Sep 23, 2016

Protoplanetary Disk Could Reveal Planetary Abortion

TW Hydrae is a young star sporting a beautiful protoplanetary disk that is almost perfectly face-on from our perspective. Within that disk, dark tracks have captivated astronomers who believe they hold clues to the earliest stages of planetary formation. In short, TW Hydrae is a stellar "petri dish," only 175 light-years away, showing us exactly where baby planets come from.

This now-famous observation, captured by the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, has gone one step further. In March, astronomers studying the emissions from the protoplanetary dust spotted something potentially groundbreaking. Close to the 10 million year-old star was a region lacking dust, possibly evidence for a world being born at roughly the same distance from the star as Earth orbits the sun. Could this be the earliest stages of the birth of an Earth-like planet? If so, the implications would be profound.

However, after carrying out computer simulations of the TW Hydrae protoplanetary disk, an international team of researchers led by Barbara Ercolano of Ludwig-Maximilians-Universität München, Germany, believes there may be another explanation for this innermost empty region and, sadly, it doesn't include a baby "Earth 2.0." In fact, the star may have aborted its birth.

When stars are young, they pump out huge amounts of ionizing radiation and blast out powerful stellar winds, vaporizing any dust that strays too close and blowing away any gases. In the case of a planet-forming disk around a young star, this could mean a region close to the star would be burnt away, leaving a gap.

ALMA observation of the TW Hydrae planet-forming disk, including the suspected Earth-like planet formation region that may, actually, be a region of photoevaporation.
The process, called "photoevaporation," could be ongoing in the TW Hydrae system, directly impacting the innermost region of the disk. The outer rings in the disk, however, are still likely being formed by accreting exoplanets sweeping up material as they orbit, it's just the inner region that is being vaporized.

Although this is obviously bad news for seeing the birth of an Earth-like exoplanent, TW Hydrae is proving an unprecedented opportunity for astronomers to study the destructive nature of a star in its earliest stages of evolution.

From Discovery News

Earth's Atmospheric Oxygen Levels Continue Long Slide

Researchers analyzed samples from ice core drilling stations in Antarctica and Greenland to evaluate the planet's atmospheric oxygen levels throughout history.
Atmospheric oxygen levels have declined over the past 1 million years, although not nearly enough to trigger any major problems for life on Earth, a new study finds.

The research behind this new finding could help shed light on what controls atmospheric oxygen levels over long spans of time, the researchers said.

Atmospheric oxygen levels are fundamentally linked to the evolution of life on Earth, as well as changes in geochemical cycles related to climate variations. As such, scientists have long sought to reconstruct how atmospheric oxygen levels fluctuated in the past, and what might control these shifts.

However, models of past atmospheric oxygen levels often markedly disagree, differing by as much as about 20 percent of Earth's atmosphere, which is oxygen's present-day concentration, the researchers said. 1 It is not even known if atmospheric oxygen levels varied or remained steady over the past 1 million years.

"There was no consensus on whether the oxygen cycle before humankind began burning fossil fuels was in or out of balance and, if so, whether it was increasing or decreasing," said study lead author Daniel Stolper, a geochemistat Princeton University in New Jersey.

In the new study, researchers calculated past atmospheric oxygen levels by looking at air trapped inside ancient polar ice samples. Specifically, they looked at samples from Greenland and Antarctica.

The new estimates suggest that atmospheric oxygen levels have fallen by 0.7 percent over the past 800,000 years. The scientists concluded that oxygen sinks — processes that removed oxygen from the air — were about 1.7 percent larger than oxygen sources during this time.

Although a drop in atmospheric oxygen levels might sound alarming, the decrease the researchers found "is trivial in regard to ecosystems," Stolper told Live Science. "To put it in perspective, the pressure in the atmosphere declines with elevation. A 0.7 percent decline in the atmospheric pressure of oxygen occurs at about 100 meters (330 feet) above sea level — that is, about the 30th floor of a tall building."

There are two hypotheses that may help explain this oxygen decline over the past million years, Stolper said.

