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Like all low- and moderate-mass stars, the Sun will burn as a red giant during its later evolution, generating of solar luminosities for some tens of millions of years. During this post-main sequence phase, the habitable (i.e., liquid water) thermal zone of our Solar System will lie in the region where Triton, Pluto-Charon, and Kuiper Belt objects orbit. Compared with the 1 AU habitable zone where Earth resides, this "delayed gratification habitable zone" (DGHZ) will enjoy a far less biologically hazardous environment - with lower harmful radiation levels from the Sun, and a far less destructive collisional environment. Objects like Triton, Pluto-Charon, and Kuiper Belt objects, which are known to be rich in both water and organics, will then become possible sites for biochemical and perhaps even biological evolution. The Kuiper Belt, with >10(5) objects > or =50 km in radius and more than three times the combined surface area of the four terrestrial planets, provides numerous sites for possible evolution once the Sun's DGHZ reaches it. The Sun's DGHZ might be thought to only be of academic interest owing to its great separation from us in time. However, approximately 10(9) Milky Way stars burn as luminous red giants today. Thus, if icy-organic objects are common in the 20-50 AU zones of these stars, as they are in our Solar System (and as inferred in numerous main sequence stellar disk systems), then DGHZs may form a niche type of habitable zone that is likely to be numerically common in the Galaxy.

NASA's New Horizons mission performed the first flyby of a small Kuiper Belt Object (KBO), (486958) Arrokoth on 1 January 2019. The fast flyby revealed a fascinating, flattened, contact binary replete with a variety of unexpected geologic terrains. However, the irregular shape and constraints imposed by the fast flyby makes it a challenge to understand these features. Here we use the latest New Horizons shape models of Arrokoth to investigate its geophysical environment, including its surface slopes, gravity field, and moments of inertia—which are critical context for understanding Arrokoth's formation, evolution, and peculiar geology. We find that Arrokoth's surface features have a complicated relationship to its geophysical environment. For example, bright material tends to be concentrated in geopotential lows (like the neck), consistent with mass wasting—however, this trend is not consistently observed across Arrokoth. Mass wasting may naturally explain some aspects of Arrokoth's geology, but the actual dynamics of material transport may be complicated owing to Arrokoth's unique shape, spin‐rate, and inferred density. While New Horizons's fast and distant flyby precluded directly measuring Arrokoth's mass, we used techniques previously pioneered for comets and asteroids to infer its density. We find that Arrokoth has a low bulk density of ρ = 235 kg/m3 (1σ range: 155–600 kg/m3). This density is low compared to previously explored small bodies, but is comparable to comets, select binary KBOs, and the ring‐moons of Saturn. This low density may be a critical data‐point for understanding the formation of planetesimals at the dawn of the Solar System. Plain Language Summary: NASA's New Horizons mission performed the first flyby of a small Kuiper Belt Object (KBO), (486958) Arrokoth on 1 January 2019. Arrokoth is a ∼30‐km long, peanut shaped, icy body in the Kuiper Belt—a reservoir of small bodies left over from the Solar System's formation. Arrokoth is the most distant, and likely the least evolved object, ever explored by a spacecraft. Arrokoth's unusual shape makes it challenging to determine how material moves across its surface. We use the latest shape models to calculate Arrokoth's gravity field. Bright material tends to collect in Arrokoth's lowest locations, perhaps indicating material can move downslope. While New Horizons was not able to measure the density of Arrokoth, we are able to infer it using techniques pioneered for studying asteroids and comets. We find that Arrokoth's density is extremely low—more akin to fluffy snow on Earth. If correct, this would be one of the lowest density objects ever explored, closer to the densities of some of the comets and small moons within Saturn's rings. Arrokoth's low density may suggests that the building blocks of planets were assembled very gently in the outer Solar System, in the first few million years of Solar System formation. Key Points: We present new models for the geophysical environment of (486958) Arrokoth based on the latest New Horizons shape modelsMany (but not all) of Arrokoth's enigmatic surface markings are correlated with its geophysical environmentAnalysis of Arrokoth's shape suggests that it has a low density of 235 kg/m3 (1σ range: 155–600 kg/m3) [ABSTRACT FROM AUTHOR]

