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Cassini points to a hidden ocean on Saturn’s icy moon

Akshat RathiLeave a comment

Finding liquid water on a celestial body within the solar system is exciting. The only thing that is probably more exciting is finding an ocean full of it. Today such news comes via Cassini, which has made measurements that show that Saturn’s moon Enceladus has a hidden ocean beneath its icy surface.

While orbiting Saturn in 2005, Cassini found jets of salty water spewing from the south polar region of Enceladus. According to Luciano Iess of Sapienza University of Rome, lead author of the new study published in Science, “The discovery of the jets was unexpected.”

Geysers require liquid water, and we wouldn’t expect Enceladus to have any. It is too far from the Sun to absorb much energy and too small (just 500km in diameter) to have trapped enough internal energy to keep its core molten. The answer to how the water got there might lie in the details of the moon’s internal structure.

Water beneath an icy crust

The data to understand Enceladus’s internal structure came from by measuring changes in Cassini’s speed as it flew close to the moon. When passing the denser parts of the moon, it sped up by a few extra thousandths of a metre per second. That minute change was tracked through recordings of the radio signals Cassini was sending to NASA’s Deep Space Network station.

In making such tiny measurements, scientists had to filter out other factors that could influence Cassini’s speed. These include pressure on the spacecraft from sunlight, the nudge from heat radiating from its nuclear-powered electrical generator, and the drag of the particles it strikes as it passes through the south polar plumes.

Iess and his colleagues have produced a model of the internal structure of Enceladus using the measurements. They conclude that there is a core that is roughly 200km in diameter; above that lies a 10km-thick layer of liquid water, which is followed by 40km of ice crust. The water layer may extend all the way to the north pole, but its thickest part lies at the south pole.

NASA/JPL-Caltech

It is possible that Saturn’s powerful gravity is responsible for the liquid water under Enceladus’s surface. Its pull could heat up the interior through a process called tidal kneading, which creates tides in the ocean causing internal friction and thus heat.

After the initial discovery of the plumes, Cassini’s minders put a lot of effort into determining Enceladus’s internal structure, but it still took nearly ten years to do so. This is because the time the spacecraft spends around Saturn is very valuable, and there are lots of other things worth studying.

Cassini can only make a handful of flybys near Enceladus while still paying attention to other moons, such as Titan. When approaching Enceladus, the controllers also had to make a choice about how to study the moon because of a limitation in how Cassini’s instruments are arranged. When making gravitational recordings it needs to point its antenna towards Earth, but in doing so all its other instruments face away from Enceladus. Of the 19 flybys, only three were used to make gravitational recordings.

“After spending eight years in the Saturnian system, one may think that the measurements are becoming repetitive and that Cassini has discovered everything in the reach of its instruments. This is far from being true,” Iess said.

Time is running out

“The evidence adds up to a large and active body of water under Enceladus’s southern polar region”, Helen Maynard-Casely of Australian Nuclear Science and Technology Organisation said. But she warned, “It is going to be a long time before we can verify if this ocean is there, if ever.”

The plutonium-powered spacecraft has enough energy to power itself till 2017. The trouble is that, in three years, it will only be able to make three more flybys of Enceladus, which is not enough to take more gravity data. Its end is slated to come when controllers drive it into Saturn’s atmosphere for incineration, because scientists are keen to avoid having it crash into Saturn’s pristine moons.

There is a push to send another mission to Saturn, but Jupiter’s moon Europa might be a better candidate to search for life. At 3,100km in diameter, it is much larger than Enceladus, and, in December, astronomers spotted water vapour coming from its south pole, as well.

The possibility of finding a large amount of liquid water is exciting because, for life to exist as we understand it, we need liquid water. Even on Earth, whenever untouched sources of liquid water, such as Lakes Vostok and Ellsworth under Antarctica, are studied, there is always the hope that we may discover new forms of life.The Conversation

First published on The Conversation. Image credit: NASA/JPL/SSI/J Major

The only reason zebras have stripes is to ward off flies

Akshat Rathi1 Comment

Zebras’ stripes have baffled biologists since Charles Darwin. Many hypotheses have been proposed regarding their purpose but, despite hundreds of years of study, there remains disagreement.

In an attempt to end the debate, researchers have pitted various models against each other and systematically analysed data from past studies. Their results reveal the one reason zebras have stripes: to ward off flies.

A handful of ideas regarding zebras’ stripes have found some support among biologists. One proposed that the dark and light bands change how air flows around a zebra’s body and helps in heat management, which could go a long way in the hot tropical areas that zebras live in.

