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A prominent feature of the interaction between a planetary moon and its magnetospheric environment is the formation of a wake cavity in the downstream hemisphere, characterized by a significant decrease of the incident plasma density. Using an analytical model of Triton's sub-Alfvénic interaction with Neptune's magnetosphere, we demonstrate that this moon's wake may be rotated away from its downstream hemisphere into a region that would be accessible to the undisturbed upstream flow. Due to the strong tilt of Neptune's magnetospheric field and the low Alfvénic Mach number of the plasma, one of Triton's Alfvén wings can penetrate into the upstream region and intercept the impinging plasma long before it reaches the moon. The interaction with this upstream wing causes the flow to be deflected toward Triton at a steep angle before being absorbed, thereby generating a wake cavity that is significantly displaced with respect to the moon's geometric plasma shadow. Along the downstream-facing wing, the flow is deflected away from Triton and therefore unable to refill the displaced wake. Since the ionospheric Pedersen conductance greatly exceeds the Alfvén conductance, this asymmetric flow deflection is particularly intense at Triton: when the plasma encounters the wings, it is channeled toward or away from the moon along the axes of the wing tubes. The deflection of the streamlines away from their upstream direction peaks in the range of plasma parameters found along Triton's orbit. Therefore, the displaced wake may be a persistent feature of this moon's plasma interaction and observable during future flybys. Plain Language Summary Many moons in the outer solar system are embedded in their parent planet's magnetospheres where they are continuously exposed to a flow of magnetized plasma. In general, the partial blockage of this flow by a moon's solid body and ionosphere generates a depletion region at the downstream side where the plasma density is significantly reduced. In addition, the interaction between a moon and its plasma environment triggers non-linear (Alfvén) waves that mainly propagate along the magnetospheric magnetic field. At many large moons of Jupiter and Saturn, the magnetospheric field is nearly perpendicular to the incident flow direction, that is, these waves cannot reach the upstream region. However, Neptune's magnetic field near Triton possesses a strong component along the plasma flow. Therefore, the waves may travel toward upstream and commence deflecting the incident flow before it even comes close to the moon. Since this deflection forces the plasma particles onto highly inclined trajectories toward Triton, their subsequent absorption generates a wake cavity that is no longer located "behind" the moon. Such a displaced wake may be observable during future missions to the Neptune-Triton system and may represent a unique feature in the diverse "zoo" of moon-magnetosphere interaction scenarios at the outer planets. [ABSTRACT FROM AUTHOR]

The tilt between Neptune's magnetic and rotational axes, along with Triton's orbital obliquity, causes a strong time variability of the moon's local electromagnetic environment. To constrain Triton's interaction with the ambient magnetospheric plasma, we apply a hybrid (kinetic ions, fluid electrons) model including the moon's ionosphere and induced field. To represent the extremes in the changes to the local electromagnetic field over a synodic rotation, we consider two orientations between the ambient magnetic field and flow velocity. For each, we first investigate the (analytical) magnetic signatures associated with the superposition of Triton's induced field and the magnetospheric field in the absence of any plasma interaction effects. To constrain the effect of Triton's ionosphere on the currents, we model the interaction between the ionospheric and magnetospheric plasma in isolation from the moon's inductive response, before combining these effects to investigate the complex scenario of plasma interaction and induction. Finally, we explore the sensitivity of the plasma interaction to changes in the ambient plasma density and the strength of Triton's inductive response. Despite plasma interaction signatures that dominate the plasma perturbations far from the moon (beyond ∼3 Triton radii), we illustrate that the induced field is clearly discernible within ∼3 Triton radii, regardless of the moon's location within Neptune's magnetosphere. We find that the orientation of the magnetospheric field and velocity vectors strongly affects Triton's plasma interaction; at times, resembling those of Jupiter's or Saturn's moons, while at others, revealing unprecedented signatures that are likely unique to moons of the ice giants. Key Points: We constrain the time variability in signatures associated with plasma interaction and induction at Triton, Neptune's largest moonDespite the presence of plasma currents far from the moon, Triton's induced field dominates the near‐surface magnetic field perturbationsThe tilt of Neptune's magnetic and rotation axes generates a plasma wake that is strongly displaced out of Triton's geometric plasma shadow [ABSTRACT FROM AUTHOR]

