Monday, September 21, 2009

A Mirror of Our Creation?

In a planetary system only 10 light years away, Spitzer has discovered that there is much more to Epsilon Eridani then a great setting for an Asimov novel. Epsilon Eridani is the star at the center of the planetary system closest to home. This Star is relatively young, perhaps less than a billion years old, and has a mass which is about .85 times the mass of the sun. As far as atomic creation goes, this sun is relatively inactive, producing not much more than Helium.

This system has been a source of great discovery; in the past it has been found to host two planets, an asteroid belt which orbits the star at a similar distance to which our asteroid belt orbits the Sun, and distant ring of dust and ice which is very similar to our Kuiper Belt. Recently, Spitzer observed that there is not just one, but two asteroid belts orbiting the not so distant star. What makes this discovery so exciting is the idea that by observing this system, we are basically looking back in history to observe our own creation.

According to the Nebular Hypothesis, solar systems are formed because massive clouds of dust and helium condense to form stars. This condensing occurs because these clouds are gravitationally unstable, so they collapse inwards into smaller clumps which accumulate to form a star, such as our Sun or Epsilon Eridani. As this star forms it sheds a disk of matter which over time begins to accumulate and form protoplanets. Although planetary formation is not well understood, it is thought by some that because of the gravitational pull of the forming star, the dense accumulating rock stays closer to and orbits the star, while the less dense gasses are able to stay further from the star in their orbit. This could be why the terrestrial planets such as Earth are closer to the Sun, while the gas giants such as Jupiter form much further out (of course there have been recent discoveries of gas giants closer to the sun than even Mercury).

Some theorize that the asteroid belts in our solar system are the result of the tidal forces produced by the gravitational pulls of the Sun and the gas giants. These tidal forces keep the rocks in the asteroid belt from coalescing to form protoplanets. The gas giants might also have another roll in solar system formation. It is possible that that the gas giants sweep out asteroids as they rotate the star, protecting the terrestrial planets from catastrophic impacts. However, some scientists also believe that the gas giants could act has a gravitational sling shot which could attract and hurl asteroids into the inner solar system.

One of the planets discovered to orbit Epsilon Eridani is located about 3.5 Au’s from the star, just outside of the range of the newly discovered asteroid belt. This is the first time a planetary system has been discovered to have an arrangement which is comparable to Jupiter and our asteroid belt.

Does this discovery prove that our solar system was formed with agreement to the Nebular Hypotheses? No, but it is defiantly worth observing this relatively young star system to see if its evolution correlates at all with any of our ideas. Who knows, maybe we could even watch as the formation of an Earthlike planet unfolds before our very eyes.

Monday, September 14, 2009

Plate tectonics may have begun 4.4 billion years ago

A new study suggests that the Earth’s tectonic activity may have begun as many as 4.4 billion years ago. The evidence stems from tiny minerals called zircons found in rocks of the Jack Hills region of Western Australia. Zircons, or zirconium silicate (ZrSiO4), are amazing minerals because of the fact that they are very widespread, and can exist in igneous, sedimentary, or metamorphic rocks.

By analyzing tiny mineral inclusions found inside seven of the zircon crystals found in Western Australia ( seven out of 400 found) scientists were able to determine that there was tectonic activity in the earliest eon of our planet, the Hadean. These inclusions allowed the scientist to determine the temperature and pressures at which the zircons formed. Six of the seven bits of zircon contained inclusions composed of the mineral muscovite (KAl2(AlSi3O10)(OH)2). The Silicon to Aluminum ratio in these muscovite inclusions suggest that the rocks formed at depths of about 25 km beneath the Earth’s surface. Because of the amount of Titanium atoms present in the zircons, the scientists were able to determine that temperature of crystallization was between 665 and 745 degrees Celsius. The seventh inclusion consisted of a mineral known as hornblende ( (Ca,Na)2-3(Mg,Fe,Al)3Si6(Si,Al)2O22(OH)2) ). After analyzing the hornblende inclusions, (using methods similar to the above methods), scientists were able to confirm the determined results of the muscovite. However, because this discovery is based only on seven samples, there is some healthy criticism.

These temperatures and pressures indicate that the temperature flux during the zircon crystallization was approximately 75 mW/m2. This flux is slightly higher than what is observed on Earth today. Because the Earth was so much hotter during its first six hundred million years, a higher paleo-flux is expected. However, the calculated flux was also determined to be about 1/5 lower than the expected flux of the hadean eon. It is because of this abnormally lower than average flux of the hadean eon zircons, that it was determined that the plate tectonics had to have begun so early in Earth’s history.

On Earth today, fluxes much lower than average occur above subduction zones, where one plate subducts beneath another. It is hypothesized that these zircons were formed as the descending plate subducted, bringing liquid water with it, where it cooled the surrounding mantle enough for the zircons, and the inclusion minerals, to crystallize out of solution. Zircon contains uranium isotopes, which allowed the year of this crystallization to be calculated using radiometric dating techniques.

This could be an important discovery because it will help us understand the evolution of terrestrial planets. Plate tectonics play a very important role in recycling the gasses which make up our atmosphere, and therefore directly affect the ability of a planet to sustain life. With the right atmospheres, Venus and Mars could have been within the habitable zone of our solar system, however neither planet is believed to have developed plate tectonics.

Besides providing clues to the development of plate tectonics, the zircons also contain oxygen isotopes that suggest that water was also present on the Earth some 4.4 billion years ago. These Western Australian zircons are the oldest minerals on Earth, and have provided us with great insight into the dawning hours of our planet.

