Thursday, April 28, 2016

Volcanoes in Space!


Volcanoes in Space! Sounds awesome right? Volcanic activity is one of the major forces that shape the surfaces of planetary bodies, along with meteorite impacts, tectonics (deformation of the planets crust) and action by an atmosphere (wind, water etc).
Aren’t there lots of volcanoes on Earth that we need to understand? Why yes, there are, and studying volcanoes on other planets helps us understand Earth volcanoes. One of the reasons I love studying things not on Earth is that they help us question our own basic understanding and assumptions about how things work on Earth. Science is a process, our understanding comes from constantly asking questions and seeking new sources of evidence and ways to test our ideas. The first step to understanding natural processes, such as volcanic eruptions, is we first have to make a few observations and come up with a really simple model of what is happening. That lava flow came out of the ground, therefore lava comes from underground. Then, as we learn more through continued observation and testing we make that model more complicated. Lava can also come out of the ground explosively, and the rocks made by those explosions have lots of holes in them (vesicles) so gas is involved in these eruptions. (These were real steps in our understanding of volcanoes, but we’ve learned a lot more about what happens since).

Alison observing lava coming out of the ground, and lots of gas. In this case SO2, sulfur dioxide. Even if we know more about volcanoes than we used to, direct observations are an important part of the process of understanding them better.

These models are based on evidence, and we must collect said evidence (observations, test results etc). Natural phenomena occur at incredibly small and incredibly large scales, and everything in between.  This makes it particularly challenging to observe everything that is happening, so we break it down into smaller pieces and then try to come up with a model that agrees with all the parts. Our ability to make observations has frequently been technology limited. Before we had the ability to blast rocks with X-rays (and now lasers, electron beams and other cool stuff) we could describe the minerals in a rock, but we couldn't say much about exactly how much of certain elements were in the rock (there was some wet chemistry that helped some, but man was it time consuming and hard to get a representative result).  We can make all sorts of educated guesses about what happens under a volcano by looking at old eroded volcano plumbing systems, but if we want to say something about a specific active volcano we need to be able to say something about what is happening underground.  We now have lots of new awesome techniques looking at small variations in gravity and how seismic ways speed up and slow down (geophysics) that help us estimate where hot rock is under volcanoes. This is a huge advance, but there are still lots of complexities about the subsurface that leave us with questions, some we didn't know to ask before. 
Technology lets us look at Earth, other planets and volcanoes in lots of new ways. At the center of the image you can spot Momotombo in Nicaragua with active lava flows in December 2015. More information on this infrared image from the ALI instrument via NASA photojournal.  

So what does this all have to do with volcanoes in space? Let’s look at some really general models of volcanoes. What is a volcano? That is easy right? It is a spot on the Earth where lava and gas comes out. Do all volcanoes look the same? Nope! They come in lots of shapes and sizes. Ok, but we still call them all volcanoes because they do the same basic thing. Geologists started looking at different volcanoes and seeing which were most alike, and which weren’t and then came up with the major types of volcanoes. So now when you take an Earth Science class you usually learn about the major shapes of volcanoes: scoria cones, stratovolcanoes, shield volcanoes, calderas, and don’t forget the maars! What makes them difference is what type of lava comes out, how much, how fast, and what it comes in contact with on the way. So that really simple definition is good, but it covers a lot of complexity.
Volcanoes come in all shapes and sizes. This is a maar (200 m across) that is inside a large caldera volcano (4000 m across), Askja in Iceland.

Volcanoes also don’t happen just anywhere on the planet, they happen in places where magma is being made in the mantle and has a way to get to the surface. We really had to adjust our understanding of volcanoes when we realized that most volcanic activity on Earth happens under water at mid-ocean ridges. Understanding how mid-ocean ridges make new crust was a really critical part of our understanding of how plate tectonics works and plate tectonics is vital to the way that we understand how magma is created and accumulated in the mantle. These simple observations are therefore fundamental to models of how the planet works! This is just one of the many ways that we improve our definitions of natural processes with new observations and tests.
Pillow lavas form when lava erupts into water and are common at mid ocean ridges. Other underwater lava types include lobate and sheet flows. There are a lot of these lavas on Earth, they are just harder to go visit. National Oceanic & Atmospheric Administration (NOAA).
So if we are going to talk about volcanoes in space, we need to ask questions like, do other planets have volcanoes that are the same size and shape and on Earth? We can even ask if it has to be lava to be a volcano? If we look at our neighbors Venus and Mars we see right away that there are lots of volcanoes. Even our moon has a volcanic history! There are just enough similarities between Venutian and Earth volcanoes (and Martian and Lunar) that we could recognize them for what they were: the products of stuff (probably hot) coming out from inside. There are also a LOT of differences. These make us ask questions about how magma/lava is made on Earth and then on other planets. Some of our basic assumptions about the Earth and its rocky neighbors is that they started out with very similar compositions, but do these different shapes mean they erupt the same things? Frequently we also look at our solar system as though we are somehow ‘normal’, mainly because we understand Earth (since we’ve had access to it longest). How similar are they?

