Tectonics on Earth's "Twin" Planet

Venus had often been called Earth’s twin planet.  While it has many similarities to Earth- size, density, composition, heat production- it turns out to be a very different world.  There are surface temperatures of 740 K (870° F, hot enough to melt lead) and an atmosphere of almost entirely carbon dioxide (a runaway greenhouse effect).   Venus is a dry planet; there is no substantial amount of water on the surface.  The atmosphere, however, has signatures of water leading to the conclusion that Venus was once a wet planet.  It seems Venus has become Earth’s evil twin. 

Fig. 1: A computer generated surface view of the volcano Maat Mons (source: NASA)

Another important difference is its absence of plate tectonics.  Earth appears to be the only planet with plate tectonics, which is the theory that Earth’s lithosphere (the crust and upper mantle) is made of individual mobile plates that can collide (to form mountains), spread apart (to create oceans and basins), or slide past one another (as in the case of the San Andrea fault).  Although Venus currently lacks plate tectonics, it is impossible to know if it existed before 500 million years ago.  It was around this time that Venus experienced a global wide volcanic event that resurfaced the planet.  Instead of plate tectonics, Venus is currently in a stagnant lid convection regime, which is a fancy way of saying that the lithosphere isn’t broken up into discrete pieces and tectonics is instead driven by the mantle convecting underneath the immobile lithosphere (Hot mantle rises and cools, then descends. The same principal as boiling water on the stove- see Fig. 2).

Fig 2: A simulation showing convection in a one-plate planet such as Venus. The light blue arrows indicate motion; warm mantle rises and cold mantle descends. (credit: Walter Kiefer and Louise Kellogg, lpi.usra.edu)

On Earth, plate tectonics require a global network of shear zones (areas of low strength and concentrated deformation, such as faults) throughout the entire lithosphere. The localization processes on Earth are dominated by water content and mineral that are layered like sheets. Venus' lithosphere is dry and lacks layered minerals, and yet the rift zones of Venus are remarkably Earth-like. The are of Beta Regio has often been compared to the East African Rift system due to their similar geologic characteristics (Fig. 3). Beta Regio has a rift called Devana Chasma, which is part of a triple junction system similar to the one seen in East Africa. For Devana Chasm to be as narrow as it is, we expect some form of localization to be active to concentrate the deformation into a localized region. It is likely that the presence of melt will have a significant impact on localization.

Fig.3:  Comparison of the East African Rift and Beta Regio; both exhibit "triple-junction" rift behavior. (credit: (top) Rathbun et al. Formation of Beta Regio, Venus. (bottom) Tesfaye, et al.  Early continental breakup boundary and migration of the Afar triple junction, Ethiopia)

It is thought that the addition of the appropriate localization mechanism(s) and including the fact that tectonics are not driven by the same forces on Venus as they are on Earth, that an agreeable model for rifts on Venus can be developed.  By understanding the tectonic regime of Venus we are better able to constrain its evolutionary history and interior processes.   Venus can also represent an appropriate analogue for early Earth, prior to the initiation of plate tectonics, which will allow us to better understand how it developed here.

Why are Martian volcanoes so lonely?

One of the first Mariner 9 images of the surface of Mars. The poor image quality is due to a planet wide dust storm occuring when the probe arrived. (Image courtesy of NASA/JPL)

Martian volcanoes were confirmed to exist in the early 1970s, from pictures taken by the Mariner 9 orbiter. These first images revealed massive shield volcanoes, like Hawaii on Earth but taller than Mount Everest and wider than Ohio.  One of the first observations was that three of these larger volcanoes, the Tharsis Montes, exist in a straight line, each separated by about 750 km. Rather than a volcanic chain like Hawaii, the line of the Tharsis Montes appears to be related to a rifting feature, but why are they spaced with a certain distance?

Topographic map of the Tharsis region of Mars. (Image courtesy of NASA)

My research focuses on simulating one idea for why the spacing exists. To start I use a computer program to replicate magma flowing up from the hot, deep inside of the planet to the cooler outside layer of the planet. The magma flows upwards because it is less dense than the rock around it, like how an inflatable beach ball raises to the surface of a pool because the air in the ball is less dense than the water. As the magma raises into the cooler layer of Mars it starts to freeze into solid rock. Eventually it freezes too much and no more magma can move up. This point is known as the permeability barrier.

Nature isn't perfect, so the permeability barrier would not be a completely flat layer. There would be points where the barrier may be a little higher than the surroundings. The magma beneath this layer would flow towards these points, like rainwater flowing towards the lowest point on the street. The barrier would be eroded by the heat of the magma at these points, raising it at some rate. All these little instabilities would grow and eat each other until only a few large ones would remain. At the peaks of the large instabilities focused magma may break through making their way to the surface, forming volcanoes.

2-D figure of volcano formation on Mars. Black dashed line represents originally flat permeability layer, solid black line represents later permeability barrier with the volcano wavelength, red lines represent magma flow direction, and green line represent heat flow where longer lines indicate more heat. (Image courtesy of Laurent Montési)

Overall the final system would look like a connected line of upside down funnels. The sides of the funnel are the permeability barrier that magma flows up and the spout is where the magma breaks through to the surface. The series of touching funnels is the wavelength, or spacing, of volcanoes. In summary, Martian volcanoes aren't really lonely, they're holding hands deep underground.

A line of funnels. The sloped funnel is like the permeability altered permeability barrier and the tip of the spout would be the volcano on the surface. (Image courtesy Walmart)

Wild images of Martian meteorites Lead to Information About Mars’s Ancient Surface and Atmosphere

Billions of years ago, a large asteroid hit mars and ejected large amounts of surface material into space. Some of the surface material traveled through space and made its way to earth and fell in Antarctica. Brave meteorite hunters found the Martian rock and delivered to me. I then looked at the rock under a microscope and took the cool pictures you see!

