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Made in Space – DNA building blocks

NASA-FUNDED RESEARCHERS HAVE UNCOVERED EVIDENCE that some building blocks of DNA—the molecule that carries the genetic instructions for life—found in meteorites, were likely formed in space.

The research gives support to the idea that a ‘kit’ of ready-made parts formed in space and delivered to Earth by meteorite and comet impacts, assisted the development of life.

Adapted from information issued by NASA / GSFC.

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Red Planet goes with the flow

Dark flows on Mars

Dark streaks that come and go on some Martian slopes, could be evidence for flows of salty water.

  • Dark, narrow features seen on Martian slopes
  • They come and go with the seasons
  • Could be caused by flows of salty water

DARK, FINGER-LIKE FEATURES that appear and extend down some Martian slopes during the warmest months of the Mars year may show activity of salty water on Mars. They fade in winter, then recur the next spring.

Repeated observations by the HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter have tracked seasonal changes in these recurring features on several steep slopes in middle latitudes of Mars’ southern hemisphere.

Some aspects of the observations still puzzle researchers.

“The best explanation we have for these observations so far is flow of briny water, although this study does not prove that,” said Alfred McEwen of the University of Arizona’s Lunar and Planetary Laboratory.

McEwen is the principal investigator for the orbiter’s High Resolution Imaging Science Experiment (HiRISE) and the lead author of a report about the recurring flows published on August 5 by the journal Science.

Other explanations remain possible, but flows of liquid brine fit the features’ characteristics better than alternative hypotheses.

More than 1,000 flows seen

Saltiness lowers the temperature at which water freezes. Some sites with the dark flows get warm enough to keep water liquid if it is about as salty as Earth’s oceans, but temperatures in those areas would not melt pure water ice.

Sites with liquid brines could be important to future studies of whether life exists on Marsand to understanding the history of water.

Dark flows on Mars

Dark streaks change with the seasons on Mars.

The features are only about 0.5 to 5 metres wide, with lengths up to hundreds of metres. That’s much narrower than previously reported gullies on Martian slopes.

They have been seen in only about one percent as many locations as larger Mars gullies, but some of those locations display more than 1,000 individual flows. Also, while gullies are abundant on cold, pole-facing slopes, these dark flows are not.

Highly seasonal

The team first discovered the strange features after University of Arizona student Lujendra Ojha, at the time a junior majoring in geophysics, used a change detection algorithm capable of identifying subtle changes occurring on the Martian surface over time in image pairs during an independent study project.

“I was baffled when I first saw those features in the images after I had run them through my algorithm,” said Ojha, who is a co-author on the Science publication. “We soon realised they were different from slope streaks that had been observed before.

“These are highly seasonal, and we observed some of them had grown by more than 200 metres in a matter of just two Earth months.”

“By comparison with Earth, it’s hard to imagine they are formed by anything other than fluid seeping down slopes,” said Mars Reconnaissance Orbiter Project Scientist Richard Zurek of NASA’s Jet Propulsion Laboratory. “The question is whether this is happening on Mars and, if so, why just in these particular places.”

More clues

Other clues help, too. The flows lengthen and darken on rocky equator-facing slopes from late spring to early autumn. Favouring warm areas and times suggests a ‘volatile’ material is involved, but which volatile? The settings are too warm for carbon-dioxide frost and, at some sites, too cold for pure water.

This suggests the action of brines with their lower freezing points. Salt deposits indicate brines have been abundant in Mars’ past. These recent observations suggest they may form near the surface today in rare times and places.

Artist's impression of MRO

The streaks were spotted by NASA's Mars Reconnaissance Orbiter spacecraft (artist's impression).

Still a mystery

However, when researchers checked some flow-marked slopes with the orbiter’s Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), no sign of water appeared. The features may quickly dry on the surface, or be mainly shallow subsurface flows.

“The flows are not dark because of being wet,” McEwen said.

A flow initiated by briny water could rearrange grains or change surface roughness in a way that darkens the appearance. How the features brighten again when temperatures drop is harder to explain.

“It’s a mystery now, but I think it’s a solvable mystery with further observations and experiments,” he said.

Adapted from information issued by the University of Arizona. Images courtesy NASA / JPL / University of Arizona.

