Massive asteroid may have kickstarted the movement of continents

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.

The only reason zebras have stripes is to ward off flies

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.

New dwarf planet found sneaking through the inner Oort cloud

A new, planet-like body has been found on the outer edges of the solar system. This object, called 2012VP113, is the second body of its class found since the identification of the dwarf planet Sedna in 2003. It joins an exclusive club composed of some of the strangest objects in the solar system.

The observable solar system can be divided into three regions: the rocky planets including the Earth and asteroids of the inner solar system, the gas giant planets, and the icy Kuiper Belt objects, which include Pluto. The Kuiper belt stretches from beyond Neptune, which is at 30 astronomical units (one astronomical unit, AU, represents the distance between the Earth and the sun), to about 50AU.

Sedna and 2012VP113 are strange objects, because they reside in a region where there should be nothing, according to our theories of the solar system formation. Their orbit is well beyond that of Neptune, the last recognised planet of the solar system, and even beyond that of Pluto, which differs from planets because of its size, unusual orbit, and composition. (Pluto, once considered a planet, is now considered the lead object of a group of bodies called plutinos.)

The closest Sedna, which is 1000km-wide, gets to the sun is about 76AU and for 2012VP113, which is 450km-wide, that distance is 80AU. Their orbits are also at weird inclinations compared to most other solar system objects.

The results of the discovery have been published in Nature. Chadwick Trujillo of Gemini Observatory in Hawaii, who was also involved in finding Sedna, and Scott Shepherd of the Carnegie Institution for Science, who found 2012VP113 with Trujillo, propose that these objects are members of the inner Oort cloud.

The Oort cloud is a hypothetical region that is thought to stretch outwards beyond the Kuiper belt. Beyond 5000AU, the Oort cloud expands out into a sphere centred on the sun. We have no direct evidence that the Oort cloud exists, but indirect evidence comes in the form of comets with extremely elongated orbits.

Oort_Cloud

Stephen Lowry at the University of Kent said: “The orbital properties of these two objects are so very different from that of the Kuiper belt objects that it wouldn’t be wrong to suggest they may be part of the inner Oort cloud.”

The fact that these objects exist is remarkable, since they exist in a region where material is thought to have been too sparse for them to form. Current thinking is that they actually formed in the giant-planet region, and that their orbits may carry the signature of whatever events caused them to scatter to such distances. It is hoped that this discovery will lead efforts to find other objects.

David Rothery of Open University said: “This is a remarkable discovery, but it is not entirely surprising. When they found Sedna, there was hope that they would find others in that region.”

But the fact that it took Trujillo, who was involved in the original team that found Sedna, more than ten years to find Sedna’s neighbour speaks to the challenge of discovery. “The farther you get from the sun, the less sunlight falls on these objects, which makes the task of locating them harder,” Lowry said.

“Worse still,” Lowry continued, “the eccentric orbits of these objects means that there is very tiny window in which they can be observed from even the most powerful telescopes on Earth. What is needed to find these objects is not just technology but persistence.” For example, Sedna gets as close as 76AU away from the sun, but at its farthest it is nearly 1000AU. Its orbital period is about 11,400 years, which means it spends lots of time too far out to be detected.

While 2012VP113 and Sedna provide some information about the inner Oort cloud, to say any more, scientists are going to need more than two data points. Next generation instruments such as the Subaru telescope in Hawaii and Large Synoptic Survey Telescope in Chile may hold the answers.The Conversation

First published on The Conversation. Images by NASA.