Meteorites and Planets and Isotopes...OH MY

Have you ever wondered why the planets in our solar system are the way they are? For example, why are the inner planets (Mercury, Venus, Earth and Mars) rocky and the outer planets (Jupiter, Saturn, Uranus and Neptune) gaseous? How did the planets get to their current locations? When did the planets form? What are the planets made of? All these questions are topics that planetary geologists spend their careers researching. But how?

Schematic of our solar system with Mercury, Venus, Mars and Earth in the inner solar system, respectively. The rocky planets are followed by the asteroid belt that separates them from the gaseous planets: Jupiter, Saturn, Uranus and Neptune. Beyond Neptune is the Kuiper belt with more asteroids.

We use these things called meteorites. Meteorites are small (but sometimes really big) objects that formed in space, traveled all the way to Earth, and survived the descent through Earth’s atmosphere to land (potentially) in our back yards. Meteorites come in all shapes and sizes. Most importantly, they are made up of a wide range of compositions. One type of meteorite useful to planetary geologists is iron meteorites, which are made up of 100% iron and nickel.

Photo of an iron meteorite.

Iron meteorites are particularly useful because they are believed to be some of the very first objects that formed in our solar system (way before the Earth) as far back as 4.5 billion years ago. We can study many things from them to learn about where they came from and when they formed. We use certain elements like tungsten (W) and molybdenum (Mo), which are very common mining products in the United States. The abundances of certain forms of these elements can tell us about an iron meteorites’ age and genetic heritage. An iron meteorites’ genetic heritage is much like our own genetic heritage. You, your brother and your sister are all descendants of one common ancestor that descended from a certain location. Many different iron meteorites can descend from their own common ancestor, a small planet and come from a certain location. 

Although almost all elements on the periodic table are found in our solar system, only several are beneficial when it comes to iron meteorites. We use select elements to help characterize iron meteorites in order to gain insight into the processes that occurred early in our solar system’s history.

Many planetary geologists think that all meteorites fall into two groups and come from two different locations. Think of these two groups as two families that came from North America and Europe. These families were never able to travel to one another because the oceans kept them apart. As a result, the families never met one another and were never able to mix their genes. This is what planetary geologists believed happened with iron meteorites. One group formed inside Jupiter's orbit (where the Earth is) and the other group formed outside Jupiter's orbit. Jupiter never allowed the meteorites to mix, causing them to be genetically different from one another. Because of this, we can use abundances of certain elements within iron meteorites to tell us about the unique processes that occurred during their formation.

When Minerals Click "Save"

When Minerals Click "Save"

How garnets act as snapshots of changing conditions in metamorphic environments.

Have you ever missed the peak of a sunset and walked out of your house right at the tail end? You have an idea that something really cool was displayed up in the sky, but there is no real way to know how bright, how pink, or how awesome it was? We have all experienced this or a similar time where the scene/person/information you wanted is not available to you because you weren't there. For many earth processes, geologists don't have direct ways to peer into the system. We can't walk into a volcano chamber to see how fast the hot rock flows in, or make your to the core of the planet to grab a piece and see what its made of (despite what Journey to the Center of the Earth would have you believe)!

Sunset mid-changing colors. The garnet picture below is from this island!

But imagine if you had a camera- and you could put it anywhere. And that camera had the ability to record lots of information. You would then have more insight into that place at that time. And if you put a bunch of those pictures and informational records together, you could piece together a story! In our sunset analogy, you would see how the colors changed or how the temperature varied. Well the mineral garnet (the pretty, pinkish-red birthstone for January), acts just like that recorder, a little spy in the heart of some geologic settings. Why are garnets so special, you may ask? Garnet is found in many metamorphic rocks, rocks that got really hot or were put under pressure and changed in response. And because these garnets are found all over the world, comparing conditions on a global scale becomes possible. The mineral structure (or how the mineral is put together on the tiny atomic scale) is particularly good in acting as a "safehouse" for rare elements which contain key information to what was going on when the garnet was forming. 

Garnet grain with different amounts of an element.

The breaking up of the grain also share insight into

how beat up the rock got.

Take a look at the picture to the right. This is a how a garnet looks up close. There is a lot this garnet is trying to tell you! First, focus on the colors. This is a sort of map, where the warmer the color, the more of a certain element there is in that region. These maps can be made for many different elements, and when that information is put together, you can tell the chemical history of the rock as the mineral grew. Next, the shape. You can see that it has a defined shape, possibly squarish or hexagonal. Because the edges aren't eaten away, geologists can say that the mineral was able to grow in its normal pattern, without any space restriction. Another aspect is how broken up the grain is. It almost looks as though you took a small hammer to it. Being able to see how the mineral has been broken and deformed is another key piece of information about what the rock had to go through from the time it was made to the time it ended up on the surface where we could grab it. When scientists can piece all that information together, they are presented with a clear view into the story of formation for that rock, and on a bigger scale, the overall region. Having a team of historians in the rock that are good at telling stories is very helpful when you're attempting to piece together the past!

