From Dust to Planets

By Dr. Paul Estrada
Planetary physicist at the Carl Sagan Center for the Study of Life in the Universe, SETI Institute, and Gail Jacobs

If planets are a dime a dozen, moons are less than a penny each. There are at least 139 moons just within our own solar system. Most of these are the property of the gas giant planets beyond Mars. More than just a nice accompaniment to planets, moons may have habitats in which liquid water could ebb and flow - and possibly be a suitable home for life. Planetary physicist Dr. Paul Estrada investigates how moons around gas giants are formed -- an important question as its answer would give us insight into the nature of moons around the myriad gas giants we know orbit other stars.

Very briefly, describe your research project.
I have multiple projects but they do have some similarities. Basically I explore how the giant planets and their intricate satellite systems were formed from the cloud of gas and minuscule particles of dust that made up the early solar nebula. How do small grains of dust which condense from the nebula gas manifest into planet-sized-objects that contain an inventory of so many different elements and materials? The question turns out to be as complicated as the process itself. There are many physical, chemical and geological processes to consider in the difficult journey from dust to planet.

Interestingly, because the discovery of many more planets around other stars is happening at a feverish pace, and many of these planetary systems are so much different in structure from our own, it becomes even more important for us to explore what complications are involved in the very early stages of planet formation. Answers to these questions can obviously reveal information about the birth of our own solar system, but even more so help in the explanation of the diversity of planetary systems we see now, or will discover in the future.

How did you come to join the SETI Institute?
I came to the SETI Institute in 2005, after working at the NRC (National Research Council) at NASA Ames for three years. I had friends and colleagues at the Institute and I had always heard good things. I felt the Institute would be the best place for me. My experience here gets better each year.

What first sparked your interest in science and astronomy in particular?
It will sound cliché, but I was a Cosmos kid. The series came out when I was around 9 years old, and I loved that show! My parents and I would watch it together every week. I truly enjoyed watching it. I may have been interested in astronomy before that, but my interest really took off from there. Needless to say, I was thrilled when I had Carl Sagan as my initial advisor while I was a student at Cornell University.

The other person who contributed to solidifying my interest would be Jeff Cuzzi. I met Jeff at a party when I was around 13. I remember this guy talking about astronomy and rings and planets and he had a whole group of kids around him. I was fascinated by that conversation. Flash forward to right before I started my undergrad program and was working at NASA Ames through an internship program for the Kuiper Airborne Observatory. I attended a talk being held in the space sciences building there, and who was giving the talk but none other than Jeff Cuzzi. Being the opportunist, I went up to him afterwards and told him the story about how we had met many years before. Then I hit him up for a job. I honestly think he felt he could not refuse! Jeff became my mentor and my relationship with him as both friend and colleague continues to this day.

What inspired you to investigate the formation and evolution of planet satellite, or moon, systems?
My interest goes back to grad school at Cornell. The problem that initially inspired me involved the outermost Galilean satellite, Callisto. The Galileo spacecraft data indicated Callisto might be what's called undifferentiated or partially differentiated. That means that essentially all of the rock has not separated from the ice and sunk to the center to form a dense core; rather, the rock and ice are still to a great degree mixed together.

Europa, Ganymede, and Callisto: Surface comparison at high spatial resolution These images show a comparison of the surfaces of the three icy Galilean satellites, Europa, Ganymede, and Callisto, scaled to a common resolution of 150 meters per picture element (pixel). Despite the similar distance of 0.8 billion kilometers to the sun, their surfaces show dramatic differences. Image credit: NASA/JPL/DLR

Satellites like Callisto and Ganymede are made of roughly 50% water/ice and 50% rock by mass. If you have a uniform mixture of rock and ice that gets thrown together fast enough, it will trap a lot of heat within the interior and eventually get hot. If it gets hot enough, the ice melts, the rock within the ice sinks, and the separation process begins. By looking at the gravitational data, it was determined that Callisto was not differentiated or separated, which piqued great interest within the scientific community. On the other hand, the gravitational data on Ganymede, the satellite interior to Callisto, was consistent with complete separation. So why is it that one satellite, similar in size and composition, is completely different from the other?

I had been working on thermal models of Callisto with Steve Squyres, my advisor at Cornell after Carl Sagan's death. I got somewhat frustrated with my numerical project, which involved a lot of debugging and coding, and I became more interested in the bigger picture. I could work on models of Callisto's internal structure and try to figure out how these states of partial differentiation occur and are maintained or I could explore the conditions of satellite formation that leads to the different states of differentiation we see. At that point, working closely with another fellow Cornellian and current SETI scientist Ignacio Mosqueira, I started focusing more on the problem of developing a unified picture that would not only explain Callisto but also find answers for satellite formation that could be applied to the other giant planets of our solar system.

