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Assistant Director and Senior Research Scientist, McDonald Observatory and University of Texas at Austin
What do you think are the most significant events that have occurred in the past fifty years of robotic planetary exploration? Why?
The most significant mission has been the Kepler mission because it has finally quantified how many planetary systems exist in the galaxy. 16 years ago, we knew of one solar system -- ours. Now we know of more than 1,000 planetary candidates that reside in other planetary systems throughout our galaxy. Characterizing these systems can tell us a great deal about planet formation.
Of course, the first time we look at any planetary body, we learn a great deal. For example, the Huygens probe on the Cassini mission was extremely important for our knowledge of Titan. Titan, the only satellite with a thick atmosphere, is critical to our understanding of nitrogen-based atmospheres.
In your field of work, what are some examples of the great achievements and discoveries in planetary science and robotic exploration throughout the past 50 years?
I work in the field of comets. The Stardust mission was a great achievement. Stardust brought back a sample of comet dust from comet Wild 2 and the analysis of this dust has led us to understand that the solar nebula was much more mixed than formerly thought possible. This means that material that was near the sun and material that was far from the sun was all mixed together very early in the history of the solar nebula.
The Deep Impact mission has led us to better understand the structure of comets, helping to confirm the dirty snowball model that Fred Whipple first proposed in 1950. However, Deep Impact also told us that comets are more homogeneous than previously thought possible.
Before Deep Impact, we thought that the outer layers of the comet might be highly changed from when the comet was formed due to heating and the loss of the most volatile (easily changed) gases. However, Deep Impact showed us that the outer layers look chemically very similar to the inner layers.
From the ground, we have studied the chemical compositions of more than 100 comets and we find evidence for at least two distinctly different reservoirs for comets.
We discover these differences in the comet's chemistry. About 70% of comets seem to have very similar compositions for their ices. The other 30% seem to be depleted in what we call "long chain carbon molecules." Long chain carbon molecules are molecules made up of many carbons (plus other things like hydrogen) strung together.
The "depleted" comets are not deficient in carbon, per se. They are deficient in carbon molecules of a particular structure. We believe this is due to where these comets formed, not how they evolved. We are studying other molecules to try to better understand what other chemical differences might exist. These chemical differences point to the formation of some comets having occurred in different pressure and temperature regions than the others. If we can figure out these differences, we will understand more about the early history of the solar system.