Enceladus: a song of ice and tides

An artist impression of Cassini diving into Enceladus water plumes. credit: NASA/JPL

Cassini will terminate its 20-odd-years-long mission in September. But it’s determined to go out with a bang. In yesterday’s press conference, NASA announced that the probe, during a 2015 flyby of Saturn’s moon Enceladus, found clues that the ocean within the icy moon has almost all we think it needs to spark life.

Enceladus is a fascinating world, with an ice version of Earth’s tectonic activity. Like Earth, Enceladus has volcanoes on its surface, but they spew water, which is what Cassini investigated. Instead of magma, in fact, its surface floats on a gigantic salty ocean. This ocean, NASA announced, seems now the place to go look for life in our solar system.

An illustration of the interior of Enceladus: its icy crust, rocky core and liquid ocean in between. credit: NASA/JPL-Caltech

But that is way off the habitable zone! Shouldn’t Enceladus and all the other ocean worlds be frozen solid all the way through?

They might just escape an icy death using an unusual tool: tides. For astronomers, tides are the difference between gravitational pulls on different sides of a planet or moon. For example, one side of Enceladus is closer to Saturn than the other, so it feels a little more gravity. Since it pulls more on one side than the other, the tidal force stretches Enceladus.

As Enceladus moves around, it gets stretched and pulled in ever-changing ways. So its crust and its interior have to rearrange themselves all the time under this force, parts move and slip on each other. The friction warms the planet up, in a process called tidal heating.

It happens to Earth as well, of course, with tidal forces from the Moon and the Sun. But our planet has an enormous amount of heat left over from its formation, enough to melt rock into magma. Tidal heating doesn’t do much here.

To keep this heat in, Enceladus’ ocean has another unusual ally: the kilometers-thick ice crust over it. Ice is a pretty good thermal insuator, and acts like a giant blanket around the ocean, keeping the freezing void out and precious heat in.

We always focus on what sort of atmosphere planets must have to harbor life, or how far they have to be from cold, dim stars. But it might turn out that a big fat ice cover and powerful tides might go quite some way.

If you want more
  • NASA, as usual, put together a great package with al lot of info on Enceladus and other ocean worlds
  • While sufficient to keep oceans all over some of Jupiter’s moons, tidal heating doesn’t seem to be enough to maintain an ocean around all of Enceladus

Cover photo: CC0 Tilgnerpictures/pixabay

What makes a planet habitable

A couple of days ago, NASA announced the discovery of seven-count-them-seven rocky planets orbiting a small star called TRAPPIST1. Three of them seem to even be in the “habitable zone”. So, have we found the aliens’ home?

Nope. However, words are a bit confusing, so let’s review what it takes for a planet to be habitable.

On planets too close to the star it’s too hot and water evaporates, on the far ones it freezes. Only those in the sweet spot stay just the right amount of wet. credit: NASA/JPL/Caltech

The key is liquid water. All life we know—from bacteria to cats, from carrots to Lionel Messi—hinges on chemical reaction that only take place in water. No water, no life.

The habitable zone of a star is the space around it where planets can have sustained liquid water on their surface. Closer, and the heat from the star will fry the planet with all the water (think Mercury), farther and the surface will freeze (think Pluto).

Now we need a surface to collect this water on. That’s why rocky planets are interesting: no rocky surface, no place for water. And again, no water, no life.

Location isn’t everything, though: the atmosphere is key too. Earth’s atmosphere keeps water on the surface, and temperatures friendly (for planetary standards). Less of it and we’d risk turning into Mars, which is in the habitable zone, but is a frozen desert, where water is more like a killer sludge. More atmosphere, and we might become Venus (also in the habitable zone), which is effectively hell, molten lead lakes and deadly acid rains included. Winds in the atmosphere also favor habitability on some exoplanets.

Then you must hold on to your atmosphere. Earth has a cozy magnetic field that deflects part of the Sun’s radiation. Mars probably had an atmosphere, when it also had oceans, but has no magnetic field. Atom by atom, the constant barraging of energy and particles from the Sun eroded it. With the atmosphere gone, so was the water. No water, no life.

How Google’s doodle portrayed the discovery

The planet discovered around TRAPPIST1 are important not because they might be habitable (though some are promising), but because TRAPPIST1 is the first star of its kind we study, and it immediately delivered several promising planets. That means it should be relatively easy to find interesting planets around that kind of star, which multiplies the chances of finding actually habitable ones.

Moreover, they are close to Earth (“just” 40 light years), so we can study their atmospheres with existing telescopes, or with the James Webb space telescope slated to launch next year. We’ll train on these for the multitude of planets we are about to find.

For now, as usual, no aliens.

If you want more
  • Do you remember what a fuss was made that time NASA found the first Earth-sized planet in a habitable zone? This time they were three at once.
  • NASA put together an impressive amount of information, graphics, and even an app. You can find everything here
  • The real star of the bunch is this phenomenal 360 video (fullscreen highly recommended!) of the view standing on TRAPPIST1d

Cover photo: CC0 David Mark/pixabay.com

The thirsty dragon and other capillarity magic

The mighty thorny dragon. Credit: wikimedia

There once was a dragon, who lived in the desert and loved to eat ants. He liked it so much that he gave up the ability to drink for that: he made his mouth perfect for eating, but useless when it came to sipping.