"The first is that global erosion rates may have increased over the past few to tens of millions of years due to, among other things, the growth of glaciers — glaciers grind rock, thereby increasing erosion rates," Stolper said.

Rising erosion rates would have exposed more pyrite and organic carbon to the atmosphere. Pyrite is better known as fool's gold, and organic carbon consists of the remains of organisms, mostly land plants and aquatic photosynthetic microorganisms such as algae. Previous research found that both pyrite and organic carbon can react with oxygen and remove it from the atmosphere.

"Alternatively, when the ocean cools, as it has done over the past 15 million years, before fossil fuel burning, the solubility of oxygen in the ocean increases. That is, the oceans can store more oxygen at colder temperatures for a given concentration of oxygen in the atmosphere," Stolper said. Oxygen-dependent microbes in the ocean and in sediments can then become more active and consume this oxygen, leaving less of the element in the atmosphere, he added.

Read more at Discovery News

'Marsquakes' Could Give Alien Life a Boost

Humans and most animals, plants and fungi get their energy mainly from chemical reactions between oxygen and organic compounds such as sugars. However, microbes depend on a wide array of different reactions for energy; for instance, reactions between oxygen and hydrogen gas help bacteria called hydrogenotrophs survive deep underground on Earth, and previous research suggested that such reactions may have even powered the earliest life on Earth.

Prior work suggested that when rocks fracture and grind together during earthquakes on Earth, silicon in those rocks can react with water to generate hydrogen gas. Study lead author Sean McMahon, a geomicrobiologist at Yale University, and his colleagues wanted to see if marsquakes could generate enough hydrogen to support any microbes that might potentially live on the Red Planet.

The scientists examined special types of rocks that are created when rocks grind against each other during earthquakes. The samples the researchers analyzed from Scotland, Canada, South Africa, the Isles of Scilly off the coast of England and the Outer Hebrides of Scotland were up to hundreds of times richer in trapped hydrogen gas than surrounding rocks that were not generated from such grinding.

"These findings were surprising and exciting because we didn't know if we were going to find anything at all," McMahon said.

The researchers said the hydrogen gas in the samples they analyzed was abundant enough to support hydrogenotrophs on Earth.

"Our findings are a contribution to a broader picture of how geological processes can support microbial life in extreme environments," McMahon told Space.com. "There's not much of what we think of as food miles below Earth's surface, but over the last few decades, scientists have found that Earth has a huge amount of biomass down there, maybe 20 percent or more of Earth's biomass."

When it comes to whether marsquakes and water might work together to generate hydrogen on Mars, previous research suggested that liquid water was once abundant on the surface of Mars. It also suggests that large reserves of liquid water may still exist underground on the Red Planet at depths of about 3 miles (5 kilometers) on average. However,Mars has much fewer quakes than Earth, because the Red Planet nowadays lacks both volcanism and plate tectonics.

Still, the researchers noted that conservative models of marsquakes based off data from NASA's Mars Global Surveyor suggest that, on average, the Red Planet experiences a magnitude-2 event every 34 days and a magnitude-7 event every 4,500 years. This means that marsquakes may on average generate less than 11 tons (10 metric tons) of hydrogen annually over the whole of Mars, which may be still enough to sporadically fuel pockets of microbial activity there, the researchers said.

Read more at Discovery News

Teeth Suggest Humans Killed Hobbits

Recreation of a Homo floresiensis male.
Humans were on hobbit turf at around the same time that the small people (Homo floresiensis) seemingly disappeared from Earth, according to new evidence presented this week in Madrid at the European Society for the Study of Human Evolution.

The evidence -- a pair of 46,000-year-old human teeth -- is described in a Nature report. The teeth were discovered at the former hobbit homeland, Liang Bua cave on the island of Flores in Indonesia.

Scientists exploring Liang Bua cave on the Indonesian island of Flores.
Earlier research determined that members of our species were living in southeast Asia by around 50,000 years ago. Around this time in Liang Bua, H. floresiensis -- and animals including giant storks, pygmy elephants and Komodo dragons -- disappeared. Scientists began to suspect that Homo floresiensis was not a stranger to Homo sapiens.