“Are those fossils of a piranha, a dragon, or an alien”? This may be the first question to pop into your mind! Your second question may be, “wait was there life on Mars”? Unfortunately, the answer to both questions is no. The images are pictures of skeletal minerals that are hosted within the Martian rock.

Although, the cool photos fail at answering the question of life on Mars, they do provide us with valuable information about how the surface of Mars interacted with the element sulfur in its atmosphere. Prior to studying the images, I measured the isotopes of sulfur. What are isotopes of sulfur? Well, sulfur can be thought of as having a personality disorder. At any given time, sulfur flaunts 4 stable personalities (aka isotopes) and depending on the activity its participating in during its lifetime, it will show different amounts of each of the 4 personalities. The personality (isotope) measurement of sulfur in this martian rock revealed that the sulfur must have been ripped apart from its original molecule by UV rays while living in the martian atmosphere. By studying the cool photos, I can get a better idea of what happened to the sulfur after it was ripped apart from it original molecule that allowed for the sulfur to make its way into the martian rock.



I made the following educated guess as to what happened to that sulfur:  After the sun ripped a bunch of sulfur molecules apart, the sulfur to fell onto the surface of mars. A volcano erupted, spewing lava over the surface, allowing the lava to capture and incorporate the sulfur into the flow (a process called "assimilation"). The lava cooled and became rock. An was then ejected off of the surface and into space by a large impact.

I was able to use a microscope with a camera to take really zoomed in pictures of the Martian meteorite and load the images into a computer program that is able to detect small differences in color (or brightness) within the image. The colors of interest in these images are the dull light/tan/gold color of the “fossil looking minerals” (highlighted in green as 'magnetite') and the really tiny bright gold/yellow specs which are the sulfur minerals (highlighted in red as 'sulfide'). A proposed chemical reaction involving sulfur's interaction with the surface suggested that we should expect a certain amount of “fossil looking minerals” to sulfur minerals. The computer software calculated the amount of “fossil looking minerals” and sulfur minerals in terms of brightness and confirmed that the hypothesis was indeed correct! Thank you for reading!

Me calculating sulfide to magnetite ratios in order to test hypothesis

  

Taking a Picture of the Earth Deep Beneath Our Feet

Taking a Picture of the Earth Deep Beneath Our Feet

Figure 1 | Poor image quality -Little information is given to aid
in the understanding of this image.

    The figure to the right  (Figure1) may look a bit incomprehensible. Even through you have more information in some locations than others, regarding the shapes and colors, it may still be puzzling to figure out what is going on in this image. The poor image quality leaves much room for interpretation, if we want a better idea of what Figure 1 is we will need more information for higher resolution. 

    In the past, lack of information to create higher resolution images has been a problem in creating seismologists’ picture of the continents.  Seismologists seek to reveal the geologic structure (the type and thickness of differing rock layers) many kilometers beneath our feet. Seismologists look into the earth with seismograms collected from seismic stations. Seismograms contain information about four main things: an earthquake, the station, background movement, and the geologic structure of the earth. If we remove the earthquake, station, and background information, we are left with information from the earth’s structure. This is one method seismologists use to “see” a little bit of the earth’s structure at each station. The effect is similar to a taking a telescope and pointing it into the ground at each station. Until recently, the images of the geologic structure from seismic stations have been fairly unclear (like Figure 1). The National Science Foundation funded project to improve the image (or resolution) of the interior of the earth called Earthscope USArray.

    Earthscope USArray densely packed seismic stations across the continental US and Alaska to collect information about the earth beneath our feet. The increased station coverage (Figure 2) allows us to gather more information and create a higher resolution image of the geologic structure. The idea is similar to increasing the resolution of an image. If we look at Figure 3, we see the higher resolution image of Figure 1. Now that we have more information we can better understand what’s going on in the picture, just like understanding the continent with Earthscope USArray.

Figure 2 | Left: Earthscope USArray station coverage Right: Prior Station coverage 

Figure 3 | Left: improved image of Figure 1with more information (Now
 we can see the image was grumpy cat) Right: Poor image quality with little
information. 

    The dense seismic stations provide the highest resolution image of any continent, allowing scientists to look at the earth in a whole new way. The vast majority of the human population lives on a continent. In fact, you are likely on one right now. It may be surprising to find then, that we know very little about the structure within these continents. In fact, scientists have found a layer present in almost all continents 100 kilometers (km) beneath our feet that has not been able to be explained –we don’t know why it is there or what the consequences of this layer have been.  Some have suggested that this layer may be related to the locations of large earthquakes that occur in the middle of the continents. In the United States most earthquakes are located on the west coast, however if we look at locations of some of the largest earthquakes and earthquake hazard areas in Figure 4, you will find bright red areas associated with high hazard in the St. Louis, MO area. If there is a correlation between some characteristic of the 100km layer within the continents and the high hazard areas within continents of the US, we may be able to where other large earthquakes in continents may occur. Even if no correlation exists, this layer might preserve billion year old secrets to continental formation, alteration, and breakup and provide insight to the past, present, and future of our continent.

Figure 4 | Hazard map from United States Geological Survey. Areas with high seismic hazard are in
warmer colors with red being the most hazardous, while cooler colors indicate low seismic
hazard  with dark blue being the least hazardous. 

For more information about Earthscope and projects being done, visit:

Earthscope - http://www.earthscope.org/ 

To make your own earthquake map (like  Figure 4)  visit:

USGS Earthquake Hazard Program - http://earthquake.usgs.gov/earthquakes/search/


References: 

"Earthquake Hazards Program." Earthquake Hazards Program. US Department of the Interior -  United States Geological Society, 26.Aug, 2014.