FIFTY YEARS OF EXPLORING PLUTO: FROM TELESCOPES TO THE NEW HORIZONS MISSION. D. P. Cruikshank1, S. A. Stern2, H. A. Weaver3, C. B. Olkin2, L. A., Young2, K. Ennico1, F. Bagenal4, New Horizons Science Team. 1NASA Ames Research Center, Moffett Field, CA, ([email protected]), 2Southwest Re- search Institute, Boulder, CO ([email protected]), 3Applied Physics Lab., Laurel, MD, 4LASP, Univ. Colorado, Boulder, CO. Discovery and early work: Pluto was discovered subsequent papers, not fully referenced here, give de- in 1930 at Lowell Observatory in the belated resump- tails. The diameters of Pluto and Charon were refined tion of a wide-field photographic search originally be- to 2376 and 1210 km, respectively, and the densities gun at Percival Lowell’s direction prior to his death in were found to be 1.854 and 1,702 g/cm3, respectively 1916. Photometry in the 1950s [1] established the ro- [13]. The complexity of Pluto’s surface and atmos- tation period of 6.4 hours and a color redder than the phere, as well as the near-space environment, revealed Sun, but the mass, density, size and albedo were un- by New Horizons far exceeded predictions and expec- known. Near-infrared photometry in 1976 indicated tations. The degree of current and recent geological the presence of CH4 frost, suggestive of a relatively activity was not anticipated for this relatively small, high surface albedo and a diameter comparable to the cold, and ancient planetary body [14]. In order to sup- Moon [2]. The large satellite Charon was discovered port the range of vertical relief of ~6 km observed in 1978 [3], followed by an epoch of mutual transits across the surface, the most probable bedrock material and occultations of Pluto and Charon from 1985 to is H2O ice. Some exposures of H2O ice are seen, but 1990, as viewed from Earth. These events resulted in most of the bedrock is overlain by a veneer of much reliable sizes and masses of the two bodies, as well as more volatile N2, CH4, and CO, all of which are spec- the orbit of Charon. The mutual events also demon- troscopically detected [15-17]. The 1000-km wide strated that Pluto and Charon are in locked synchro- nitrogen glacier (Sputnik Planitia) is being fed by ice nous rotation and revolution, a configuration unique flows from nearby highland sources, while the glacier among the planets [4]. The atmosphere of Pluto was itself is slowly convecting [18], with surface expres- discovered in 1988 from a stellar occultation observed sions of convective cells, wind-blown dunes of dark from the Kuiper Airborne Observatory and ground material [19], and no evidence on the surface of impact stations [5,6], with indications of a haze layer (or a craters larger than a few hundred meters. Sputnik temperature inversion) in the lower atmosphere. Sub- Planitia formed as a large impact basin, thinning the sequent stellar occultations showed that the extent of planet’s crust and creating a gravity anomaly [20]. The the atmosphere is variable on a timescale of a few anomaly was enhanced as solid N2 accumulated in the years. The spectroscopic detection of N2 and CO ice in basin [21]. Although there are other scenarios for the 1993 [7] demonstrated that the atmosphere must be evolution of Sputnik Planitia, the net effect of the al- primarily composed of N2, with CH4 and CO as minor tered gravity field was to cause Pluto to reorient so as components; the spectroscopic detection of gaseous to line up exactly with Charon at the antipode of the CH4 was reported in 1994 [8]. planet’s opposite hemisphere. Pluto appears to have New Horizons Mission: Discussions with NASA once had a global subsurface ocean, which may persist for a spacecraft mission to Pluto began in 1989 [9-11], to the present, with some regions of the surface sug- and after several studies, proposals, and programmatic gesting relatively recent (<109 y) episodes of ejection changes, a mission to the Kuiper Belt and Pluto was of cryovolcanic fluids from subsurface reservoirs. recommended by the first Planetary Decadal Survey at Pluto’s multiple layers of atmospheric haze and lower the highest priority for NASA’s first New Frontiers escape rate were unexpected [22]. The yellow and red- mission [12]. New Horizons was selected in 2001 in brown colors widely distributed on Pluto’s surface are competition with other mission proposals. suggestive of a tholin composed of complex organic Scientific Results: NASA’s New Horizons mission chemicals, some of which have