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Fertile ground for other Earths

Artist's impression of a white dwarf

Astronomers have suggested that white dwarfs—old, cool, burned out stars—could be good places to look for orbiting habitable planets. (Artist's impression)

  • White dwarfs are Sun-like stars reaching the end of their lives
  • Small and cool, they could be ideal places for habitable planets
  • Sky surveys should show if white dwarfs have such planets

PLANET HUNTERS HAVE FOUND hundreds of planets outside the Solar System in the last decade, though it is unclear whether even one might be habitable.

But it could be that the best place to look for planets that can support life is around dim, dying stars called white dwarfs.

In a new paper published online in The Astrophysical Journal Letters, Eric Agol, a University of Washington associate professor of astronomy, suggests that potentially habitable planets orbiting white dwarfs could be much easier to find—if they exist—than other exoplanets located so far.

White dwarfs, cooling stars believed to be in the final stage of life, typically have about 60 percent of the mass of the Sun, but by volume they are only about the size of Earth.

Though born hot, they eventually become cooler than the Sun and emit just a fraction of its energy, so the habitable zones for their planets are significantly closer than Earth is to the Sun.

“If a planet is close enough to the star, it could have a stable temperature long enough to have liquid water at the surface—if it has water at all—and that’s a big factor for habitability,” Agol said.

A planet so close to its star could be seen using an Earth-based telescope as small as one metre wide (the largest telescope are now 8-10 metres), as the planet passes in front of, and dims the light from, the white dwarf, he said.

Red giant to white dwarf

White dwarfs evolve from stars like the Sun. When such a star’s core can no longer produce nuclear reactions that convert hydrogen to helium, it starts burning hydrogen outside the core.

That begins the transformation to a red giant, with a greatly expanded outer atmosphere that typically envelops—and destroys—any planets as close as Earth.

Finally the star sheds its outer atmosphere, leaving the glowing, gradually cooling, core as a white dwarf, with a surface temperature around 5,000 degrees Celsius.

Life cycle of a sun-like star

A star like our Sun goes through many different stages during its life, ending up as a white dwarf surrounding by a cloud of gas.

At that point, the star produces heat and light in the same way as a dying fireplace ember, though the star’s ember could last for three billion years.

Once the red giant sheds its outer atmosphere, more distant planets that were beyond the reach of that atmosphere could begin to migrate closer to the white dwarf, Agol said.

New planets also possibly could form from a ring of debris left behind by the star’s transformation.

In either case, a planet would have to move very close to the white dwarf to be habitable, perhaps 800,000 to 3.2 million kilometres from the star. That’s less than one percent of the distance from Earth to the Sun (150 million kilometres) and substantially closer than Mercury is to the Sun.

“From the planet, the star would appear slightly larger than our Sun, because it is so close, and slightly more orange, but it would look very, very similar to our Sun,” Agol said.

The planet also would be ‘tidally locked’, so the same side would always face the star and the opposite side would always be in darkness. The likely areas for habitation, he said, might be toward the edges of the light zone, nearer the dark side of the planet.

How to find other Earths

Candidate white dwarf in NGC 6397.

A candidate white dwarf star (marked in red) within the star cluster NGC 6397.

The nearest white dwarf to Earth is Sirius B at a distance of about 8.5 light years (a light year is about 9.5 trillion kilometres). It is believed to once have been five times more massive than the Sun, but now it has about the same mass as the Sun packed into the same volume as Earth.

Agol is proposing a survey of the 20,000 white dwarfs closest to Earth. Using a 1-metre telescope, he said, one star could be surveyed in 32 hours of observation.

If there is no telltale dimming of light from the star in that time, it means no planet orbiting closely enough to be habitable is passing in front of the star so that it is easily observable from Earth.

Ideally, the work could be carried out by a network of telescopes that would make successive observations of a white dwarf as it progresses through the sky.

“This could take a huge amount of time, even with such a network,” he said.

The same work could be accomplished by larger specialty telescopes, such as the Large Synoptic Survey Telescope that is planned for operations later this decade in Chile, of which the UW is a founding partner.

If it turns out that the number of white dwarfs with potential Earth-like planets is very small—say one in 1,000—that telescope still would be able to track them down efficiently.

Finding an Earth-like planet around a white dwarf could provide a meaningful place to look for life, Agol said. But it also would be a potential lifeboat for humanity if Earth, for some reason, becomes uninhabitable.

“Those are the reasons I find this project interesting,” he said. “And there’s also the question of, ‘Just how special is Earth?'”

Adapted from information issued by the University of Washington. Images courtesy ESA / Hubble Information Centre / ESO / NASA / G. Bacon (STScI) / S. Steinhöfel.

