Archive for August, 2011
The Earth-Moon system is unique, and it believed that without our Moon life would not have had a chance to form. However how the Moon came to exist is a bit of a mystery. One theory is they formed togetherÂ each sweeping up material as the pair orbit the Sun. Yet another theory is a large impact on Earth tore away a nice sized chunk of debris that later evolved toÂ become our Moon. A new theory has been published in Nature Magazine in that a large Mars sized body struck the Earth and two large chunks appeared eventually coming together to form our Moon. This could explain why the side of the MoonÂ facing us has many low lying areas and the far side of the Moon has a thicker crust ans is more cratered. Below is the article from Nature.
Earth once had two moons, which merged in a slow-motion collision that took several hours to complete, researchers propose inÂ Nature today.
Both satellites would have formed from debris that was ejected when a Mars-size protoplanet smacked into Earth late in its formation period. Whereas traditional theory states that the infant Moon rapidly swept up any rivals or gravitationally ejected them into interstellar space, the new theory suggests that one body survived, parked in a gravitationally stable point in the Earthâ€“Moon system.
Several such ‘Lagrangian’ points exist, but the two most stable are in the Moon’s orbit, 60Â° in front or 60Â° behind.
Traces of this ‘other’ moon linger in a mysterious dichotomy between the Moon’s visible side and its remote farside, says Erik Asphaug, a planetary scientist at the University of California, Santa Cruz, who co-authored the study with Martin Jutzi, now of the University of Berne.
The Moon’s visible side is dominated by low-lying lava plains, whereas its farside is composed of highlands. But the contrast is more than skin deep. The crust on the farside is 50 kilometres thicker than that on the nearside. The nearside is also richer in potassium (K), rare-earth elements (REE) and phosphorus (P) â€” components collectively known as KREEP. Crust-forming models show that these would have been concentrated in the last remnants of subsurface magma to crystallize as the Moon cooled.
What this suggests, Asphaug says, is that something ‘squished’ the late-solidifying KREEP layer to one side of the Moon, well after the rest of the crust had solidified. An impact, he believes, is the most likely explanation.
“By definition, a big collision occurs only on one side,” he says, “and unless it globally melts the planet, it creates an asymmetry.”
Asphaug and Jutzi have created a computer model showing that the Moon’s current state can be explained by a collision with a sister moon about one-thirtieth the Moon’s mass, or around 1,000 kilometres in diameter.
Such a moon could have survived in a Lagrangian point long enough for its upper crust and that of the Moon to solidify, even as the Moon’s deeper KREEP layer remained liquid.
Meanwhile, tidal forces from Earth would have been causing both moons to migrate outward. When they reached about one-third of the Moon’s present distance (a process that would take tens of millions of years), the Sun’s gravity would have become a player in their orbital dynamics.
“The Lagrange points become unstable and anything trapped there is adrift,” Asphaug says. Soon after, the two moons collided. But because they were in the same orbit, the collision was at a relatively low speed.
“It’s not a typical cratering event, where you fire a ‘bullet’ and excavate a crater much larger than the bullet,” Asphaug says. “Here, you make a crater only about one-fifth the volume of the impactor, and the impactor just kind of splats into the cavity.”
Like A Pancake
In the hours after the impact, gravity would have crushed the impactor to a relatively thin layer, pasted on top of the Moon’s existing crust. “You end up with a pancake,” Asphaug says. The impact would have pushed the still-liquid KREEP layer to the Moon’s opposite side.
Apshaug’s theory isn’t the only attempt to explain the lunar dichotomy. Others have invoked tidal effects from Earth’s gravity, or convective forces from cooling rocks in the Moon’s mantle.
“The fact that the nearside of the Moon looks so different to the farside has been a puzzle since the dawn of the space age,” says Francis Nimmo, one of the authors of a 2010 paper inÂ Science proposing tidal forces as the cause.
But despite his competing model, Nimmo (a colleague of Asphaug’s at Santa Cruz, but not an author of the new study) calls the new theory “elegant”.
And Peter Schultz of Brown University in Providence, Rhode Island, calls it “interesting” and “provocative”, despite his own theory involving a high-angle collision at the Moon’s south pole, which he believes would have pressed crustal material northward to form the farside highlands.
“All this is great fun and tells us that there are very fundamental questions that remain about the Moon,” he says.
NASA’s upcoming GRAIL mission, designed to probe the Moon’s interior using precise measurements of its gravity, may help figure out what happened billions of years ago. “But in the end,” Schultz says, “new lunar samples may be necessary.”