This possibility was deemed a "smoking gun" by Bert Roberts of the University of Wollongong earlier this year, but he had not yet found the "bullet" linking the two human groups.

"The exact cause of the demise of the hominids and associated animals is not yet understood, but in my view, may be related to the appearance in the area of the most aggressive of all hominin species, Homo sapiens, modern humans," said Donald Johanson, founding director of the Institute of Human Origins at Arizona State University.

Recreation of a Homo floresiensis individual.
The 46,000-year-old human teeth, consisting of an upper premolar and a lower molar, support Johanson's view. Roberts, archaeologist Thomas Sutikna and their team found the teeth while excavating the hobbit cave.

The researchers also found freshwater mollusk shells, which are commonly associated with early Homo sapiens sites in Europe, Africa and other parts of Asia. Stone tools made from a hard rock known as chert were additionally unearthed, as was evidence for fire hearths. All are typical of early human settlements.

Read more at Discovery News

Sep 22, 2016

Exotic Star System Discovered via Spacetime Warp

For the vast majority of exoplanetary discoveries, one or more planets are found orbiting one star. However, there are a few exotic exoplanets orbiting two stars and the Hubble Space Telescope has helped confirm the discovery of a unique star system.

Way back in 2007, a ground-based system looking for transient brightenings -- called "microlensing" events -- detected a peculiar signal.

Microlensing events are caused when a massive object, like a planet or star, passes in front of a more distant background star. As predicted by Einstein's theory of general relativity, the lensing object will warp spacetime, causing any light passing by to slightly change its path. Should the alignment between distant star, lensing object and Earth be just right, the object can create a spacetime lens -- akin to passing a magnifying lens in front of a candle flame.

The result is a short-lived brightening of the background star. By studying the microlensing event light-curve (i.e. how the brightening fluctuates with time), we can learn many things about the object(s) creating the lens.

But in the case of the 2007 event, the ground-based Optical Gravitational Lensing Experiment (or "OGLE") detected something else in the light-curve that confused matters. This wasn't a single object, it was a whole star system -- with a twist.

"A detailed analysis revealed a third lensing body in addition to the star and planet that were quite obvious from the data," said astronomer David Bennett, of NASA's Goddard Space Flight Center, in a statement.

But what does this mean? Through analysis of the signal, called OGLE-2007-BLG-349, there were two explanations. According to Bennett, there was either "a Saturn-mass planet orbiting a close binary star pair or a Saturn-mass and an Earth-mass planet orbiting a single star."

As microlensing events are, by their nature, one-offs, astronomers needed another way to confirm the nature of OGLE-2007-BLG-349 and Hubble has been used to zoom in on the star system that triggered the 2007 brightening.

It turns out that OGLE-2007-BLG-349 was caused by a planet orbiting two stars, both tiny red dwarfs, drifting in front of a more distant bright star. This is the first time a binary system plus single exoplanet has been discovered through microlensing. "We were helped in the analysis by the almost perfect alignment of the foreground binary stars with the background star, which greatly magnified the light and allowed us to see the signal of the two stars," Bennett added.

Read more at Discovery News

Wanna-Be Dino? Prehistoric Reptile Looked Like a Dinosaur

The preserved remains of Triopticus (left) show the evolution of a thickened domed skull in the Triassic Period, 150 million years before the evolution of the famous dome-headed pachycephalosaur dinosaurs, such as Stegoceras (right). The background image shows the field site in Texas where WPA crews in 1940 found the curious fossils of Triopticus.
A reptile that lived before the Dinosaur Age resembled a dino, suggesting that iconic dinosaur body shapes were present long before the dinosaurs themselves actually emerged.

The newly identified reptile, described in the journal Current Biology, resembled pachycephalosaur dinosaurs that lived more than 100 million years later. Other extinct animals found with the reptile looked like later dinos too.

The reptile has been named Triopticus primus, meaning the "First of Three Eyes" because the natural pit at the top of its skull lends the appearance of an extra eye.

