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Skywatching

Gauging ‘planetquakes’ and ‘moonquakes’

Planetary shaking

When the Apollo astronauts went to the Moon, among the instruments they installed were seismometers, devices for detecting vibrations in the ground—“moonquakes.”

Mars, the planet currently inhabited mainly by robots from Earth, also hosts a seismometer or two. Obviously these instruments are deemed important tools for understanding the worlds they are sitting on.

Our Earth is in a state of constant change. Tectonic plates are colliding and riding up over one another. These forces cause cracking in the rocks: "faulting" in geological terms.

Molten magma coming from below causes cracking and compression as it injects itself between the layers or forces its way to the surface, where it forms volcanoes. The stresses that build up in the rocks due to these processes may be slow, but when the stresses become too much the release of energy can be very rapid, ranging from scarcely discernible ground tremors to huge earthquakes.

These launch seismic waves that travel and echo through the whole body of the Earth. When they are detected by a distant seismometer they will have been changed by what they passed through: molten rock, rock softened by heat, soft rock, hard rock and so on. Therefore, if we observe an earthquake with a global network of seismometers, or observe lots of earthquakes with one or two seismometers, we can determine what our planet is like inside.

The geologically active world we live on means there are lots of seismic events to observe. Seismic science has told us a lot about the interior workings of our world; what can it tell us about the other bodies in the Solar System?

The Apollo seismometers on the Moon show it to be a geologically quiet place. There is still the odd “moonquake” that might be due to remaining stresses being released in rocks, but most events seem to be due to impacts: meteoric material hitting the Moon's surface at tens of kilometres a second.

On Earth, we have an atmosphere that makes most of these objects burn up. The Moon has no atmosphere, so incoming objects can impact the surface. There are no plate tectonic processes on the Moon, and the occasional seismic waves passing through it suggest that world has cooled, and is now a solid, rocky sphere.

The Earth has a strong magnetic field, generated by flows of molten iron and nickel in its deep interior. Mars is smaller than Earth and cooled off a lot faster. We believe its core has solidified. This idea is supported by there being no strong, global magnetic field, because there is no longer any circulation inside to generate one.

There is a lot of evidence on the surface of Mars suggesting that billions of years ago, it was geologically very active, making mountains and huge volcanoes. However, there are no signs of new mountains being built.

Our seismometers certainly show Mars is geologically quiet. However, as time passes, the story is becoming more complicated. There is evidence that major volcanic activity occurred on Mars less than 100,000 years ago. This is a mere instant in geological time, so today Mars cannot be as quiet as it seems. Keeping a seismic ear to the ground on Mars could give us a few surprises.

Venus is a puzzle in that it is volcanic but there are no signs of plate tectonics. However, until we can produce seismometers that can survive temperatures high enough to melt tin, and an atmospheric pressure 90 times the pressure at the Earth's surface, that puzzle will remain.

However, the Solar System is loaded with other, easier to handle worlds likely to be seismically interesting. For example, there is Mercury, the closest planet to the Sun. My personal choice would be to put one on Io, one of Jupiter's moons, and the most geologically active object in the Solar System.

•••

• Saturn and Jupiter are in the sky after sunset. Mars rises three hours later.

• The Moon will be reach its first quarter on Oct. 2.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.





The North Star is not the constant we think is

The 'temporary' North Star

Most of us know how to find the Pole, or North Star.

We find the Dipper and follow the line indicated by the pair of stars opposite the handle. It is the star that does not appear to move, while the rotation of the Earth carries all the other stars in circles around it.

This star is overhead at the North Pole, which is why it is called the Pole Star, or Polaris, which is its "star name". It is also called the North Star, because for anyone in the Northern Hemisphere, not standing at the pole, looking in its direction means we are looking North. In addition, measuring its angle of elevation from the northern horizon tells us our latitude.

A few centuries ago, when we had no accurate way of determining our longitude (our position east-west), we would navigate our way across the oceans by using Polaris. We sailed north or south until we were at the latitude of our destination, and then sailed east or west, depending on where we were going, keeping Polaris at the same angle above the horizon. The Pole, or North Star, has been so fundamental, for so long that it is easy to assume it is a constant thing. It is not.