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Poison could have led to life

Artist's impression of a young planetary system

Organic molecules on Earth could have had their beginning in chemical reactions involving formaldehyde in the infant days of the Solar System.

FORMALDEHYDE, A POISON and a common molecule found throughout the universe, is likely the source of the Solar System’s organic carbon solids—abundant in both comets and asteroids—according to new research.

Scientists have long speculated about how organic, or carbon-containing, material became a part of the Solar System’s fabric.

Now new research from the Carnegie Institution’s George Cody, along with Conel Alexander and Larry Nittler, shows that these complex organic solids were likely converted from formaldehyde that existed in the primitive Solar System.

“We may owe our existence on this planet to interstellar formaldehyde,” Cody said. “And what’s ironic about it is that formaldehyde is poisonous to life on Earth.”

During the early period of the inner Solar System’s formation, much of the organic carbon that wasn’t trapped in primitive bodies like asteroids was lost into space, along with much of the water.

To find out where the organics came from, Cody, of Carnegie’s Geophysical Laboratory, along with Alexander and Nittler, of the Department of Terrestrial Magnetism, and the team decided to study primitive Solar System bodies using advanced methods.

Cross-section of a chondritic meteorite

Cross-section of a chondritic meteorite

What they discovered clearly pointed to a substance formed from formaldehyde.

They tested their conclusion with experiments that aimed to reproduce the type of organic matter found in carbonaceous chondrites—a type of organic-rich meteorite—starting with formaldehyde.

They found that their formaldehyde-synthesised organic material was not only similar to that found in carbonaceous chondrites, but also similar to organic material found in a comet named 81P/Wild 2, pieces of which were collected in space by NASA’s Stardust mission, as well as in interplanetary dust particles, or particles from space that likely originated from comets and asteroids.

They say their results make sense, because formaldehyde is relatively abundant throughout the galaxy and the conversion process would have been possible under the conditions prevailing in the primitive Solar System.

Their work was published online April 4 in the Proceedings of the National Academy of Sciences.

Adapted from information issued by the Carnegie Institution. Planetary system artwork courtesy ESA, NASA and L. Calçada (ESO).

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Do aliens eat arsenic for breakfast?

Artist's impression of an exoplanet

The discovery of weird, arsenic-eating life on Earth raises questions about what varieties of life might exist elsewhere in the cosmos. Artist's impression, courtesy G. Bacon / STScI / AVL.

  • Micro-organism found to live on deadly arsenic
  • Raises the stakes on what potential alien life could be like
  • Scientists think there could be more “weird” life here on Earth

CAN YOU IMAGINE eating toxic waste for breakfast? Researchers have discovered a bacterium that can live and grow entirely off arsenic, reports a new study.

The findings point for the first time to a micro-organism that is able to use a toxic chemical (rather than the usual phosphate) to sustain growth and life.

Arsenic is normally highly toxic to living organisms because it disrupts metabolic pathways, but chemically it behaves in a similar way to phosphate.

Scientists have previously found organisms that can chemically alter arsenic; and these organisms have been implicated in ground water poisoning events in Bangladesh and other places in Asia when people have shifted to using borehole or well water to avoid cholera.

Felisa Wolfe-Simon and Ronald Oremland

Felisa Wolfe-Simon, right, a NASA astrobiology research fellow, and Ronald Oremland, an expert in arsenic microbiology, examine sediment from Mono Lake. Photo by Henry Bortman.

Now, Felisa Wolfe-Simon, a NASA astrobiology research fellow in residence at the US Geological Survey, and colleagues have found a bacterium able to completely swap arsenic for phosphorus to the extent that it can even incorporate arsenic into its DNA.

“Life as we know it requires particular chemical elements and excludes others,” says Arizona State University (ASU) professor Ariel Anbar, a biogeochemist and astrobiologist who directs the astrobiology program at ASU.

“But are those the only options? How different could life be?” Anbar and Wolfe-Simon are among a group of researchers who are testing the limits of life’s chemical requirements.

The salt-loving bacteria, a member Halomonadaceae family of proteobacteria, came from the toxic and briny Mono Lake in California.

In the lab, the researchers grew the bacteria in Petri dishes in which phosphate salt was gradually replaced by arsenic, until the bacteria could grow without needing phosphate, an essential building block for various macromolecules present in all cells, including nucleic acids, lipids and proteins.

Using radio-tracers, the team closely followed the path of arsenic in the bacteria; from the chemical’s uptake to its incorporation into various cellular components.