Triton, the largest Solar System satellite beyond Saturn, was probably captured from the Transneptunian population by Neptune. It is mainly covered by N 2 , CO, CO 2 , CH 4 and H 2 O in solid state and, except for H 2 O and CO 2 , these species are also present in gas phase. Sublimation and recondensation of the volatile species may lead to geographical and temporal variation on the surface composition, and could participate to the formation of complex chemical compounds formed from photochemistry occurring in the atmosphere (Krasnopolsky and Cruikshank, 1999) or from irradiation of N 2 : CH 4 : CO layers (Moore and Hudson, 2003). We present new near-IR observations performed at the VLT-ESO with SINFONI in 2010, 2011 and 2013, from which band depths, areas and positions of the main ice features are determined at different longitudes. We also re-reduce and re-analyze earlier data obtained during the last 20 years. Models based on the Hapke theory (Hapke, 1993) are developed to constrain the abundance, grain size, temperature, and state of mixing of the different ices (N 2 , CH 4 , CO, CO 2 , H 2 O) as well as attempt to identify other species. For this purpose, we present and use new optical constants of CO 2 measured at 35 and 54 K. Our analyses confirm the longitudinal variation of the N 2 and CO surface abundances previously evidenced, and suggest additional latitudinal and/or temporal variability of these two species. We confirm the presence of deep N 2 layers in which CO and CH 4 are diluted. In particular, we demonstrate that CO is present in diluted (as opposed to pure ice) form. In contrast, our models support the presence of small and longitudinally variable amounts of pure CH 4 ice, providing an explanation to the enhanced atmospheric CH 4 /N 2 ratio over expectations based on an ideal N 2 -CH 4 -CO mixture. They also suggest significant smaller particles of H 2 O and CO 2 than reported previously in Quirico et al. (1999), with CO 2 being probably distributed over a large area of the surface. We find that the 2.40 µm band is primarily due to 13 CO ice, with a telluric value of the 13 CO/ 12 CO ratio, and not to ethane. We infer a N 2 ice temperature of 37.5  ±  1K, suggesting that the atmospheric pressure over 2010–2013 was similar to that during the Voyager 1989 epoch. [ABSTRACT FROM AUTHOR]

Triton's surface shows clear evidences of resurfacing processes such as volcanism, tectonic structures and a very low presence of impact basins, which indicates that Triton's interior could remain active currently. It is generally assumed in icy satellites that the depth of faulting associated with large faults corresponds to the brittle-ductile transition (BDT), and the properties of large faults penetrating to the BDT depth can be used to obtain information on the mechanical behavior of the lithosphere, and potentially can contribute to constrain the thermal state of a planetary body. Here we present a detailed study of Raz Fossae, suggesting that this structure is not only limited to the graben originally described in previous works. Our study shows that north of the classical Raz Fossae there is another trough set with similar structural characteristics that can be interpreted as part of the same system of faults. We have calculated surface heat flows for Triton from the depth of the BDT under both structures: Raz Fossae and the new northern trough set from a detailed analysis of the troughs widths, taking into account the possible composition of the ice shell (H 2 O and NH 3 ·2H 2 O). Our results show surface heat flows values much higher than those estimated in previous studies by modeling radiogenic production and tidal dissipation for fixed orbital eccentricities. Furthermore, our results suggest regional differences in the pattern of heat loss throughout Triton's lithosphere. [ABSTRACT FROM AUTHOR]

Context. Both Pluto and Triton possess thin, N2-dominated atmospheres controlled by sublimation of surface ices. Aims. We aim to constrain the influx and ablation of interplanetary dust grains into the atmospheres of both Pluto and Triton in order to estimate the rate at which oxygen-bearing species are introduced into both atmospheres. Methods. We use (i) an interplanetary dust dynamics model to calculate the flux and velocity distributions of interplanetary dust grains relevant for both Pluto and Triton and (ii) a model for the ablation of interplanetary dust grains in the atmospheres of both Pluto and Triton. We sum the individual ablation profiles over the incoming mass and velocity distributions of interplanetary dust grains in order to determine the vertical structure and net deposition of water to both atmospheres. Results. Our results show that <2% of silicate grains ablate at either Pluto or Triton while approximately 75% and >99% of water ice grains ablate at Pluto and Triton, respectively. From ice grains, we calculate net water influxes to Pluto and Triton of ~3.8 kg day−1 (8.5 × 103 H2O cm−2 s−1) and ~370 kg day−1 (6.2 × 105 H2O cm−2 s−1), respectively. The significant difference in total water deposition between Pluto and Triton is due to the presence of Triton within Neptune's gravity well, which both enhances interplanetary dust particle (IDP) fluxes due to gravitational focusing and accelerates grains before entry into Triton's atmosphere, thereby causing more efficient ablation. Conclusions. We conclude that water deposition from dust ablation plays only a minor role at Pluto due to its relatively low flux. At Triton, water deposition from IDPs is more significant and may play a role in the alteration of atmospheric and ionospheric chemistry. We also suggest that meteoric smoke and smaller, unablated grains may serve as condensation nuclei for the formation of hazes at both worlds. [ABSTRACT FROM AUTHOR]