Another proposed the stripes were used by zebras as a way of social interaction. They may use them to identify other zebras and for bonding as a group in the wild.

A third proposal suggested zebras used the stripes as camouflage. While stripes are clearly visible in the day, there some thought that it helped at dawn, dusk, and in the night.

All these ideas were shot down when tested rigorously. Two others, however, remained intriguing.

Now, how do I get rid of these ants?
dkeats, CC BY

The first was that the stripes were used to dodge predators. It is called the “motion dazzle hypothesis”, and it suggests predators are confused by zebras’ stripes and cannot understand their movement. Research published in the journal Zoology in 2013 used a simulated visual system to show that zebra stripes do interfere with visual perception. But this is a difficult hypothesis to test in the field.

Martin Stevens at the University of Exeter has researched the motion dazzle hypothesis by getting human subjects to catch moving stripy objects on a computer. “It’s an artificial experimental system,” he admitted.

The second proposal was that stripes helped keep flies at bay. Zebras are especially susceptible to biting flies due to their geographic spread. These flies, which include the tsetse fly, stomoxys stable flies, and tabanid biting flies, are particularly prevalent in areas with high temperature and humidity – exactly the areas where zebras are normally found.

Bites from these flies can be nasty and, quite literally, draining. About thirty flies feeding for six hours on just one horse can draw as much as 100mL of blood. Usually the flies can number in the hundreds around one animal.

Zebras have shorter hair than other equids – the family that includes horses, donkeys and zebras – which may also increase their susceptibility to attack. Also, four diseases which are fatal to equids have been found in Africa. This could mean that investing in anti-biting defenses such as stripes is especially important for zebras compared to non-African equids.

It is possible that the dazzle effect acts on flies, rather than larger predators, and deter them from biting. “Stripes clearly have a number of functions,” Stevens said, “and these could be interacting in zebras.”

Revealing maps

In the new research, just published in Nature Communications, Tim Caro and his colleagues at the University of California in Davis, didn’t perform experiments. Instead they used ecological and observational data on zebras’ geographical locations and related factors. It is the first time that a comparative approach has been applied to find the reasons for zebras’ characteristic colouration. Caro thinks his findings may have nailed the answer at last.

Caro looked at seven species of equids and scored them for number and intensity of stripes. Just to be sure, they tested all five hypotheses regarding zebra stripes’ use: camouflage, predator avoidance, heat management, social interaction, and warding off flies. The extent of overlap between the geographic distribution of striped equids with each of these five measures was calculated.

E. greyvi, E. burchelli and E. zebra have stripes on all their bodies. Other equids don’t.
Caro, Izzo, Reiner, Walker and Stankowich

“The results were a shock to me,” said Caro. Of these five proposals, only warding off flies had statistical support. He had not expected such a clear-cut answer to the question. As the map shows, the only places where flies and equids live together are areas that are populated by striped equids.

The exact mechanism by which stripes deter flies remains unknown, but experimental studies performed by researchers at Lund University in 2012 have found support for this proposal. They created striped surfaces and stuck glue on them. Based on the number of flies on the surfaces with different thicknesses of stripes, they concluded that these flies stayed away from stripes as thin as those found on zebras.

“As is normal in science you get a solution that asks more questions,” Caro said. It is time to hand the problem over to vector biologists, who can understand the susceptibility of horses to biting flies.

In Darwin’s days, people didn’t consider animal colouration with respect to fitness advantages. “People thought that animal colouration existed simply to please humans or was caused directly by the environment,” Caro said.

Darwin “would be delighted” that researchers are now considering animal colouration as a functional trait, he said. We might not have all the answers regarding zebra stripes – but it seems we may be looking through the right lens.The Conversation

Written with Angela White. This article was originally published on The Conversation. Header picture credit: eoghann, CC-BY-NC.

The greatest mass extinction may have been the doing of microbes

Akshat RathiLeave a comment

The worst time to be alive in Earth’s history is unarguably the end-Permian, about 250 million years ago. It is the period when the greatest-ever extinction event recorded took place, killing 97% of all species, an event so severe it has been called The Great Dying.

This event has generally been blamed on massive volcanic eruptions that took place at the same time. But now, in a new analysis, researchers at the Massachusetts Institute of Technology (MIT) argue that the mass extinction event may have been instigated by microbes. These microbes led to a perturbation of the carbon cycle that caused environmental shocks, such as global warming and ocean acidification. The shocks wiped out species in great numbers over a period of tens of thousands of years – a blip on geological scales.