What tools and information do you use that allows you to conduct your research?
Those of us involved in this field of study are fortunate to be associated with missions that send spacecraft to the outer planets. Initially the Voyager and Pioneer missions flew by the outer planets. Now we have special missions, such as Galileo that orbited Jupiter and Cassini that is orbiting Saturn. These spacecraft were able to spend a great deal of time conducting multiple fly-bys of the satellites, studying the systems and the planets and their atmospheres. Scientists can extract so much information from that data! We can study the atmosphere of Jupiter and look at the concentration of atmospheric elements to learn more about the initial state of the solar system, and the conditions that may have been present during the formation of the Galilean moons from Jupiter's disk of gas and dust.

I'm interested in what this data tells us about the kind of materials embedded into the objects that went into making the solid parts of these planets and their satellites. We see a lot of diversity in the way the satellites look. For example, we know that water is not only just a very important constituent, its ubiquitous and easy to track down. Other constituents, such as ammonia -- not so easy to find even though most of us expect that it is there. Jupiter System Montage (left): Image credit: NASA/JPL

Much of what I do is theoretical so I develop theoretical models. I take the data we see and construct a picture based on that data. If it's possible to relate what we see at Jupiter to what we see at Saturn or Uranus, I want to understand both the similarities and the differences.

Do you study both satellite and planet formation?
In looking at the satellite systems of the giant planets, we look for trends; what are the sizes of the moons, how are they distributed, what are their bulk properties? Are there some rules of formation or overall process at work that applies to all of the systems, thereby leading us to believe it's not a completely random process? It's complicated because, for one, you have to know something about how the disc of gas forms around the giant planet and what that structure is. This disk of gas and dust, or subnebula as it is called, is where the moons will eventually form. In a sense, the giant planet's satellite system is like a mini-solar system initially involving the same process of growth from dust to larger bodies like its solar nebula counterpart.

We're fortunate that the Galileo and Cassini missions have provided us with so much data with which to work. We can use the trends that we see, as well as the properties of the moons and the composition of the giant planet's atmosphere to construct a somewhat complete picture of what that early subnebula was like. On the other end, theorists can simulate the formation of the giant planet and its subnebula and we can compare the outcomes to see if our picture is right.

There are obviously many factors involved in the process of growth in the solar nebula and giant planet subnebula. I'm particularly interested in studying the process that begins with dust that evolves into objects that are known as planetesimals, which are essentially the building blocks of the planets. One of the various difficulties involved in getting from dust grains to planetesimal, or in the case of the subnebula, satellitesimal, has to do with the actual state of the nebula gas and whether it is turbulent or calm.

Within the nebula gas are lots of dust grains of different sizes floating around at different relative speeds with respect to each other. If they're small enough, they're part of the gas and move within its eddies. At this stage, grains stick together without much difficulty because their relative speeds are low. As the particles get larger, however, they are less influenced by the gas motion and begin to jump from eddy to eddy and their relative speeds are higher, especially with respect to each other. For these larger particles, the gas is acting more like a headwind trying to slow the particle down. We've found that once particles grow to something close to a meter in size, the headwind tends to be the highest. The relative speed between these objects can get so high in fact that a barrier is reached -- these objects find it very difficult to grow further and instead are easily fragmented back into small grains. If this happens, growth stalls. Thus one of the main mysteries of planet formation is trying to explain how to overcome this barrier.

What is one of the coolest things about your project?
The state of differentiation of Saturn's moon, Titan, is between that of Callisto and Ganymede. I'm working on developing a consistent thermal evolutionary model that could explain the formation process for Ganymede, Titan and Callisto. I'd like to show you can get their different states over the age of the solar system.

Cassini Gazes at Veiled Titan (rendering): Artist's concept of Cassini's Sept. 24, 2010, flyby of Saturn's moon Titan. Image credit: NASA/JPL-Caltech

Right now, I'm doing internal modeling of these objects based on formation times that are consistent with our satellite formation models and are consistent with the size of objects from which we would expect these large satellites to accrete. Just after they complete their formation, there is still internal heat from accretion, and heat generated from the gravitational release of energy when rock separates from the ice. It turns out that the surface layers of Titan and Callisto are separated, but their deep interiors are not. On top of that, there is also internal heat from the radioactive elements which still provide heat to the present day If I can show certain parameter situations in which objects like Callisto or Titan are still differentiating, albeit very slowly, that would be pretty cool.

But perhaps the coolest thing about my projects in general is having actual data -- it's tangible. You can access the data from which you can draw these conclusions. It's almost like being there and taking a sample.

What is one of the most interesting discoveries you've made about the compositional evolution of Saturn's rings?

When I started working with Voyager data, my project involved looking at the rings and coming up with models to try and explain their composition. One of the most interesting things about the rings is they're made up almost entirely of ice. However, for anyone who has ever seen a true color picture of the rings, they certainly do not look like pure ice. It turns out it takes very little impurity, less than a fraction of a percent of material, to give the rings their distinct color. Ringscape in color (left:) Image credit: NASA/JPL/Space Science Institute

The model was based on the idea that because the rings' surface area is so huge, they are much more susceptible to micro-meteorite bombardment. Even now, there are lots of dust particles floating around in the outer solar system which pass with some regularity through the planetary systems. These dust grains impact on the satellites as well, but because the rings' surface area to mass ratio is so huge relative to a moon of the same mass, the dust actually makes a significant difference over time. Thus, micrometeorite bombardment is an extremely important piece of the rings' evolutionary puzzle because these impurities cause them to darken over time. The impacts also lead to structural changes in the rings because material gets thrown around from one ring region to another. Most of the dust particles impacting on the rings are thought to be mostly neutral and dark in color, similar to cometary dust (e.g. Haley's Comet).