Thursday, September 10, 2009

Large deposits of wate-ice found on Mars

On November 20th, 2008, NASA confirmed that the Mars Reconnaissance Orbiter has discovered ancient glaciers of water-ice preserved under a layer or dust and rock. These subsurface glaciers are located at altitudes much lower than any previously discovered layers of ice, and also contain more water-ice than any other region on Mars, including the poles.

These glaciers were discovered underneath a formation that had been puzzling geologists for years. These formations are known as “aprons” because of the way they gently slope upwards. The glaciers were discovered after ground penetrating radars were pointed at these “aprons” because the radio waves were reflected without a significant loss of energy shortly after penetrating the surface. The radio waves that are not reflected travel through these formations with an apparent velocity, which is consistent with the composition of water-ice.

Hundreds of these apron-like features are located in latitude bands between 35 and 60 degrees on either side of the Martian equator. They are also commonly located beneath cliffs and are typically tens of kilometers long, and may be the remnants of a giant ice sheet, which may have at one time engulfed these mid-altitude regions. Many scientists believe that Mars was once tilted in such a fashion that the poles pointed toward the Sun, leaving the mid-latitude regions in a much cooler climate. This discovery offers proof to this hypothesis.

Studying these ice-sheets could help us understand processes that effect climate change, which is a poorly understood phenomenon, here on Earth. Our last glacial Maximum occurred about 20,000 years ago, when much of the North American and Eurasian continents were covered in an ice sheet over 3 kilometers thick. There are many factors which are thought to cause the onset of an ice age, such as: changes in the atmosphere, tectonic geography, variation in the Earth’s orbit, and variations in solar energy. The changes in the atmosphere which effect the onset of glaciers is not well understood, although, there is some proof that CO2 levels shrink during the onset of an ice age, and increase during interglacial periods. The effects of increased CO2 on the climate have long been a subject of great debate. Tectonic geography affects the onset of ice ages by arranging the continents in such a way that they prevent the flow of warm water from the equator to the poles; this allows the formation of ice sheets. These ice-sheets increase the planets albedo, which decreases the amount of solar radiation, which is absorbed. This decrease in absorbed energy allows the ice sheets to expand. There are three known configurations which block or reduce this flow of warm water--two of which exist today. A continent sits on top of a pole, such as Antarctica. Or a polar sea, such as the Arctic Ocean, is nearly land-locked. The third configuration consists of a single mass continent which covers much of the equator. Such a mass continent existed between 850 and 635 million years ago during the Cryogenian period, and was called Rodinia. Variations in the Earth’s orbit, called Milankovitch cycles, suggests that major ice ages occur every 100,000 years due to periodic changes in Earth’s eccentricity, axial tilt, and orbital periods; however, how these variations effect the climate are not well understood.

The discovery of these large volumes of water-ice on Mars will be very important to the future colonization, and manned missions to mars. This ice will serve as drinking water, and as a source of energy, which will be used in hydrogen fueled vehicles. This will defiantly reduce costs for such future missions because fewer supplies will have to be shipped to the red planet.

On Earth, such buried glaciers in Antarctica preserve traces of ancient organisms. So these Martian glaciers might also serve as a place to look for fossil evidence of past life on Mars. In addition to fossil life, localized heating due to volcanism may have melted some ice, which could provide an environment for microorganisms to evolve.

The discovery of subsurface glaciers on Mars will help us understand the processes which evoke climate change on Earth, provide a place to gather food and fuel for future missions to Mars, and could be one of the best places to look for signs of ancient organisms. Indeed, these will be places which will be thoroughly explored by rovers, Landers, and eventually, by mankind.

Wednesday, September 2, 2009

Could life exist on Super-Earths?

The search for extraterrestrial life within our solar system has mainly been focused on Mars, and there has been speculation that some the moons of the outer solar system may also be a good place to look for life. Outside of our solar system, planet hunters and astrobiologists have been searching for Earth-like planets to help answer one of mankind’s most profound questions, “are we alone?” To date, no such planets have been discovered, so a team of scientists have now set their sights on a relatively abundant group of extrasolar planets known as “super-Earths”.

The term “super-Earth” is slightly misleading because the only thing that these planets have in common with the Earth is the fact that they are terrestrial. A super-Earth is typically classified as a terrestrial planet with a mass of 5 to 10 Earth masses. Thus far, Super Earths have not been found within the habitable zone of their host star, with orbits much too far or much too close to sustain life as we know it. The super-Earths with orbits far from their host star are the places that astrobiologists now believe could harbor some form of life.

It is estimated that one-third of all solar systems contain super-Earths, and some scientists believe that it may be possible to find some that have liquid water either on the surface, or below a thick layer of ice. This water could theoretically exist on a super-Earth if one of three conditions were met. 1) If the planet had a thick enough atmosphere it may be possible that enough solar radiation could be by greenhouse gases to prevent water from completely freezing. 2) If the planet was massive enough or young enough, there may still be enough primordial heat available to sustain some amount of liquid water.

Currently, the best technique for discovering super-Earths is by using gravitational microlensing. This phenomena occurs when an object in the foreground has enough mass, its gravitational field will bend the incoming light of a much more distant object. This results in the magnification of the distant object, no matter how faint it may seem.

It is not unfathomable to predict that an extrasolar super-Earth outside of its host stars habitable zone could contain water, at least as ice. Much of the ice in our own solar system is located outside of the habitable zone. There is no super-Earth in our solar system, but there are icy bodies that could contain liquid oceans. It is hypothesized that Jupiter’s moon, Europa, may have enough heat due to tidal flexing to permit a liquid ocean.

Traveling amongst the stars and exploring extrasolar planets is unfortunately not in the near future, but we can test hypothesis such as this one by exploring the planets within our solar system, and isn’t it about time we send a probe to Europa?