jupiter – Rising Ape Collective

By Jonathan Farrow of the Thoughtful Pharaoh

With this post, rising-ape.com is now caught up with my website, thoughfulpharaoh. From now on, I will be posting articles simultaneously on both sites, on Wednesdays.

Thanks to everyone for following along and as always, if there is a topic you have in mind, don’t hesitate to leave a comment below.

And now for this week’s article: exomoons.

There are 8 planets in our Solar System (sorry Pluto). Most of these planets have companions that follow them around, like obedient pets and criminal records. The total count of these moons is 181. We are all quite familiar with the big shiny one that orbits Earth (that may or may not be made of cheese), but what many people don’t know is the sheer number of other moons that exist in our Solar system.

Just like planets, these moons come in all different shapes and sizes. S/2009 S1 is only 400m across and orbits in one of Saturn’s rings, making it the very smallest moon. Ganymede, the solar system’s largest moon, measures in at about 5300km across, almost half the size of earth.

One of the biggest findings to come from the Kepler mission is that most of the stars in the galaxy have planets. In other words, our solar system isn’t unique. That means our Solar System probably isn’t the only whose planets have moons. If our system, with 22 times more moons than planets, is any indication, there are a lot of moons to find.

This presents two immediate problems: firstly, why should we want to find them? Secondly, how do we go about finding them?

Why find an exomoon?

The same thing that makes seawater rise and fall twice a day, tidal forces, can heat up a moon. Tides are a result of the fact that the strength of the force of gravity is related to the distance between two objects. On Earth, the water on the side close to the moon gets pulled out towards the moon stronger than the water on the other side, this creates bulges of water that move around as the earth spins: tides.

The Earth is too small and our moon is too far away for much more than sea level change to happen, but Io, one of Jupiter’s moons, has over 400 active volcanoes caused by extreme tides from the gravity of its host planet Jupiter. In this case, it’s not just bulges of water that are created, but bulges in the crust of the moon itself. This creates an immense amount of friction and heat. Europa, another moon of Jupiter, gets enough energy to keep a planet-wide ocean of water liquid under its icy crust. Some people think Europa might be habitable, even though it is so far away from the Sun.

If there are moons here in our Solar System that can be habitable at Jupiter-like distances, there could be moons in other systems that orbit planets much closer, at Earth- or Mars-like distances. Some people, like Rene Heller at McMaster University’s Origins Institute (a fine institution, if I do say so mystelf *full disclosure: I did my undergrad there*), think exomoons might be our best shot for finding habitable places in the galaxy simply because of their abundance relative to planets (remember, there are 22 times more moons in our system than planets).

How to find an exomoon

This is the tricky part. It was hard enough finding exoplanets. Finding a transiting exoplanet is often compared to looking for the effect of a mosquito passing in front of a car’s headlight. In that analogy, finding an exomoon would be like finding out how many legs it has. No easy task.

It’s not impossible, though. Moons do have effects on their planets and if we look carefully enough, we can find them.

One way to find exomoons in transit data takes advantage of the fact that, viewed edge-on, a moon will appear more often at the edges of its orbit.

If you capture many transits over time, you can begin to see these wingtips in the transit data.

The grayscale bar in the image above represents the average effect of a moon orbiting a planet. What astronomers can look for in the transit data is a preliminary dip (1) that starts off severe then levels off, followed by the regular planetary transit (2), followed by another characteristic dip (3) as the other wingtip passes in front of the star.

This method only works if you have a lot of data, but luckily Kepler was operational for four years and gathered just the right kind of data.

So now the search will begin. Who will find the first exomoon? And what if it turns out to be “no moon” at all?

An artist's impresison of a view from an exomoon with a triple star system. Far out, dude. Image by NASA/JPL-Caltech — An artist’s impresison of a view from an exomoon with a triple star system. Far out, dude. Image by NASA/JPL-Caltech

By Jonathan Farrow from the Thoughtful Pharaoh

If you look up in the night’s sky and point even a simple pair of binoculars at Jupiter, like Galileo did with a rudimentary telescope 405 years ago, you will see what he did: a reddish-pink planet with swirling masses of clouds. These clouds are beautiful in their own right, but there is one particular feature that has drawn the eyes and the fascination of people for over four centuries. The Great Red Spot.

This swirling, gurgling red super-storm could fit three earths inside of it and has been raging on the gas giant ever since we’ve been keeping records. How it has lasted for so long and why it has such a different colour has long been a mystery.

In 2000, on the way to Saturn, an ESA mission called Cassini aimed to give us some clues when it flew within 9.7 million kilometres of Jupiter and looked more closely at the spot than we had ever done before or since. 9.7 million kilometres sounds like a lot, but consider that is only about 1% of the distance between Jupiter and Saturn. (As a side note, did you know that Jupiter and Saturn are further away from each other than Jupiter and Earth? I didn’t!).

It might have looked kind of like this, but with Jupiter instead of Saturn in the background. Cassini's main mission was to Saturn and its moons. — It might have looked kind of like this, but with Jupiter instead of Saturn in the background. Cassini’s main mission was to study Saturn and its moons. Image by NASA

With its flyby, Cassini found out that the clouds that form the spot are up to eight kilometres higher than the surrounding clouds and started to understand the chemical composition of the clouds.

14 years later, in November 2014, NASA scientists released results that combine data from the Cassini flyby with lab experiments on Earth. They showed that the colour must come from the interaction of ultraviolet (UV) light from the sun and the ammonia and acetylene in the top layers of the storm. Once the red particles are produced, they are trapped by the circular winds of the storm. This overturns the previous theory that it was the bottom clouds which provided the colour. The NASA scientists compare the colour of the storm to a sunburn rather than a blush.

So, thanks to NASA and the Cassini mission, we have a better idea about how the spot gets its colour, but last spring the astronomy world was in a tizzy because news came that the spot has been shrinking.

That's some pretty serious shrinkage! — That’s some pretty serious shrinkage! Image by NASA

Since it is so noticeable, the storm has been recorded as far back as the 1800s, when it was believed to be about 41000 km across (roughly equal to the circumference of Earth). The most recent image, from 2014, puts the size at only 16500 km (less than the length of the great wall of China). Not only is it much smaller than it used to be, but the rate of shrinkage is increasing.

Totally, 100% peer-reviewed, recollection of secondary school math to figure out when the spot will disappear. — Totally 100% peer-reviewed recollection of secondary school math to figure out when the spot will disappear. Image my own

If my calculations are correct, and if the storm keeps shrinking the way it has been, it will disappear entirely in about 35 years, in 2059. That means we may be among the last people to ever see the spot that Galileo spied on that fateful night in 1610.

Instead of relying on me and my calculations, however, NASA sent a spacecraft to go investigate. Juno left in 2011 and by now is more than halfway to its destination. When it arrives in July 2016, Juno will study the gas giant in a variety of ways and hopefully get the bottom of this whole shrinking storm mystery.