Lava flow on Mars looks a lot like lava flows on Earth, just with more impact craters. Mars Orbital Camera from NASA photojournal.
Mars is home to the tallest volcano in our solar system: Olympus Mons. This volcano is shaped like the Hawaiian volcanoes like Mauna Kea, Hawaii, but it is more than twice as tall and would cover the entirety of the state of Arizona or most of Germany. So it is a LOT bigger than a Hawaiian volcano. It has long lava flows and shallow slopes similar to the Hawaiian volcanoes suggesting that it was built by repeated eruptions of runny basaltic lava. So does this size thing matter? There are several really really big volcanoes on Mars, and probably thousands of small ones that we are only beginning to catalog. But we don’t have anything on Earth quite like Olympus Mons and its buddies like Arsia and Ascraeus Mons.  So what is the difference?
Mars topography from the Mars Orbital Laser Altimeter. The highest elevations (in reds and white) are volcanoes. You can see Olympus Mons on the far left and its neighbors (Albus, Arsia, Ascreus and Pavonis). The Elysium volcanoes are the round peaks on the right hand side of the image. If we made a similar image on Earth it would be much harder to identify individual volcanoes!
This makes us ask questions like 'So why can’t we get volcanoes that would cover European countries on Earth?' The answer is plate tectonics. Large volcanoes need a good supply of magma to be able to grow, but on Earth our crust is broken into plates that are moving (about as fast as your finger nails grow). So if we have a nice supply of magma, say in the form of a Hot Spot, like is under Hawaii and the Galapagos, if the plate moves (even slowly) the lava that reaches the surface will be more spread out. Since Mars doesn't have active plate tectonics all the magma comes out at the same spot over and over again.  On Earth we do have plate tectonics, so instead of one jumbo volcano, we get a chain, usually islands. Now, while we knew about hot spot volcanism from observations on Earth, this information from comparing other planets provides further evidence (which is always a good thing!).
Lots of overlapping lava flows on the flanks of Olympus Mons. NASA Photojournal.
There are also volcanoes on Venus, but they have very different shapes from those on Earth or Mars. The lava flows can frequently be more than 1000 km long. Venus also has lots of short squat pancake shaped domes on that we just don’t see on Earth. What is the difference? Is it the material itself, for example, does Venus produce something that is just hotter and runnier than what we get on Earth? Or is it the environment on the Venutian surface? The atmosphere on Venus is not very friendly, because it has 90 times the pressure and  the temperature is 400 degrees Celsius hotter than  Earth. So while that isn’t so great for carbon based life-forms, it does help lava stay hot and travel further. To see if the lengths and shapes observed on Venus were possible with compositions of lava that matched Earth we really needed to be sure we understood how lava flows worked on Earth! At the same time there are more familiar volcanoes on Venus that might be shield-like volcanoes like those of Hawaii and Mars, so this tells us this process is easy to do in a range of environments. The long lava flows and domes (next to normal shields) suggest that Venus has more than one composition of lava: some that are runny and some that are sluggish. These lavas are likely very similar in chemistry to those on Earth, they just behave different in such a different environment. The atmosphere on Venus, however, has a bad habit of destroying spacecraft, which means we don't have near as much data for Venus as we do Mars or Earth. We still have a lot to learn about our bright neighbor.

Lava flows on Venus look like lava flows on Earth, but can be 1000's of kilometers long (an order of magnitude bigger than our most impressive lava flows on Earth). Radar image from Magellan spacecraft, NASA photojournal.
In the case of the Moon we have not only observed lava compositions and landforms remotely, but we have samples of those lavas collected by the Apollo astronauts that reveal just how familiar these features are. Next time you look up at the moon you can appreciate that the those dark patches are basaltic lava. You may also notice in maps and images of the moon that even though we see lava all over the place, there aren't any large pointy landforms like Mt. Fuji. So the picturesque volcanoes of Earth are not common away from Earth. From a quick tour of our neighbors we see that lava flows are a pretty common thing, but the places where lava piles up (volcanoes) come in a wide variety of shapes. Our definition of volcanism now includes even more variety than we expected from first glance on Earth.
Stacks of basaltic lava on the Moon exposed in an impact crater.
So let us keep following this path. Do volcanoes only erupt lava? On Earth we have a things we call mud volcanoes, which is a pile of mud made by hot mud and water erupting out of the Earth’s crust. What do volcanoes oozing/exploding lava have in common with mud volcanoes? The process: the transport of stuff from inside out. So if we take this concept and apply it to other celestial bodies can volcanoes erupt water, or even salt? Alright, so maybe we can make a bigger definition of volcanism that includes ‘hot stuff coming out of a planet’. So do we see that process on other planets? Why yes! A term that you hear a lot when studying the outer solar system is Cryovolcanism. Cryo means ice, and I know, ice doesn't make you think of 'hot', but temperature is relative in space. Cryovolcanism happens on icy satellites that have liquid water hiding under their surface instead of hot rock like we have on Earth. So this liquid ocean is hotter than the surrounding icy crust, and thus buoyant,  and can erupt to form plumes, form new crust, and form slowly rising plugs of ice, not unlike what we are used to on Earth, only with ice. So does ice volcanism work the same way as magma made from silicate rock? Not quite, but there are important similarities.

