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.

Links

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

Over and Oort on the comet’s tale

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An enormous asteroid or comet smashing into the Earth 65 million years ago killed off the dinosaurs. But, according to a new study by US scientists, published in the wake of an impact event on Jupiter, cometary collisions with Earth probably didn’t cause any more than one other extinction event during life’s history.

US researchers have looked to the Oort Cloud to help them work out just how many comets may have struck the earth during the time complex life has existed. The Oort Cloud is a remnant of the nebula from which the solar system formed 4.5 billion years ago. It starts about 93 billion miles from the sun (a thousand times as far from the sun as the Earth) and stretches to an incredible three light years away. It is thought that the Oort Cloud contains billions of comets but most are so small and distant that we will never see them.

Now, Thomas Quinn, Nathan Kaib and colleagues at the University of Washington, have used computer models to simulate the evolution of comet clouds in the solar system over the last 1.2 billion years. They found that even outside the periods of comet showers, the inner Oort Cloud was a major source of long-period comets. Some of these can cross Earth’s path but the simulations show that no more than two or three comets could have struck the Earth even during the most powerful comet shower of the last 500 million years.

Astronomers have known for some time that the inner solar system of rocky planets – Earth, Mars, Venus, and Mercury – is protected from comets by the gas giants Saturn and Jupiter. The enormous gravitational fields of these planets essentially eject comets into interstellar space or act as traps for them. This latter point was reinforced on 20th July when a huge scar appeared on Jupiter’s surface, likely evidence of a cometary impact.

There are about 3,200 known long-period comets of which Hale-Bopp is perhaps the most famous as it was visible with the naked eye for much of 1996 and 1997. Some long-period comets take thousands if not millions of years to make a single orbit of the sun. Comet Halley by contrast, which reappears every 75 years, is a short-period comet, which originate in a different part of the solar system known as the Kuiper Belt and are much more predictable. The computer simulation by Quinn and colleagues assumed that the inner Oort Cloud is the only source of potentially dangerous long-period comets.

The long-period comet 2001 RX14
The long-period comet 2001 RX14

“For the past 25 years, the inner Oort Cloud has been considered a mysterious, unobserved region of the solar system capable of providing bursts of bodies that occasionally wipe out life on Earth,” Quinn explains, “We have shown that comets already discovered can actually be used to estimate an upper limit on the number of bodies in this reservoir.”

With three major impacts taking place almost simultaneously, researchers had suggested that the minor extinction event of 40 million years ago resulted from a comet shower. Kaib and Quinn’s research implies that if that relatively minor extinction event was caused by a comet shower, then that was probably the most-intense comet shower since the fossil record began.

Further resources

Science Express, 2009, Jul 30, online

Tom Quinn’s home page

Comet shower movie from Nathan Kaib

It’s life Jim, but not as we know it

Would we recognize life if we found it on other planets? Are the weird imaginings of decades of science fiction writers enough to provide the necessary clues for finding ET?

Exobiology is an odd field of science. Research in this fascinating field relies on one of the biggest assumptions we make about the universe – that we are not alone, that there could be life on worlds other than the Earth. However, while life on earth is incredibly diverse, fundamentally, almost every single species of plant, virus, fungus, bacterium, fish, mammal, insect boils down to one of two nucleic acids – DNA or RNA. There is no reason to presume that life on another planet would use molecules resembling the nucleic acids. So, any discussion of exobiology always comes back to that first question – would we recognize alien life if we saw it?

Professor John A. Baross

Professor John A. Baross

A new report published by the US National Research Council (NRC) suggests that scientists focus at least some of their efforts on weird life, that is, the possibility of life that does not confirm to the standard terrestrial chemical blueprint. The NRC team suggests that the fundamental requirements for life on Earth – liquid water, as a biosolvent, carbon-based metabolism, molecular system capable of evolution, and the ability to exchange energy with the environment – may not be the only biochemical options for life. Our investigation made clear that life is possible in forms different than those on Earth, explains committee chair John Baross, of the University of Washington, Seattle.

The report states that the discovery of extraterrestrial life would have an enormous and inspirational impact on our position in the cosmos, but nothing would be more tragic than if we failed to recognize it.

Life elsewhere in the universe needn’t resemble life on earth.

Life elsewhere in the universe needn’t resemble life on earth.

Until now, the search for ET, has homed in on putative habitats that share at least some of the features of Earth, such as liquid water. However, Baross and his colleagues suggest that other liquids such as ammonia or formamide could also work as biosolvents, although the biochemistry of an organism using those materials would inevitably be very different. That said, Saturn’s moon Titan is thought to have a liquid water-ammonia mixture within and so is close to the top of the list of likely homes for weird life beyond Earth.

It is critical to know what to look for in the search for life in the Solar system, explains Baross, The search so far has focused on Earth-like life because that’s all we know, but life that may have originated elsewhere could be unrecognizable compared with life here. Life forms that use a replication chemistry unrelated to nucleic acids could well exist, there are suggestions that spiral entities that form spontaneously from cosmic dust, replicate and change, could be defined as somehow alive. There is also perhaps the entirely non-carbon organisms of science fiction fame, the silicon-based lifeform. It looks like a rock, Jim but double check those vital signs on the tricorder just in case

Further reading

The Limits of Organic Life in Planetary Systems
http://www.nap.edu/catalog.php?record_id=11919

Professor John A. Baross
http://oceanweb.ocean.washington.edu/ocean_web/about/faculty/profiles/BARJOF.html

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