Postdoc Spotlight: Planetary Scientist Jessica Arnold

Jessica Arnold
An artist’s impression of a white dwarf (burned-out) star accreting rocky debris left behind by the star’s surviving planetary system. Credit: NASA, ESA, and G. Bacon (STScI)

Monday, February 26, 2018

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We know of space dust reflecting and absorbing light from stars. But do we really know how dust grains and starlight interact in space?

Planetary scientists such as Jessica Arnold, a postdoc at Carnegie's Department of Terrestrial Magnetism (DTM), are currently trying to answer that question and find out what the irregular shapes of dust grains might do to light from a star. Arnold's research on light scattering and optics can contribute to the work of astronomers trying to better understand debris disks, a type of circumstellar disk with high concentrations of dust. In this Postdoc Spotlight, Arnold discusses her research and its importance for our understanding of how planets—including our own—form.

Jessica Arnold joined DTM's astronomy group as a postdoctoral associate in summer 2017. Credit: Roberto Molar Candanosa, DTM

DTM: What exactly do you do?

JA: Using computer models and sometimes lab experiments, I study what happens when light hits space dust. Yes, I study space dust, but I promise it's not boring. I want to find out how planets and moons are made.

The Solar System is a very dusty place. The surfaces of the Moon, Mars, other terrestrial planets, and asteroids are all covered in dust in the form of regolith, a layer of rocky material (dust, broken rock, soil) covering bedrock. Comet tails contain dust. And outside our Solar System, dust occurs in circumstellar disks and other environments. This dust is somewhat different than the dust that might collect on your bookshelf. It's made up of various minerals, metals, carbon and ice. Its shape ranges from compact to very fluffy and irregular. It's subject to processes like space weathering, which is a catch-all term for interaction with solar wind particles, exposure to cosmic rays, and bombardment by meteorites and micrometeorites.

Remote sensing instruments on spacecraft and telescopes collect different wavelengths of light to examine the composition of planetary surfaces via electronic transitions and vibrational modes of chemical bonds. This is known as spectroscopy, and I focus on trying to make computer models of such spectra in the infrared. Models for interpreting remote sensing data are important because even for a relatively nearby object like the Moon that we have actual samples of, the samples we have represent a limited portion of the surface.

Clockwise from upper left: (1) Microscope image of Apollo 12 soil. Credit L. Taylor, D. S. McKay; (2) Single lunar regolith grain. Credit: D. S. McKay NASA/JSC. (3) Microscope image of a ground mineral sample. Credit: Koike et al. 2010. (4) Interplanetary dust particle. Credit: NASA.

Why is planetary science important?

We can't understand how the Earth formed and came to be suited for life by studying it in isolation. Many missions to other Solar System bodies as well as space-based and ground-based telescopes include spectroscopic instruments in order to find out what planetary surfaces and circumstellar disks are made of. The purpose of my work is to help figure out how to interpret that data in order to provide better estimates of the rock types or chemical components that occur on the regoliths of rocky Solar System objects and within debris disks. This in turn informs petrologists and geophysicists who study how the internal conditions of Solar System objects lead to the observed rock types and chemistry. From there, we start to piece together answers to questions such as: What were the formation conditions that created contents of the Solar System as we observe it today?

How did you become interested in your field of research?

My interest in science started very early on. I think that was because there were so many different hands-on science activities for kids, and I really liked anything like that. I was also into different kinds of crafts like beading and had a very well-used Erector set. I grew up near New York City, and my parents would occasionally take us to museums like the American Museum of Natural History. I remember being captivated by just about everything there, but especially the Hayden Planetarium. The planetarium was great because finding dark, cloudless skies around a city like that can be challenging. When I became interested in astronomy, my parents also took me a couple of times to observing parties held by a local amateur astronomy club. I also remember that I was just as interested in the telescopes themselves as what we were looking at with them, so maybe it's not a surprise that my research involves light scattering and optics.

I didn't think about a career in planetary science until college. In addition to astronomy, I was also interested in the Earth sciences but never really considered majoring in any of those subjects. When I found out I could combine space science with geology, I was pretty much sold.

If you weren't a planetary scientist, what else would you like to be?