Instead, he learned something way cooler: how to conjure water from sand itself, summoning a force even stronger than gravity.

It’s actually a true story: the thorny dragon is a majestic, 20cm long Australian lizard. A recent study revealed that, to drink, it uses a complicated network of narrow channels that start from its feet, suck water from the ground. Then, snaking between its scales, they deliver the water directly into the lizard’s throat.

We bottled that force into wondrous technologies such as… er… paper towels.

Seriously, though, paper towels do suck water against gravity. Since nothing is pushing the water up, that’s more or less sorcery. However, scientists prefer to call it capillarity, and it’s fascinating.

You probably noticed that water sticks to stuff. It’s scientifically called—I kid you not—wetting. But water also sticks to itself, and these two things combined make capillarity work.

In a way it’s like keeping up a hammock. The net is made to stay together, it’s suspended from the poles, and it sags in between under its own weight. In a tube with water it’s much the same: the tube walls act as many tiny poles around the water surface hammock.


Water in a capillary tube is like a hammock, but the poles around it grow a little on their own. credit: Stefanie Laubscher/pixabay

Water has an important difference from the hammock net: it self-sticks to the tube walls. That means it can climb (a little) up the wall. That’s why capillarity works so well in narrow channels, with walls close to each other. Basically, the distance between the “hammock poles” is small, so there’s less space for the net to sag.

A paper towel and its maze of narrow tubes under the microscope. credit: Mattheyses Lab/Emory University

Though it seems a little more mundane, paper towels work the same way. Inside their mesh hides an intricate network of tiny tubes through which water can climb. A little dragon magic in our kitchens.

If you want more
  • Water sticks to stuff because of van der Waals forces. They’re very cool, but they deserve their own thing.
  • Now that we know how to get stuff wet, can we do the opposite? Well, yes… stay tuned!!

Cover photo: CC0 Tom Mathews/pixabay

Every snowflake is unique

No Christmas landscape is complete without snow. Lots of snow. And every little snowflake is unique, everyone knows that! How come, tho?

Snow is nothing else than teeny tiny ice crystals that form in the clouds and stay solid all the way down to the ground. Water crystallizes around microscopic imperfections, like dust particles floating in the clouds. Once the initial nucleus is formed, the microscopic droplets gather around it very rapidly.

Even though all snowflakes are somewhat hexagonal (due to the geometry of water molecules), each of them grows in slightly different conditions. Some had more water droplets close by, some other was in a portion of space a fraction of a degree warmer. Each and every factor counts: the final shape of the crystal is sensitive to exactly everything.


Snowflake Sculpture 3, CC-BY-NC Julie Falk, via Flickr.

So the shape of each snowflake is random, it’s like rolling a die with infinitely many faces. You never know what will come out and all outcomes are different. If you want to put it in more physics-pompous terms, the formation of snowflakes is a stochastic process.

In the end, each snowflake is a picture of the exact conditions in which it formed. And since it’s impossible to reproduce the exact same conditions twice, each of them paints a slightly different picture.

Like pictures, snowflakes too come out better if the scene doesn’t move too much. Indeed, if conditions don’t stay relatively constant around the budding crystals, anything can happen. Most of the times, several crystals aggregate in one big snowflake, sort of a little snowball, which look a lot more like each other.

Regardless of the conditions, it’s really hard to tell them apart anyway.

Happy holidays from amorefisico!

Cover photo: Snow leopards playing in the snow, CC-BY-ND Tambako The Jaguar, via Flickr. Some rights reserved.

Frozen bubbles

In the cold of Canada, a man blows a soap bubble, which immediately freezes. Since that’s awesome, the guy makes a nice video about it.

Who knows, maybe he even knew about all the physics that was going on in front of his camera.

Let’s start from the easiest thing: why does the bottom of the bubble freeze last? The answer is gravity: the outermost layers of the bubble slide down, so the base just has more water to freeze. And it takes longer.

But why does it freeze in spots, instead of just from the top down?
Because cold is not the whole story. Water needs a point to start building its crystals, if it can’t find one, it stays liquid well below zero degrees (Celsius), but is very unstable and freezes at the slightest disturbance.

In a stunning turn of events, a soap bubble has plenty of soap molecules floating around, and they are excellent starting points for the ice crystals we see growing on the surface.

Finally, why does the bubble pop instead of just staying there? In the comments to the video, the author says he popped it, but I do believe he could have just waited a little.

The air he blew in the bubble came from his lungs, so it was over 30 degrees. Pressure inside the bubble, initially, is the same as the atmospheric one, but it drops as the air cools down. This puts a lot of stress on that thin and stiff ice surface. Sooner rather than later it was bound to collapse anyway.

All this magnificent physics in less than 30 seconds. What a wonderful world…


Foto: Frozen, CC-BY-NC-ND Benjamin Lehman, via Flickr. Some rights reserved.