The New Horizons spacecraft returned images and compositional data showing that terrains on Pluto span a variety of ages, ranging from relatively ancient, heavily cratered areas to very young surfaces with few-to-no impact craters. One of the regions with very few impact craters is dominated by enormous rises with hummocky flanks. Similar features do not exist anywhere else in the imaged solar system. Here we analyze the geomorphology and composition of the features and conclude this region was resurfaced by cryovolcanic processes, of a type and scale so far unique to Pluto. Creation of this terrain requires multiple eruption sites and a large volume of material (>104 km3) to form what we propose are multiple, several-km-high domes, some of which merge to form more complex planforms. The existence of these massive features suggests Pluto's interior structure and evolution allows for either enhanced retention of heat or more heat overall than was anticipated before New Horizons, which permitted mobilization of water-ice-rich materials late in Pluto's history. Giant icy volcanos (cryovolcanos) on Pluto are unique in the imaged solar system and provide evidence for unexpected, active geology late in Pluto's history. [ABSTRACT FROM AUTHOR]

Recent analysis of New Horizons data revealed an ∼1,000‐km‐diameter region of large‐scale lineations antipodal to Sputnik Planitia, the 1,200 × 2,000 km elliptical impact basin on Pluto's anti‐Charon hemisphere. At the available resolution, these lineations are similar to antipodal terrains associated with large impact basins elsewhere in the Solar System. Here, we simulate the Sputnik Planitia‐forming impact and track stress waves through a Pluto‐like target body to the antipode. We find that the Sputnik Planitia‐forming impact is capable of producing significant antipodal terrains and the observed lineations may be extensional graben. The extent and mode of deformation at the antipode, however, is sensitive to ocean thickness and core composition. Simulations that best reproduce the observed terrains imply that Pluto may have hosted a >150‐km‐thick ocean and a hydrated core at the time of impact. Plain Language Summary: Pluto's interior structure remains largely a mystery, including the composition of the core and the existence and extent of a subsurface ocean. Though seismological study is not yet possible on Pluto, the reaction of a planet's interior structure to stress waves produced by large impacts can be used to indirectly probe the planet's interior—seismology by impact. We use impact modeling to simulate the formation of Sputnik Planitia, Pluto's massive impact basin, to determine how the dwarf planet's interior controls transmission of stress waves to the opposite hemisphere. Our results suggest that a large subsurface ocean beneath the ice shell, as well as a core that has experienced substantial alteration from interactions with an ocean above, are critical to match the size of the lineated terrain observed antipodal to Sputnik Planitia. Key Points: We model the formation of Pluto's Sputnik Planitia basin from an impact origin to assess the dwarf planet's interior structureOur simulations illustrate the Sputnik Planitia‐forming impact is capable of producing significant antipodal terrains on PlutoThe extent and mode of deformation at the antipode is most consistent with a thick (>150‐km) ocean and a hydrated core [ABSTRACT FROM AUTHOR]

Collisional modeling has been a fertile area of Kuiper Belt research for almost a decade. Such modeling has yielded important results concerning expected KBO surface properties, the KBO size distribution, the origin of KBOs and the properties of the primoridal Kuiper Belt, and most recently, the formation of KBO satellites. In what follows we briefly review some isolated aspects of these research results. A far more comprehensive, but older review of this topic was provided by Farinella et al. (in: Protostars and Planets IV, Mannings et al. (Eds.), University of Arizona Press, 2001). To cite this article: S.A. Stern, S.J. Kenyon, C. R. Physique 4 (2003). [Copyright &y& Elsevier]