Author: Richard Lovett
These two images, taken about a week apart by NASA’s Hubble Space Telescope, show four moons orbiting the distant, icy dwarfÂ planet Pluto. The green circle in both snapshots marks the newly discovered moon, temporarily dubbed P4, found by Hubble in June. P4 is the smallest moon yet found around Pluto, with an estimated diameter of 8 to 21 miles (13 to 34 km). By comparison, Pluto’s largest moon Charon is 648 miles (1,043 km) across. Nix and Hydra are 20 to 70 miles (32 to 113 km) wide. The new moon lies betweenÂ the orbits of Nix and Hydra, two satellites discovered by Hubble in 2005. P4 completes an orbit around Pluto roughly every 31 days.
Launched in March 2009, NASA’s Kepler observatory has become synonymous with the search forÂ Â Â Â Â Â Â extra-solar planets. But that’s not all it’s been doing up in space. The spacecraft has also been recording the gentle pulsations of starsâ€”the small variations in their brightness caused by sound waves throbbing outward from the stellar core to the surface (seen in the yellow star in illustration). In the latest issue of Science, researchersÂ report measuring these pulsations for some 500 sun-like stars, which isÂ Â Â Â Â Â Â enabling statistical studies of stellar characteristics like mass, radius and age and test models of stellar evolution. In another paper in the same issue, a different research team reports using Kepler data to detectÂ aÂ system of three stars, which includes a red giant star and two red dwarfs. Although astronomers thought that the red giant would show sun-like oscillations caused by waves from within, they found that the star’s pulsationsÂ Â Â Â Â Â Â were being driven by the waxing and waning of gravity from the orbital motion of the two red dwarfs. Researchers hope to use these observations to gain new insights into the formation of stellar systems, as well as the evolution of stars.
The first stars born in the universe are believed to have been massive objects, up to hundreds of times bigger than the sun. They were also spinning tops or “spinstars,” according to a new study, that spent their lives whirling at incredible speeds. If true, astronomers may one day be able to glimpse the final days of these early suns.
Astronomers have yet to catch a glimpse of the earliest stars, which formed some 300 million years after the Big Bang and burned out by the time the universe was 1 billion years old. Made entirely of hydrogen and helium, these stars produced heavier elements in the process of consuming their fuel and ultimately died in explosions that spewed out the newly forged elements into interstellar space. Those elements were incorporated into later generations of stars. By measuring the relative proportions of heavy elements in second- and third-generation stars, many of which have survived to this day, astronomers can make inferences about their now-extinct ancestors.
That’s exactly what a team led by Cristina Chiappini of the Leibniz Institute for Astrophysics in Potsdam, Germany, set out to do by analyzing the ratio of different elements in eight stars from NGC 6522, one of the oldest globular clusters in the Milky Way. The cluster is more than 12 billion years old, which means the stars that the researchers looked at formed only a few hundred million years after the death of first-generation stars. Studying the spectra of the stars, the researchers found unexpectedly high abundances of the heavy elements strontium (Sr) and yttrium (Y). Based on the other characteristics of the stars, it was evident that these two elements had not been made within the stars themselves but were likely present in the interstellar clouds from which the stars originated.
The researchers knew from theory that rare elements such as Sr and Y are forged at higher rates in rotating stars as a result of mixing between outer and inner layers of gas within the star. The nuclear reactions that result from this gas mixing produce a large supply of neutrons that are captured by the nuclei of heavy elements such as iron to make Sr and Y. Chiappini and her colleagues found that the best way to explain the pattern of abundances they had observed was to apply a stellar model involving a spinning velocity of 500 kilometers per second at the surface. In other words,Â the ancestral stars that spawned the eight stars observed in the study were likely to have been spinning at that velocity, the authors report online today in Nature. That’s 250 times as fast as the sun.
If the first stars were indeed rapid spinners, they are likely to have ended their lives with a huge Gamma Ray Burst (GRB), producing an enormous flash of high-energy radiation. That augurs well for astronomers hoping to watch the first stars in the act of dying. “I think we have little hope ofÂ detecting individual first stars directly in the distant universe, but GRBs can be seen much further away than individual stars,” says Jason Tumlinson, an astronomer at Space Telescope Science Institute in Baltimore, Maryland. “A higher frequency of bursts increases the chances of seeing the firstÂ generations directly,” he says.
Volker Bromm, a theoretical astrophysicist at the University of Texas, Austin, says rapid rotation “could also lead to a special class of energeticÂ supernova explosions called hypernovae, with unusual chemical abundance patterns.” And, he says, high rates of spinning would induce deep mixing of currents inside the star, causing the star to evolve into a chemically homogeneous object. Future missions such as the Joint Astrophysics Nascent Universe Satelliteâ€”a small explorer mission being considered by NASAâ€”could give astronomers their first look at gamma-ray bursts produced by these first-generation stellar objects.
Science Magazine: by Yudhijit Bhattacharjee