Arsenic had completely replaced phosphate in the molecules of the bacteria, right down to its DNA.

Weird life could be all around us

Cosmologist and ASU professor Paul Davies has previously speculated that forms of life different from our own, dubbed “weird life,” might even exist side-by-side with known life on Earth, in a sort of “shadow biosphere.”

Halomonadaceae proteobacteria

Halomonadaceae proteobacteria have been found to eat deadly arsenic. Image courtesy of Science/AAAS.

The particular idea that arsenic, which lies directly below phosphorous on the periodic table, might substitute for phosphorus in life on Earth, was proposed by Wolfe-Simon and developed into a collaboration with Davies and Anbar.

Their hypothesis was published in January 2009, in a paper titled “Did nature also choose arsenic?” in the International Journal of Astrobiology.

Davies predicts that the new organism “is surely the tip of a big iceberg, and so has the potential to open up a whole new domain of microbiology.”

It is not only scientists, however, who will be interested in this discovery.

“Our findings are a reminder that life-as-we-know-it could be much more flexible than we generally assume or can imagine,” says Wolfe-Simon, noting that because microbes are major drivers of biogeochemical cycles and disease this study may open up a whole new chapter in biology textbooks.

“Yet, this story isn’t about arsenic or Mono Lake,” Wolfe-Simon says. “If something here on Earth can do something so unexpected, what else can life do that we haven’t seen yet? Now is the time to find out.”

Adapted from information issued by Arizona State University / Science / AAAS / Henry Bortman.

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Life on Titan: up in the air?

Titan, Epimetheus and Saturn's rings

Saturn's moon Titan looms large behind the planet's rings. (A smaller moon, Epimetheus, is in the foreground.) Chemical reactions in Titan's upper atmosphere could form molecules that are the precursor of life.

  • Titan’s atmosphere simulated in the lab
  • Chemical reactions produce amino acids
  • Key ingredients for life as we know it

While simulating possible chemical processes that could occur in the hazy atmosphere of Titan, Saturn’s largest moon, a University of Arizona-led planetary research team found amino acids and nucleotide bases in the mix—the most important ingredients of life on Earth.

“Our team is the first to be able to do this in an atmosphere without liquid water. Our results show that it is possible to make very complex molecules in the outer parts of an atmosphere,” said Sarah Hörst, a graduate student in the University of Arizona’s (UA) Lunar and Planetary Lab, who led the international research effort together with her adviser, planetary science professor Roger Yelle.

The molecules discovered include the five nucleotide bases used by life on Earth to build the genetic materials DNA and RNA: cytosine, adenine, thymine, guanine and uracil, and the two smallest amino acids, glycine and alanine. Amino acids are the building blocks of proteins.

Reaction chamber

A window into Titan’s atmosphere: Energised by microwaves, the gas mix inside the reaction chamber lights up like a pink neon sign. Thousands of complex organic molecules accumulated on the bottom of the chamber during this experiment.

The results suggest not only that Titan’s atmosphere could be a reservoir of pre-biotic molecules that serve as the springboard to life, but they offer a new perspective on the emergence of terrestrial life as well: Instead of coalescing in a primordial soup, the first ingredients of life on our planet may have rained down from a primordial haze high in the atmosphere.

Oddball of the Solar System

Titan has fascinated—and puzzled—scientists for a long time.

“It’s is the only moon in our Solar System that has a substantial atmosphere,” Hörst said. “Its atmosphere stretches out much further into space than Earth’s. The moon is smaller so it has less gravity pulling it back down.”

Titan’s atmosphere is much denser, too—on the surface, atmospheric pressure equals that at the bottom of a 5-metre-deep pool on Earth.

“At the same time, Titan’s atmosphere is more similar to ours than any other atmosphere in the Solar System,” Hörst said. “In fact, Titan has been called ‘Earth frozen in time’ because some believe this is what Earth could have looked like early in time.”

Saturn's moon Titan

Saturn's moon Titan has a thick, hazy atmosphere.

When the Voyager I spacecraft flew by Titan in the 1970s, the pictures transmitted back to Earth showed a blurry, orange ball.

“For a long time, that was all we knew about Titan,” Hörst said. “All it saw were the outer reaches of the atmosphere, not the moon’s body itself. We knew it has a an atmosphere and that it contains methane and other small organic molecules, but that was it.”