Both atmospheres of Pluto and Neptune's largest satellite Triton have cold surfaces with surface gravitational accelerations and atmospheric surface pressures of comparable magnitude. To study their atmospheres we have updated Zhu et al. (2014) model for Pluto's atmosphere by adopting Voigt line profiles in the radiation module with the latest spectral database and extended the model to Triton's atmosphere by including additional parameterized heating due to the magnetospheric electron transport and energy deposition. The CH 4 mixing ratio profiles play central roles in differentiating the atmospheres of Pluto and Triton. On Pluto the surface CH 4 mole fraction is in the range of 0.3-0.8%, sufficiently high to ensure that it is well mixed in the lower atmosphere and not subject to photochemical destruction. Near the exobase CH 4 attains comparable density to N 2 due to gravitational diffusive separation and escapes at 500 times the N 2 rate (= 1 × 10 23 N 2 s − 1 ). In Triton's atmosphere, the surface CH 4 mole fraction is on the order of 0.015%, sufficiently low to ensure that it is photochemically destroyed irreversibly in the lower atmosphere and that N 2 remains the major species, even at the exobase. With solar EUV power only, Triton's upper thermosphere is too cold and magnetospheric heating, approximately comparable to the solar EUV power, is needed to bring the N 2 tangential column number density in the 500–800 km range up to values derived from the Voyager 2 UVS observations (Broadfoot et al. , 1989). Due to their cold exobase temperatures relative to the gravitational potential energy wells that N 2 resides in, atmospheric escape from Triton and Pluto is not dominated by N 2 Jeans escape but by CH 4 from Pluto and H, C, N and H 2 from Triton. The atmospheric thermal structure near the exobase is sensitive to the atmospheric escape rate only when it is significantly greater than 2 × 10 27 amu s −1 , above which enhanced Jeans escape and larger radial velocity adiabatically cools the atmosphere to a lower temperature. Finally we suggest that Pluto's thermosphere is a cold ∼ 70 K due to ablation of H 2 O molecules from the influx of dust grains detected by New Horizons Student Dust Counter. [ABSTRACT FROM AUTHOR]

755 new CCD observations during the years 2014-2016 have been reduced to derive the precise positions of Triton, the first satellite of Neptune. The observations were made by the 1-m telescope at Yunnan Observatory over 15 nights. The positions of Triton are measured with respect to the stars in Gaia DR1 star catalogue. The theoretical position of Triton was retrieved from the Jet Propulsion Laboratory (JPL) Horizons System that includes the satellite ephemeris nep081xl, while the position of Neptune was obtained from the JPL ephemeris DE431mx. Our results show that the mean (O -- C)s (observed minus computed) are 0.042 and -0.006 arcsec in right ascension and declination, respectively. The dispersions of our observations are estimated at about 0.012 arcsec in each direction. [ABSTRACT FROM AUTHOR]

Several distant icy worlds have atmospheres that are in vapor-pressure equilibrium with their surface volatiles, including Pluto, Triton, and, probably, several large KBOs near perihelion. Studies of the volatile and thermal evolution of these have been limited by computational speed, especially for models that treat surfaces that vary with both latitude and longitude. In order to expedite such work, I present a new numerical model for the seasonal behavior of Pluto and Triton which (i) uses initial conditions that improve convergence, (ii) uses an expedient method for handling the transition between global and non-global atmospheres, (iii) includes local conservation of energy and global conservation of mass to partition energy between heating, conduction, and sublimation or condensation, (iv) uses time-stepping algorithms that ensure stability while allowing larger timesteps, and (v) can include longitudinal variability. This model, called VT3D, has been used in Young (2012a, 2012b ), Young (2013), Olkin et al. (2015), Young and McKinnon (2013), and French et al. (2015). Many elements of VT3D can be used independently. For example, VT3D can also be used to speed up thermophysical models (Spencer et al., 1989) for bodies without volatiles. Code implementation is included in the supplemental materials and is available from the author. [ABSTRACT FROM AUTHOR]

Haiyang-2 (HY-2) is the first marine dynamic environmental satellite of China, which was launched on August 16, 2011. The scanning microwave radiometer (RM) onboard HY-2 has low-frequency channels with the capability of observing sea surface temperature (SST) from space. In this paper, the Level 2A (L2A) SST products of HY-2 RM are evaluated. The global HY-2 RM L2A SST products are compared with the buoy SST measurements, WindSat SST, and National Oceanic and Atmospheric Administration Optimum Interpolation (OI) weekly SST products for the period from January 2012 to December 2014. The collocations of HY-2 RM, WindSat, and buoy SST data are generated with the spatial window of 0.25° and the temporal window of 0.5 h. The biases are −0.45 °C (RM minus buoy) and −0.41 °C (RM minus WindSat) and the corresponding standard deviations are 1.73 °C and 1.72 °C. The comparisons of the weekly averaged HY-2 RM and OI SST show that the biases of each week difference are from −1.06 °C to 0.48 °C with the mean value of −0.30 °C. The standard deviations of the SST difference are from 0.83 °C to 1.47 °C with the mean value of 1.05 °C. The relationships between SST difference and the sea surface and atmospheric parameters, such as wind speed, wind direction, SST, and water vapor are investigated. [ABSTRACT FROM PUBLISHER]

