Pluto System After New Horizons 2019 (LPI Contrib. No. 2133) 7062.pdf REVISIONS TO THE ONLINE TEXTBOOK EXPLORING THE PLANETS (EXPLANET.INFO): PLUTO. B.C. Spilker1, E.H. Christiansen1, and J. Radebaugh1, 1Brigham Young University, Department of Geological Sci- ences, Provo, UT 84602. [email protected] Introduction: Exploring the Planets (http://ex- the planets of the Solar System will find this book planet.info) [1] is a free online college textbook cover- helpful. ing the basic concepts of planetary science, and the character and evolution of the planetary bodies in the Solar System (including the planets, important moons, asteroids, and Kuiper Belt Objects). The latest edition (3rd edition) was published online in 2007 by Eric H Christiansen. Earlier paper editions were published by Prentice Hall in 1990 and 1995. Exploring the Planets approaches an introductory study of the solar system mainly through basic geolog- ical principles. Compared with other introductory plan- etary geology texts, such as Planetary Sciences by de Pater and Lissauer [2], Introduction to Planetary Sci- ence by Faure and Mensing [3], The New Solar System Figure 1. Pluto as imaged by New Horizons at its closest by Beatty, Petersen, and Chaikin [4] or Earth, Evolu- approach in July of 2015. (A) A semi-true color image tion of a Habitable World by Lunine [5], this is the of Pluto, similar to what the naked eye would see. (B) A only book with a basic geology approach. It is intended color enhanced image reveals a geologically diverse to be used as a primary or supplementary source in in- planet that has undergone many geologic processes, with troductory science courses (geology or astronomy). some still continuing today. Perhaps the most spectacu- lar part of the image is Sputnik Planitia, a basin filled The MS thesis project of the lead author was to up- with a large ice sheet at the heart of Pluto. This image date the section for Pluto using all the new data from shows the location of the areas that are discussed in the New Horizons. We describe the main sections included chapter. (NASA/Johns Hopkins University Applied Phys- and the reasons for focusing on those for the intended ics Laboratory/Southwest Research Institute). audience. Significance: The modern results based on recent Methods: In to order to produce instructive chap- investigations of Pluto are critical for our understand- ters about Pluto, we used effective pedagogical princi- ing of the nature and history of this body, icy bodies, ples of transfer [7], cognitive learning [8], and cogni- and the Solar System as a whole. Pluto is an end mem- tive apprenticeship [9] to organize, construct figures, ber in a spectrum of planetary objects in the Solar Sys- and write text. We implemented these principles to tem. Pluto is cold, icy, distant from the sun, and a rep- help students recall previous information, present new resentative object of the vast Kuiper Belt, and is unlike information, guide students in developing critical the terrestrial bodies of the inner Solar System. This thinking skills, and elicit and assess performance. body refines models of how icy planets evolve over Chapter Sections: The Planet Pluto and the Kui- time, and how our Solar System has evolved. per Belt discusses the physical charactersiscts of Pluto For these reasons, it is important to update Explor- along with a discussion of the definition of a planet, ing the Planets to summarize the current understanding Pluto’s place in the Solar System, and the Kuiper Belt. of the geology of Pluto. This way, students can better A section on Pluto’s Spin and Climate follows and de- understand the formation and evolution of small icy scribes how the axial tilt of Pluto affects the planets bodies and the implications for the evolution of our So- unique climate of long summers and winters, and over- lar System. all patterns of sublimation and deposition of volatile Objectives: We completed our main objective of ices. Pluto’s Ices outlines the major ices and tholins producing a pedagogically and scientifically sound observed on the surface of Pluto and their distributions. chapter for Exploring the Planets, on Pluto. The chap- The section on the Atmosphere describes the composi- ter is comprehensive in scope, accurate, and easy to tion of the atmosphere and its relationship to the subli- understand, and contains helpful figures and anima- mation of the volatile surface ices. Geological Prov- tions to facilitate learning. Instructors and students will