In the meantime, scientists learned that Titan’s haze consists of aerosols, just like the smog that cloaks many metropolitan areas on Earth. Aerosols, tiny particles about a quarter millionth of an inch across, resemble little snowballs when viewed with a high-powered electron microscope.

The exact nature of Titan’s aerosols remains a mystery. What makes them so interesting to planetary scientists is that they consist of organic molecules—potential ingredients for life.

“We want to know what kinds of chemistry can happen in the atmosphere and how far it can go.” Hörst said. “Are we talking small molecules that can go on to becoming more interesting things? Could proteins form in that atmosphere?”

What it takes to make life’s molecules

For that to happen, though, energy is needed to break apart the simple atmospheric molecules—nitrogen, methane and carbon monoxide—and rearrange the fragments into more complex compounds such as pre-biotic molecules.

“There is no way this could happen on Titan’s surface,” Hörst said. “The haze is so thick that the moon is shrouded in a perpetual dusky twilight. Plus, at -124 degrees Celsius, the water ice that we think covers the moon’s surface is as hard as granite.”

However, the atmosphere’s upper reaches are exposed to a constant bombardment of ultraviolet radiation and charged particles coming from the sun and deflected by Saturn’s magnetic field, which could spark the necessary chemical reactions.

Smog-like particles

Tiny particles are thought to create the smog-like haze that enshrouds Saturn's moon Titan.

To study Titan’s atmosphere, scientists have to rely on data collected by the spacecraft Cassini, which has been exploring the Saturn system since 2004 and flies by Titan every few weeks on average.

“With Voyager, we only got to look,” says Hörst. “With Cassini, we get to touch the moon a little bit.”

During fly-by manoeuvres, Cassini has gobbled up some of the molecules in the outermost stretches of Titan’s atmosphere and analysed them with its on-board mass spectrometer. Unfortunately, the instrument was not designed to unravel the identity of larger molecules—precisely the kind that were found floating in great numbers in Titan’s mysterious haze.

“Cassini can’t get very close to the surface because the atmosphere gets in the way and causes drag on the spacecraft,” Hörst said. “The deepest it went was 900 kilometres (560 miles) from the surface. It can’t go any closer than that.”

To find answers, Hörst and her co-workers had to recreate Titan’s atmosphere here on Earth. More precisely, in a lab in Paris, France.

“Fundamentally, we cannot reproduce Titan’s atmosphere in the lab, but our hope was that by doing these simulations, we can start to understand the chemistry that leads to aerosol formation,” Hörst said. “We can then use what we learn in the lab and apply it to what we already know about Titan.”

Like a spy in a movie

Hörst and her collaborators mixed the gases found in Titan’s atmosphere in a stainless-steel reaction chamber and subjected the mixture to microwaves causing a gas discharge—the same process that makes neon signs glow—to simulate the energy hitting the outer fringes of the moon’s atmosphere.

The electrical discharge caused some of the gaseous raw materials to bond together into solid matter, similar to the way UV sunlight creates haze on Titan. The synthesis chamber, constructed by a collaborating group in Paris, is unique because it uses electrical fields to keep the aerosols in a levitated state.

“The aerosols form while they’re floating there,” Hörst explains. “As soon as they grow heavy enough, they fall onto the bottom of the reaction vessel and we scrape them out.”

“And then,” she added, “the samples went on an adventure.”

To analyse the aerosols, Hörst had to use a high-resolution mass spectrometer in a lab in Grenoble, about a three-hour ride from Paris on the TGV, France’s high-speed train.

“I always joke that I felt like [I was in ] a spy in a movie because I would take our samples, put them into little vials, seal them all up and then I’d get on the TGV, and every 5 minutes I’d open the briefcase, ‘Are they still there? Are they still there?’ Those samples were really, really precious.”

Analysing the reaction products with a mass spectrometer, the researchers identified about 5,000 different molecular formulas.

Sarah Hörst

“When I came back and looked at the screen, I thought: That can’t be right,” said graduate student Sarah Hörst.

“We really have no idea how many molecules are in these samples other than it’s a lot,” Hörst said. “Assuming there are at least three or four structural variations of each, we are talking up to 20,000 molecules that could be in there. So in some way, we are not surprised that we made the nucleotide bases and the amino acids.”

“The mass spectrometer tells us what atoms the aerosols are made of, but it doesn’t tell us the structure of those molecules,” Hörst said. “What we really wanted to find out was, what are all the formulas in this mass spectrum?”

“On a whim, we said, ‘Hey, it would be really easy to write a list of the molecular formulas of all the amino acids and nucleotide bases used by life on Earth and have the computer go through them.’”

