Viranga Perera

Postdoc at the Johns Hopkins University Applied Physics Lab

Research

Cooling of the Moon after formation

Lunar Magma Ocean.jpg

The Moon is thought to have formed from debris after a planetary body collided with the early Earth about 4.5 billion years ago. This formation scenario combined with a specific rock type (anorthosite) found on the Moon led to the idea in 1970 that the early Moon was covered by a global ocean of magma. CONTENT CURRENTLY BEING UPDATED (see Our 2018 JGR-Planets publication).

 

BRAZIL NUT EFFECT ON ASTEROIDS

As part of my dissertation work, I ran computer models to understand the interior structure of asteroids. Many asteroids are considered to be rubble-piles in that they are a collection of objects held together largely by their self-gravity. As such asteroids likely behave like granular material. Granular material, like sand, can act both like a solid (you can make a pile of sand) and a liquid (sand flows inside an hourglass).

Using a discrete element modeling code called pkdgrav, I modeled an asteroid as a three-dimensional collection of smaller objects. The video on the right shows an asteroid-like object that has an overall radius of about 800 meters and is made of large red balls (80 meters in radius) and small yellow balls (40 meters in radius). Though not exactly like a real asteroid, the end product is similar to an asteroid in that it is made of a collection of smaller objects and thus allows me to study both surface and internal properties of asteroids. Of course real asteroids are not that spherical and are not made of smaller perfect spherical objects. However, to make these simulations more realistic the pkdgrav code not only incorporates gravity, but also friction (which is important since a ball rolling on a surface will not roll forever) and loss of energy due to collisions (since a ball that is dropped on the floor will not keep bouncing forever). The addition of certain physical aspects to these simulations make them more realistic.

In the video every time you see a jolt it is because at that moment each particle was assigned a random velocity (both a random speed and a random direction). That was done to mimic impacts onto this modeled asteroid. As asteroids are shaken by impacts, they should size sort with larger objects rising to the surface while smaller objects sink toward the center. This process is known as the Brazil Nut Effect and has been argued to occur on asteroids such as Itokawa. One of the findings of our work is that while asteroids may be size-sorted near their surfaces, their interiors are unlikely to be size-sorted (see our 2016 Icarus publication).

Role of emotion in science education

HabWorlds.png

Part of my dissertation work involved trying to understand the role of emotions on student learning. Specifically, I considered how students' attitudes towards science may affect their course performance. I did statistical analyses on surveys taken by students of the Habitable Worlds online science course offered by Arizona State University. Habitable Worlds is built on the Smart Sparrow architecture and is an adaptive learning environment that aims to help students learn by asking them to tackle one of humanities biggest questions: are we alone in the universe? As such the course is organized around the Drake equation.

Students were given the survey at both the beginning of the course and at the end. Students were asked to choose responses based on a Likert scale. For example, they were asked to rate how much they agreed or disagreed with items such as "Creativity does not play a role in science" and "I can do well in science courses.” One of the interesting findings of this work is that students enrolled in fully-online degree programs have better attitudes towards science than their traditional in-person degree counterparts (see our 2017 CBE-Life Sciences Education publication).

the Shape of the moon

As part of my master's degree, I worked on the lunar asymmetry problem (see our 2011 LPSC abstract). Due to the fact that the Moon is tidally locked we can only see the nearside of the Moon (left side of the images) from the Earth while the farside of the Moon (right side of the images) faces away from us. It has been long known that the nearside and the farside of the Moon are geologically distinct. For example, the darker areas that you see when you look at the Moon at night are the result of ancient volcanism and are concentrated on the nearside. In addition, certain interesting chemical elements, such as thorium, are also concentrated on the nearside. On the other hand, the farside topography is on average higher and the crustal thickness on the farside is on average greater than the nearside. Even though these features have been observed for a long time there has not been a comprehensive explanation as to why the Moon is asymmetric.

Our work involved analyzing topography and gravity data to understand what processes would have established the large-scale features of the Moon. Our work shows that the large-scale topography and gravity of the Moon are due to two reasons: (1). The Moon freezing into a certain shape at a time when the Moon was hotter and closer to the Earth (2). The crust of the Moon being tidally heated, thus making regions near the equator thicker while the regions near the poles thinner (see our 2014 Nature publication).