Modelling the motion of Triton, the main satellite of Neptune, is specific. Earlier researchers built Triton's ephemeris by numerical integration of the equations of its motion. However, these ephemeris can be accessed only by using online ephemeris server or by borrowing a special calculating program and huge data file from authors of the ephemeris. In addition, the interval of the earlier ephemeris is limited. In this paper, simple and easily programmable formulae are presented for computing Triton's ephemeris for any instant of time. The formulae are based on a new analytical theory of Triton's motion all necessary perturbing factors being taken into consideration. The parameters of the theory are fit to all published observations made from 1847 to 2012 (10 254 observations in total). After the parameters were fit to observations, the root-mean-square residuals were 0.228 arcsec, the weighted average residual being 0.036 arcsec. The new ephemeris of Triton slightly differs from those produced by other authors because of differences in the sets of used observations. The new ephemeris of Triton are put on our online ephemeris server. It is shown that the available observations do not allow to determine reliably the quadratic term in the orbital longitude of Triton. Such a term would be an additional indicator of the accuracy of the theory and observations. [ABSTRACT FROM AUTHOR]

Compositional analyses of Pluto’s surface ice in the literature typically include large areas on the body where CH 4 and other volatiles are segregated in the pure form from the solid solution N 2 :CH 4 in which CH 4 is diluted. However, the existence of continent-size areas of pure CH 4 are in conflict with both of the alternative models that successfully explain the enhancement of CH 4 in Pluto’s atmosphere, the Detailed Balancing thermal equilibrium model and the Hot Methane Patch model. Pluto’s spectrum includes an apparently unshifted CH 4 component while Triton’s does not, and 93% of the concentration range of the binary phase diagram at 38 K shows that these species exist as a mixture of two saturated solid solution phases. Recognizing this, we propose that both of these saturated phases are present on Pluto and the CH 4 -rich phase of the mixture, CH 4 :N 2 , is the source of the relatively unshifted CH 4 spectrum attributed to pure CH 4 . We also propose that CH 4 is less abundant in Triton’s ice to the point where either the ice is not saturated or the saturated CH 4 :N 2 phase has not been detected. In this scenario, the partial vapor pressures do not change when the relative proportions of these saturated phases are varied in the mixture. Thus, the partial vapor pressures are independent of N 2 –CH 4 concentrations if both saturated phases are present. Accordingly, the longitudinal and seasonal variations of CH 4 and N 2 features in Pluto’s spectrum would be attributed to spatial variations in the relative proportions of these species. This may occur during volatile transport in the sublimation wind through extensive influences. The lower, unsaturated, values of the mole fraction of CH 4 in the ice reported by Owen et al. (Owen et al. [1993]. Science 261, 745–748) and Cruikshank et al. (Cruikshank, D.P., Rush, T.L., Owen, T.C., Quirico, E., de Bergh, C. [1998]. The surface compositions of Triton, Pluto, and Charon. In: Solar System Ices. Astrophysics and Space Science Library Series, vol. 227. Kluwer Academic Publishers, Dordrecht), and by Doute et al. (Doute, S., Schmitt, B., Quirico, E., Owen, T.C., Cruikshank, D.P., de Bergh, C., Geballe, T.R., Roush, T.L. [1999]. Icarus 142, 421–444) based on a compositional analysis of Pluto’s surface, were not obtained using optical constants for components consistent with the constraints of the phase diagram. [ABSTRACT FROM AUTHOR]

The current Cassini–Huygens Mission to the Saturn system provides compelling evidence that the present state of Titan’s dense atmosphere is unsustainable over the age of the Solar System. Instead, for most of its existence, Titan’s atmosphere might have been in a Snowball state, characterized by a colder surface and a smaller amount of atmospheric CH 4 , similar to that of Pluto or Triton. We run a 1-D chemical transport model and show that the rates of organic synthesis on a Snowball Titan are significantly slower than those on present-day Titan. The primary method of methane destruction—photosensitized dissociation in the stratosphere—is greatly dampened on Snowball Titan. The downward flux of higher-order molecules through the troposphere is dominated not by hydrocarbons such as ethane, as is the case on Titan today, but by nitriles. This result presents a testable observation that could confirm the Snowball Titan hypothesis. Because Pluto’s atmosphere is similar to Titan’s in composition, it serves as a basis for comparison. Future observations of Pluto by the New Horizons Mission will inform photochemical models of Pluto’s atmosphere and can help us understand the photochemical nature of paleo-Titan’s atmosphere. [ABSTRACT FROM AUTHOR]

