Tag Archives: planets

Saturn’s rings get spontaneously shaken up

From far away Saturn’s rings look pretty solid – I’m sure I’m not the only person who, as a child, imagined it’d be possible to skate around the planet on them. In reality, though, they’re made up of millions and millions of bits of ice and dust, ranging in size from micrometres to metres. Until recently, scientists thought that the occasionally odd behaviour of the most massive ring, known as the B ring, was solely due to the pull of one of Saturn’s moons, Mimas. However, new research published in the December issue of the Astronomical Journal explains that Mimas is not the only reason for the variations that we see in this ring…

Saturn as seen by the Cassini Orbiter. Image: NASA

Joseph Spitale and Carolyn Porco from the Space Science Institute at Boulder, Colorado looked at four years worth of images of Saturn’s rings from the Cassini mission. They saw evidence of wave patterns in the B ring that seemed to have arisen spontaneously – without being forced by Mimas. The waves are thought to come about because of the high density of the B ring, and are given a boost by its sharp edge which reflects and amplifies the waves. Spitale and Porco also found small moons, known as “moonlets”, near the outer edge of the B ring.

Cassini image of Saturn's B ring, taken in 2009. Image: NASA

The small chunks of ice and dust that make up Saturn’s rings may be left over from the formation of the planet itself, or could be all that is left of a moon that strayed too close to its parent and got broken up by Saturn’s gravity.* Either way, these new findings show that the rings are anything but the static bands of ice we sometimes imagine them to be, and that their motion doesn’t even always come from outside influences.

But these findings don’t just tell us about the behaviour of Saturn’s rings. They also offer insight into other systems in the universe that may have similar oscillations, such as spiral galaxies and protoplanetary disks. This is an example of one of the amazing things about physics. By observing something close to us, we can learn about the behaviour of systems on the other side of the universe.

*There’s something known as the Roche limit that dictates how close a moon can get to its planet before it’s broken up by tidal forces caused by the planet itself.

References:
Joseph N. Spitale, & Carolyn C. Porco (2010). Free Unstable Modes and Massive Bodies in Saturn’s Outer B Ring Astron.J.140:1747-1757,2010 arXiv: 0912.3489v2

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Solar system might be older than we thought…

Researchers from Arizona State University have found the oldest solar system object ever discovered. In fact, it’s so old that it formed up to two million years before the solar system did, according to current estimates. It might be time for a rethink of when and how our little place in the Universe came into existence…

Planets and dwarf planets in the solar system. Sizes to scale, distances (obviously) not. Image: NASA

Coming up with a successful model for the formation of the solar system is not an easy task. Such a model must explain everything we know about the solar system today, from the fact that all the planets revolve the same way around the Sun and in the same plane, to the composition of the planets themselves.

The most generally accepted model is the Solar Nebula Disk Model (SNDM), which is a modern variant of the Nebular Hypothesis originally put forward by Laplace and Swedenborg in the 16th Century. In the SNDM, stars form in huge rotating clouds of molecular hydrogen. Our own Sun started out its life as a proto-star in one of these clouds, and formed when a small part of the cloud underwent gravitational collapse. Most of the collapsing mass went into the formation of the proto-Sun, with the rest making the protoplanetary disk that surrounded it. Next came planetesimals, which are believed to be the starting point of planets. It is thought that they grow when bits of material in the disk stick together after collisions, and once they reach a certain size, around a kilometer across, gravity takes over and they attract more and more mass. Not all planetesimals become fully-fledged planets; only the largest are able to survive long enough to make it.

When our solar system was evolving, the planetesimals that didn’t get swept up to form planets likely became asteroids instead. It is these asteroids that large meteorites found on earth are believed to originate from. Because they were created at the birth of the solar system, meteorites can give us some clues about its formation and age.

Artists impression of a protoplanetary disk. Image: NASA

Audrey Bouvier and colleague Meenakshi Wadhwa looked at something known as the calcium-aluminium rich inclusions (CAIs) in a meteorite found in the Sahara desert. The CAIs range in size from a few centimeters down to sub-millimeter lengths, and are believed to have formed in the protoplanetary disk as the solar system was beginning to take shape.

Several different radioactive decays can be investigated to determine how old a piece of rock is. The half-life of the each decay is the key to finding out the rock’s age. Researchers can look at how much of an isotope is present in the sample and compare it with how much there is of whatever it decays into, and then use the decay’s half-life to find the age of the sample. By looking at several different decays and combining the age estimates found for each it’s possible to get an even more precise estimate.

Bouvier and Wadhwa did this for the CAIs in their meteorite and found that it was 4,568.2 million years old. That’s between 0.3 and 1.9 million years older than previous research suggests the solar system is.

During their research, Bouvier and Wadhwa also learnt about how the solar system started. By comparing the time of formation of the CAIs with the time of formation of small, round grains of rock known as chondrules, they were able to determine the concentration of an isotope of iron, Fe-60, at the beginning of the solar system. Pushing back the formation of the solar system means that the concentration of Fe-60 at its beginning was twice as much as estimated using previous knowledge. Fe-60 is only made in the end stages of a star’s life, and is then scattered into space when the star dies. The high concentration found makes it very likely that the source of the Fe-60 was the death of a nearby star: our solar system evolved out of the remnants of a supernova.

Reference:
Audrey Bouvier, & Meenakshi Wadhwa (2010). The age of the Solar System redefined by the oldest Pb–Pb age of a meteoritic inclusion Nature Geoscience : 10.1038/ngeo941

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