At some point, we must all have played with spinning tops. If we set them spinning fast enough, they would stay upright, balancing on their tips. We found that it is very hard to set a top spinning exactly upright, and that when we failed to do so, the top would describe slow, circular wobbles, but not fall over. This wobbling process is known as precession and is due to the interplay between the spinning motion and gravity trying to pull that non-upright top over. The Earth is in a similar situation.

Our planet spins on its axis once a day. However, a uniform, fixed spin is made impossible by the Earth not being perfectly spherical. It bulges at the equator, and has the Sun and Moon gravitationally tugging at that bulge. The result is that, like the top, our spinning planet wobbles; it precesses. This means Polaris has not always been the Pole Star and won't remain so into the future.

If we stand at the North Pole and look upwards to the zenith—the point exactly overhead—we will be looking along the line of the Earth's axis of rotation, and see that the axis points very close to Polaris. This means that as the Earth rotates, that star stays where it is, and all the other stars appear to circle around it. As the Earth's rotation precesses, the axis of rotation will describe a circle among the stars.

It takes 26,000 years to complete each lap. Back in the days of the ancient Egyptians, the axis pointed at Thuban, in the constellation of Draco, "The Dragon". Since then it moved until it pointed close to Polaris. Now it is slowly moving away and in the direction of the star Alderamin, in the constellation of Cepheus (a constellation named after the husband of Queen Cassiopeia), so in 7500 AD that will be our North Star.

The constellation of Cepheus looks rather like a house, and none of the stars in it, including Alderamin are very bright.

Around 10,000 AD the pole position will lie in the constellation of Cygnus, the Swan. In 13,700 AD the North Star will be the bright, bluish star Vega, which at the moment lies almost overhead in the evening. Around 23,000 AD Thuban will be the Pole Star again, and by 27,000 AD Polaris will be back on the job.

One interesting consequence of precession is that the signs of the Zodiac are slowly slipping backwards.

The first sign of the Zodiac is the constellation sitting at the point where the Sun crosses the celestial equator in the spring. That used to be Aries. The first sign is now Pisces. However, in just under 26,000 years, when the Earth starts its next wobble, it will be Aries again.

•••

• On Sept. 23, the Sun will cross the equator, heading south, marking the autumn equinox. It will be in front of the constellation of Virgo.

•Saturn and Jupiter are in the sky after sunset. Mars rises three hours later.

• The Moon will be new on the Sept. 25.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



Large asteroids are a danger to earth

Killer asteroids

At 07:17 local time on June 30, 1908, something entered the atmosphere over Tungusca, Siberia and exploded.

The blast, equivalent to about 17 million tonnes of TNT, flattened around 80 million trees over an area of more than two thousand-square-kilometres. The body is suspected to have been a small asteroid, with a diameter of around 50 metres or maybe a small comet.

If it had arrived a few hours later, it would have exploded over Europe, with devastating results.

Sixty-five million years ago, at the end of the Cretaceous period, ammonites, creatures related to the modern nautilus, filled the seas, and dinosaurs ruled the land. Both had ruled the planet for hundreds of millions of years and were in decline, probably due to increased volcanism and environmental change.

The coup de grace was provided by an asteroid between 10 and 15 kilometres across, which crashed down in the Gulf of Mexico. Soon after, the dinosaurs, ammonites, together with about 75% of living species, were gone. Mammals survived, leading to our presence on Planet Earth today.

The Barringer Crater in Arizona, and others around the world, show our planet has been hit many times throughout its history. Actually, erosion and plate tectonics have erased most traces of cosmic impacts.

A better understanding comes from having a telescopic look at the Moon. There has been little erasure of impact craters there, showing the countless number of times the Moon has been hit. Almost certainly, we have been hit at least as many times. The main process by which lunar craters disappear is obliteration by impacts and being overwritten by new craters.

Today, with our planet on the verge of not being able to provide the requirements of life for us and the species with which we share the world, the last thing we want is to be hit by an asteroid, even a small one. What can we do to minimize the chance of this happening?

The first task is to identify potentially threatening bodies. Since many asteroids are dark grey or even black, and we are looking for them against a black sky, this is not easy. Telescopes with high-sensitivity imagers are now dedicated to asteroid spotting, and they are getting better and better.

The key thing is to spot the threats early enough to do something to mitigate them.

A five-kilometre, in diameter, asteroid would weigh about 65 billion tonnes if it is made of ice, around 180 billion tonnes if made of basalt rock and roughly 500 billion tonnes if made of nickel iron. In reality, most asteroids are a mixture of these ingredients.