We investigate the origins of Triton’s deformed and young surface. Assuming Triton was captured early in Solar System history, the bulk of the energy released during capture will have been lost, and cannot be responsible for its present-day activity. Radiogenic heating is sufficient to maintain a long-lived ocean beneath a conductive ice shell, but insufficient to cause convective deformation and yielding at the surface. However, Triton’s high inclination likely causes a significant (≈0.7°) obliquity, resulting in large heat fluxes due to tidal dissipation in any subsurface ocean. For a 300 km thick ice shell, the estimated ocean heat production rate (≈0.3 TW) is capable of producing surface yielding and mobile-lid convection. Requiring convection places an upper bound on the ice shell viscosity, while the requirement for yielding imposes a lower bound. Both bounds can be satisfied with an ocean temperature ≈240 K for our nominal temperature-viscosity relationship, suggesting the presence of an antifreeze such as NH 3 . In our view, Triton’s geological activity is driven by obliquity tides, which arise because of its inclination. In contrast, Pluto is unlikely to be experiencing significant tidal heating. While Pluto may have experienced ancient tectonic deformation, we do not anticipate seeing the kind of young, deformed surfaces seen at Triton. [ABSTRACT FROM AUTHOR]

The planet Neptune and its largest moon Triton hold the keys to major advances across multiple fields of Solar System science. The ice giant Neptune played a unique and important role in the process of Solar System formation, has the most meteorologically active atmosphere in the Solar System (despite its great distance from the Sun), and may be the best Solar System analogue of the dominant class of exoplanets detected to date. Neptune׳s moon Triton is very likely a captured Kuiper Belt object, holding the answers to questions about the icy dwarf planets that formed in the outer Solar System. Triton is geologically active, has a tenuous nitrogen atmosphere, and is predicted to have a subsurface ocean. However, our exploration of the Neptune system remains limited to a single spacecraft flyby, made by Voyager 2 in 1989. Here, we present the high-level science case for further exploration of this outermost planetary system, based on a white paper submitted to the European Space Agency (ESA) for the definition of the second and third large missions in the ESA Cosmic Vision Programme 2015–2025. We discuss all the major science themes that are relevant for further spacecraft exploration of the Neptune system, and identify key scientific questions in each area. We present an overview of the results of a European-led Neptune orbiter mission analysis. Such a mission has significant scope for international collaboration, and is essential to achieve our aim of understanding how the Solar System formed, and how it works today. [ABSTRACT FROM AUTHOR]

Astrometric positions of the Neptunian satellite Triton with a visual magnitude of 13.5 were obtained during three successive oppositions in 2007, 2008 and 2009. A total of 1095 new observed positions of Triton were collected during 46 nights of observations, involving eight missions and three telescopes. We compared our observations to the best ephemerides of Triton available now. This comparison has shown that our observations present a high level of accuracy as they provide standard deviations of residuals hardly higher than 50 mas and mean residuals lower than 30 mas, corresponding to about only 500 km in the position of the very distant satellite Triton. Moreover, we have compared most of the different planetary ephemerides of Neptune available now as well as two recent orbit models of Triton. These new comparisons have clearly shown the differences between all of these ephemerides which can be significant and that are presented in this work. [ABSTRACT FROM AUTHOR]

The article presented a study on Neptune's atmosphere, its viscous shock layer and stagnation-region heating environment. It presents an overview of the chemical kinetics and radiation phenomena occurring in the shock layer of outer planets and the aerocapturing flight if one hopes to be captured into an orbit around the planet that passes by Neptune's moon Triton. Ionization of the shock-layer flow and thermochemical processes producing electrons are also discussed.

In this paper, we used the 3108 Earth-based astrometric observations from the Natural Satellite Data Center over more than 30 yr time span from 1975 to 2006 for determining the epoch state vectors of the Neptunian largest satellite Triton. These observations almost contain all modern photo and CCD observations available. In integrating perturbation equation, the barycentric frame of Neptune–Triton system is adopted, and in considering the oblateness perturbation due to Neptune, a revised pole model describing the precession of the Neptune's pole is used in our calculation. [ABSTRACT FROM AUTHOR]

