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Skywatching

Volcanos in space

Volcanoes are important things.

They play a part in recycling the surface rocks of a world, and in building new land. For example, all the Hawaiian Islands are actually active or extinct volcanoes.

Here on Earth, volcanoes come in two main types: shield and plinian.

Shield volcanoes are formed from lava that comes from the Earth's mantle. It is mainly basalt, which runs smoothly, like molasses. It forms flat, mound-shaped volcanoes, looking like a shield in cross section. They rarely erupt explosively.

Plinian volcanoes form over subduction zones, where an oceanic plate is being pushed down and under a continental one. The descending plate material takes with it part of the continental shelf, seabed rocks and a lot of seawater. The two ingredients that make plinian volcanoes behave very differently from shield volcanoes are silica (sand) and water. The material carried down gets mixed with molten basalt and other rocks, producing new, less dense rocks, which then bubble upward to the surface, building volcanoes along the line of the subduction zone. The silica makes the lava very thick and viscous, so it tends to plug up its vents. The water, rising along with the lava, is in the form of superheated steam. The result is a colossal build up of pressure in plugged vents, causing explosive eruptions.

Mount Vesuvius is one of these plinian volcanoes. Its eruption in 79 AD, which buried Pompeii and Herculaneum, was observed by Pliny the Elder, who made the mistake of being too curious and getting too close, and described by his surviving son Pliny the Younger. That is why we refer to volcanoes that erupt explosively as "plinian." 

The volcanoes making up the "Ring of Fire" around the Pacific Ocean are plinian, and associated with subduction zones. This includes Mount St. Helens and the other volcanoes along the West Coast.

Venus is dotted with volcanoes and enormous lava flows. The volcanoes are almost all round, flat mounds – shield volcanoes. There are no signs of plinian volcanoes, which is consistent with our having as yet seen no signs of plate tectonics on that world. This is really interesting, because Venus is almost the same size as the Earth, formed from the same stuff, but so different geologically.

Maybe oceans are an important factor. Venus is a dry furnace of a world. Mars has the biggest known shield volcano in the solar system, Olympus Mons (Mount Olympus). It is almost 22 kilometres high. There are other shield volcanoes, together with large lava flows and plains, but again, no plinian volcanoes. Once Mars had oceans, but as yet no signs have been found of that world having plate motions and subduction.

The most volcanically active world in the Solar System is Io, Jupiter's closest moon. The continuous tidal "kneading," due to being close to a giant planet, is producing huge amounts of heat, leading to almost continuous eruptions of sulphur-laden magma onto the surface of the moon.

Lava is molten rock. In the outer reaches of the solar system, water is just another rock mineral. In fact, ice is a major component of those rocks. The result is mud volcanoes. When tidal distortions or other things cause the ice to melt, mud erupts onto the surface and flows across the surface as a lukewarm lava, which then freezes.

Even further out, on worlds like Pluto, where it is even colder, there are eruptions of liquid nitrogen lava.

It's likely that at some point in their lives, all worlds with solid surfaces had volcanoes. Therefore, why is it that despite the huge lava flows and lava plains on the moon, there are no large volcanoes?

  • Mercury, the closest planet to the Sun, hides low in the sunset glow.
  • Before dawn, Jupiter and Saturn are close together in the south and Mars in the southeast.
  • The moon will be full on the 5th. 




A long and winding road

Life here on Earth depends on our supply of heat and light from the sun. A question with a suprising answer is how long does the energy produced in the core of the sun take to reach us?

Let's see. The sun is about 150 million kilometres away, and light and heat travel at almost 300,000 kilometres a second. So it would take about 8.3 minutes. That is indeed how long that light and heat takes to reach us from the surface of the sun, but to get to us from the sun's core, we need to add almost a million years to that answer!
 
The sun has a diameter of about 1.5 million kilometres. The energy is produced in the core, which has a diameter of around 300,000 kilometres. In that part of the sun, the temperature is around 15 million degrees Celsius and the pressure about 250 billion times the pressure at the Earth's surface. Under these conditions atoms come apart and rearrange themselves. The most abundant element in the sun is hydrogen, and under these extreme pressures and temperatures, four hydrogen atoms combine to form an atom of helium, releasing a lot of energy. This nuclear fusion reaction is by far the source of the sun's energy output. Around four million tonnes of the hydrogen is annihilated and turned into energy every second. The rest is turned into helium. The energy is released in the form of photons, pulses of electromagnetic waves. Gamma rays, X-rays, ultraviolet, visible light, infrared and radio waves are all forms of electromagnetic waves, and come in the form of streams of photons.