Majestic Saturn, in the Infrared This false-color composite image, constructed from data obtained by NASA's Cassini spacecraft, shows Saturn's rings and southern hemisphere. The composite image was made from 65 individual observations by Cassini's visual and infrared mapping spectrometer in the near-infrared portion of the light spectrum on Nov. 1, 2008. The observations were each six minutes long. Image credit: NASA/JPL/University of Arizona

If you have an idea of what the impact rate of this dusty material is, you can simulate the evolution of the rings over time and extrapolate how long it would take to start from some initial condition involving just ice and a very small fraction of some initially unknown native, spectrally absorbing material and get the coloring that we now see. From that, we can extract information about the materials that gives the rings their characteristic color.

What do you currently consider your biggest challenge?
Funding is always a difficult process. Getting people to accept your ideas can also be a challenge. Inevitably if you're working on a problem that is important in the field, there is going to be a lot of competition for grants. In this field, people can be married to their ideas and it's hard to convince people if you have a different idea.

Why should the general public care about your research? In your opinion, what is the potential impact?
That's a difficult question to answer. There is value in pure science and research. For anyone interested in furthering the general collective knowledge of humanity, we need to know how planetary objects were formed. Once we know that, we can predict how objects in other planetary systems were formed. At some point, the answers we discover now may prove even more valuable for future research and explorations.

What motivates you?
Solving the mystery. Creating models that lead to solutions and results keeps my interest high. It may not be the final answer but it's an answer that takes us in the right direction. Science is a building process and we keep building on what was discovered years ago.

With the rapid rate of technological advancements, can the answers you're seeking be found in the next 20 years or so?
I think we can get closer to the answers. It could be that we substantiate that someone was right a long time ago but didn't have the means to model it. Obviously with technology, the ability to do things has improved but there are always more problems and they tend to increase in complexity. Qualitatively, we're always getting closer to the answers; we're probably not far off.

I'm trying to advance current knowledge significantly. Generally when you try to do this, you inevitably run into a lot of scrutiny. If you try to move in a certain direction that is quite different than someone else who has been working on the same problem for a decade, you're going to run into resistance and that's just the nature of the field. You have to be tough skinned to blaze new trails.

The Birth of Stars This new Hubble photo is but a small portion of one of the largest seen star-birth regions in the galaxy, the Carina Nebula. Towers of cool hydrogen laced with dust rise from the wall of the nebula. Image Credit: NASA, ESA, and M. Livio and the Hubble 20th Anniversary Team (STScI)

What historic personality do you admire most and why?
I admire people who espouse or inspire free thought. I always admire people, and especially the people in the past, who were willing to go against the establishment because they believed in what they were doing. People like Darwin, Galileo or Copernicus did serious research, reasoned things out in their head, and certainly in the case of Darwin, struggled with it for years before finally going public with his theories. I admire that those men challenged current thinking in a time when it was very dangerous to do so.

What was your dream job as a child?
When I think back on my youth, a love of science and astronomy was always a constant. I went through a stage where I wanted to be an astronaut; but as a kid, I wanted to be the astronaut who was traveling to other planets. Once I got old enough, I realized that was not the reality. Other interests would come and go, but astronomy was the one constant, so in a sense, I'm doing what I always wanted to do.

What is your philosophy of life?
My philosophy is to enjoy my work and what I have in my life as best I can.

How do you spend your free time?
I spend it with my family and friends. I have a 3-year old daughter, and my wife and I just had a son who is just three weeks old. My family also travels to Japan at least once a year to visit my wife's family.

Is there someone you would like to swap roles with for a day?
Strangely, I've thought about swapping roles with the President of the United States for just a day. I'm sure he has this really structured day, which is, of course, the total antithesis of my day, and he deals with so many different pressures. It must be so hectic. It would be interesting to see what it's like, especially with our current president.

What is your favorite vacation destination?
I love going to Japan, of course. Although I've been many places around the world, I found Maui to be a wonderful place. Being on a small island and feeling somewhat isolated was certainly appealing , at least for a short while. It was amazing to be on top of the Haleakala volcano, looking down to see the confines of where I was, with just ocean and the other islands beyond. That was fascinating.

If time wasn't an issue, what would you still like to learn?
I'm always interested in learning other languages. I speak Spanish and continuously trying to improve my Japanese, but learning Chinese might be practical. It would also be interesting to learn more about the medical field and become a doctor or a medical researcher. Another frontier, of course, is the ocean and there is so much that is interesting to study there as well.