Postcards from Pluto

The New Horizons mission has provided a lot of observations about Pluto, the farthermost known celestial object in our Solar System. Thanks to these observations, astronomers can now describe some interesting Pluto's characteristics.

The Pluto system is surprisingly complex, comprising six objects that orbit their common center of mass in approximately a single plane and in nearly circular orbits. (...)
All four of Pluto's small moons are highly elongated objects with surprisingly high surface reflectances (albedos) suggestive of a water-ice surface composition. Kerberos appears to have a double-lobed shape, possibly formed by the merger of two smaller bodies. Crater counts for Nix and Hydra imply surface ages of at least 4 billion years. Nix and Hydra have mostly neutral (i.e., gray) colors, but an apparent crater on Nix's surface is redder than the rest of the surface; this finding suggests either that the impacting body had a different composition or that material with a different composition was excavated from below Nix's surface. All four small moons have rotational periods much shorter than their orbital periods, and their rotational poles are clustered nearly orthogonal to the direction of the common rotational poles of Pluto and Charon.

Pluto's small moons exhibit rapid rotation and large rotational obliquities, indicating that tidal despinning has not played the dominant role in their rotational evolution. Collisional processes are implicated in determining the shapes of the small moons, but collisional evolution was probably limited to the first several hundred million years after the system’s formation. The bright surfaces of Pluto's small moons suggest that if the Pluto-Charon binary was produced during a giant collision, the two precursor bodies were at least partially differentiated with icy surface layers.

Weaver, H., Buie, M., Buratti, B., Grundy, W., Lauer, T., Olkin, C., Parker, A., Porter, S., Showalter, M., Spencer, J., Stern, S., Verbiscer, A., McKinnon, W., Moore, J., Robbins, S., Schenk, P., Singer, K., Barnouin, O., Cheng, A., Ernst, C., Lisse, C., Jennings, D., Lunsford, A., Reuter, D., Hamilton, D., Kaufmann, D., Ennico, K., Young, L., Beyer, R., Binzel, R., Bray, V., Chaikin, A., Cook, J., Cruikshank, D., Dalle Ore, C., Earle, A., Gladstone, G., Howett, C., Linscott, I., Nimmo, F., Parker, J., Philippe, S., Protopapa, S., Reitsema, H., Schmitt, B., Stryk, T., Summers, M., Tsang, C., Throop, H., White, O., & Zangari, A. (2016). The small satellites of Pluto as observed by New Horizons Science, 351 (6279) DOI: 10.1126/science.aae0030

Also the composition of the Pluto's atmosphere is really interesting: it was dominates by molecular nitrogen, N₂, but there are also methane, CH₄, acetylene, C₂H₂, ethylene, C₂H₄, and ethane, C₂H₆. New Horizons has observed also some hazes on the surface:

The observations revealed major surprises, such as the unexpectedly cold upper atmosphere and the globally extensive haze layers. The cold upper atmosphere implies much lower escape rates of volatiles from Pluto than predicted and so has important implications for the volatile recycling and the long-term evolution of Pluto’s atmosphere.