Explaining Highway-Speed Magma Ascension

Geologists think magma travels from where it originates beneath the oceanic floor to where it erupts in only 1000 years. While 70km / 1,000 years is orders of magnitude slower than the speed we drive, or even walk, in terms of geologic speeds this is quite rapid. Many geologic processes, such as the uplift of mountains, takes millions of years, not thousands! This ascension is, fundamentally, driven by pressure; much in the way air leaves a balloon when squeezed, or what drives water up a well.

How magma can segregate and ascend so rapidly has been a question geologists have been working to answer for decades. There must be processes which explain the limited observations that exist of the volcanic features underwater where juvenile oceanic floor is created, and where life could have originated billions of years ago. Not only could these underwater volcanic regimes have impacted the evolution of primitive bacteria billions of years ago, but they play an active role in Earth's climate, oceanic and atmospheric chemistry, and the ability for Earth to continue sustaining life. 

Photograph showing volcanic gas venting near the Mid-Atlantic Ridge, where some of the most primitive life forms are thought to have evolved. Image from New York Times (https://www.nytimes.com/2016/01/12/science/midocean-ridges-volcano-underwater.html)

Explaining magma movement here is not as simple as explaining other surficial or sub-aerial geologic processes, largely because observations are rather limited. The magmatic regions beneath these volcanic settings are deeper than anything humans can directly sample. The observations that exist are largely indirect, and mostly fall under three groups of evidence geologists have grown over the past several decades. 

1.) Rocks that form from the magma that travels upward beneath the sea floor can provide estimates on how quickly the magma that forms these rocks traveled. The chemical composition of these rocks suggest magma moves very quickly, yielding the estimate of ~1000 years from magma origin to eruption along the oceanic floor.

2.) Waves generated by the vibration from earthquakes that can inform scientists about how much magma is beneath the ocean surface. Much in the same way that your voice sounds different underwater as it does otherwise, the waves generated from an earthquake behave differently depending on the material they travel through. This distinction of how waves behave differently through various materials can provide an estimate of how much magma is in a specific area of the Earth. 

3.) The observation of electric currents can also provide estimates of how much magma is beneath the sea floor. Did you know how an electric thermometer works? Slight fluctuations in temperature impact how quickly an electric current can travel, and from this change in speed of the current, the thermometer can yield a precise measure of temperature. Material properties also affect the speed of currents; otherwise, we would not make wires out of gold and copper, if a cheaper metal would work just as well. Using this relationship, scientists can estimate the amount of magma beneath the sea floor.

         However, while the geochemical evidence suggests magma travels extremely quickly, the waves generated by earthquakes suggest the regions beneath the sea floor contain less than two percent magma. How magma ascends this quickly with such little magma present is not clear. Meanwhile, the percent of magma estimated in these regions is different depending on if you look at the vibrational waves from earthquakes, or the data from how quickly an electric current can travel in this region. 

Simple mid ocean ridge diagram, where magma shown in orange, rises from a large region of melt origin at the bottom of the diagram, to a very narrow region at the surface of the sea-floor (shown in blue). Image from https://divediscover.whoi.edu/mid-ocean-ridges/types-of-ridges/

           Geologists have suggested several ways magma might achieve rapid ascension in these regions. One could be that, at shallow depths, the oceanic floor is cracked, and magma travels quickly through these cracks. Another, is that some regions beneath the sea floor are chemically distinct. This could enable some regions to melt more magma initially, which could promote rapid magma movement. There are several other factors that impact rapid magma ascension, but all suggest magma would have to ascend at geological highway-like speeds.

          Better experimental constraints that simulate magma movement and how magma might develop "highways", and more highly resolved mid ocean ridge observations, will help explain these geologic structures which are the interface between the Earth’s surface and the deep Earth. These are interfaces where processes occur that directly influence oceanic sea life, the ocean’s chemistry, and Earth’s climate, and as such, are paramount in understanding Earth as an entire system. 

The Copycat Core: Recreating Earth's Magnetic Field in the Lab

If you have ever used a compass, you have experienced the effects of Earth’s magnetic field—in fact, you experience the effects every day without even realizing it! The magnetic field shields us all from harmful solar radiation and allows for our electronics to function properly, but have you ever wondered why Earth has a magnetic field in the first place, or how we study it? Earth scientists explain its presence with something called the geodynamo, and build laboratory experiments to simulate it.

