Post contributed by Dr Veronica Bray, Lunar and Planetary Laboratory, University of Arizona.

Images of Pluto coming back from NASA’s New Horizons spacecraft have revealed many unexpected landforms and show extreme albedo and compositional variations across the dwarf planet’s surface. This blog post concentrates on one high-resolution swath across the boundary between the cratered terrains of Viking and Voyager Terra and the smoother ices of Sputnik Planum (see Figure 1). Take time to scroll down this long image (Figure 2), that covers ~530 km of Pluto’s surface at around 30°N.

The high albedo highlands at the top of Figure 2 are cut by large fracture systems and multiple impact craters, showing them to be the oldest unit in this image. Darker/redder material is also present and picks out the finer-scale surface features.

This area of Pluto is one of the locations identified as water ice. As the spectral signature of water ice aligned with some of the redder areas in lower resolution imagery it prompted the question: “Was water on Pluto red?!” The answer came from the high resolution analysis of the unit ‘next door’…

The mid-section of Figure 2 shows a chaotic terrain of red and white tilted blocks. These mountains are thought to be gigantic icebergs that have calved off the edge of Voyager Terra and spread into the soft nitrogen ice of Sputnik Planum. The blocks, some of them 100s of metres high, have tilted to expose brighter ice beneath the redder material. Spectral analysis of this area indicates a water ice bedrock with a surface coating of red tholins (organic molecules created by the break down of methane and nitrogen molecules by sunlight), not red water ice.

At the bottom of Figure 2 lies the smoother terrain of Sputnik Planum (SP). SP is covered with polygons tens of kilometers wide. Each polygon shows some surface pitting, probably due to sublimation, and is bounded by shallow troughs of 2-3km in width. The polygon edges are smoother and more curved than typical contraction crack or desiccation polygons seen on the Earth and Mars. Instead, our current understanding is that they mark the top of convection cells within a slowly churning mass of nitrogen, methane and carbon monoxide ices. SP is geologically young based on the lack of identified impact craters. It is possible that this unusually active region is the result of a large impact; larger than would be expected to have occurred late in solar system history… But, as with all fun science, that argument is ongoing…

Further Reading:

Stern, S. A. et al., The Pluto system: Initial results from its exploration by New Horizons. Science 16. Vol. 350 no. 6258. DOI: 10.1126/science.aad1815. Full Article

S. A. Stern, The New Horizons Pluto Kuiper Belt mission: Overview with historical context. Space Sci. Rev. 140, 3–22 (2008). Abstract

H. A. Weaver, et al., Overview of the New Horizons science payload. Space Sci. Rev. 140, 75–92 (2008). Abstract

J. M. Moore et al., Geology before Pluto: Pre-encounter considerations. Icarus 246, 65–81 (2015). Abstract