Infrared spectra and radiation chemical behavior of [N.sub.2]-dominated ices relevant to the surfaces of Triton and Pluto are presented. This is the first systematic IR study of proton-irradiated [N.sub.2]-rich ices containing C[H.sub.4] and CO. Experiments at 12 K show that HCN, HNC, and diazomethane (C[H.sub.2][N.sub.2]) form in the solid phase, along with several radicals. N[H.sub.3] is also identified in irradiated [N.sub.2] + C[H.sub.4] and [N.sub.2] + C[H.sub.4] + CO. We show that HCN and HNC are made in irradiated binary ice mixtures having initial [N.sub.2]/C[H.sub.4] ratios from 100 to 4, and in three-component mixtures have an initial [N.sub.2]/(C[H.sub.4] + CO) ratio of 50. HCN and HNC are not detected in [N.sub.2]-dominated ices when C[H.sub.4] is replaced with [C.sub.2][H.sub.6], [C.sub.2][H.sub.2], or C[H.sub.3]OH. The intrinsic band strengths of HCN and HNC are measured and used to calculate G(HCN) and G(HNC) in irradiated [N.sub.2] + C[H.sub.4] and [N.sub.2] + C[H.sub.4] + CO ices. In addition, the HNC/HCN ratio is calculated to be ~ 1 in both icy mixtures. These radiolysis results reveal, for the first time, solid-phase synthesis of both HCN and HNC in [N.sub.2]-rich ices containing C[H.sub.4]. We examine the evolution of spectral features due to acid-base reactions (acids such as HCN, HNC, and HNCO and a base, N[H.sub.3]) triggered by warming irradiated ices from 12 K to 30-35 K. We identify anions (OC[N.sup.-], C[N.sup.-], and [N.sup.-.sub.3]) in ices warmed to 35 K. These ions are expected to form and survive on the surfaces of Triton and Pluto. Our results have astrobiological implications since many of these products (HCN, HNC, HNCO, N[H.sup.3], N[H.sub.4]OCN, and N[H.sub.4]CN) are involved in the syntheses of biomolecules such as amino acids and polypeptides. Keywords: Radiation chemistry; Pluto; Triton; Spectroscopy; Ices

The ephemerides of satellites of major planets are needed in planning spacecraft missions both for studying the satellites themselves and for navigational support during the flights of spacecraft in the vicinity of planets. In addition, accurate numerical theories of motion of the natural satellites of major planets make it possible to increase the accuracy of the ephemerides of their central planets based on positional (photographic and CCD) observations of the satellites. Numerical theories of Neptune’s satellites, Triton and Nereid, constructed within the framework of the ERA software package developed at the Institute of Applied Astronomy of the Russian Academy of Sciences are presented. [ABSTRACT FROM AUTHOR]

Abstract: Model-independent constraints for the neutron–triton and proton–Helium-3 scattering lengths are calculated with a leading-order interaction derived from an effective field theory without explicit pions. Using the singlet neutron–proton scattering length, the deuteron-, and triton binding energy as input, the predictions , , , and are obtained. The calculations employ the resonating group method and include the Coulomb interaction when appropriate. The theoretical uncertainty is assessed via a variation of the regulator parameter of the short-distance interaction from 400 MeV to 1.6 GeV. The phase-shift and scattering-length results for the proton–Helium-3 system are consistent with a recent phase shift analysis and with model calculations. For neutron–triton, the results for the scattering lengths in both singlet and triplet channels are significantly smaller than suggested by R-matrix and partial-wave-analysis extractions from data. For a better understanding of this discrepancy, the sensitivity of the low-energy four-body scattering system to variations in the neutron–neutron and proton–proton two-nucleon scattering lengths is calculated. Induced by strong charge-symmetry-breaking contact interactions, this dependence is found insignificant. In contrast, a strong correlation between the neutron–triton scattering length and the triton binding energy analogous to the Phillips line is found. [Copyright &y& Elsevier]

Abstract: We present a 3D general circulation model of Pluto and Triton’s atmospheres, which uses radiative–conductive–convective forcing. In both the Pluto and Triton models, an easterly (prograde) jet is present at the equator with a maximum magnitude of 10–12ms−1 and 4ms−1, respectively. Neither atmosphere shows any significant overturning circulation in the meridional and vertical directions. Rather, it is horizontal motions (mean circulation and transient waves) that transport heat meridionally at a magnitude of 1 and 3×107 W at Pluto’s autumn equinox and winter solstice, respectively (seasons referenced to the Northern Hemisphere). The meridional and dayside–nightside temperature contrast is small (⩽5K). We find that the lack of vertical motion can be explained on Pluto by the strong temperature inversion in the lower atmosphere. The height of the Voyager 2 plumes on Triton can be explained by the dynamical properties of the lower atmosphere alone (i.e., strong wind shear) and does not require a thermally defined troposphere (i.e., temperature decreasing with height at the surface underlying a region of temperature increasing with height). The model results are compared with Pluto stellar occultation light curve data from 1988, 2002, 2006, and 2007 and Triton light curve data from 1997. [Copyright &y& Elsevier]