inces lays out the major geologic regions of Pluto and be able to use this book as a resource in introductory the geologic processes that occur in each region. This college courses for non-science majors. The next edi- section includes an extensive discussion about Sputnik tion of Exploring the Planets will be freely available at Planitia and Tombaugh Regio. explanet.info. Anyone who desires to learn more about Pluto System After New Horizons 2019 (LPI Contrib. No. 2133) 7062.pdf Figure 3. The thermal history of Pluto starts with the accretion of frigid ice (water, methane, and nitrogen) and silicate-dominated materials in the distant reaches of the disk of debris around the ancient Sun. Early on, accretionary and radiogenic heat allowed dense silicate Figure 2. On Sputnik Planitia, each polygonal cell forms rock to separate from ice and sink to the core. Part of by convection as warm ice rises at the center, reaches the the water ice mantle also melted and created a layer of surface, and flows laterally as it cools, ultimately return- liquid water above the rocky core. The creation of this ing to the dark interior of the ice sheet from whence it liquid water may have led to contractional tectonics came. The energy that drives this convection is thought to (since liquid water is denser than water ice) though be from radiogenic or solar heating. The black lines mark there are not contractional features on the surface (per- some of the many, unexpected, transverse dunes on Sput- haps erased by later cryovolcanism and extensional tec- nik Planita. The source of the “sand” is probably methane tonics). As Pluto lost its heat, the liquid water began to ice that precipitates on the top of al-Idrisi Montes and is freeze forming a thicker icy lithosphere and leading to a then blown off the mountain and onto Sputnik Planitia. change in tectonic regime. As the ice froze, extensional (NASA/Johns Hopkins University Applied Physics Labora- tectonism dominated. Today, Pluto may still have a 100 tory/Southwest Research Institute). km thick liquid water layer, and as it continues to freeze, Pluto will keep experiencing extensional tecton- Impact Craters are an important part of any plane- ics, similar to Charon. tary body. The most interesting fact about the craters on Pluto is the lack of them seen on Sputnik Planitia, Conclusion: Exploring the Planets is a valuable indicating recent resurfacing of the basin. The only textbook for explaining basic geological and planetary type of Tectonic Features imaged on Pluto were exten- concepts to college students and so, it is important to sional. These grabens and rifts formed by exapansion keep the textbook updated. To provide a complete up- as the water on Pluto froze. Two possible Volcanic to-date and free resource on the planets, we have up- Features have been identified on Pluto. These are large dated the chapter on Pluto by summarizing the new mounds with central depressions that reveal that there scientific literature on this planet by rewriting the must be internal planetary heat. Pluto’s Internal Struc- chapters and introducing new images/figures. ture is similar to the icy moons of the outer planets, with a rocky core, a liquid water layer, a water-ice References: [1] Christiansen E.H., and Spilker mantle, with a surface veneer of volatile ices. There are B.S. (2017) explanet.info [2] Pater I. and Lissauer J.J. five Moons of Pluto with most of the information gath- (2010) Planetary Sciences. Cambridge University ered for Charon, the main moon of Pluto. This body is Press, United Kingdom [3] Faure, G. and Mensing now geologically dead but it has experienced cryovol- T.M. (2007) Springer, Netherlands [4] Beatty, J.K. et canism, extensional tectonics, and tholin deposition. al. (1999) Sky Publishing Corporation, Massachusetts The Geologic Evolution of Pluto (and Charon) is simi- [5] Lunine J.I. (2013) Cambridge University, United lar to other small icy bodies. It began with accretion of Kingdom [6] Russel C.T. (2008) Space Science Re- ices and silicates and perhaps a major collision that views, 140, 1. [7] Bransford J.D. et al (1999) National formed Charon and the other icy moons. Pluto differ- Academy of Sciences. [8] Hunt R.R. and Ellis H.C. entiated quickly, with the rocky material sinking to the (2004) McGraw Hill, Boston [9] Lave J. and Wegner center of the planet, followed by cooling of Pluto that E. (1991) Cambridge University Press, New York. caused a thickening of the lithosphere and extensional tectonics. .