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Entries in Sistema Solar (13)

Saturday
Feb232013

NASA Deciphering the Mysterious Math of the Solar Wind

A constant stream of particles and electromagnetic waves streams from the Sun toward Earth, which is surrounded by a protective bubble called the magnetosphere. A scientist at NASA Goddard has recently devised, for the first time, a set of equations that can help describe waves in the solar wind known as Alfven waves. Credit: European Space Agency (ESA). › View larger
Many areas of scientific research -- Earth's weather, ocean currents, the outpouring of magnetic energy from the Sun -- require mapping out the large scale features of a complex system and its intricate details simultaneously.

Describing such systems accurately, relies on numerous kinds of input, beginning with observations of the system, incorporating mathematical equations to approximate those observations, running computer simulations to attempt to replicate observations, and cycling back through all the steps to refine and improve the models until they jibe with what's seen. Ultimately, the models successfully help scientists describe, and even predict, how the system works.

Understanding the Sun and how the material and energy it sends out affects the solar system is crucial, since it creates a dynamic space weather system that can disrupt human technology in space such as communications and global positioning system (GPS) satellites.

However, the Sun and its prodigious stream of solar particles, called the solar wind, can be particularly tricky to model since as the material streams to the outer reaches of the solar system it carries along its own magnetic fields. The magnetic forces add an extra set of laws to incorporate when trying to determine what's governing the movement. Indeed, until now, equations for certain aspects of the solar wind have never been successfully devised to correlate to the observations seen by instruments in space. Now, for the first time, a scientist at NASA's Goddard Space Flight Center in Greenbelt, Md., has created a set of the necessary equations, published in Physical Review Letters on Dec. 4, 2012.

"Since the 1970s, scientists have known that movement in the solar wind often has the characteristics of a kind of wave called an Alfvén wave," says Aaron Roberts, a space scientist at Goddard. "Imagine you have a jump rope and you wiggle one end so that it sends waves down the rope. Alfvén waves are similar, but the moving rope is a magnetic field line itself."

The Alfvén waves in this case tended to have great consistency in height -- or amplitude, which is the common term when talking about waves -- but they are random in direction. You might think of it like a jump rope twirling, always the same distance from center, but nonetheless able to be in many places in space. Another way scientists have envisioned the waves is as a "random walk on a sphere." Again, always the same distance from a given center, but with a variable placement.

Such metaphorical descriptions are based on what instruments in space have, in fact, observed when they see magnetic waves go by in the solar wind. But it turns out that the equations to describe this kind of movement -- equations necessary to advance scientific models of the entire system -- were not easily found.

"The puzzle has been to figure out why the amplitude is so constant," says Roberts. "But it's been very difficult to find equations that satisfy all the characteristics of the magnetic field."

Similar waves are, in fact, seen in light, known as polarized waves. But magnetic fields have additional constraints on what shapes and configurations are even possible. Roberts found a way to overlap numerous waves of different wavelengths in such a way that they ultimately made the variation in amplitude as small as possible.

To his surprise, the equations Roberts devised matched what was observed more closely than he'd expected. Not only did the equations show waves of constant amplitude, but they also showed occasional random jumps and sharp changes -- an unexplained feature seen in the observations themselves.

"Overlapping the waves in this way gives us a way of writing down equations that we didn't have before," says Roberts. "It also has this nice consequence that it is more realistic than we expected, since it shows discontinuities we actually see in the wind. This is important for simulations and models where we want to start with initial conditions that are as close to the observed solar wind as we can get."

Of course, having an equation doesn't yet tell us the reason why the waves in the solar wind are shaped in this way. Nonetheless, equations that describe how the waves move open the door to increasingly accurate simulations that may well help explain such causes. By alternately improving models and improving observations, scientists continue the cyclic nature of such research, until just what physical action on the sun causes these curiously-shaped Alfvén waves someday becomes clear.

 

Saturday
Feb232013

Icy Titan Spawns Tropical Cyclones

Object: Mini-hurricanes of methane rain
Location: North Pole of Saturn's moon Titan

With a maximum surface temperature of -180 °C, Saturn's icy moon Titan is no tropical paradise, at least by earthly standards. But it may still have tropical cyclones, and at what sounds like the unlikeliest of places – near its north pole.

These mini-hurricanes have never been observed anywhere but Earth. If they exist on Titan, that would add to a growing list of features that the distant moon shares with our planet, from lakes, hills, caves and sand dunes to fog, mist, smoggy haze and rain clouds.