In high densities in the sun's core, almost immediately after an energy photon is produced, it collides with a particle and gets absorbed. Then it gets reradiated in some random direction. This process of absorption and reradiation goes on over and over again. Try this thought experiment. You are in the middle of a field and you want to leave the area. However, after each step or two you pick a new, totally random direction, take a couple more steps, pick a new random direction and so on. How long do you think it would take for you to get off that field? This sort of moving around is known as a "random walk." This is what those photons in the sun are doing. The part where those photons are random walking around is known as the radiative zone. As the lucky photons work their way outwards, the density drops, so they get to move further before being bounced into some new direction. Eventually, after bouncing around for around a million years on average, they find themselves around 70% of the way to the solar surface. This is an important place because at that level the density has fallen to the point where collisions become unimportant, and another transport process takes over, convection.

Here on Earth, we know that hot air rises. This is because it is warmer than its surroundings and less dense, so it floats upwards. As it rises, it cools. However, as long as it is warmer than its surroundings it will continue to rise. This condition exists in the outer 30% of the sun; from this point the energy is carried upwards as flows of hot, rising gas. This arrives at the surface, the photosphere, a layer a few hundred kilometres thick, which we generally think of as the solar "surface." Here that gas radiates its energy into space, cools, and sinks down again, to the base of the convection zone, where it heats, rises, and repeats the process. The result is a pattern of rising and forming currents, forming convection cells, just like what we see in a pan of heating oil when about to make some french fries. Once radiated into space, those important energy photons are on the last leg of their trip to us: 150 million kilometres in 8.3 minutes.

  • Venus is hard to see, low in the sunset glow.
  • Look for Mars low in the southeast before dawn, and, to its right, Jupiter and Saturn, close together.
  • The moon will reach first quarter on the 29th.


A surprise in the attic

An international team of astronomers may have discovered the closest black hole to us so far.

A thousand light years away might sound like a huge distance, but in cosmic terms it is in our backyard. The object has about four times the sun's mass, and was probably once a star itself.

Massive stars can become black holes; less massive ones, neutron stars; and stars like the sun just end their lives as white dwarfs.

This object lies in the direction of the constellation of Telescopium, "The Telescope," and is only visible from the Southern Hemisphere. That is an odd name for a constellation. Our northern sky is filled up with mythical beasts and heroes from Greek, Arab and other ancient cultures. Cassiopeia, Andromeda, Hercules and Perseus are all there. There are also oddballs, such as Triangulum, "The Triangle."

The southern skies are different. Mixed in with the usual mythical characters such as Centaurus, "The Centaur" is an assortment of constellations that are definitely not of mythical origin. In addition to Telescopium, there is also Microscopium, Antlia (the pump), Fornax (the furnace), Octans (the Octant – a measuring instrument). Horologium (the clock), Circinus (the compass), Mensa (the table) and many others. Triangles are obviously important, so there is one in the southern sky too – Triangulum Australe.

The southern sky has been likened to the attic of a retired scientist. Finally, to go with a collection of old instruments in a dusty attic, there is the constellation of Musca, "The Fly."

The reason for this intriguing difference is quite simple. We, along with the Greeks, Arabs and others who set up our familiar constellations, live in the Northern Hemisphere. There is a good chunk of the southern sky that never rises above the horizon in our northern mid-latitudes, and consequently never got organized. There were some gaps that were filled later, and there was a bit of "tidying up" done later, but usually by astronomers who had a classical education.

The 18th Century was a time of a great explosion in science and the quest for knowledge. Some of that need was practical; because the world was being opened up for trade, navigation was critically important. At that time French astronomer Abbé Nicolas-Louis de Lacaille wanted to measure the distances of the planets. The method he intended to use was a common surveying technique called triangulation. He wanted to make position measurements of the planets compared with the background stars, and to do this from widely separated geographic locations. He picked as an observing site the Cape of Good Hope, near the southern tip of Africa, well south of the equator.
 