"Triopticus is an extraordinary example of evolutionary convergence between the relatives of dinosaurs and crocodylians and later dinosaurs that is much more common than anyone ever expected," co-author and project leader Michelle Stocker, a Virgnia Tech College of Science researcher, said in a press release. "What we thought were unique body shapes in many dinosaurs actually evolved millions of years before in the Triassic Period, about 225 million years ago."

Convergence refers to when distantly related animals evolve to look very similar to each other. A classic example of this is a bird wing and a bat wing. Both animals use their wings for flight, yet the inner details of their wings are different and evolved independently.

Three Eyes dates to about 230 million years ago, according to the researchers. The reptile's partial skull was originally collected at a site called Otis Chalk near Big Spring, Texas, by the Works Progress Administration in 1940.

President Franklin Roosevelt had initiated a monumental effort to put Americans back to work at end the Great Depression. With so much digging going on, numerous fossils were unearthed. In fact, so many fossils were found during such a short time span that several of them were just put into storage uncleaned.

Such was the case for Three Eyes, whose skull was eventually sent to the Texas Vertebrate Paleontology Collections in 2010. It is there that Stocker and her team rediscovered and analyzed the specimen.

They determined that the reptile had an extremely thickened skull roof, just like the very distantly related pachycephalosaur dinosaurs that lived more than 100 million years after the lifetime of Three Eyes.

"CT scanning showed us that the similarity of Triopticus with the much later dome-headed pachycephalosaur dinosaurs was more than skin deep, extending to the structure of the bone and even the brain." co-author Lawrence Witmer of Ohio University's Heritage College of Osteopathic Medicine said.

It was not a coincidence that the reptile and dinosaurs resembled each other.

"After the enormous mass extinction 250 million years ago, reptiles exploded onto the scene and almost immediately diversified into many different sizes and shapes," co-author Sterling Nesbitt of Virginia Tech said. "These early body shapes were later mimicked by dinosaurs."

Read more at Discovery News

Dead Sea Scroll 'Virtually Unwrapped'

Shown is the completed virtual unwrapping for the En-Gedi scroll.
The En-Gedi scroll, a text that includes part of the Book of Leviticus in the Hebrew Bible that was ravaged by fire about 1,400 years ago, is now readable, thanks to a complex digital analysis called "virtual unwrapping."

Rather than physically unfurl the scroll, which would have destroyed the crumbling artifact, experts digitally scanned the document, and then virtually flattened the scanned results, allowing scholars to read its ancient text.

"We're reading a real scroll," lead study author Brent Seales, professor and chairman in the department of computer science at the University of Kentucky, said in a news conference yesterday (Sept. 20). "It hasn't been read for millennia. Many thought it was probably impossible to read."

Archaeologists found the scroll in 1970 in En-Gedi, where an ancient Jewish community thrived from about the late 700s B.C. until about A.D. 600, when a fire destroyed the site, the researchers said. Excavations of the synagogue's Holy Ark, a chest or cupboard that holds the Torah scrolls, revealed charred scrolls of parchment, or animal skin. But each scroll was "completely burned and crushed, had turned into chunks of charcoal that continued to disintegrate every time they were touched," the researchers wrote in the study.

The En-Gedi scroll is different than the original Dead Sea Scrolls, which a young shepherd discovered in caves near Qumran in the Judean Desert in 1947. However, Dead Sea Scroll has become an umbrella term for many ancient scrolls found in the area, and some researchers also call the En-Gedi artifact a Dead Sea Scroll.

The scorched En-Gedi scroll fragments sat in storage for more than 40 years until experts decided to give them another look, and try the newly developed "virtual unwrapping" method for the first time on the scroll.

Potential scroll fragments from En-Gedi that are severely burned.
The virtual journey began in Israel, where experts digitally scanned the rolled-up scroll with X-ray-based micro-computed tomography (micro-CT). At this point, they weren't sure whether the scroll had text within it, said study co-author Pnina Shor, curator and head of the Dead Sea Scrolls Projects at the Israel Antiquities Authority. So, they increased the spatial resolution of the scan, allowing them to capture whether or not each layer had detectable ink.