Though cyclones - a large family of storms in which winds spiral inward to a low-pressure zone, such as the eye of a hurricane or tornado – have been glimpsed on Mars and Saturn, a tropical cyclone is a special case that is driven by the heat of evaporation from a warm sea. These storms involve a lot of rain as well as gale-force winds, and have not been glimpsed anywhere but Earth.

As Titan is the only body in the solar system apart from Earth known to have liquid on its surface and, therefore, rain (Titan is so cold that its rain is in the form of liquid methane, not water), Tetsuya Tokano of the University of Cologne in Germany decided to calculate what it would take for Titan to have its own mini-hurricanes.

Methane seas

The first thing that would be required, he says, is the right blend of hydrocarbons in the moon's lakes and seas. "We know ethane is present, and methane probably is," he says. The methane is crucial because it evaporates much more readily, and could deliver the heat needed to drive the storm.

Assuming the methane fraction is large enough, Tokano calculated the heat it would carry and how that would be converted into kinetic energy to power a storm. He reckons that the resulting storm would not be as powerful as hurricanes or typhoons on Earth, but that they could produce surface winds of up to 20 metres per second (72 kilometres per hour). That's 10 times the average wind velocity on Titan: on Earth, it's equivalent to the wind speeds of a midsize tropical storm – and two-thirds those needed for a full-scale hurricane.

Tokano also looked at where these could storms could form – and discovered that the 1200-kilometer-long Kraken Mare, and the smaller Ligeia and Punga mares, are the only seas on Titan large enough to support the growth of a tropical cyclone. All three are situated near Titan's North Pole, making a contrast to the tropical cyclones on Earth.

As on Earth, however, any mini-hurricanes on Titan would be seasonal. Tokano says the storms could form in Titan's northern summer, lasting up to 10 days and reaching hundreds of kilometres in diameter, limited by the size of the lakes.

Spectacular storm

It's now spring on northern Titan, and solar warming of the north pole should make the storms possible from 2015 to 2021. (Because Titan is so much further away from the sun than Earth, its year – and therefore its seasons - are much, much longer.)

That means that when mini-hurricane season next returns to Titan, the Cassini spacecraft, which started orbiting it in 2004, will still be watching. The craft's orbit gives it a better view of Titan's poles than terrestrial telescopes, and its mission is scheduled to continue until 2017.

"It would be spectacular to see this kind of storm over Kraken Mare," says Elizabeth Turtle of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. "This gives us a specific type of storm to look for."

Failure to spot a storm during this period would not tell us much, however, says Tokano, as any of a number of factors might cause Cassini to miss it, or it might just be a slow season.

Because of its similarities to Earth, Titan looks like a good place to hunt for extra-terrestrial life. Though Tokano wouldn't be drawn on how tropical cyclones might feed into this picture, one thing is clear: the frigid moon is certainly living up to its reputation as one of the most intriguing places in our solar system.

Source: http://www.newscientist.com/article/dn23209-astrophile-icy-titan-spawns-tropical-cyclones.html

Saturday
Feb232013

Rare Asteroid Observed with Comet-like Dust Trail

Spanish researchers have observed a rare asteroids from the Gran Telescopio Canarias (Spain) and have discovered that something happened around the 1st July 2011 causing a comet-like trail to appear: maybe internal rupture or collision with another asteroid. To date, ten asteroids have been located to date that at least at one moment have displayed a trail like that of comets. They are named main-belt comets (MBC) as they have a typical asteroidal orbit but display a trail at the same time, which means that their dust (and possibly gas) emission activity is similar to that of comets.

One of these objects, baptised as P/2012 F5 (Gibbs), was discovered in March 2012 from the Mount Lemmon Observatory in Arizona (USA). In May and June of that year, Spanish astrophysicists from the Gran Telescopio Canarias tracked it and have discovered when the trail was born using mathematic calculations.

"Our models indicate that it was caused by an impulsive short-lived event lasting just a few hours around the 1st July, 2011, with an uncertainty of 20 days," as explained to SINC by Fernando Moreno, researcher at the Astrophysics Institute of Andalusia (CSIC). In collaboration with other colleagues from the Astrophysics Institute of the Canary Islands and the University of La Laguna, the data have been published in the 'The Astrophysical Journal Letters'.

The telescope images reveal "a fine and elongated dust structure that coincides exactly with the synchrone of that day," says Moreno. For a given observation date, a synchrone is the position in the sky of the particles emitted from these types of objects with zero speed in an instant of time. In this case the synchrone of the 1st July, 2011 is the best adapted to the fine trail.