As he proceeded southward, more and more unfamiliar sky emerged above the horizon, so he filled his time with organizing it into new constellations. It was he who decided to fill the southern sky with scientific instruments.

Actually though, he was not deviating from our approach to naming constellations. The ancients wrote their contemporary culture in the sky. The culture of the 18th Century was one of an enthusiastic interest in science. Lacaille did was what the Greeks did. He put the current culture in the sky.

Today, many of the devices Lacaille put in the sky, such as the octant, are no longer used, and may indeed find their way into a dusty attic. Calling a constellation "The Table" is unusual though. It  suggests Lacaille was definitely someone with an eminently practical type of mind. 

  • Venus still dominates the western sky after sunset, but is slowly sinking back into the sunset glow.
  • Mercury is there too, but is hard to see. Mars, Jupiter and Saturn lie low in the southeast before dawn.
  • Mars lies further to the left; Jupiter and Saturn lie close together, further to the right.
  • The moon will be new on the 22nd.  




How bad can sun behave?

We depend on the sun. Without it our world would be very cold, dark, and lifeless. It provides our light and heat. Since we are so dependent on it, we are also vulnerable to any fickle behaviour it might get up to.

We are familiar with its more or less regular sunspot cycle, where the level of magnetic activity and consequently the number of sunspots rises and falls over a period of 10-13 years. However, at various times in the past, such as the period 1645-1715, its behaviour changed; its magnetic activity decreased dramatically, sunspots became rare and our star got slightly dimmer, resulting in a cooling climate and weather cold enough for the River Thames in London to freeze over. This is very unusual. Then, around 1715 the cycle restarted and we returned to the behaviour pattern we are familiar with today. 

Solar magnetic activity drives solar storms, great clouds of ionized gas thrown off by the sun (better referred to as coronal mass ejections), and blasts of high energy particles and radiation can degrade or completely disrupt our technical infrastructure. So the big question is whether what we have seen the sun do since we started watching it is typical, or can it get worse.

The sun is 4.5 billion years old and about halfway through its life. What we have seen over a few hundred years and in more detail over a few decades is not really representative. However, as we exploit our planet's resources more completely, and become more and more dependent on technical infrastructure, it becomes increasingly important that we find out.

The sun is a fairly average yellow dwarf star, powered by nuclear fusion – the conversion of hydrogen into helium. Solar rotation and complex flows of extremely hot gas and magnetic fields inside the sun generate enormous electrical currents and in the process generate more magnetic fields. This process is known as the solar dynamo. These magnetic fields permeate the interior of the sun and erupt through the surface and out into space. Patches of strong magnetic field on the solar surface are known as active regions.

In some places the magnetic fields are so strong they interfere with the outward flow of energy, creating cooler patches, known as sunspots. The magnetic fields are the troublemakers. They can change the efficiency with which the sun radiates energy into space and, being elastic, enormous amounts of energy can be stored in them through stretching and twisting. Energy stored over days can be released in seconds, providing the energy for solar flares and coronal mass ejections. We need to know whether the patterns of solar behaviour we know of are typical. Can it get worse, or better? One approach being investigated is to look at sun-like stars. 

The sun is a fairly average yellow dwarf star, and our neighbourhood in the Milky Way contains a fair number of similar stars, some older, some younger, and some of these stars have been monitored for many years. Some of them show similar magnetic activity cycles, and some don't. Maybe these stars are doing what the sun did in the late 17th Century. Interestingly though, it looks as though in general those sun-like stars are more magnetically active than ours has been. 

The engine driving the magnetic activity is driven by the sun's rotation. For some reason, as yet unknown, the sun is rotating more slowly than those other sun-like stars, which would explain why they are more active, but it does not really help us with the fundamental question, namely how badly can the sun behave. We need to know.

  • Venus still dominates the western sky after sunset, but is slowly sinking back into the sunset glow.
  • Mars, Jupiter and Saturn lie low in the southeast before dawn. Mars is to the left; Jupiter and Saturn lie close together.
  • The moon will reach last quarter on the 14th.


More Skywatching articles

About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]



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The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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