Analysis of Convection Cells on Sputnik Planum on Pluto Foteine Dimitracopoulos April 26, 2017 Dr. Karen Prestegaard Dr. Nicholas Schmerr GEOL394 Abstract Pluto is a little-known icy dwarf planet located approximately 5.9 billion kilometers from the Sun (Frances, et. al, 2008). The New Horizons mission revealed new discoveries about Pluto, especially information about its geology, its atmosphere, and the geology and physical properties of its moons. One of the more anomalous features on Pluto are the ice polygons in Sputnik Planum, a young and geologically active icy plain. These ice polygons resemble the surface expression of Rayleigh-Bernard convection cells, and consist of volatile ices that make up nearly all of Sputnik Planum. The goal of this research was to measure the size and shape of these polygons and to use these measurements to evaluate the Rayleigh number and the minimum temperature difference required to drive convection. Measurements of the width and length of selected polygons show that they are of similar size and shape to one another, and that the shape tends to be elliptical. I assumed that the length of the polygons represented the size of the convection cells and used the average size to determine the dimensionless wavenumber k and the critical Rayleigh number for the onset of convection. These calculations suggest that the convection would require a small temperature difference to initiate, and that polygon formation is consistent with convection. I calculated convection initiation for convection cells with a range of aspect ratios, but the temperature difference required for convection is very small for all simulations. In addition, calculations were done to determine the minimum and maximum renewal rate of the cells using a maximum time limit of ten million years. Results showed that the renewal rate is likely on a scale of millimeters of movement per year, similar to plate tectonics. 1 Introduction and Background Pluto’s existence was first predicted in 1905 by Percival Lowell and later discovered in 1930 by amateur astronomer Clyde Tombaugh (McBride, et. al, 2011). Until the recent NASA New Horizons mission, this icy object could only be observed through telescopes that provided low resolution images. Pluto has been of great interest to astronomers and planetary scientists since its discovery. New images of Pluto, sent back in the summer of 2015, revealed diverse, unusual features on Pluto’s surface. These features include highly cratered areas, pitted terrain, icy plains, and features resembling the ice flow structures observed on glaciers on Earth. Many of these features were unexpected, due to Pluto’s size and location in the Solar System (Figure 1). (Figure 1: False color image of Pluto, courtesy of NASA. The heart shaped region is the Tombaugh Regio.) Since Pluto’s discovery, researchers have determined many fundamental physical and orbital characteristics of this icy object. Pluto is approximately 5.9 billion kilometers from the Sun and has a highly elliptical orbit. Due to its distance from the Sun and the shape of its orbit, one Plutonian year is equal to about 248 Earth years, and has an estimated rotation period of little 2 more than 6 Earth days (Frances, et. al 2008). Pluto rotates on its orbital plane with an axial tilt of 122 degrees and an axial inclination of 119.6 degrees, which results in a retrograde rotation. This in turn results in extreme seasonal variations of temperature on Pluto’s surface, which is estimated to be approximately 40 K (McBride, et. al, 2011). In addition, this tilt seasonally exposes Pluto’s polar regions to the faint light that it receives from the Sun. Compared to the planets in the Solar System, it is very small in size, with a radius of 1,152 kilometers (Frances, et. al, 2008). Pluto also has five moons; Charon (the largest moon), Styx, Nix, Kerberos, and Hydra. Previous studies done by Moore and McKinnon in 2016 have speculated that according to its calculated density and observed albedo, Pluto is composed of silicate rock, water ice, and various organic compounds in the form of volatile ices. However, not much information was known about Pluto prior to the New Horizons mission, due to its inaccessibility. In order to gain access to Pluto, NASA launched the New Horizons spacecraft on January 19th, 2006. This was the first reconnaissance mission of Pluto and the Kuiper Belt. Its purpose is to explore icy objects such as Pluto, and to determine how they may have evolved over time. In the summer of 2015, the New Horizons spacecraft performed a six-month long flyby study of Pluto and its moons. It is currently traveling farther into the Kuiper Belt to study other icy worlds at the edges of the Solar System with the goal of performing another flyby study in 2019. The New Horizons spacecraft uses several different instruments to acquire