Secrets of the Titan landing

The landing of the Huygens probe on Titan, Saturn’s moon, in January 2005 was the most spectacular single event in our exploration of the outer solar system. But it has taken until now for mission scientists to work out exactly what happened during the most distant touchdown of a man-made spacecraft on an alien world.

Their analysis shows that the 200kg probe “bounced, slid and wobbled its way to rest in 10 seconds” after parachuting down through the moon’s thick and hazy atmosphere. Huygens was made by the European Space Agency and brought to Titan by Nasa’s Cassini spacecraft, which is still orbiting Saturn.

The findings, published in the journal Planetary and Space Science, provide a new insight into Titan’s texture, which is soft with a firm surface – like damp sand that has dried on top or snow with a frozen crust.

Scientists reconstructed the chain of events by analysing data from all the instruments active during the landing. Their data were compared with computer simulations and drop tests using a model of Huygens.

The probe hit the ground with an impact speed similar to a ball dropped from about a metre on Earth. On first contact Huygens dug a shallow hole about 12cm deep, then bounced out and slid for 35cm across the surface. At its final resting place it wobbled back and forth five times, till all motion ceased.

“A spike in the acceleration data suggests that during the first wobble, the probe likely encountered a pebble protruding by around 2cm from the surface of Titan, and may have even pushed it into the ground, suggesting that the surface had a consistency of soft, damp sand,” says Stefan Schröder of the Max Planck Institute for Solar System Research in Germany.

Had the probe hit a wet, mud-like substance, its instruments would have recorded a “splat”, with no further indication of bouncing or sliding. The surface must have therefore been soft enough to allow the probe to make a sizeable depression, but hard enough to support Huygens rocking back and forth.

“It is like snow that has been frozen on top,” says Erich Karkoschka of the University of Arizona. “If you walk carefully, you can walk on it as on a solid surface, but if you step on the snow a little too hard, you break in very deeply.”

The landing data also show evidence of a dry, dust-like material thrown up by the impact. Although liquid hydrocarbons (methane and ethane) sometimes fall from Titan’s clouds – forming streams, rivers and lakes and giving the landscape a surprisingly Earth-like appearance – it had evidently not rained on the landing site for some time.

Could Twitter be the ultimate exit poll?

When Barack Obama was voted into office, Twitter was an emerging technology, not yet two years old, Denise Roland writes. Last spring, the most dramatic moment of his presidency, the killing of Osama bin Laden, was tweeted across the globe before the White House made an official announcement.

As the November presidential election approaches, researchers are asking whether Twitter is powerful enough to predict the result before polling stations have finished counting the votes.

Twitter has already been used by researchers to successfully predict the winner of reality talent show American Idol. “We started this study almost as a game. We wanted to see how much we were able to predict an outcome using unsophisticated algorithms in as little time as possible,” says Fabio Ciulla, who led the research from Northeastern University. The results appear online in the journal EPJ Data Science.

The scientists assumed that the number of tweets mentioning a participant correlated with popularity, which, together with geographical information on the origin of the tweet, provided enough information to predict results. Ciulla cautions that a more sophisticated approach will be required to predict the outcome of political elections.

Nonetheless, Twitter has already played a central role in this year’s US campaign, and social media generally has made the views of individual voters more public than ever. It can affect their behaviour, too – a single non-partisan “Get out the vote” Facebook message on the day of the US congressional elections in 2010 increased voter numbers by 340,000, according to one study.

Ambitious Twitter trackers will be making their predictions as polling day approaches. Whether or not they succeed, little doubt remains about the political power contained in just 140 characters.

A 24,000-year-old leaf

Japanese lake holds carbon dating key

Radiocarbon measurements from sediment beneath a Japanese lake will give archaeologists a more accurate way of dating objects up to 53,000 years old.

An international research team headed by Oxford university’s Radiocarbon Accelerator Unit extracted cores of superbly preserved sediment from the bottom of Lake Suigetsu, where organic material from plants and algae has accumulated in distinguishable annual layers for tens of thousands of years.

Radiocarbon dating is based on the amount of C-14, the radioactive isotope of carbon, present in archaeological materials. Living organisms incorporate C-14 from the environment into their bodies as they grow; after they die, the radioactivity decays at a known rate, so the amount of C-14 left in a sample indicates its age.

However, the amount of C-14 in the environment varies over time. So the dating scale needs to be calibrated from materials whose age is known as accurately as possible. The Lake Suigetsu results provide the best calibration so far, according to a paper in the journal Science.

“This record will not result in major revisions of dates,” says Christopher Bronk Ramsey of Oxford. “But, for example, in prehistoric archaeology, there will be small shifts in chronology in the order of hundreds of years. Such changes can be very significant when you are trying to examine human responses to climate that are often dated by other methods.”

Artists more prone to mental illness

A huge new study in Sweden has confirmed the idea that people in creative professions are treated more often for mental illness than the general population – and that there is a particularly strong connection between writing and schizophrenia.

Last year researchers at the Karolinska Institute in Stockholm used Sweden’s comprehensive health records to show that artists and scientists are more common among families where bipolar disorder and schizophrenia are present, compared with the population at large.

Now they have expanded their study to many more psychiatric diagnoses – including depression, anxiety, anorexia nervosa – as well as cases of suicide, and incorporated both inpatients and outpatients.

The new study, published in the Journal of Psychiatric Research, tracked almost 1.2 million patients and their relatives.

The results showed, for instance, that authors were more likely than the general population to suffer from a wide range of psychiatric diseases and were almost 50 per cent more likely to commit suicide.

“If one takes the view that certain phenomena associated with the patient’s illness are beneficial, it opens the way for a new approach,” says Simon Kyaga of the Karolinska. “In that case, the doctor and patient must come to an agreement on what is to be treated, and at what cost. In psychiatry and medicine generally there has been a tradition to see the disease in black-and-white terms and to endeavour to treat the patient by removing everything regarded as morbid.”

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