03Apr2012

Not A Snowball’s Chance In Hell…

Hale-Bopp over Stonehenge

Over the last three decades astronomers have seen thousands of comets falling into the Sun.  For the most part, they’re too small to survive a close approach – let alone re-emerge.   It has only been within recent years that these icy travelers have been observed near the glaring Sun.  Now a team of scientists led by Professor Emeritus John Brown, Astronomer Royal for Scotland and former Regius Professor of Astronomy at Glasgow University, have been able to pinpoint which ones are destroyed high above the solar atmosphere, which ones are annihilated just above the surface and which ones make it through their hellish journey. 

The comet story is a well-known one.  Born from the solar nebula, these icy dust balls originate far outside our solar system in a region known as the Oort Cloud.  They’re happy in their distant homes… until their orbits are disrupted.  Then they begin their journey towards our nearest star.  It’s a journey that lasts of tens of thousands of years.   When they reach the inner solar system, they become volatile.  Their ices turn to gases and form huge tails that are blown away from the nucleus by solar winds and sunlight.  Some are huge, like the famous comet Hale-Bopp.  It was estimated to have a nucleus which spanned tens of kilometers across and a mass of a million million tons.  Thanks to their enormous size, comets of this type lose only a small portion of their total mass and live to orbit the Sun another day.  However, those that measure in the neighborhood of only 10 meters in diameter and 1000 tons in mass aren’t so lucky.  As they make a close pass to the Sun, they don’t stand a snowball’s chance in hell.  They get vaporized.

View from the STEREO satellite showing a comet plunging into the Sun.

As a result of several years of work, Professor Brown and his colleagues have been able to predict how comets lose mass and encounter total destruction in the solar atmosphere.  This work also encompasses differing orbital paths – ones which only reach the Sun’s lower atmosphere and those which encounter the photosphere.  The equations also include other factors, such as the various ways that comets loos their mass, momentum and energy to the Sun’s atmosphere according to their height.  Like Dante’s Inferno, there are different levels of hell.  Above 7000 km the cometary nuclei are slowly vaporized by sunlight…their gases peeled back like seared skin to form a coma and tail as they lose momentum due to drag.  But why settle for exquisite slow torture when you can go out with a bang?  Comets that encounter the low solar atmosphere aren’t stripped away by the pretty sunlight – they encounter the drag of the solar gas and explode from the ram pressure force of solar atmosphere as they sink into the dense layers.

The research team divided the comets into three types:  survivors, sunskimmers and sunplungers.  The sunskimmers never get closer than 7,000 km away from the Sun and they go out with a “fizzle” lasting anywhere from hundreds to thousands of seconds dependent on their size.  At their demise, they could emit weak but detectable extreme ultraviolet (XUV) radiation.   However, sunplungers don’t go quietly.  Their end might only last a few seconds as they dive into the dense lower solar atmosphere, but they send out a death scream with their resulting explosion.  These encounters produce effects as strong as solar flares and sunquakes on the surface.  And what of the giants?  You know what happens when a tiny drop of water encounters a hot surface.  Now imagine that on a much grander scale. Should a huge comet directly impact the Sun it would create a tremendous  – and violent – explosion just above the photosphere!

According to the press release, in July and December last year the NASA Solar Dynamics Observatory (SDO) satellite made the first direct observations of comets making close approaches to the Sun. The first comet, C/2011 N3 (SOHO) was completely destroyed after passing 100,000 km above the photosphere whilst the second and larger comet, C/2011 W3 (Lovejoy) survived a close approach to a similar distance (140,000 km) although it lost a significant fraction of its mass in the process. Both events were in line with the predictions of Professor Brown and his collaborators. They emphasize that, contrary to some news releases, the death or attrition of such comets has nothing to do with the high temperature (two million degrees Celsius) of the outer solar atmosphere since, though hot, it is so tenuous that it contains little heat.

Professor Brown comments: “In modeling how icy comets behave in this extreme environment, we really are starting to understand what happens to these ‘supersonic snowballs in hell’ when they make a close approach to the Sun. The two sunskimmers seen last year have already given us a new insight into the Sun’s atmosphere and it’s only a matter of time before we see the flare from a sunplunger — with a low enough orbit to reach the lower atmosphere of the Sun. When that happens we will be able to analyze the light from the resulting ‘cometary flare’ and find out even more about the composition of the interiors of comets.” Credit: Tammy Plotner  Original Story Source:  Royal Astronomical Society New Release