Basaltic lava on Earth. In planetary geology we call it a silicate lava to contrast it with other potential 'lavas'. Silicate means silica is one of the major components. Silicate minerals all contain a silica tetrahedron of silica and oxygen and can bond with lots of other important elements like Mg, Fe, Ca, Na, Al and so on. It is a good generic term when we don't want to limit the composition we are talking about, or don't know any more!
Volcanism is how a body loses heat and that is accomplished by moving less viscous (i.e. more runny/liquid) material from inside to the outside. What is different is how that liquid is formed, i.e. what is the energy that makes the liquid. Things melt when we raise the temperature, decrease the pressure, or change the composition of what is being melted. But for any of these to work the material we are melting (rock, ice, chocolate... mmmm chocolate volcano) must be reasonably close to its melting temperature in the first place (that is why volcanism is a process of cooling off the body). The melting temperature of silicate rocks like we have on Earth are about 900-1200 C (1600-2200 F). Basalt lava, the most common type on Earth that I've mentioned a lot in this post is at the higher end. These temperatures are high enough that the inside of the planet has to be hot in the first place so that melting isn't too difficult, but not so hot that our planet is just a ball of magma. On Earth a lot of our planet's internal heat comes from the decay of radioactive isotopes (the rest is left over from formation). This means even as our planet  gets older there is an ongoing process to keep the inside warm. Without that internal heat all the other processes that help melt the mantle and make magma wouldn't function (i.e. plate tectonics again). We will continue to have active volcanoes on Earth for a while.
Enceladus, a moon of Saturn, is mostly made out of ice. It is pretty cold, -198 C (-324.4 F) on its surface.  So how do we melt ice that far away from the sun? The process is called tidal heating. Basically the gravity of the other satellites and Saturn all pull on Enceladus and cause it to deform and stretch. You can think of it as whole moon friction. If you were to squish a tennis ball repeatedly it would eventually get warm. This friction melts the ice beneath Enceladus' surface and we now have a way to make a liquid ocean under this crust. Water occasionally bursts through the crust and forms plumes, giving us evidence of this liquid beneath the surface.
An artists rendering of the plumes from Enceladus with possible hydrothermal systems driving water from the subsurface ocean to the surface to form plumes. NASA photojournal more description here.
Are there any other unique types of volcanoes hanging around our solar system? We have several candidates for salt bearing domes on Pluto and Ceres (a dwarf planet in the asteroid belt between Mars and Jupiter who's name is pronounced 'series'). Wait, salt? But does that happen on Earth? Yup, sort of. But volcanologists don’t usually look at them, they are called salt tectonics, or salt diapirs. The salt occurs in pockets in our crust (so much shallower than where most magma is made) and rather than hot it is just less dense than surrounding rock. Because it is less dense it rises from depth and then can flow out on the surface. There are really neat examples, called salt glaciers, from the Zagros Mountains in Iran. These differ from our usual definition of volcanism because there really isn't any melting involved, it is more a result of layers of salt that were deposited at the Earth's surface and then buried, then squished, then escaped at the surface. That is when the layers get compressed during mountain building the salt starts to collect and rise locally and can eventually reach the surface.

Salt flows in the Zagros Mountains, Iran are examples of extrusive salt tectonics on Earth. Scale bar is 10 miles. Image from GoogleEarth.
The features on Ceres and Pluto are potential volcanic features is because there is evidence that material is rising endogenously (from inside) the planet.  Compositional data from satellites suggests that the features on Ceres is some combination of ice and salts. Because we don't fully understand how material is distributed inside these dwarf planets and what drives the material to the surface we have more questions than answers. The whole extrusion of stuff from inside means we can compare it to volcanism for now. As we get more data back from ongoing missions looking at both of these bodies we’ll learn more about the relationship between these potential volcanoes and their surroundings and maybe more about how they formed.

Ahuna Mons on Ceres appears to be endogenic (sourced from inside the planet). It is taller than Mt. Rainier, but not shaped like it... and is a good candidate for volcanism. Image credit NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
What is even more awesome is that some of these are not just ancient processes that are done and dead; in many cases we can watch this wide range of volcanism in action! Enceladus is active, and we’ve sent a spacecraft through the plume! There are even active lava flows made of silicates like on Earth, on Jupiter’s moon Io! So really there is a lot to look at off of Earth. The variety and the similarities are all useful in helping us understand how our own planet works, and how we can better investigate it in light of the constant surprises we find off Earth.
Active volcanism in our solar system, a plume from Io, a moon of Jupiter. NASA photojournal.