“I was sitting in front of my computer one day—I had just written up the list—and I put the file in, hit ‘Enter’ and went to go do something,” she said. “When I came back and looked at the screen, it was printing a list of all the things it had found and I sat there and stared at it for a while. I thought: That can’t be right.”

“I ran upstairs to find Roger, my adviser, and he wasn’t there,” Hörst said with a laugh. “I went back to my office, and then upstairs again to find him and he wasn’t there. It was very stressful.”

“We never started out saying, ‘we want to make these things,’ it was more like ‘hey, let’s see if they’re there.’ You have all those little pieces flying around in the plasma, and so we would expect them to form all sorts of things.”

In addition to the nucleotides, the elements of the genetic code of all life on Earth, Hörst identified more than half of the molecular formulas for the 22 amino acids that life uses to make proteins.

Titan: A window into Earth’s past?

In some way, Hörst said, the discovery of Earth’s life molecules in an alien atmosphere experiment is ironic.

Here is why: The chemistry occurring on Titan might be similar to that occurring on the young Earth that produced biological material and eventually led to the evolution of life. These processes no longer occur in the Earth’s atmosphere because of the large abundance of oxygen cutting short the chemical cycles before large molecules have a chance to form. On the other hand, some oxygen is needed to create biological molecules. Titan’s atmosphere appears to provide just enough oxygen to supply the raw material for biological molecules, but not enough to quench their formation.

“There are a lot of reasons why life on Titan would probably be based on completely different chemistry than life on Earth,” Hörst added, “one of them being that there is liquid water on Earth. The interesting part for us is that we now know you can make pretty much anything you want in an atmosphere. Who knows this kind of chemistry isn’t happening on planets outside our Solar System?”

Adapted from information issued by UA / S. Hörst / NASA.

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Moon gases: life starters?

Titan, Saturn’s largest moon

To simulate sunlight hitting Titan's atmosphere to see what chemicals were produced, researchers zapped nitrogen and methane in a laboratory with high-energy UV light. Shown here are some of Saturn's rings, the tiny moon Epimetheus, and the much larger Titan in the background.

  • Exposing nitrogen and methane to UV light
  • Resulted in nitrogen-containing “brown gunk”
  • Such chemicals might be found on Saturn’s moon Titan

The first experimental evidence showing how atmospheric nitrogen can be incorporated into organic macromolecules has been reported by a University of Arizona (UA) team.

The finding suggests the type of organic molecules that might be found on Titan, the moon of Saturn that scientists think is a model for the chemistry of pre-life Earth.

Earth and Titan are the only known planetary-sized bodies that have thick, predominantly nitrogen atmospheres, said Hiroshi Imanaka, who conducted the research while a member of UA’s chemistry and biochemistry department.

How complex organic molecules become “nitrogenated” in settings like early Earth or Titan’s atmosphere is a big mystery, Imanaka said.

“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,” said Imanaka, now an assistant research scientist in the UA’s Lunar and Planetary Laboratory. “Nitrogen is an essential element of life.”

Nitrogen fluorescing blue under UV light

Nitrogen fluorescing blue under UV light

Smog world could hide life

However, not just any nitrogen will do. Nitrogen gas must be converted to a more chemically active form that can drive the reactions that form the basis of 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 UV rays. The laboratory set-up was designed to mimic how solar radiation affects Titan’s atmosphere.

Most of the nitrogen moved directly into solid compounds, rather than gaseous ones, said Smith, a UA professor and head of chemistry and biochemistry. Previous models predicted the nitrogen would move from gaseous compounds to solid ones in a lengthier stepwise process.

Titan looks orange in colour because a smog of organic molecules envelops the planet. The particles in the smog will eventually settle down to the surface and may be exposed to conditions that could form life, said Imanaka, who is also a principal investigator at the SETI Institute.

However, scientists don’t know whether Titan’s smog particles contain nitrogen. If some of the particles are the same nitrogen-containing organic molecules the UA team created in the laboratory, conditions conducive to life are more likely, Smith said.

Laboratory observations such as these indicate what the next space missions should look for and what instruments should be developed to help in the search, Smith said.

Brown gunk held the key

The UA researchers wanted to simulate conditions in Titan’s thin upper atmosphere because results from the Cassini Mission indicated “extreme UV” radiation hitting the atmosphere created complex organic molecules.