Their exhaustive attention to detail paid off: There was ink, and it likely contained metal, such as iron or lead, because it showed up on the micro-CT scan as a dense material, the researchers said.

However, the text was illegible. So Shor and her colleagues in Israel sent the digital scans to Seales in Kentucky so he and his team could try the new "virtual unwrapping" technique.

"It was certainly a shot in the dark," Shor said.

This new method marks the first time that experts have virtually unrolled and noninvasively studied a severely damaged scroll with ink text, Seales said.

The unwrapping took time and involved three steps: segmentation, texturing and flattening, he said.

With segmentation, they identified each segment, or layer, within the digital scroll, which had five complete revolutions of parchment in the scroll. Then, they created a virtual geometric mesh for each layer made of tiny, digital triangles. They were able to manipulate this mesh, which helped them "texture" the document, or make the text more visible.

"This is where we see letters and words for the first time on the recreated page," the researchers wrote in the study.

Finally, they digitally flattened the scroll, and merged the different layers together into one, flat 2D image that could easily be read.

The scroll holds the beginning of the Book of Leviticus, the third of the five books of Moses (known as the Pentateuch) that make up the Hebrew Bible, biblical scholars said. In fact, the En-Gedi scroll contains the earliest copy of a Pentateuchal book ever found in a Holy Ark, the researchers said.

The virtual unwrapping revealed two distinct columns of text that include, in total, 35 lines of Hebrew. Each line has 33 to 34 letters. However, there are only consonants, no vowels. This indicates that the text was written before the ninth century A.D., when Hebrew symbols for vowels were invented, said study co-author Emanuel Tov, a professor emeritus in the department of Bible at Hebrew University of Jerusalem.

Radiocarbon dating places the scroll in the third or fourth century A.D., but studies based on historical handwriting place it at either the first or second century A.D., the researchers said. Regardless, the data suggest that it was written within the first few centuries of the Common Era, they said.

Read more at Discovery News

Earth Wobbles May Have Driven Ancient Humans Out of Africa

 A computer model simulated human density 80,000 years ago, showing the arrival of humans in eastern China and southern Europe as well as migrations out of Africa along vegetated paths in Sinai and the Arabian Peninsula. 
Ancient human migrations out of Africa may have been driven by wobbles in Earth's orbit and tilt that led to dramatic swings in climate, a new study finds.

Modern humans first appeared in Africa about 150,000 to 200,000 years ago. It remains a mystery as to why it then took many millennia for people to disperse across the globe. Recent archaeological and genetic findings suggest thatmigrations of modern humans out of Africa began at least 100,000 years ago, but most humans outside of Africa most likely descended from groups who left the continent more recently — between 40,000 and 70,000 years ago.

Previous research suggested that shifts in climate might help explain why modern human migrations out of Africa happened when they did. For instance, about every 21,000 years, Earth experiences slight changes to its orbit and tilt. These series of wobbles, known as Milankovitch cycles, alter how much sunlight hits different parts of the planet, which in turn influences rainfall levels and the number of people any given region can support.

Now scientists have developed a new computer simulation of Earth to pinpoint how these changes in orbit and solar radiation levels might have affected rainfall, temperature, sea levels, glacial ice, vegetation, carbon dioxide levels and global modern human migration patterns over the past 125,000 years. The researchers noted that this model's predictions agree well with previous findings regarding ancient climates.

The model suggests that modern humans dispersed from Africa in multiple waves across the Arabian Peninsula and the area known as the Levant, the eastern Mediterranean region that includes Israel and Syria. These results closely align with previous estimates garnered from archaeological and fossil data of when modern humans arrived in areas such as the Middle East, Europe, Asia, Australia and the Americas.

"Earth's wobble with a periodicity of 21,000 years played a huge role in our dispersal across the planet and most likely also in our evolution and adaptation," said study lead author Axel Timmermann, a climate researcher at the University of Hawaii at Manoa. "If the climate had been constant over the past 125,000 years, we would have evolved in a very different way."