How can we deal with something that big, because at some point we are likely to have to? Doing what they do in the movies, which is blowing up the asteroid, would turn a single impact into a shotgun blast of impacts. A five-kilometre asteroid can be broken into a lot of large and very dangerous fragments.

The solution getting most attention is to change the orbit of the object so that it does no hit us.

The most powerful rocket motors we have would take weeks or months to deflect a threatening object. In addition, those rocket motors can only be fired for a few minutes. A low-thrust engine that can run for months or years, preferably using the asteroid as fuel is feasible.

However, this means we have to identify the threats years in advance. Once we have done so, it will take months or even years to get to the asteroid to install the engine. Predictions that far in advance are not easy, because asteroid orbits are being constantly perturbed by the gravitational pulls of giant planets such as Jupiter.

However, we have a strong incentive to solve this problem.

•••

• Saturn and Jupiter are in the sky after sunset. Mars rises three hours later followed, deep in the dawn glow, by Venus.

• The Moon will reach its last quarter on Sept. 17

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



199220


Searching for the 'holy grail' of planets

An ocean planet?

The holy grail in our search for exoplanets (planets orbiting other stars) is to find one with water and at the right distance from its star for that water to be present on its surface as a liquid.

Now it looks as though such a planet may have been found. It is early days yet, but the evidence looks convincing.

Around 100 light years away, in the constellation of Draco, "The Dragon", there are two red dwarf stars orbiting each other. Orbiting one of these stars is a very interesting planet, which has been given the romantic name of TOI-1452 b, which could be just what we have been hoping to find. Draco sits in the northern sky, more or less wrapping closely around the constellation of Ursa Minor, "The Little Bear", which has the Pole or North Star marking the end of its tail.

So this pair of stars and that intriguing planet are high in our skies every night.

The usual way we search for exoplanets is to observe stars for a long time, looking for minute dimmings as their planets pass in front of them. This might sound a bit "dodgy" but it works. Thousands of planets have been discovered, with some of them being found by backyard astronomers.

From the spacings between the dimmings, their durations and how the brightness rises and falls at the beginning and end of each dimming, it is possible to estimate the orbit, the size and the mass of the planet.

The orbit tells us how far it is from its star, which in turn gives us an idea of its surface temperature. This particular planet lies in its star's "Goldilocks Zone", the range of distances giving surface temperatures that would allow liquid water to exist there.

This is an exciting result in itself, but things get more interesting when we look at the size and mass of the planet. It is nearly five times the mass of the Earth, but its mass is too small for its size.

Rocky planets like the Earth, Mars or Venus are basically big lumps of basalt rock with iron cores in the middle.

We can interpret the average density of a planet (mass divided by volume) in terms of different combinations of iron and basalt. However, the density of the new planet is too low to interpret in terms of any combination of these ingredients.

It needs a third ingredient, one that is a lot less dense than basalt. That ingredient is likely to be water. Basalt has a density of about 2.7 gm/cc (grams per cubic centimetre). Water's density is 1 gm/cc. Even so, to account for the density of that new planet, it has to be about 30% water. That is an amazing amount of water.

Our planet looks blue from space because 70% of its surface is covered by water. However, that layer is pretty thin compared with the diameter of the planet. Our world is only about 1% water. The rest is the usual iron and basalt mixture. Ours is quite a dry planet.

That huge relative amount of water suggests TOI-1452 b is covered by an ocean that is many kilometres deep everywhere, not just in occasional deep spots or trenches as we have here.

Europa and Enceladus, moons in our Solar System have deep oceans, but in their cases they are tidally heated and covered by enormously thick layers of ice.

On this new world, the ocean would be open to the sky, and illuminated by the planet's red sun. This means, just as in our oceans, photosynthesis could be possible, which means seaweeds and phytoplankton could be floating around. However, at this point, the low density might have another explanation, namely that it is a sort of mini-Jupiter, with a rocky core surrounded by a dense, opaque atmosphere, so we are estimating the size of the atmosphere, not the size of the planet.

More work is needed to see which of these possibilities is the case. On the other hand, this discovery is one of the most exciting ones yet.

•••

• Saturn and Jupiter are in the sky after sunset. Mars rises three hours later followed, just as the sky starts to brighten for dawn, by Venus.

• The Moon will be full on Sept.10.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.



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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]



197601
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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