The width and varying brightness of the head to the end of the trail allowed for the researchers to deduce the physical properties of the particles and proportions of their different sizes.* As for the maximum size and speed values of the liberated particles, the team has calculated that the asteroid should have a radius of between 100 m and 150 m and the dust mass emitted should be about half a million tonnes.

Researchers juggled two possible theories for the origin of the P/2012 F5 trail: "It could have arisen from collision with another asteroid or rather a rotational rupture." The second mechanism consists of material gradually breaking free after partial fragmentation of the asteroid.

In turn, the rapid spinning of the asteroid, "like an accelerating carousel" causes pieces to break off. The rotation speed of small asteroids can increase over time due to the Yarkovsky effect (YORP effect for short). This can induce acceleration due to the thermal differences of the different surface regions of the asteroid, eventually leading to rupture.

Moreno indicates that, from the brightness distribution of the trail, "we have verified that the dependence of the speed of particle ejection on size is very weak, in accordance with what we already obtained for the other asteroid of this group: 596 Scheila, which probably suffered a collision."

MBCs are main-belt asteroids situated at a distance of between 2 and 3.2 astronomical units, which is the average distance between the Earth and the Sun. For some reason they become active and emit dust. For now they have not been found to generate gas but this could be due to the fact that they are weak at the very moment of observation.

Since the first discovery of an MBC in 1996, the 133P/Elst-Pizarro, a total of ten have now been found. The presence of a trail in some has lasted for a relatively long period of a few months, like in the cases of 2006 VW139 and P/2010 R2 (La Sagra). The latter was discovered from an observatory of the same name in Granada. Its activity could have been due to an ice sublimation which could have released the gas, although this has not been detected.

In other cases, however, activity developed during a short period of time, like in the case of 596 Scheila. Its dust cloud dissipated very quickly in a matter of hardly three or four weeks following its detection.

There are also examples of MBCs that have shown recurrent activity, like 133P/Elst-Pizarro and 238P, which have displayed a trail on more than one occasion.

In the case of P/2012 F5, it is still unknown what group it belongs to. More data will be available when it can be observed again in good conditions next year in around July or August 2014.

The last documented MBC to date is the so-called P/2012 T1 (PANSTARRS), which Spanish astrophysicist are also analyzing. Similar to what has happened with exoplanets, many more main-belt comets will start appearing in the coming years.

Source: http://www.dailygalaxy.com/my_weblog/2013/02/rare-asteroid-observed-with-comet-like-dust-trail-.html

Sunday
Jan272013

Titan's disappearing craters

A relatively "fresh" crater called Sinlap (left) and an extremely degraded crater called Soi (right). Image credit: NASA/JPL-Caltech/ASI/GSFC

New reserarch using observations from NASA's Cassini spacecraft suggest that Saturn's largest moon Titan may look much younger than it really is because its craters are getting erased as dunes of exotic, hydrocarbon sand are slowly but steadily filling in the craters.

"Most of the Saturnian satellites, Titan's siblings, have thousands and thousands of craters on their surface. So far on Titan, of the 50 percent of the surface that we've seen in high resolution, we've only found about 60 craters," said Catherine Neish, a Cassini radar team associate based at NASA's Goddard Space Flight Center. "It's possible that there are many more craters on Titan, but they are not visible from space because they are so eroded. We typically estimate the age of a planet's surface by counting the number of craters on it (more craters means an older surface). But if processes like stream erosion or drifting sand dunes are filling them in, it's possible that the surface is much older that it appears."

Neish and her team compared craters on Titan to craters on Jupiter's moon Ganymede. Ganymede is a giant moon believed to have a water ice crust, similar to Titan, so craters on the two moons should have similar shapes. However, Ganymede has almost no atmosphere and thus no wind or rain to erode its surface.

This research is the first quantitative estimate of how much the weather on Titan has modified its surface

Titan on the other hand is the only moon in the solar system with a thick atmosphere, and the only world besides Earth known to have lakes and seas on its surface. However, with surface temperatures of around minus 290 degrees Fahrenheit (94 kelvins), the rain that falls on Titan is not water but liquid methane and ethane, compounds that are normally gases on Earth.

"This research is the first quantitative estimate of how much the weather on Titan has modified its surface," said Neish.

The source of Titan's methane remains a mystery as methane in the atmosphere is broken down over relatively short timescales by sunlight. Fragments of methane molecules then recombine into more complex hydrocarbons in the upper atmosphere, forming a thick, orange smog that hides the surface from view. Some of the larger particles eventually rain out on to the surface, where they appear to get bound together to form the sand.