We present a concept study for a distinctive robotic spacecraft mission that combines both the exploration of a never-before-visited class of planetary bodies and cutting-edge astrophysical investigations. The planetary targets are a class of objects called Centaurs that have relatively recently escaped from the Kuiper Belt and currently orbit closer to the Sun among the giant planets. We developed a trajectory that would visit several Centaurs, including the second largest known Centaur, 2060 Chiron, which displays enigmatic coma activity at large heliocentric distances and orbiting ring or dust structures. This concept takes advantage of the cruise times between planetary encounters to conduct nearly continuous astrophysical observations at wavelengths that are not accessible by ground-based facilities. Additionally, ride-along cubesats included aboard can be deployed at different points in the mission to perform various experiments or observations. This mission concept achieves its objectives with solar power (MegaFlex arrays), no new technology development, and within the approximate budget of a NASA New Frontiers class mission. The mission design accomplishes its objectives using solar electric propulsion and the recently developed NASA Evolutionary Xenon Thruster (NEXT) ion engines. • This mission concept combines astrophysical, planetary, and cubesat science. • The spacecraft would flyby the never-before visited Centaur population. • The large (~220 ​km diameter) Centaur Chiron is the principal flyby target. • Astrophysical science would be conducted during much of the 10-year flight. • The mission is accomplished with solar power and NASA NEXT ion engines. [ABSTRACT FROM AUTHOR]

The article discusses a study which examines the proportions of nitrogen and methane on the two largest dwarf planets in Solar system, Pluto and Eris. Findings of the study reveal that the abundances of nitrogen and methane in both planets were nearly similar with 90 percent and 10 percent on Eris and 97 percent and three percent on Pluto. It indicates that similar results were also depicted on the dwarf planet and Kuiper-belt escapee Triton.

We conducted an extensive search for dust or debris rings in the Pluto-Charon system before, during, and after the New Horizons encounter in July 2015. Methodologies included attempting to detect features by back-scattered light during the approach to Pluto (phase angle α  ∼ 15°), in situ detection of impacting particles, a search for stellar occultations near the time of closest approach, and by forward-scattered light imaging during departure ( α  ∼ 165°). An extensive search using the Hubble Space Telescope (HST) prior to the encounter also contributed to the final ring limits. No rings, debris, or dust features were observed, but our new detection limits provide a substantially improved picture of the environment throughout the Pluto-Charon system. Searches for rings in back-scattered light covered the range 35,000–250,000 km from the system barycenter, a zone that starts interior to the orbit of Styx, the innermost minor satellite, and extends out to four times the orbital radius of Hydra, the outermost known satellite. We obtained our firmest limits using data from the New Horizons LORRI camera in the inner half of this region. Our limits on the normal I / F of an unseen ring depends on the radial scale of the rings: 2 × 10 − 8 (3 σ ) for 1500 km wide rings, 1 × 10 − 8 for 6000 km rings, and 7 × 10 − 9 for 12,000 km rings. Beyond  ∼ 100, 000 km from Pluto, HST observations limit normal I / F to ∼ 8 × 10 − 8 . Searches for dust features from forward-scattered light extended from the surface of Pluto to the Pluto-Charon Hill sphere ( r Hill = 6.4 × 10 6 km). No evidence for rings or dust clouds was detected to normal I / F limits of ∼ 8.9 × 10 − 7 on  ∼ 10 4 km scales. Four stellar occulation observations also probed the space interior to Hydra, but again no dust or debris was detected. The Student Dust Counter detected one particle impact 3.6 × 10 6 km from Pluto, but this is consistent with the interplanetary space environment established during the cruise of New Horizons. Elsewhere in the solar system, small moons commonly share their orbits with faint dust rings. Our results support recent dynamical studies suggesting that small grains are quickly lost from the Pluto-Charon system due to solar radiation pressure, whereas larger particles are orbitally unstable due to ongoing perturbations by the known moons. [ABSTRACT FROM AUTHOR]