Gladstone, G., Stern, S., Ennico, K., Olkin, C., Weaver, H., Young, L., Summers, M., Strobel, D., Hinson, D., Kammer, J., Parker, A., Steffl, A., Linscott, I., Parker, J., Cheng, A., Slater, D., Versteeg, M., Greathouse, T., Retherford, K., Throop, H., Cunningham, N., Woods, W., Singer, K., Tsang, C., Schindhelm, E., Lisse, C., Wong, M., Yung, Y., Zhu, X., Curdt, W., Lavvas, P., Young, E., Tyler, G., , ., Bagenal, F., Grundy, W., McKinnon, W., Moore, J., Spencer, J., Andert, T., Andrews, J., Banks, M., Bauer, B., Bauman, J., Barnouin, O., Bedini, P., Beisser, K., Beyer, R., Bhaskaran, S., Binzel, R., Birath, E., Bird, M., Bogan, D., Bowman, A., Bray, V., Brozovic, M., Bryan, C., Buckley, M., Buie, M., Buratti, B., Bushman, S., Calloway, A., Carcich, B., Conard, S., Conrad, C., Cook, J., Cruikshank, D., Custodio, O., Ore, C., Deboy, C., Dischner, Z., Dumont, P., Earle, A., Elliott, H., Ercol, J., Ernst, C., Finley, T., Flanigan, S., Fountain, G., Freeze, M., Green, J., Guo, Y., Hahn, M., Hamilton, D., Hamilton, S., Hanley, J., Harch, A., Hart, H., Hersman, C., Hill, A., Hill, M., Holdridge, M., Horanyi, M., Howard, A., Howett, C., Jackman, C., Jacobson, R., Jennings, D., Kang, H., Kaufmann, D., Kollmann, P., Krimigis, S., Kusnierkiewicz, D., Lauer, T., Lee, J., Lindstrom, K., Lunsford, A., Mallder, V., Martin, N., McComas, D., McNutt, R., Mehoke, D., Mehoke, T., Melin, E., Mutchler, M., Nelson, D., Nimmo, F., Nunez, J., Ocampo, A., Owen, W., Paetzold, M., Page, B., Pelletier, F., Peterson, J., Pinkine, N., Piquette, M., Porter, S., Protopapa, S., Redfern, J., Reitsema, H., Reuter, D., Roberts, J., Robbins, S., Rogers, G., Rose, D., Runyon, K., Ryschkewitsch, M., Schenk, P., Sepan, B., Showalter, M., Soluri, M., Stanbridge, D., Stryk, T., Szalay, J., Tapley, M., Taylor, A., Taylor, H., Umurhan, O., Verbiscer, A., Versteeg, M., Vincent, M., Webbert, R., Weidner, S., Weigle, G., White, O., Whittenburg, K., Williams, B., Williams, K., Williams, S., Zangari, A., & Zirnstein, E. (2016). The atmosphere of Pluto as observed by New Horizons Science, 351 (6279) DOI: 10.1126/science.aad8866

Observations of Pluto and Charon obtained that day reveal regionally diverse colors and compositions. On Pluto, the color images show nonvolatile tholins coating an ancient, heavily cratered equatorial belt. A smooth, thousand-kilometer plain must be able to refresh its surface rapidly enough to erase all impact craters. Infrared observations of this region show volatile ices including N₂ and CO. H₂O ice is not detected there, but it does appear in neighboring regions. CH₄ ice appears on crater rims and mountain ridges at low latitudes and is abundant at Pluto's high northern latitudes. Pluto’s regional albedo contrasts are among the most extreme for solar system objects. Pluto's large moon Charon offers its own surprises. Its H₂O ice–rich surface is unlike other outer solar system icy satellites in exhibiting distinctly reddish tholin coloration around its northern pole as well as a few highly localized patches rich in NH₃ ice.

Grundy, W. M., Binzel, R. P., Buratti, B. J., Cook, J. C., Cruikshank, D. P., Dalle Ore, C. M., ... & Olkin, C. B. (2016). Surface compositions across Pluto and Charon. Science, 351 (6279), DOI: 10.1126/science.aad9189

The geology of Pluto and Charon are really very complex. On the surface of the first one, astronomers had identified the Cthulhu Regio (CR):

CR is a large dark area that covers a swath from ~15°N to ~20°S, bordering TR at 160°E, and stretching westward almost halfway around the planet to 20°E. Eastern CR is not a distinct physiographic province, but instead a region of dark mantling thin enough to preserve underlying topography, superimposed upon various geological terrains, including dendritic valleys, craters, fossae (long, narrow troughs), and retreating scarps. The dark coating is likely the result of atmospheric tholin deposition. CR contains striking correlations between color/albedo and topography: Bright material is correlated with high elevations in some areas and with north-facing slopes in others. This may result in part from insolation-dependent deposition of the bright material on the dark landscape. Other western low-latitude terrains are discussed in the supplementary materials.

Moore, J. M., McKinnon, W. B., Spencer, J. R., Howard, A. D., Schenk, P. M., Beyer, R. A., ... & Stern, S. A. (2016). The geology of Pluto and Charon through the eyes of New Horizons. Science, 351 (6279), 1284-1293. DOI: 10.1126/science.aad7055