Tag: allotrope

Massive asteroid may have kickstarted the movement of continents

Akshat RathiLeave a comment

Earth was still a violent place shortly after life began, with regular impactors arriving from space. For the first time, scientists have modelled the effects of one such violent event – the strike of a giant asteroid. The effects were so catastrophic that, along with the large earthquakes and tsunamis it created, this asteroid may have also set continents into motion.

The asteroid to blame for this event would have been at least 37km in diameter, which is roughly four times the size of the asteroid that is alleged to have caused the death of dinosaurs. It would have hit the surface of the Earth at the speed of about 72,000kmph and created a 500km-wide crater.

At the time of the event, about 3.26 billion years ago, such an impact would have caused 10.8 magnitude earthquakes – roughly 100 times the size of the 2011 Japanese earthquake, which is among the biggest in recent history. The impact would have thrown vapourised rock into the atmosphere, which would have encircled the globe before condensing and falling back to the surface. During the debris re-entry, the temperature of the atmosphere would have increased and the heat wave would have caused the upper oceans to boil.

AGU

Donald Lowe and Norman Sleep at Stanford University, who published their research in the journal Geochemistry, Geophysics, Geosystems, were able to say all this based on tiny, spherical rocks found in the Barberton greenstone belt in South Africa. These rocks are the only remnants of the cataclysmic event.

According to Simon Redfern at the University of Cambridge, there are two reasons why Lowe and Sleep were able to find these rocks. First, the Barberton greenstone belt is located on a craton, which is the oldest and most stable part of the crust. Second, at the time of the event, this area was at the bottom of the ocean with ongoing volcanic activity. The tiny rocks, after having been thrown into the atmosphere, cooling, and falling to the bottom of the ocean, then ended up trapped in the fractures created by volcanic activity.

This impact may have been among the last few major impacts from the Late Heavy Bombardment period between 3 and 4 billion years ago. The evidence of most of these impacts has been lost because of erosion and the movement of the Earth’s crust, which recycles the surface over geological time.

However, despite providing such rich details about the impact, Lowe and Sleep are not able to pinpoint the location of the impact. It would be within thousands of kilometres of the Barberton greenstone system, but that is about all they can say. The exact location may not be that important, Lowe argued: “With this study, we are trying to understand the forces that shaped our planet early in its evolution and the environments in which life evolved.”

One of the most intriguing suggestions the authors make is that this three-billion-year-old impact may have initiated the the movement of tectonic plates, which created the continents that we observe on the planet.

The continents ride on plates that make up Earth’s thin crust; the crust sits on top of the mantle, which is above a core of liquid iron and nickel. The heat trapped in the mantle creates convection, which pushes against the overlying plates.

All the rocky planets in our solar system – Mercury, Venus, Earth and Mars – have the same internal structure. But only Earth’s crust shows signs of plate motion.

A possible reason why Earth has moving plates may be to do with the heat trapped in the mantle. Other planets may not have as much heat trapped when they formed, which means the convection may not be strong enough to move the plates.

However, according to Redfern: “Even with a hot mantle you would need something to destabilise the crust.” And it is possible that an asteroid impact of this magnitude could have achieved that.The Conversation

First published on The Conversation.

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.