Nitrogen-fixing aliens

Scientists hope that Titan, a moon of Saturn, with its nitrogen-rich atmosphere, could act as a model system for terrestrial chemistry before life began on our planet. Now, another step towards that goal has emerged as researchers at the University of Arizona have incorporated atmospheric nitrogen into organic macromolecules under conditions resembling those on Titan.

“Titan is so interesting because its nitrogen-dominated atmosphere and organic chemistry might give us a clue to the origin of life on our Earth,” explains Hiroshi Imanaka, who is an assistant research scientist in the UA’s Lunar and Planetary Laboratory. “Nitrogen is an essential element of life.” Titan looks orange through a telescope because its atmosphere is a rich smog of organic molecules. Particles in the smog could settle on the surface and be exposed to conditions that might eventually create life, said Imanaka.

Saturn's A and F rings, the small moon Epimetheus and the smog-enshrouded Titan, Saturn’s largest moon. (Credit: NASA/JPL/Space Science Institute)
Saturn's A and F rings, the small moon Epimetheus and the smog-enshrouded Titan, Saturn’s largest moon. (Credit: NASA/JPL/Space Science Institute)

Of course, nitrogen alone is not enough, nitrogen molecules must be converted to a chemically active form that can drive the necessary biochemical reactions that underpin biological systems.

Imanaka and Mark Smith converted a nitrogen-methane gas mixture similar to Titan’s atmosphere into a collection of nitrogen-containing organic molecules by irradiating the gas with high-energy ultraviolet light. The laboratory set-up was designed to mimic how solar radiation affects Titan’s atmosphere.

Most of the nitrogen simply formed solid compounds directly, rather than gaseous ones, explains Smith, whereas previous theories suggested that nitrogen would move from gaseous compounds to solid ones in stepwise process. But, those settling particles may not contain nitrogen at all. If some of the particles are the same nitrogen-containing organic molecules created by the UA team in the laboratory then it would suggest that conditions conducive to life might just exist on Titan, Smith says.

These and other laboratory observations help scientists planning future space missions to decide on what to look for on other worlds that might hint at life and what instruments should be developed to help in the search.


Proc Natl Acad Sci, 2010, online
Mark A. Smith homepage
UA lunar and planetary laboratory

Yet another supernova

Just when you’d given up hope of another starburst, a third type comes along unannounced! This third class of previously unidentified supernova could help explain some anomalous observations in the night sky and even how our bodies come to contain so much calcium.

Until recently, astronomers had assumed there were just two types of supernovae. The first two types of supernova are either hot, young giants that explode on to the scene violently as they collapse under their own weight, or old, dense white dwarves (type a1) that undergo a thermonuclear explosion to briefly add their light to the night sky.

However, a third class appeared in telescope images in early January, 2005 and scientists, seeing that it had recently begun the process of exploding, started collecting and combining data from different telescope sites around the world, measuring both the amount of material thrown off in the explosion and its chemical composition.

Avishay Gal-Yam and colleagues at the Weizmann Institute in Israel and teams in Canada, Chile, Italy, UK, and USA, soon realised that the new supernova was neither old and dense nor young and hot.

There was too little material being ejected by the 2005 supernova for it to be an exploding giant, but its remote location from stellar nurseries suggested it was old. Moreover, its chemical makeup did not match the second type of supernova. The scientists turned to a computer simulation to see if they could figure out what kind of stellar processes could give rise to this anomalous kind of starburst.

Type Ia supernovae are primarily composed of carbon and oxygen as seen in their spectra, but the newly discovered supernova has unusually high levels of calcium and titanium which derive from nuclear reactions of helium not carbon and oxygen. However, the astronomers were initially at a loss to explain the source of the helium. Their simulations suggested that a pair of white dwarves might have been involved, with one assimilating helium from the other. When the thief star’s helium load rises past a certain point, the explosion occurs. “The donor star is probably completely destroyed in the process, but we’re not quite sure about the fate of the thief star,” says Gal-Yam.

Helium theft may have led to a third class of supernova that gives rise to the calcium in your bones and the titanium in a replacement hip! (Credit: Gal-Yam, Weizmann Institute of Science.

These new supernovae are relatively dim, so may not be as rare as they at first seem. This might explain why calcium is so prevalent in the universe and so in life on earth. The existence of radioactive titanium from these supernovae might also preclude the need for exotic explanations, such as invoking dark matter, of positrons at the heart of our galaxy. “Dark matter may or may not exist,” says Gal-Yam, “but these positrons are perhaps just as easily accounted for by the third type of supernova.”


Avishay Gal-Yam homepage

Interplanetary storm

A meteoric storm raged over the Earth 13,000 years ago as thousands of pieces of rock each the size of the Tunguska comet rained down over the course of an hour. The end result was a dramatic cooling of the planet, according to astronomer Bill Napier of the Cardiff University Astrobiology Centre.

Writing in the journal Monthly Notices of the Royal Astronomical Society, Napier suggests that the temperature drop was as high as 8 Celsius and interrupted global warming at the end of the last ice age, causing glaciers to re-advance.

Scientists have puzzled over a boundary layer marked by the occurrence of a “black mat” tens of millimetres thick present at sites throughout the United States, which contains high levels of soot from continental-scale wildfires and nanoscopic hexagonal diamonds found only in meteorites or impact craters. The evidence hinted at a catastrophic change at that time caused by the impact of an asteroid or comet 4 km across on the Laurentide ice sheet, which at that time covered what would become Canada and the northern part of the United States.

Napier points out that the cooling lasted a more than a millennium and led to the rapid extinction of 35 genera of North American mammals, as well as the disruption of the Palaeoindian culture. However, the chances of an asteroid impacting the Earth during that period were extremely low. Moreover, the heat generated by the rising fireball would be limited by the curvature of the horizon and could not have led to the continent-wide occurrence of wildfires.

Napier has now devised a model that can account for the evidence.
According to Napier’s model, the Earth ran into a dense trail of material from a large disintegrating comet. He points out that there is compelling evidence that such a comet entered the inner planetary system between 20,000 and 30,000 years ago and has been fragmenting ever since, giving rise to a number of closely related meteor streams and asteroids known as the Taurid Complex.

As the comet disintegrated, the Earth would have ploughed through at least one dense swarm of cometary fragments over an hour-long period.
Thousands of individual impacts would have occurred across what is now continental America, each releasing the energy of a megaton atomic bomb and triggering extensive wildfires.

2005 Hubble Space Telescope image of the breakup of a comet (73/P Schwassmann-Wachmann 3).
2005 Hubble Space Telescope image of the breakup of a comet (73/P Schwassmann-Wachmann 3). Credit: NASA / ESA / H.Weaver (JHU/APL) / M. Mutchler / Z.Levay (STScI)

“A large comet has been disintegrating in the near-Earth environment for the past 20,000 to 30,000 years, and running into thousands of fragments from this comet is a much more likely event than a single large collision. It gives a convincing match to the major geophysical features at this boundary,” says Napier. Indeed, a recent meteorite which may have come from this giant comet progenitor fell on Yukon Territory in January 2000 and has the highest abundance of nanodiamonds of any meteorite so far analysed.

Monthly Notices Royal Astronom Soc, 2010, in press Preprint link
Cardiff staff