Felt like the end of time

The end-Permian extinction, which took place about 250 million years ago, is the most severe of five known mass extinction events. It killed off the last of the trilobites – a hardy marine species that had survived two previous mass extinction. While land plants survived, almost all forests disappeared. Worse of all, it is the only known extinction event where even insects weren’t spared.

For an event of this size to take place, a lot of things would have had to go wrong. At the time the world was made up of a single supercontinent called Pangea. This large landmass, by altering the dynamics of how carbon is cycled with subducting plates, may have pushed global temperatures to the highest they had ever been.

Then, over the course of about a million years, huge eruptions in Siberia created basalts that cover an area that was about seven times the size of France. This may have pushed the environment past a tipping point by sending even more carbon dioxide into the atmosphere. That would have caused the oceans to acidify, killing more marine life, and heat up, releasing frozen methane. The upshot of all this would have been a “runaway” climate that kept heating up and removing more oxygen from the environment.

The mighty microbe

But Daniel Rothman of MIT thinks that the numbers don’t add up. “The changes in the carbon cycle globally are difficult to reconcile with only volcanic activity in Siberia,” he said.

His calculations, just published in the Proceedings of the National Academy of Sciences, were hinting that something else must have caused the runaway event. One hypothesis was that microbial life may have been responsible for that.

“This hypothesis is not as outrageous as it seems. After all, about 2.4 billion years ago, it was microbes in the form of cyanobacteria that gave our atmosphere all of its oxygen,” Rothman added. That period, called the Great Oxygenation Event, also killed most organisms that were adapted to the lack of oxygen and began one of the longest cold periods in Earth’s history. So microbes can certainly have global impact.

With colleagues at MIT, Rothman looked at the evolutionary history of Earth and spotted the rise of a particular type of microbe that occurred around the time of the Great Dying. That microbe, called Methanosarcina, had the ability to digest organic matter to produce methane. (Molecular biologists at MIT have shown that Methanosarcina evolved this ability thanks to the transfer of a single gene from the Clostridia class of bacteria.)

Rothman knew that the chemical process involved in creating the methane relied on the metal nickel. He went looking for evidence that Methanosarcina was thriving at the time in the sedimentary layer of the Meishan region of China. If the environment at that time had any more nickel than normal, then the sediments would hold the record of it.

Rothman chose the Meishan region to look for nickel because it is a particularly well-studied region. Its sedimentary layers have been used to mark and standardise different periods of Earth’s geological history, and they span the period of the Great Dying.

The search was successful. There was indeed a higher amount of nickel in the sediments deposited during that period. Methanosarcina would not have just been effective at creating methane – they would have flourished.

The nickel, Rothman suggests, would have been added to the oceans, where Methanosarcina lived and grew, by the continuous volcanic activity occurring in Siberia. The growing amount of nickel, transported by ocean currents, would have allowed more Methanosarcina to convert organic matter into methane, which would be converted to carbon dioxide through reactions with oxygen. This would have meant increased global temperatures and acidification of the oceans. The latter would have combined with the loss of oxygen (used up in creating the carbon dioxide) to accelerate the extinction in the oceans. And the dead organisms would have provided Methanosarcina with more organic matter to digest.

In short, a microbial innovation may have tipped over the balance to cause the Great Dying.

Marc Reichow at the University of Leicester remains sceptical of these results. He argues that there is no evidence that the increased nickel came from Siberian volcanoes. Rothman agrees that current data cannot identify the source of the nickel.

“This is an interesting hypothesis, but I think that Great Dying was the doing of many ‘kill mechanisms’ rather than just a single mechanism suggested here,” Reichow said.

There is also doubt over the exact period in which Methanosarcina actually evolved. Current techniques for estimating its origins based on DNA sequence differences have a huge error margin, which means it could have been well before or after the Great Dying.

Rothman concedes that there are limitations. “We believe that volcanism alone could not have caused this extinction event. Instead, what we have done is broadened the conversation by suggesting that it is possible that microbes may have caused it to happen.”

“The implications for today are that there many ways in which natural fluctuations can happen in Earth’s carbon cycle. When studying the changes happening to the carbon cycle now, we should try to take into consideration as many of those as possible to make future predictions.”The Conversation

This article was originally published on The Conversation. Image credit tjt195 (CC-BY-NC).