Abstract: The highest resolution (pixel scale 30 km) images of Ceres to date have been acquired by the Advanced Camera for Surveys onboard Hubble Space Telescope, through three wide band filters, centered at 535, 335, and 223 nm, covering more than one rotation of Ceres. The lightcurve at 535 nm agrees with earlier observations at V-band [Tedesco, E.F., Taylor, R.C., Drummond, J., Harwood, D., Nickoloff, I., Scaltriti, F., Schober, H. J., Zappala, V., 1983. Icarus 54, 23–29] in terms of magnitude, amplitude, and shape. The 0.04 magnitude lightcurve amplitude cannot be matched by Ceres'' rotationally symmetric shape, and is modeled here by albedo patterns. The geometric albedos at the above three wavelengths are measured to be , , and , respectively. V-band geometric albedo is calculated to be , consistent with earlier observations [Tedesco, E.F., 1989. In: Binzel, R.P., Gehrels, T., Matthews, M.S. (Eds.), Asteroids II. Univ. of Arizona Press, Tucson, pp. 1090–1138]. A strong absorption band (30%) centered at about 280 nm is observed, but cannot be identified with either laboratory UV spectra or the spectra of Europa or Ganymede. The single-scattering albedo has been modeled to be , , and , respectively. The photometric roughness of Ceres'' surface is found to be about from photometric modeling using Hapke''s theory, consistent with earlier radar observations [Mitchell, D.L., Ostro, S.J., Hudson, R.S., Rosema, K.D., Campbell, D.B., Velez, R., Chandler, J. F., Shapiro, I.I., Giorgini, J.D., Yeomans, D.K., 1996. Icarus 124, 113–133]. The first spatially resolved surface albedo maps of Ceres at three wavelengths have been constructed from HST observations, as well as the corresponding color maps. Eleven surface albedo features are identified, ranging in scale from 40–350 km. Overall the range of these albedo and color variations is small compared to other asteroids and some icy satellites. [Copyright &y& Elsevier]

The dominant topographic features on two-lobed Kuiper Belt object (486958) Arrokoth (provisionally designated 2014 MU 69) are scattered, small, circular depressions or pits up to ~1.0 km across and curvilinear troughs observed near the terminator during the New Horizons encounter of 01 January 2019. With important exceptions, evidence for an endogenic origin for pits is lacking and impact remains the most likely origin. Pit depths relative to the local surface are shallower than hypervelocity simple craters on icy moons and consistent with low velocity (~300 m/s) impacts in an icy target (the role of porosity and cohesion in cratering on Arrokoth being uncertain). There is, however, a large scatter in observed d/D, with some pits too shallow to measure reliably. The range of preservation states observed for both pits and troughs is consistent with slow but persistent degradation on the surface of this (and perhaps other) small primitive planetesimal(s) in the Kuiper Belt by agents such as micrometeorite bombardment or volatile loss, indicating that degradation does indeed occur on such bodies. Arrokoth pits could also be related to circular depressions on comet 9P/Tempel (though comparisons of surface features on Arrokoth and comets are intriguing they are limited by resolution differences and the evolved state of cometary surfaces). Two of the curvilinear troughs occur along the terminator of the Large Lobe. One trough is a shallow depression devoid of resolvable structures, the other features several linear scarps a few 10s of meters high, suggesting that coherent failure can occur on the surfaces of small Kuiper Belt objects such as Arrokoth. Several of the deepest small pits are also clustered within this walled trough, similar to tectonically associated pit chains on other planets. These pits and an obliquely viewed linear chain of elongate depressions remain the best candidates for endogenic features on Arrokoth. Subtle undulations and linear features no more than a few 10's of meters in amplitude (and even a few candidate positive relief features) are also evident in the terminator regions, indicating that the long-term evolution of this body may have been complex. • Circular pits on the KBO Arrokoth are shallower than simple craters on icy moons. • Range of observed pit depths suggests slow degradation of Arrokoth over much of Solar System's age. • Linear troughs on the terminator are shallow, <45 m deep. • Linear pit chains are best candidates for endogenic activity. • Whether these are related to linear and circular features on comets is unknown [ABSTRACT FROM AUTHOR]

The dusty jovian ring system must be replenished continuously from embedded source bodies. The New Horizons spacecraft has performed a comprehensive search for kilometer-sized moons within the system, which might have revealed the Larger members of this population. No new moons were found, however, indicating a sharp cutoff in the population of jovian bodies smaller than 8-kilometer-radius Adrastea. However, the search revealed two families of clumps in the main ring: one close pair and one cluster of three to five. All orbit within a brighter ringlet just interior to Adrastea. Their properties are very different from those of the few other clumpy rings known; the origin and nonrandom distribution of these features remain unexplained, but resonant confinement by Metis may play a role. [ABSTRACT FROM AUTHOR]