What is the geodynamo, anyway?

A depiction of Earth's magnetic field. Blue lines represent
magnetic field lines. Figure credit: Hyperphysics

The geodynamo is Earth’s magnetic field engine, similar to the engine in your car. Instead of using little explosions to convert gasoline into motion the way your car does, however, it uses swirling liquid metal to convert electricity into a magnetic field. The idea is that Earth has a solid core, with liquid metal material surrounding it. Since the Earth is spinning, the liquid metal sloshes around like a fish tank on a turntable, and that turbulent motion throws off electrons from the metal. Electrons in motion create an electric current which induces a magnetic field. Once the process gets started, it can keep going and make the magnetic field bigger by cloning itself into lots of tiny engines, increasing the overall field strength.

Sounds nice…but can you prove it?

Despite what Hollywood wants us to think, we cannot travel to the core to confirm if the magnetic field engine idea is correct. Instead, we build experiments that simulate the Earth to test and study the phenomena indirectly.

Meet the 3-meter Geodynamo Experiment

A 3d depiction of the Geodynamo experiment. 

Figure credit: Laurent Hindryckx. 

The largest experiment studying how the core makes a magnetic field is nearly 10 feet in diameter and lives in a hangar at the University of Maryland. It consists of an interior sphere, meant to represent the solid inner core, encased in a larger sphere, with 24,000 lbs of liquid sodium metal between the two, meant to represent the outer core (that's approximately the weight of 2 helicopters!). Motors on the spheres allow for each to rotate independently and copy the way the Earth spins.

So far, the experiment has been partially successful at making an Earth-like magnetic field, but only with extra equipment to start the process. In order to better match real-world conditions, the entire setup is being given a rough makeover. Right now, the inner sphere has a smooth surface, so even though the metal is flowing in a chaotic way, it is not quite enough for the magnetic engine to start on its own. It is like trying to strike a match against a smooth surface (or start a car without spark plugs)--we do not have the right surface to create a spark. Adding roughness to the inner sphere increases friction so the engine can turn over and start.

How rough is rough enough?

The final design for the inner sphere of the 3-meter
Geodynamo experiment. Each bend helps increase
turbulent flow when the experiment is running.
Photo and design credit: Ruben Rojas

Based on smaller experiments and simulations, the winning design for the inner sphere looks similar to the ridges of a basketball, with each bend helping to increase the friction (turbulence) for the liquid sodium.

It is a logistical and safety challenge coordinating the modifications, but they are currently on track to be complete by summer 2020. Stay tuned!

For more information, click here.

Proposal Figures: Seismic Anisotropy in MTZ

Figure 1. The phase diagram of mineralogy and thermal gradient of top 1000 km of Earth’s mantle. The mantle transition zone is marked by two phase changes at 410 and 660km discontinuities. The 410 km discontinuity is caused by a phase change from olivine to wadsleyite. The 660 km discontinuity is caused by a phase change from ringwoodite to perovskite and manesiowustite.  (Bellini et al., 2013)

Figure 2. The minerals in mantle transition zone are aligned by the subducting flow. The mineral alignment can produce seismic anisotropy in the mantle transition zone.

Figure 3. The ray path of SS phase and SS precursors. (A) The whole ray path of SS phase and SS precursor with epicentral distance 125 degree and 180 degree. (B) The ray path beneath the SS bounce points. The polarization of SV wave and SH wave is shown as blue arrows and red arrows respectively. This study will only use SH wave to look at the azimuthal anisotropy.

Figure 4. The illustration of shear wave splitting in anisotropic materials. Shear wave can be split into two orthogonally polarized waves (blue and red) after traveling through anisotropic minerals. (Ed Garnero, http://garnero.asu.edu/research_images/images_anisotropy.html)

Figure 5. A map of earthquake event and station locations. The red circles are earthquakes and blue triangles are stations.

Figure 6. The map of SS bounce point locations and azimuths. The azimuths of bounce points are plotted as different colors. The black boxes are the locations with enough bounce point density and azimuthal coverage.

Figure 7. The  S410S travel time (top) and S660S travel time (bottom) are plotted against azimuth. The grey circles and error bars are the travel time measurements and uncertainties from individual azimuthal bins. The red circles and error bars are the average of individual measurements  within every 30 degree.

Figure 8. The  differential travel time between S410S and S660S are plotted against azimuth. The grey circles and error bars are the travel time measurements and uncertainties from individual azimuthal bins. The red circles and error bars are the average of individual measurements  within every 30 degree.