Solving the puzzle of liquid water on a frozen planet

Geysers on a frozen world

Photo: Pixabay

The James Webb Space Telescope continues to show the Outer Solar System in ways that originally required space probes.

A few days ago it detected a huge geyser—a high-pressure jet of liquid water—ejected from Enceladus, one of Saturn's moons. Jets of water ejected from that strange world have been known for some years. However, the big puzzle is why they are happening.

To better understand Enceladus, let's look at our moon.

Our moon is an airless rock ball. It is quite dark and absorbs about 85% of the solar energy falling on it. Its average temperature is about -48 C. The reason our world is warmer is the Earth has an atmosphere and the Moon does not.

The greenhouse effect makes our world liveable. Enceladus lies around 9.6 times farther from the sun than our moon and, consequently, it receives only around 1.1% of the solar energy. That is not the end of the story, because Enceladus is also covered with ice. That is far more reflective than the basalt rock covering our moon, so it captures less energy, making it colder—a frigid -200 C.

At that distance from the sun, water should be a permanently frozen rock mineral, with little prospect of it ever being a liquid. If there is liquid water on Enceladus, the heat must be from another source—inside.

Two possibilities are the decay of radioactive materials and tidal heating.

Our planet is hot in the middle for two main reasons. When it formed, it collected from its birth dust and gas cloud a collection of heavy, radioactive elements. Those sank down into the core while the earth was still molten, and they are producing heat as they decay.

There is also heat remaining from when the Earth was born. On average, a cubic centimetre of our planet weighs in at about 5.5 grams, that is, its density is 5.5 grams per cubic centimetre. Enceladus is far less dense, with an average density of 1.61 grams per cubic centimetre. Because rocks such as basalt have densities of around 2.9 grams per cubic centimetre, Enceladus must be made mostly of something less dense, such as ice. Such a low density also suggests it does not have a concentration of heavy, radioactive elements in its core, so heating by radioactive decay is probably not important.

The most likely candidate is tidal heating. Saturn, the planet it orbits, weighs in at the equivalent of about 95 Earths, and Enceladus orbits relatively close by, so tidal distortions are likely to be far more severe than the Moon inflicts on us, or us on the Moon.

Since Enceladus is probably mostly ice, tidal heating would have a lot to work on. So a possible picture for that interesting world would be a core consisting of rock and ice, surrounded by an ocean of liquid water, and sheathed in a shell of ice. Water does not tidally heat very much. It just happily slops around as it does in a bath. Most of the heat is probably generated in the flexing and cracking of the core of the body. There is probably a balance. If the core melts too much, tidal heating decreases and it freezes again.

Europa, one of Jupiter's moons, is another world sheathed in ice with an ocean underneath, due to tidal heating. Getting a sample of water from that ocean to see what it's like and to search for life involves a lander and a long drill.

Enceladus' geysers offer an easier option. The Cassini spacecraft flew though some of these plumes and took water samples. The information is still being analyzed, but we now know there is a deep, salty ocean under the ice, and that there are simple organic (carbon-based) chemicals in that water. These could be signs of life, or ingredients for life, but so far the jury is still out. Even if we search for just extraterrestrial carbon-based life, like life on Earth, it seems there are many exotic possibilities.

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• Venus and Mars lie close together in the west after sunset.

• Saturn rises in the early hours, and Jupiter appears low in the sky before dawn glow.

• The Moon will be new on June 17.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory near Penticton.

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

The time when our fundamental understanding of the universe changed

Celestial discoveries

Photo: Contributed

A few centuries ago, many believed our planet was the centre of creation and everything we see in the sky orbits around us.

Then, we found our world is just one of a number of planets orbiting the sun. When, in 1785, William Herschel made the first ever map of the Milky Way, he assumed we live in the centre. However, by the time of the “Great Debate,” we were further demoted to the suburbs of the Milky Way, well out from the centre, with our sun just one star out of billions.

Our next demotions started to come together in 1920. Before then most astronomers believed the Milky Way was alone in the universe. As telescopes improved, astronomers found an increasing number of fuzzy, spiral things. It was widely accepted this was the spiralling in of material in the process of forming new stars and planetary systems, and was located within the Milky Way. However, by 1920, a substantial minority of astronomers suggested those spirals were other galaxies, lying well outside ours.

