded world that had eluded close scrutiny until Huygens and the Cassini mothership teamed up for a series of observations.
The landing site
Titan's surface was unveiled as Huygens plunged through the clouds and fired up its Descent Imager/Spectral Radiometer (DISR). While these images did not show the pools of liquid hydrocarbon that many scientists expected, they do show the signs of flowing liquid in the form of brightness variations around the outlines of "ponds" and a "coastline" as well as what appear to be slopes carved by liquid drainage.
Images taken after landing show several small stones and pebbles and that the general topography of the region is fairly flat, varying only 3 feet or so in height. Images and other instruments ruled out the presence of extensive methane ground fogs in the landing site.
The surface of the landing site was neither hard nor fluffy soft, instead having characteristics similar to wet clay, lightly packed snow, or wet or dry sand, into which the probe sank a tiny bit after landing. The composition of surface vapors near the probe showed that the surface was wet with methane, which evaporated as the warm probe landed in the cold soil. The surface was also rich in organic compounds – such as cyanogen and ethane – not detected in the atmosphere.
Some scientists speculated that a tide had just gone out and the probe landed on the still-wet beach.
Scientists had long suspected that Titan's atmosphere was moving around the moon faster than the moon was rotating – a physical characteristic known as superrotation and previously observed on Venus. Now, data from the probe's DISR instrument and the Doppler Wind Experiment have confirmed that Titan's methane filled clouds do indeed superrotate.
While the uppermost clouds – about 75 miles above the surface – spin around the moon at about 270 miles per hour, wind speeds gradually decline as the near the surface. Here generally weak winds, gusting no more than a few feet per second, were observed in the lowest 3 miles of the probe's descent.
The probe passed through one other region of near zero wind speeds, from altitudes 62 to 37 miles. Scientists cannot explain this yet.
During its descent, Huygens provided the first in situ look at what chemicals exist in Titan's atmosphere. The atmosphere is mainly nitrogen and methane, but scientists didn't know how these chemicals originated – did they arrive in their present form or were they originally part of other molecules and were chemically altered to the states seen today.
The gas argon 36 was detected by the Gas Chromatograph Mass Spectrometer in very low abundance, and scientists say this indicates that nitrogen was originally present as part of ammonia. Also, the early atmosphere of the moon was at least five times denser with nitrogen than it is now, suggesting that some of the gas has been lost into space.
Ratios of carbon isotopes also indicate that the atmosphere is also leaking methane, and that there must be some period method for replenishment, although none was observed. Some researchers predicted there would be a large methane surface or subsurface reservoir that replenished the atmosphere, although that too was not seen. Also missing from the observations was the signs of methane rain, which scientists believed showered the moon's surface.
It could be that cyrovolcanoes – similar to terrestrial volcanoes except they spew liquid water and ammonia from subsurface reservoirs – could be providing both the atmosphere and the surface with nitrogen and methane.
"The current theories are that there is a region of water-ammonia mix in liquid form at some depth below Titan's surface – this is equivalent to the magma on Earth," study co-author Andrew Ball told SPACE.com. "These are one possible source for renewable methane on Titan."
Aerosol clumps, freezing temperatures, and maybe some lightning
The Aerosol Collector and Pyrolyser (ACP) instrument analyzed particles as it passed through the clouds and reported the presence of nitrogen-containing organic compounds, which may include amino, imino, and nitrile groups – key components to protein formation here on Earth.
These particles also clump together, perhaps providing a foundation for cloud formation, and affect temperatures and wind speeds throughout the atmosphere. They also fall as kind of steady organic rain on Titan's surface, the researchers write, and could produce a global blanket with a potential thickness of half a mile or more.
Researchers also used the Huygens Atmospheric Structure Instrument (HASI) during the descent to measure the atmosphere's temperature from the upper layers all the way down to the surface. Temperature at the surface averaged around -289 degrees Fahrenheit, which is warm enough for the existence of lakes and rivers of liquid natural gas. Atmospheric temperatures ranged from -333 degrees Fahrenheit at the highest elevations – 870 miles above the surface – to about -154 degrees Fahrenheit in the stratosphere, about 25 miles from the surface.
The HASI instrument also detected electric activity that is similar to lightning's signature. This was spotted around 37 miles above the moon's surface, which is also the region where the wind speed dropped to near zero.