Specifically, the researchers found that intensified rainfall in northern Africa, the Arabian Peninsula and the Levant would have generated habitable green corridors for modern humans to migrate through the Sahara and Arabian deserts. These corridors would have been open during four distinct times — about 106,000 to 94,000 years ago; 89,000 to 73,000 years ago; 59,000 to 47,000 years ago; and 45,000 to 29,000 years ago — "enabling Homo sapiens to leave northeastern Africa and embark onto their grand journey into Eurasia, Australia and the Americas," Timmermann told Live Science.

The model suggests these migrations were not one-way in nature away from Africa, "as is often portrayed in schematics," Timmermann said. "A green migration corridor between Africa and the eastern Mediterranean meant that Africans were migrating into Eurasia, and Eurasians were moving into Africa. The backflow of Homo sapiens into certain regions and the corresponding backflow of genes may be crucial for understanding who we are, why we are, where we are."

Read more at Discovery News

This Is the Iceman’s Voice

The 5,300-year-old mummy's speech is recreated by scientists.

The "best possible approximation" of Ötzi the Iceman's voice has been reconstructed by scientists who aimed to discover the timbre and color of his Stone Age vowels.

Ötzi broke his silence with a deep male voice after 5,300 years at the EURAC Institute for Mummies and the Iceman in Bolzano.

He spoke Italian -- but just vowels, as you can hear below.

The experiment was presented on Wednesday morning during a major congress to celebrate the 25th anniversary of the mummy's discovery in the Ötztal Alps in South Tyrol.

"We can't say we have reconstructed Ötzi's original voice because we miss some crucial information from the mummy," Rolando Füstös, chief of the ENT department at Bolzano's General Hospital, told Discovery News.

"But with two measurements, the length of both the vocal tract and the vocal cords, we have been able to recreate a fairly reliable approximation of the mummy's voice. This is a starting point for further research," he added.

Füstös and colleagues Francesco Avanzini, ENT specialist and phoniatrician at the city's General Hospital, Piero Cosi, at the Institute of Cognitive Sciences e Technology, National Research Council in Padova, Andrea Sandi, at SINTAC Biomedical Engineering in Padova,and others based their research on the CT scans of the mummy to create a model of his vocal tract.

"The vocal chords are the source of the vocal sound, but the main contribution to it is given by the selective filtering accomplished by the vocal tract configuration," Füstös said.

The researchers had to face several challenges as they worked to reconstruct the structure of the 5,300-year-old mummy's vocal tract.

"We had to deal with Ötzi's position, whose arm is covering his throat," Avanzini told Discovery News. "For our project this is the worst position you can imagine. Moreover, the hyoid bone, or tongue-bone, was party absorbed and dislocated."

With special software, the researchers moved Ötzi's arm, repositioned his skull in the erect position, reconstructed his vertebrae, from the first one (C1) closest to the skull, to the first thoracic vertebra (T1), and reconstructed and repositioned the hyoid bone, which supports the tongue.

They ended up with a complete model of the vocal tract, including the vocal cords and mouth, though they missed important data such as the tension and density of the vocal cords or the thickness and composition of the soft tissues that affect the human voice.

MRI scans would have helped the researchers getting more insights, but the technology could not be used because of the condition of Ötzi's mummified body.

"We had to rely on mathematical models and a software that simulates the way the vocal tract works," Cosi said.

He explained that calculations of vocal area functions applied on the structure of the vocal tract produce a certain vocal sound.

"We ... tried to extract the formants of a synthesized sound 'injected' in the reconstructed vocal tract," Cosi said.

Taking into consideration that Ötzi had a rather large head and slender body, the researchers concluded that his voice likely had a fundamental frequency between 100 Hz and 150 Hz, in line with today's average male.

Read more at Discovery News

Sep 21, 2016

NASA scientists find 'impossible' cloud on Titan -- again

The hazy globe of Titan hangs in front of Saturn and its rings in this natural color view from NASA's Cassini spacecraft.
The puzzling appearance of an ice cloud seemingly out of thin air has prompted NASA scientists to suggest that a different process than previously thought -- possibly similar to one seen over Earth's poles -- could be forming clouds on Saturn's moon Titan.