The discussion of the two ideas culminated in what became known as the "Great Debate", when, on April 28, 1920, the two factions got together to argue it out. Some, including Harlow Shapley, hung onto the idea those spirals lay within our galaxy. Others, such as Heber Curtis, maintained the spirals were other galaxies, "island universes" in their own right.

Over the following decade, the evidence of the "other galaxies idea" proved correct and became conclusive. The universe was filled with galaxies, some larger than ours, some smaller, extending out into space as far as we could see at the time. Our concept of the universe had fundamentally changed. We now orbited a star that was one of billions, in a galaxy that is just one of billions. The picture changed again in the early 1930s, when evidence started to appear that the matter making up us, and the observable universe, is actually a very minor ingredient. The main ingredient is invisible.

Astronomer Fritz Zwicky found himself a puzzle. It was 1933, and he was observing a distant cluster of galaxies known as the Coma Supercluster. The cluster lies about 321 million light years away, far outside our galaxy. Zwicky measured the speeds the galaxies were orbiting around each other, and how far apart they were. He estimated the numbers of stars and from these he obtained a value for the masses of the galaxies.

Newton's theory of gravitation ties masses, separations and orbital speeds together. If we have two of these quantities, we can calculate the other. So Zwicky used the orbital speeds and separation distances to calculate the masses. In measurements and calculations like this we expect there to be errors or discrepancies. However, the estimated mass of the cluster was about ten times larger than he estimated from what he could see.

The galaxies were moving so fast that the gravitational attraction of the visible matter could not hold the cluster together. This discrepancy was far too big to be an error. There was something fundamentally wrong. He concluded there was a lot of matter - some 90% of the matter in the cluster - that was invisible. Being a native German speaker, he called this unseen matter "Dunkle Materie" , which in English means "Dark Matter".

Since the 1930s, we have found countless galaxies extending out into space and back in time to around a billion years after the Big Bang, which happened just under 14 billion years ago. Dark matter is a major ingredient in galaxies. Moreover, it seems that without this massive fraction of dark matter, the galaxies would not even have formed.

That decade between the early 1920s and 1930s, between the two World Wars, fundamentally changed our view of the universe.

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• Venus and Mars lie close together in the west after sunset.

• Saturn rises in the early hours, and Jupiter appears low in the sky before dawn.

• Mercury lies very low and is hard to see in the dawn glow.

• The Moon will reach its last quarter on June 10.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory near Penticton.

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

Long, long ago, in a distant galaxy

Death of stars from the past

Photo: Wikipedia

Twenty-one million years ago, a star in a distant galaxy exploded.

The galaxy, known as Messier 101, is located in the constellation of Ursa Major, "The Great Bear". At the time of the explosion, the earth was in the Miocene period. Huge flows of lava were in the process of covering much of the southern interior of British Columbia, forming a great plateau. Volcanic activity was intense.

As time passed, the light from that stellar explosion spread out in all directions into intergalactic space. It so happens that M101 is 21 million light years from earth, so the light from that explosion - a supernova - has just arrived here.

If you have a telescope, you might be able to see it for yourself. Find the Big Dipper, which is the brightest part of Ursa Major. The star at the end of the handle is Alkaid, and the next star heading towards the bowl is Mizar, with its close partner Alcor. From a point halfway between Alkaid and Mizar, scan upwards about two-thirds the distance between the stars.

This supernova marks the end of a giant star. Stars obtain energy by fusing small atoms, like hydrogen, into bigger ones, like carbon, oxygen and so on. This happens in the cores of the stars where pressures and temperatures are high.

The end comes when there are no atoms left in the core that can be used to produce energy. At that point, the star collapses. Ironically, at that final moment, the star still has lots of fuel available, but it lies in the cooler, much less compressed outer layers.

Extensive study and observations have shown that there is almost no mixing between the core and outer layers, so that fuel is not available for energy production. However, in the collapse it tumbles down into core region and gets compressed and heated by the fall. Runaway nuclear fusion takes place, releasing a huge pulse of energy that helps blow the star apart.

Ironically though, this lack of mixing is extremely useful to astronomers. It means the surface layers of a star, which we can observe, are preserved samples of the material from which the star formed. In other words, each star tells us something about the evolution of the universe.

In the beginning the only elements were hydrogen and helium. These were in the form of giant clouds which over billions of years provided the raw materials to make new stars. Over time, these clouds have been increasingly enriched or polluted by the elements forged in stars as by-products of their energy production.