Therefore, Imanaka and Smith used the Advanced Light Source at Lawrence Berkeley National Laboratory’s synchrotron in Berkeley, California, to shoot high-energy UV light into a stainless steel cylinder containing nitrogen-and-methane gas held at very low pressure.

Hiroshi Imanaka inside the Advanced Light Source

Hiroshi Imanaka stands next to the experiment inside the Advanced Light Source

The researchers used a mass spectrometer to analyse the chemicals that resulted from the radiation.

Simple though it sounds, setting up the experimental equipment is complicated. In addition, many researchers want to use the Advanced Light Source, so competition for time on the instrument is fierce. Imanaka and Smith were allocated one or two time slots per year, each of which was for eight hours a day for only five to 10 days.

Completing all the necessary experiments took years.

At the beginning, they analysed only the gases from the cylinder. But he didn’t detect any nitrogen-containing organic compounds.

Imanaka and Smith thought there was something wrong in the experimental set-up, so they tweaked the system. But still no nitrogen.

“It was quite a mystery,” said Imanaka. “Where did the nitrogen go?”

Finally, the two researchers collected the bits of brown gunk that gathered on the cylinder wall and analysed it with what Imanaka called “the most sophisticated mass spectrometer technique.”

Imanaka said, “Then I finally found the nitrogen!”

Imanaka and Smith suspect that such compounds are formed in Titan’s upper atmosphere and eventually fall to Titan’s surface. Once on the surface, they contribute to an environment that is conducive to the evolution of life.

Adapted from information issued by University of Arizona / Hiroshi Imanaka / Doug Archer / NASA / JPL / Space Science Institute.

Was Venus once habitable?

Artist’s concept of lightning on Venus

If Venus had more water in its distant past, could it have been a habitable planet like Earth?

  • Venus might once had have more water
  • Water split by sunlight; hydrogen/oxygen escaped to space
  • If it was wetter, could it have had life?

The Venus Express spacecraft is helping planetary scientists investigate whether Venus once had oceans. If it did, it may even have begun its existence as a habitable planet similar to Earth.

These days, Earth and Venus seem completely different. Earth is a lush, clement world teeming with life, whilst Venus is hellish, its surface roasting at temperatures of a furnace.

Venus in the ultraviolet

Sunlight breaks up water molecules in Venus' clouds, letting hydrogen and oxygen atoms to escape into space.

But underneath it all the two planets share a number of striking similarities. They are nearly identical in size and now, thanks to the European Space Agency’s (ESA) Venus Express orbiter, planetary scientists are seeing other similarities too.

“The basic composition of Venus and Earth is very similar,” says Håkan Svedhem, ESA Venus Express Project Scientist.

One difference stands out—the planet has very little water. Were the contents of Earth’s oceans to be spread evenly across Venus, they would create a layer 3km deep. If you were to condense the current amount of water vapour in Venus’ atmosphere onto its surface, it would create a global puddle just 3cm deep.

Water lost into space

Yet there is another similarity here. Billions of years ago, Venus probably had much more water. Venus Express has confirmed that the planet has lost a large quantity of water into space.

This happens because ultraviolet radiation from the Sun streams into Venus’ atmosphere and breaks the water molecules into their atoms—two of hydrogen and one of oxygen. These then escape to space.

Venus Express has measured the rate of this escape and confirmed that roughly twice as much hydrogen is escaping as oxygen. It’s therefore thought that water is the source of these escaping atoms.

It has also shown that a heavy form of hydrogen, called deuterium, is enriched in the upper echelons of Venus’s atmosphere, because the heavier hydrogen finds it harder to escape the planet’s grip.

Artist's impression of the Venus Express spacecraft

The Venus Express spacecraft is helping scientists study the water history of Venus.

“Everything points to there being large amounts of water on Venus in the past,” says Colin Wilson, Oxford University, UK. But that doesn’t necessarily mean there were oceans on the planet’s surface.

No oceans, but life anyway?

Eric Chassefière, Université Paris-Sud, France, has developed a computer model that suggests the water was largely atmospheric and existed only during the very earliest times, when the surface of the planet was completely molten.

As the water molecules were broken into atoms by sunlight and escaped into space, the subsequent drop in temperature probably triggered the solidification of the surface. In other words, no oceans.

Although it is difficult to test this hypothesis, it does raise a key question. If Venus ever did possess surface water, could planet have had an early habitable period?

Even if true, Chassefière’s model does not preclude the chance that colliding comets might have brought additional water to Venus after its surface solidified, and these could have created bodies of standing water in which life may have been able to form.