"Since the sand appears to be produced from the atmospheric methane, Titan must have had methane in its atmosphere for at least several hundred million years in order to fill craters to the levels we are seeing," says Neish. However, researchers estimate Titan's current supply of methane should be broken down by sunlight within tens of millions of years, so Titan either had a lot more methane in the past, or it is being replenished somehow.

The difference in depth between craters on Titan and Ganymede is best explained by filling from windblown sand, although erosion from liquids and viscous flow might contribute to the modification of Titan's craters. The team thinks these considerations need further investigations.  A paper about this research was published online in the journal Icarus December 3, 2012.

Source: http://www.sen.com/news/titan-s-disappearing-craters.html

Tuesday
Dec182012

Did a Massive Supernova Explosion Trigger the Formation of Our Solar System?

A new study published by University of Chicago researchers challenges the notion that the force of an exploding star prompted the formation of the solar system. In this study, published online last month in Earth and Planetary Science Letters, authors Haolan Tang and Nicolas Dauphas found the radioactive isotope iron 60—the telltale sign of an exploding star—low in abundance and well mixed in solar system material. As cosmochemists, they look for remnants of stellar explosions in meteorites to help determine the conditions under which the solar system formed. Some remnants are radioactive isotopes: unstable, energetic atoms that decay over time. Scientists in the past decade have found high amounts of the radioactive isotope iron 60 in early solar system materials.

"If you have iron 60 in high abundance in the solar system, that's a 'smoking gun'—evidence for the presence of a supernova," said Dauphas, professor in geophysical sciences. Iron 60 can only originate from a supernova, so scientists have tried to explain this apparent abundance by suggesting that a supernova occurred nearby, spreading the isotope through the explosion.

But Tang and Dauphas discovered that levels of iron 60 were uniform and low in early solar system material. They arrived at these conclusions by testing meteorite samples. To measure iron 60's abundance, they looked at the same materials that previous researchers had worked on, but used a different, more precise approach that yielded evidence of very low iron 60.

Previous methods kept the meteorite samples intact and did not remove impurities completely, which may have led to greater errors in measurement. Tang and Dauphas' approach, however, required that they "digest" their meteorite samples into solution before measurement, which allowed them to thoroughly remove the impurities. This process ultimately produced results with much smaller errors.

"Haolan has dedicated five years of very hard work to reach these conclusions, so we did not make those claims lightly. We've been extremely careful to reach a point where we're ready to go public on those measurements," Dauphas said.

To address whether iron 60 was widely distributed, Tang and Dauphas looked at another isotope of iron, iron 58. Supernovae produce both isotopes by the same processes, so they were able to trace the distribution of iron 60 by measuring the distribution of iron 58.

"The two isotopes act like inseparable twins: Once we knew where iron 58 was, we knew iron 60 couldn't be very far away," Dauphas explained. They found little variation of iron 58 in their measurements of various meteorite samples, which confirmed their conclusion that iron 60 was uniformly distributed.

To account for their unprecedented findings, Tang and Dauphas suggest that the low levels of iron 60 probably came from the long-term accumulation of iron 60 in the interstellar medium from the ashes of countless stars past, instead of a nearby cataclysmic event like a supernova. If this is true, Dauphas said, there is then "no need to invoke any nearby star to make iron 60."

However, it is more difficult to account for the high abundance of aluminum 26, which implies the presence of a nearby star. Instead of explaining this abundance by supernova, Tang and Dauphas propose that a massive star (perhaps more than 20 times the mass of the sun) sheds its gaseous outer layers through winds, spreading aluminum 26 and contaminating the material that would eventually form the solar system, while iron 60 remained locked inside the massive star's interior. If the solar system formed from this material, this alternate scenario would account for the abundances of both isotopes.

"In the future, this study must be considered when people build their story about solar system origin and formation," Tang said..

The image at the top of the page shows massive stars that exploded as supernovae, creating superbubbles in the surrounding gas. X-Ray (blue), Optical (yellow/green), Infrared (red) composite of N44 in the Large Magellanic Cloud

For more information: "Abundance distribution, and origin of 60Fe in the solar protoplanetary disk," by Haolan Tang and Nicolas Dauphas, Earth and Planetary Science Letters, December 2012. Journal reference: Earth and Planetary Science Letters

Source: http://www.dailygalaxy.com/my_weblog/2012/12/did-the-explosion-of-a-massive-alien-star-trigger-the-formation-of-our-solar-system.html