Located in Titan's stratosphere, the cloud is made of a compound of carbon and nitrogen known as dicyanoacetylene (C4N2), an ingredient in the chemical cocktail that colors the giant moon's hazy, brownish-orange atmosphere.

Decades ago, the infrared instrument on NASA's Voyager 1 spacecraft spotted an ice cloud just like this one on Titan. What has puzzled scientists ever since is this: they detected less than 1 percent of the dicyanoacetylene gas needed for the cloud to condense.

Recent observations from NASA's Cassini mission yielded a similar result. Using Cassini's composite infrared spectrometer, or CIRS -- which can identify the spectral fingerprints of individual chemicals in the atmospheric brew -- researchers found a large, high-altitude cloud made of the same frozen chemical. Yet, just as Voyager found, when it comes to the vapor form of this chemical, CIRS reported that Titan's stratosphere is as dry as a desert.

"The appearance of this ice cloud goes against everything we know about the way clouds form on Titan," said Carrie Anderson, a CIRS co-investigator at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the study.

The typical process for forming clouds involves condensation. On Earth, we're familiar with the cycle of evaporation and condensation of water. The same kind of cycle takes place in Titan's troposphere -- the weather-forming layer of Titan's atmosphere -- but with methane instead of water.

A different condensation process takes place in the stratosphere -- the region above the troposphere -- at Titan's north and south winter poles. In this case, layers of clouds condense as the global circulation pattern forces warm gases downward at the pole. The gases then condense as they sink through cooler and cooler layers of the polar stratosphere.

Either way, a cloud forms when the air temperature and pressure are favorable for the vapor to condense into ice. The vapor and the ice reach a balance point -- an equilibrium -- that is determined by the air temperature and pressure. Because of this equilibrium, scientists can calculate the amount of vapor where ice is present.

"For clouds that condense, this equilibrium is mandatory, like the law of gravity," said Robert Samuelson, an emeritus scientist at Goddard and a co-author of the paper.

But the numbers don't compute for the cloud made from dicyanoacetylene. The scientists determined that they would need at least 100 times more vapor to form an ice cloud where the cloud top was observed by Cassini's CIRS.

One explanation suggested early on was that the vapor might be present, but Voyager's instrument wasn't sensitive enough in the critical wavelength range needed to detect it. But when CIRS also didn't find the vapor, Anderson and her Goddard and Caltech colleagues proposed an altogether different explanation. Instead of the cloud forming by condensation, they think the C4N2 ice forms because of reactions taking place on other kinds of ice particles. The researchers call this "solid-state chemistry," because the reactions involve the ice, or solid, form of the chemical.

The first step in the proposed process is the formation of ice particles made from the related chemical cyanoacetylene (HC3N). As these tiny bits of ice move downward through Titan's stratosphere, they get coated by hydrogen cyanide (HCN). At this stage, the ice particle has a core and a shell composed of two different chemicals. Occasionally, a photon of ultraviolet light tunnels into the frozen shell and triggers a series of chemical reactions in the ice. These reactions could begin either in the core or within the shell. Both pathways can yield dicyanoacteylene ice and hydrogen as products.

The researchers got the idea of solid-state chemistry from the formation of clouds involved in ozone depletion high above Earth's poles. Although Earth's stratosphere has scant moisture, wispy nacreous clouds (also called polar stratospheric clouds) can form under the right conditions. In these clouds, chlorine-bearing chemicals that have entered the atmosphere as pollution stick to crystals of water ice, resulting in chemical reactions that release ozone-destroying chlorine molecules.

"It's very exciting to think that we may have found examples of similar solid-state chemical processes on both Titan and Earth," said Anderson.

The researchers suggest that, on Titan, the reactions occur inside the ice particles, sequestered from the atmosphere. In that case, dicyanoacetylene ice wouldn't make direct contact with the atmosphere, which would explain why the ice and the vapor forms are not in the expected equilibrium.

"The compositions of the polar stratospheres of Titan and Earth could not differ more," said Michael Flasar, CIRS principal investigator at Goddard. "It is amazing to see how well the underlying physics of both atmospheres has led to analogous cloud chemistry."

Read more at Science Daily