Adapted from information issued by ESA / MPS / DLR / IDA / J. Whatmore.

Life on Titan could eat acetylene

Artist's concept of a lake on the surface of the moon Titan

This artist concept shows a mirror-smooth lake on the surface of the smoggy moon Titan. Cassini scientists have concluded that at least one of the large lakes observed on Saturn's moon Titan contains liquid hydrocarbons, and have positively identified ethane. This result makes Titan the only place in our Solar System beyond Earth known to have liquid on its surface.

  • Chemicals are disappearing on Titan
  • Could be food for primitive life

Strange chemistry on Saturn’s moon Titan could indicate the presence of primitive life, say scientists.

While non-biological chemistry offers one possible explanation, some scientists believe the chemical indications bolster the argument for a primitive, exotic form of life or precursor to life on Titan’s surface.

One key finding shows hydrogen molecules flowing down through Titan’s atmosphere and disappearing at the surface. Another is that maps of hydrocarbons on the surface show a lack of acetylene, commonly known as welding gas.

The lack of acetylene is important because that chemical would likely be the best energy source for a methane-based life on Titan, said Chris McKay, an astrobiologist at NASA Ames Research Centre, who proposed a set of conditions necessary for this kind of methane-based life on Titan in 2005.

One interpretation is that the acetylene is being consumed as food. But McKay said the flow of hydrogen is even more critical because all of the proposed life mechanisms involved the consumption of hydrogen.

Titan as seen by the Cassini spacecraft

Saturn's moon Titan is very cold and smothered in hydrocarbon smog.

“We suggested hydrogen consumption because it’s the obvious gas for life to consume on Titan, similar to the way we consume oxygen on Earth,” McKay said. “If these signs do turn out to be a sign of life, it would be doubly exciting because it would represent a second form of life independent from water-based life on Earth.”

Life in deep-freeze

To date, methane-based life forms are only hypothetical. Scientists have not yet detected this form of life anywhere, though there are liquid-water-based microbes on Earth that thrive on methane or produce it as a waste product.

On Titan, where temperatures are around 90 Kelvin (minus 283 degrees Celsius), a methane-based organism would have to use a substance that is liquid for living processes, but not water itself. Water is frozen solid on Titan’s surface and much too cold to support life as we know it.

“Scientific conservatism suggests that a biological explanation should be the last choice after all non-biological explanations are addressed,” said Mark Allen, principal investigator with the NASA Astrobiology Institute Titan team.

“We have a lot of work to do to rule out possible non-biological explanations. It is more likely that a chemical process, without biology, can explain these results—for example, reactions involving mineral catalysts.”

Adapted from information issued by NASA / JPL.

Earth attacks!

Image of the surface of Mars taken by one of NASA's Viking landers, showing parts of the lander in the foreground.

Could our microbes survive on the surface of Mars? It's not impossible, say University of Central Florida scientists.

  • Could Earth bacteria survive on Mars?
  • Spacecraft need to be sterilised

Bacteria commonly found on spacecraft may be able to survive the harsh environs of Mars long enough to inadvertently contaminate the Red Planet with terrestrial life, according to research published in the journal Applied and Environmental Microbiology.

The search for life on Mars remains a stated goal of NASA’s Mars Exploration Program and Astrobiology Institutes. To try and prevent the Martian surface from being contaminated by Earth microbes, spacecraft are subjected to sterilisation procedures.

But despite the efforts made to reduce the “bioload” on spacecraft, recent studies have shown that diverse microbial contaminations remain at the time of launch. And the sterile nature of spacecraft assembly facilities ensures that only the most resilient species survive, including acinetobacter, bacillus, escherichia, staphylococcus and streptococcus.

To see how well Earth microbes might survive on Mars, researchers from the University of Central Florida replicated Mars-like conditions in the laboratory — extreme dryness, low air pressure, low temperatures and UV irradiation.

During the weeklong study they found that Escherichia coli bacteria, a potential spacecraft contaminant, might likely survive but not grow on the surface of Mars … if it were shielded from solar UV irradiation by thin layers of dust or in UV-protected niches in spacecraft.

The researchers say that if long-term microbial survival is possible on Mars, then past and future spacecraft explorations could provide the microbial “seed” for contaminating Mars with terrestrial life. They conclude that “a diversity of microbial species should be studied to characterise their potential for long term survival on Mars”.

Adapted from information issued by American Society For Microbiology / NASA.