That means the surface layers of the very oldest stars would contain nothing other than hydrogen and helium. Succeeding generations would have surface layers containing increasing amounts of elements formed by earlier generations. It means we can put together a sort of chronology of generations of stars. Our sun's surface layers show it is not an old star, but not a teenager either. This is good because both very young and very old stars can be unstable. The young ones require time to settle down and stabilize. The older ones are unstable too, because they are running out of fuel.

That supernova in M101 has ejected its material into nearby clouds of gas and dust. We can see nearby parts of the cloud material glowing in the radiation from newly born stars. Of course we are seeing the situation as it was 21 million years ago. By now, that material could be helping to make new stars and planets.

Life on Earth seems to have appeared around half a billion to a billion years after the Solar System formed. So by now, 21 million years later, any new planets forming from the elements released will still be far too young. However, clearly the process of world creation and the possible appearance of living things go on.

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• Venus and Mars lie close together in the west after sunset. Saturn rises in the early hours, and Jupiter appears low in the sky before dawn.

• Mercury lies very low and hard to see in the dawn glow.

• The Moon will be full on June 3.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory near Penticton.

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

How a planet comes to the end of its life

When a star eats a planet

Photo: Wikipedia/NASA/JPL-Caltech

Imagine a planet slowly spiralling in towards its star.

As it gets closer the increasing heat, together with the star's equivalent of the solar wind, blast away the atmosphere. Then, as what is left of the planet continues its inward spiral, it disintegrates into a shower of falling fragments.

For the first time, astronomers have actually observed this happening. The star in question, poetically named ZTF SLRN-2020, lies in the constellation of Aquila, "The Eagle", at a distance of some 12,000 light years. It might be a sobering thought that the Earth will share a similar fate in a few billion years.

Imagine a system of planets orbiting another star. For most of its life, the star, like our sun, behaves itself, shining reasonably steadily, and keeping any life-bearing planets comfortable. Then, like all stars, it starts to run low on fuel. This, paradoxically, causes it to expand into a red giant, and to radiate more energy. Planets that orbit close in find themselves having to shoulder their way through the gas and dust making up the star's extended envelope, instead of moving more or less frictionlessly through a near vacuum.

The drag sucks energy from the planet and its orbit turns into an inward spiral. Ploughing through the gas and dust, together with the increasing heat from the star, gradually strips away the planet's atmosphere and boils away any oceans it might have had, turning it into a dead world. Intriguingly though, the story does not end there.

When we swing a weight on a piece of string in circles around our heads, we feel an outward pull on the string. This is actually the inertia of the weight resisting being pulled into a curved path rather than doing what it really wants to, which is fly off in a straight line. We have come to refer to this outward pull, not completely accurately, as "centrifugal force".

This "force" is related to two things: the speed the object is moving and the diameter of the circle in which it moves. The smaller the circle, or the higher the speed, the stronger the force.

A planet orbiting a star in a more or less circular orbit is moving so that the inward-directed gravitational pull of the star is balanced by the outward-directed centrifugal force.

Actually, the balance of forces occurs only at the planet's centre of gravity. Half the planet is closer to the star than the centre of gravity. The other side is further away. Since the planet is a solid object, all parts of it are moving at the same speed.

Therefore, the outer part of the planet is moving faster than is needed, and the centrifugal force is bigger than the pull of the star's gravity, pulling that part of the planet outwards. Similarly, the half of the planet closest to the star is not moving fast enough, and there is a net inward force due to gravity being stronger than the centrifugal force.

The result is the part of the planet closest to the star is being pulled inward, and the outer part pulled outwards. This stretching force is often referred to as a tidal force, because it is the reason we have ocean tides here on earth.

Normally, as we can see with the planets in the Solar System, this tidal force is too small to endanger the planet. However, the gravitational attraction rises rapidly as we get closer to the star, quadrupling each time we halve the distance.

The result is that, as our ill-fated planet spirals in closer and closer to its star, the tidal force increases rapidly.

Rock is really good at resisting compression, which is why we make pyramids out of it. However, it is not very good at handling stretching. Eventually, for our planet, the tidal forces become too great and the planet disintegrates, with its fragments falling down into the star.

This will probably be the ultimate fate of our planet, although not for billions of years.

•••

Venus shines very brightly in the west after sunset. Mars, much less bright, and reddish, lies a little higher. Saturn, golden coloured and moderately bright, lies low in the dawn glow.

The Moon will reach last quarter on May 27.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory near Penticton.

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