What exactly is a rising star? Well, by whichever definition you choose, Briley Lewis certainly fits the bill. A senior at Columbia College, recipient of multiple awards and fellowships, published author of multiple astronomy articles, researcher at the AMNH as well as the NASA/Goddard Institute for Space Science and the Space Telescope Science Institute, she was recently named a National Science Foundation Fellow and will as such pursue her PhD next Fall at UCLA’s astronomy department.
On Friday evening, April 20, a packed Pupin Hall at Columbia was treated to a glimpse of this particular star, when Lewis gave a talk entitled, “New Horizons: Understanding Pluto.” At hand were several AAA members, including Faissal Halim, as well as this article’s author.
Lewis began by comparing Pluto to Earth. On our planet, water exists as a solid, liquid and a gas, with the hydrological cycle creating our daily weather. On Pluto, water ice is the bedrock, and in its place it is nitrogen that exists in all three states of matter. Lewis compared the Earth’s internal structure, with its iron core, mantle and crust, to that of Pluto, which has a rocky core, layers of ice and liquid water, and a surface composed of water- and other ices. We see familiar formations and processes on Pluto, including mountains (composed of ice rather than rock), frozen lakes, nitrogen glaciers which expand and contract with the seasons and which calve (break apart) as on Earth, even a suspected cryovolcano (Wright Mons), from which it is ice, rather than lava, which we believe has erupted to harden into a large cone shape.
Although Pluto was discovered by Claude Tombaugh in 1930, we really didn’t understand much about it until 2015, when New Horizons first visited the dwarf planet. At a distance of almost 6 billion km from the Sun, it takes a long time to get there: nine years in the case of the speedy New Horizons. But the satellite’s short visit of Pluto furnished us with 50 GB of data, which took 16 months to transmit back to Earth. That said, there is still much we have yet to understand about Pluto.
Lewis then gave her audience a guided tour of Pluto’s regions, illustrated by striking photos from New Horizons. Pluto’s topography includes methane and nitrogen ice, impact craters, Sputnik Planitia (the left side of Pluto’s great “heart”), which is fringed by 3km tall ice mountains. Sputnik Planitia is actually an impact basin filled in with ices, which form convection cells in a process similar to lava lamps, albeit in extreme slow-motion. Pluto has fossae (or canyons) like Earth, although we don’t know by what process they formed in the absence of rivers. Pluto also has bladed terrain similar to jagged ice formations as found in southern Chile, east of Tombaugh Regio (the unofficial name given to the entire “heart”). Pluto’s frost cycle is observable at Cthulhu Macula, the dark-colored whale-shaped region to the left of Sputnik Planitia, whose surface color is caused by the presence of tholins, complex hydrocarbons created by the atmosphere’s methane and nitrogen reacting with sunlight and cosmic rays. As Lewis demonstrated through her tour of New Horizons images, Pluto is a highly complex world with active geology.
What about atmosphere? Yes, Pluto has one, although Lewis noted that it is 100,000 times thinner than Earth’s, and is composed mainly of nitrogen, carbon monoxide and methane, with a smattering of other hydrocarbons including hydrogen cyanide and ethane. The atmosphere is too thin to generate winds, but it does produce nitrogen frost. And this atmosphere creates a multi-layered haze blanketing the planet, although we do not yet know what causes the haze to form layers.
New Horizons was not able to study all five of Pluto’s moons during its quick flyby, but it did get a good look at Charon, Pluto’s oversized moon. Charon is so large relative to Pluto that the system has at times been called a binary planet. Charon has a canyon running across its equator believed to be caused by flex stress due to the moon’s close proximity to Pluto’s gravity, as well as a tholin-covered reddish region appropriately named Mordor, which we suspect is caused by migration of material from the dwarf planet to its moon.
Finally, Lewis discussed the highly controversial demotion of Pluto from a planet to a dwarf planet, and the reasons thereof. In the end, it might be simpler just to refer to Pluto as a Kuiper Belt Object, along with thousands of neighboring icy bodies greater than 100km in diameter and likely trillions of smaller ice balls. Lewis concluded by noting that one of these with the unforgettable name MU69 will be the next stop on the continuing saga of New Horizons, with the encounter scheduled for New Years Day, January 1, 2019.