The great thing about planetary science is how interdisciplinary it is, so you don't need to pick just one kind of science. Planetary science incorporates the geosciences, physics, chemistry, and astronomy. But I think if I had to pick something far from my own field, I'd be interested in being a dietician. When one of those many this-or-that-food-causes-cancer articles come up on the news, I sometimes find it interesting to track down the original paper and read it.

What projects are you working on now at DTM?

A better understanding of the composition of dust grains in debris disk gives us context for what components might have been around when the planets in our own Solar System formed. We know that the shape of the dust affects how we interpret what it is made of. But because it can take quite some time for computers to do these computations (up to a couple of months for a large dust grain), it is usually assumed that all the particles in the disk are spherical. I'm working on figuring out how much of that assumption matters.

In addition to modeling light scattered by dust in debris disks, I'm also studying how these fluffy dust grains are affected by radiation pressure. Despite the fact that photons have no mass, they still carry momentum, so the starlight can actually nudge small grains out of the disk. How effective this radiation pressure is at removing dust depends on what the dust is made of, how porous it is, and how large the grains are (as well as on the properties of the central star itself).

Examples of simulated dust grains ranging from spherical to irregular. Credit: J. Arnold and E. Zubko

What do you think will be different in your field of study in 100 years from now?

This is such an interesting question because it depends on two very different factors: technological advancement (what's possible?) and political motivation (how much money do we have and how do we want to spend it?). Every few years NASA produces a document outlining what they think the answers to those questions will be over the next few decades. I found one such document from the month and year I was born (May, 1986) that lists "potential space science headlines." Of course, not all of these came to pass, but some of the ones that did are amazing! Detection of gravitational waves, first planet discovered outside the Solar System, ice discovered at the lunar poles

One thing that will inform the direction of planetary science in 100 years will be current, planned, and proposed missions returning planetary samples to Earth. OSIRIS-REx and Hayabusa 2 will be returning to Earth with asteroid samples in 2023 and 2020, respectively. Several space agencies (NASA, ESA, Roscosmos, CNSA, ISRO) are thinking about additional lunar sample return missions from terrains that haven't been visited previously, but could tell us a lot about the formation of the Moon. I think a Mars sample return mission is definitely possible within the 100-year time frame. The lunar samples that were already collected during the Apollo and Luna missions have been keeping scientists busy for the last 50 years, so I think the results of all these missions could easily keep us busy for another 100!

Where do you see yourself in 20 years?

While my current work mostly involves computer models, in the future I'd like to continue connecting such models to lab measurements. If we can't understand simple samples under known conditions, then how can we understand complex planetary materials made up of many different minerals and elemental compositions?

Who is the most memorable scientist you've ever met?

A couple of years ago I met Jack Schmitt (geologist and Apollo 17 crewmember) at a conference during an event for early career planetary scientists interested in lunar exploration. What struck me was how interested he was in hearing about what all of the students and postdocs were working on. Some of the students there were working with Apollo samples, and if it was one he collected, he recalled exactly at what point in the mission it was collected, what the surrounding terrain was like, and so on.

If you could meet one of your science icons, dead or alive, who would it be?

Dorothy Hodgkin, the x-ray crystallographer who won the 1964 Nobel Prize in Chemistry for discovering the structure of vitamin B12. She was diagnosed with arthritis in her 20s, and as this got progressively worse, she eventually had to spend a lot of time in a wheelchair. Despite her disability, she still managed to remain a prolific scientist. I want to ask: "Where did you find the energy and tenacity to do that!?"

What does it mean to be a really good scientist?

A lot of people think of scientists as people who solve really hard problems, but I think of scientists as people who ask really good questions.

What word of advice do you have for young scientists?

Even if you think you know what field of science you'd like to go into and even though you are just starting out, take some time to do a pretend job search as if you already hold your degree. Don't wait until junior or senior year to visit your campus career center. High school coursework only exposes students to a small slice of STEM topics. You might have an idea of what your skills are and what you enjoy doing, and there may be careers out there you don't even know exist that are a good fit for you. If the academic department your major is in has seminars with speakers from other institutions, go to some of them, find out what different types of scientists are up to. That said, my other piece of advice is to not think you'll be stuck doing whatever you get your degree in. There are always opportunities to change direction.

—Roberto Molar Candanosa