fallen from the atmos- to the Kuiper Belt and Pluto was launched on January phere as haze particles, with some created by sunlight 19, 2006, and after flying by Jupiter some 13 months and solar wind particles acting on the CH4 and N2 ices later, passed by Pluto at a distance of 12,500 km and covering much of the surface. The color properties of Charon at a distance of 28,800 km on July 14, 2015. much of Pluto’s surface are closely similar to those of Four small satellites previously discovered with the tholin produced in the lab by energetic processing of a Hubble Space Telescope were imaged, and their rota- Pluto-mix of ices (CH4, N2, CO), and consisting of a tion periods measured. Details of the spacecraft, in- mix of many organic molecular structures [23-25]. struments, and the flyby are given in [9-11], with an Pluto shows evidence of major changes in atmos- early report on the science results in [13], while many pheric pressure on Milankovich (~million-year) time- scales due to precession of the rotational pole, possibly planetary bodies in their size range, demonstrating that resulting in past episodes of running or standing liquid geological activity on both large and small scales can on the surface [26, 27]. The atmosphere is in vapor occur long after the epoch of planet formation. A few pressure equilibrium with the volatile surface ices and other dwarf planets comparable in size to Pluto and is well mixed horizontally above ~10 km. The tropo- Charon are known in the transneptunian population, pause is variable over location and time of day. The and the extraordinary discoveries by New Horizons photochemical products C2H2, C2H4, and C2H6 were inform open questions of their origins and physical- detected. As many as 20 haze layers were seen; the chemical characteristics that will remain until they, too, haze may contribute to cooling the atmosphere [28]. In can be explored. forward scattering the atmosphere is blue as a result of Supported by NASA's New Horizons mission. Rayleigh scattering. Before the New Horizons flyby the estimates of the References: [1] Walker, M. F. and Hardie, R. atmospheric escape (presumed to be mostly N2 mole- (1955) PASP 67, 224-228. [2] Cruikshank D.P. et al. cules) ranged from as low as 1.5 x 1025 s-1 to as high as (1976) Science 194, 835-837. [3] Christy, J. W. and 2 x 1028 s-1. Combining these atmospheric escape rates Harrington, R. S. (1980) Icarus 44, 38-40. [4] Tholen, with Voyager and New Horizons observations of the D. J., and Buie, M. W. (1997) In Pluto and Charon (S. solar wind at 33 AU produced estimates of the scale of A. Stern and D. J. Tholen, editors. Univ. Ariz. Press. the interaction region from 7 to 1000 Pluto radii [29]. [5] Elliot, J. L. et al. (1989) Icarus 77, 148-170.. [6] Observations by the SWAP instrument revealed a sur- Millis, R. L. et al. (1993) Icarus 105, 282-297. [7] prisingly small interaction region, confined on its up- Owen T. C. et al. (1993) Science 261, 745-748. [8] wind side to within ~6 Pluto radii [30, 31]. The occul- Young L.A. et al. (1994), Bull. AAS, 26, 1168 (abstract tation measurements by New Horizons revealed Pluto’s #26, 28.02). [9] Stern S. A. (2008) Space Sci. Rev. atmosphere to be colder and less extended than ex- 140, 3-22. [10] Stern S. A. and Grinspoon D. (2018) 25 pected, which reduces the escape rate to only 6 x 10 Chasing New Horizons. Picador, New York, 295 pp. -1 molecules s and the main escaping gas is now thought [11] Cruikshank D. P. and Sheehan, W. (2018) Discov- to be mostly methane rather than nitrogen [32]. The ering Pluto. U. Ariz. Press, Tucson, 475 pp. [12] New surprisingly small size of the interaction region is con- Frontiers in the Solar System: An Integrated Explora- sistent with a reduced atmospheric escape rate as well tion Strategy (2015) Nat'l. Acad. Press. Washington, as a particularly high solar wind flux, enhanced by a DC. [13] Stern, S. A. et al. (2015) Science 350, 292. factor of ~4 due to a passing compression region in the [14] Moore J. F. et al. (2016) Science 351, 1284-1293. solar wind. [15] Grundy W. M. et al. (2016) Science 351 issue The cratering record on both Pluto and Charon 6279, aad9189-8. [16] Protopapa S. et al. (2017) Ica- demonstrates a paucity of small impactors in their re- rus 287, 218-228. [17] Schmitt B. et al. (2017) Icarus gion of the Solar System, suggesting an early depletion 287, 229-260. [18] McKinnon W. B. et al. 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