data. These instruments include: Alice, Ralph, SDC, LORRI, SWAP, PEPSSI, and REX (Stern, 2008). Alice, known as the Ultraviolet Imaging Spectrometer, is an instrument used to detect wavelengths in the ultraviolet region of the electromagnetic spectrum. Its main purpose is to scan Pluto’s atmosphere and determine its composition and atmospheric structure. Ralph is an imaging telescope used to take images in visible and near infrared wavelengths. It is separated into two components – the Multispectral Visible Imaging Component (MVIC) and the Linear Etalon Imaging Spectral Array (LEISA). Both of these components take high resolution pictures of the surfaces of Pluto and Charon, and contribute to the study of their geology, geomorphology, and surface composition (Reuter, et. al, 2008). The SDC is the Student Dust Counter; an instrument developed by students at the University of Colorado, meant to detect dust grains in space as the spacecraft travels through the Solar System (Stern, 2008). This information would also provide a better understanding of collision rates in the outer edges of the Solar System. LORRI stands for 3 Long-Range Reconnaissance Imager, and it is another high resolution camera attached to the spacecraft. Its purpose is to take pictures of the surfaces of Pluto and Charon to determine if there is any evidence of geologic activity (Cheng, et. al, 2007). Another instrument is Solar Wind at Pluto (SWAP), whose main purpose is to study how Pluto interacts with solar wind at its distance from the Sun. The Pluto Energetic Particle Spectrometer Science Investigation instrument (PEPSSI) is used to study particles involved with the interaction between the solar wind and Pluto’s atmosphere. Another instrument is the Radio Science Experiment (REX), which takes measurements for temperature and pressure in Pluto’s atmosphere (Stern, 2008). The pictures taken by the imaging systems revealed that the geology of Pluto and its moon Charon is far more complex than expected. There appears to be a variety of organic compounds in the form of ices, including water ice, nitrogen ice, methane ice, and carbon monoxide ice. Some regions display active ice flow and cryovolcanism, while others are heavily cratered with an estimated crater retention age of at least four billion years (Moore, et. al 2016). Several papers, such as Geomorphological Mapping of the Encounter Hemisphere on Pluto (White, et. al, 2016) and The geology of Pluto and Charon through the eyes of New Horizons (Moore, et. al, 2016) have taken a special interest in a heart-shaped icy plain, named Tombaugh Regio. While the eastern lobe is intriguing in its intricate bladed terrain, the focus of this interest goes to the western lobe of Tombaugh Regio – a flat plain called Sputnik Planum (SP). This region consists mainly of nitrogen ice, with some small amounts of water and carbon monoxide ice. It has an estimated area of 870,000 km2 and is interpreted to be an unusually young surface (see Figure 1). Studies done by Moore, McKinnon, and Grundy have not confirmed any impact craters on the surface of SP, which suggests that the icy plain is very young and estimated to have a crater retention age of less than ten million years (Moore, et. al 2016; McKinnon, et. al 2016; Grundy, et. al 2016). Images from the New Horizons spacecraft revealed that SP displays a peculiar cellular pattern, similar to ice polygons. Each of these cells are surrounded by troughs that have been estimated to be one hundred meters deep, with the center of these cells raised by about fifty meters, relative to the edges of the cells. The western border of SP comprises a broken mountain chain of angular blocks, possibly composed of volatile ices that have broken apart and reformed (Moore, et. al, 2016). 4 The discovery of these features raised many questions about the geologic processes that occur on or near the surface of icy worlds, especially those lying in the Kuiper Belt at the outer edges of the Solar System. What kind of geologic processes could generate the features such as the ice polygons in Sputnik Planum? Researching Pluto and its moons is critical to understanding icy objects and will help us understand the processes that formed such features in the past, and will continue to modify our understanding of it in the future. Hypotheses The focus of my research is the polygonal features of Sputnik Planum. I had several different hypotheses on the formation process of the features: 1. The polygonal features in SP are formed by convection cells with a characteristic length scale. 2. For surface convection to occur on Pluto, the critical temperature difference required for the onset of convection must be small. To test for this, the polygon size can be used to determine the critical Rayleigh number for convection, and therefore determine the critical temperature difference.