Abstract: This is an experimental study of ice grain ejection when trapped gases are released from water ice. When ice is formed by adherence of water molecules at low temperatures, it forms an amorphous structure with many pores, where gas molecules can reside. When further ice layers are formed, the gases are trapped in the ice. Upon its warming-up, the ice structure changes, releasing fractions of the trapped gas. If they do not encounter obstacles, they are released quiescently by dynamic percolation. In a non-dense ice a huge flux of ice grains emanates from the ice, propelled by gas jets and covering its entire surface. When the overlying ice is denser, due to back-migration of water vapor during its sublimation, gas trying to escape from below cannot penetrate the dense ice and breaks it, producing non-circular craters and a chaotic terrain, as observed experimentally and in close encounters with Comets Wild 2, Tempel 1 and Hartley 2. These experimental findings explain several observations of Solar System bodies: ice grain ejection from Comets Temple 1 and Hartley 2. Also explained are the dark jets observed on Triton, where their ejection speed suggests a deep source. On Mars, dark streaks are observed in the southern pole in spring, most likely by plumes carrying dark dust, carried by winds and falling on the surface. As found by us experimentally, only frozen CO2 covered by water ice or mixed with it will work to form jets, whereas pure frozen CO2 will sublimate quiescently. [Copyright &y& Elsevier]

Ice grains are ejected whenever gases are flowing from gas-laden amorphous ice upon its warming-up. We elucidated the mechanism of gas trapping when flowing onto a gas-free ice layer. The observations on ice grain ejection from comets, Enceladus, Triton and the Martian South Polar region are explained as well. [ABSTRACT FROM AUTHOR]

Abstract: We present a study of coupled thermal and structural evolution of Neptune’s moon, Triton, driven by tidal dissipation and radiogenic heating. Triton’s orbital history likely involves capture from a binary system by Neptune, followed by a period of circularization. This work investigates Triton’s evolution past its circularization. We examine the rate of ice shell growth as a function of different orbital eccentricities, in the presence of radiogenic heating. Tidal dissipation in the ice shell, proportional to orbital eccentricity squared, concentrates heating near the base, reducing the basal heat flux. As the growth of the ice shell is proportional to the basal heat flux, increased tidal heating creates a blanketing effect, reducing the rate of ice shell growth. Radiogenic heating from Triton’s core is the other, more dominant, source of heat to the shell. Despite being several orders of magnitude higher than the tidal dissipation, radiogenic heating alone fails to sustain an ocean within Triton over 4.5Ga. For orbital eccentricities of 5×10−7 and 3×10−5 it takes approximately 2Ga and 3Ga, respectively, to completely freeze the ocean. For higher values of orbital eccentricities, an ocean can be sustained in Triton’s interior over 4.5Ga. If Triton’s history past circularization involves a slow decrease in orbital eccentricity to the current value, a thin, possibly NH3-rich ocean exists beneath Triton’s icy shell. [Copyright &y& Elsevier]

Abstract: We report photochemical studies of thin cryogenic ice films composed of N2, CH4 and CO in ratios analogous to those on the surfaces of Neptune’s largest satellite, Triton, and on Pluto. Experiments were performed using a hydrogen discharge lamp, which provides an intense source of ultraviolet light to simulate the sunlight-induced photochemistry on these icy bodies. Characterization via infrared spectroscopy showed that C2H6 and C2H2, and HCO are formed by the dissociation of CH4 into H, CH2 and CH3 and the subsequent reaction of these radicals within the ice. Other radical species, such as C2, , CN, and CNN, are observed in the visible and ultraviolet regions of the spectrum. These species imply a rich chemistry based on formation of radicals from methane and their subsequent reaction with the N2 matrix. We discuss the implications of the formation of these radicals for the chemical evolution of Triton and Pluto. Ultimately, this work suggests that , CN, HCO, and CNN may be found in significant quantities on the surfaces of Triton and Pluto and that new observations of these objects in the appropriate wavelength regions are warranted. [Copyright &y& Elsevier]

