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Planets Get Heat With a Little Help From Their Friends

How might worlds orbiting other stars stay warm enough to support life?

Tidal heating – the inspiration for my research – may be the answer!

Artist's conception of planets orbiting a low-mass star. (image courtesy of NASA/JPL)

“Let’s be honest for a second. The search for planets in the galaxy is about the search for life. It is awesome that we’re learning about thousands of other planetary systems. We’re making discoveries – some surprising, some reassuring – about the nature and evolution of the universe. This is great. But one look at the hyperbolic headlines about Earth-like planets and Earth twins reveals why, in the hearts and minds of the public, and maybe of a lot of researchers, the search for planets is the search for potential homes for life.

In that case, our understanding of exoplanets is about finding promising candidates so that, once we have the means to look for evidence of life, we’ll have a shortlist.”

Reposted from astrobites.org.

Making Ice – Mother Nature makes it look so easy!

With heaps of ice crystals falling from the sky, it seems an appropriate time to answer one of my readers’ questions: How do I make the ice that I torture during my experiments? Unfortunately, it’s not as straightforward as putting a tray in the freezer and coming back a few hours later for cubes.

Making ice for scientific experiments requires me to precisely control not only the cleanliness of the ice, but the size of the crystals, and the methods we use to control crystal size – or “grain size” in scientific lingo – are pretty exciting.

The first step in any of my experiments is to decide “how” I want my ice to deform. Ice is a non-newtonian fluid (Wikipedia has a great article for anyone who wants to delve into that) whose properties can be strongly influenced by grain size. As such, I decide what sort of properties I want to investigate, and then choose a grain size that will allow me to do so.

Choosing the grain size, in a way, determines how I will make my ice.

For large grains (that is, bigger than 100 microns or so), I use what I like to call the alien lander. It really does look like Dr. Goldsby stole this thing from the set of “War of the Worlds:”

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The process goes from frozen, deionized water, through a blender and a set of sieves (imagine a red-faced blonde leaning over into a chest freezer for about an hour, banging a set of sieves against the inside, and you’ll get the idea), packing the resulting snow into a mold and flooding it with ice-cold water. The latter process involves a small-scale “Polar Bear Plunge” where I reach into a bucket of ice water to move valves on the alien lander.

After all that, the flooded powder is left to freeze overnight. I take the frozen sample out of the mold, cut it down to size, and voila! Coarse-grained ice sample, ready to go.

If I need grains smaller than 100 microns, I have to get creative. The process involves spraying a fine mist of deionized water into a dewar filled with liquid nitrogen (at about -200⁰ C). This forms a slurry of liquid nitrogen and very small particles of ice, which I then pour through a shaker to separate. As the shaker runs, I must continually douse the mixture with liquid nitrogen to keep anything from melting or growing. Finally, the resulting powder gets packed into a mold.

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 The result of these efforts is a beautiful piece of polycrystalline ice, ready for science!

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How Doth Ice Creepeth?

This is a blog about ice. That’s right – ice! Other topics will probably work their way in here (namely, SPACE!), but I’ll try to keep more or less on topic. So: ice. The same stuff that was in your soda at lunch, yet at the scale of planets… well, I’m getting a little ahead of myself.

I’m a graduate student who lives in “The Ice Lab:” a small, windowless room in the basement of the Geochemistry building, full of things that sow unspeakable terror in the hearts of ice samples everywhere. When grown up ice samples want their little ice samples to behave, they threaten to send them to my lab.

It is here that I slowly, relentlessly, crush ice. Very slowly: at about 10-5 millimeters per second. The pressure and temperature at which I do so are chosen to elicit a specific response from the ice, which I carefully measure before taking the experiment apart and examining the ice under an electron microscope. I am looking for “subgrain boundaries” – the alignment of tiny defects within ice crystals – and seek to link their size to the rate at which the ice dissipates energy.

Here is where the planets come in.

Why do we care about how ice dissipates energy?

There are many moons in the Outer Solar System – orbiting Jupiter, Saturn, Uranus, and Neptune – that are completely encased in ice. We even have spacecraft data which tell us that beneath the ice, some of these moons have liquid oceans. Water, tens of kilometers deep! To maintain liquid water through billions of years in the utter cold of the Outer Solar System, though, these moons need a heat source. One such source is the dissipation of tides inside the ice.

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When planetary scientists try to model the amount of heating occurring inside the ice shells, though, they are stymied by an incomplete knowledge of what causes dissipation (a.k.a. attenuation) at the extremely low frequency of this “tidal forcing.” Much of our knowledge comes from studies of attenuation at frequencies similar to that of earthquakes – because understanding seismic-frequency attenuation helps us interpret seismic returns from deep inside the Earth! – but planetary scientists must take that data and extrapolate it to the conditions of an icy moon, across huge differences in frequency. Such extrapolations are dangerous in science, especially when we don’t quite understand the processes we are trying to model.

This is where my lab work comes into play. By performing attenuation experiments on ice that has been slowly deformed under a constant weight (a process called “creep,” which is how we expect the interiors of these moons to deform) I can then take that ice, put it under an electron microscope, and look at the crystal structure related to the measured attenuation. Hopefully, this will give me a more complete picture of the microscopic processes causing attenuation at tidal frequencies, allowing planetary scientists to model the (very macroscopic) processes occurring inside moons in the Outer Solar System.

I’ll try to update this blog as often as possible on what it’s like to live and work in the Ice Lab, and the fates of my samples as they are slowly and steadily tortured. I hope that you enjoy learning a little bit about the science of ice!