Abstract: The origin of Neptune’s large, circular but retrograde satellite Triton has remained largely unexplained. There is an apparent consensus that its origin lies in it being captured, but until recently no successful capture mechanism has been found. Agnor and Hamilton (Agnor, C.B., Hamilton, D.P. [2006]. Nature 441, 192–194) demonstrated that the disruption of a trans-neptunian binary object which had Triton as a member, and which underwent a very close encounter with Neptune, was an effective mechanism to capture Triton while its former partner continued on a hyperbolic orbit. The subsequent evolution of Triton’s post-capture orbit to its current one could have proceeded through gravitational tides (Correia, A.C.M. [2009]. Astrophys. J. 704, L1–L4), during which time Triton was most likely semi-molten (McKinnon, W.B. [1984]. Nature 311, 355–358). However, to date, no study has been performed that considered both the capture and the subsequent tidal evolution. Thus it is attempted here with the use of numerical simulations. The study by Agnor and Hamilton (Agnor, C.B., Hamilton, D.P. [2006]. Nature 441, 192–194) is repeated in the framework of the Nice model (Tsiganis, K., Gomes, R., Morbidelli, A., Levison, H.F. [2005]. Nature 435, 459–461) to determine the post-capture orbit of Triton. After capture Triton is then subjected to tidal evolution using the model of Mignard (Mignard, F. [1979]. Moon Planets 20, 301–315; Mignard, F. [1980]. Moon Planets 23, 185–201). The perturbations from the Sun and the figure of Neptune are included. The perturbations from the Sun acting on Triton just after its capture cause it to spend a long time in its high-eccentricity phase, usually of the order of 10Myr, while the typical time to circularise to its current orbit is some 200Myr, consistent with earlier studies. The current orbit of Triton is consistent with an origin through binary capture and tidal evolution, even though the model prefers Triton to be closer to Neptune than it is today. The probability of capturing Triton in this manner is approximately 0.7%. Since the capture of Triton was at most a 50% event – since only Neptune has one, but Uranus does not – we deduce that in the primordial trans-neptunian disc there were some 100 binaries with at least one Triton-sized member. Morbidelli et al. (Morbidelli, A., Levison, H.F., Bottke, W.F., Dones, L., Nesvorný, D. [2009]. Icarus 202, 310–315) concludes there were some 1000 Triton-sized bodies in the trans-neptunian proto-planetary disc, so the primordial binary fraction with at least one Triton-sized member is 10%. This value is consistent with theoretical predictions, but at the low end. If Triton was captured at the same time as Neptune’s irregular satellites, the far majority of these, including Nereid, would be lost. This suggests either that Triton was captured on an orbit with a small semi-major axisa ≲50R N (a rare event), or that it was captured before the dynamical instability of the Nice model, or that some other mechanism was at play. The issue of keeping the irregular satellites remains unresolved. [Copyright &y& Elsevier]

Abstract: Triton, the large satellite of Neptune, was imaged by the Voyager 2 spacecraft in 1989 with dark plumes originating in its volatile-rich south polar region. Southern summer solstice, a time when seasonal volatile transport should be at a maximum, occurred in 2001. Ground-based observations of Triton’s rotational light curve obtained from Table Mountain Observatory in 2000–2004 reveal volatile transport on its surface. When compared with a static frost model constructed from Voyager images, the light curve shows an increase in total amplitude. An earlier light curve obtained in 1992 from Mauna Kea Observatory is consistent with the static frost model. This movement of volatiles on the surface agrees with recent imaging results from the Hubble Space Telescope (Bauer, J.M., Buratti, B.J., Li, J.-Y., Mosher, J.A., Hicks, M.D., Schmidt, B.E., Goguen, J.D. [2010]. Astrophys. J. 723, L49–L52). The changes in the light curve can be explained by the transport of nitrogen frost on the surface or by the uncovering of bedrock of less volatile methane. We also find that Triton exhibits a large opposition surge at solar phase angles less than 0.1°. This surge cannot be entirely explained by the effects of coherent backscatter. [Copyright &y& Elsevier]

Abstract: We present here a search for solid ethane, C2H6, on the surfaces of Pluto and Triton, based on near-infrared spectral observations in the H and K bands (1.4–2.45μm) using the Very Large Telescope (VLT) and the United Kingdom Infrared Telescope (UKIRT). We model each surface using a radiative transfer model based on Hapke theory (Hapke, B. [1993]. Theory of Reflectance and Emittance Spectroscopy. Cambridge University Press, Cambridge, UK) with three basic models: without ethane, with pure ethane, and with ethane diluted in nitrogen. On Pluto we detect weak features near 2.27, 2.405, 2.457, and 2.461μm that match the strongest features of pure ethane. An additional feature seen at 2.317μm is shifted to longer wavelengths than ethane by at least 0.002μm. The strength of the features seen in the models suggests that pure ethane is limited to no more than a few percent of the surface of Pluto. On Triton, features in the H band could potentially be explained by ethane diluted in , however, the lack of corresponding features in the K band makes this unlikely (also noted by Quirico et al. (Quirico, E., Doute, S., Schmitt, B., de Bergh, C., Cruikshank, D.P., Owen, T.C., Geballe, T.R., Roush, T.L. [1999]. Icarus 139, 159–178)). While Cruikshank et al. (Cruikshank, D.P., Mason, R.E., Dalle Ore, C.M., Bernstein, M.P., Quirico, E., Mastrapa, R.M., Emery, J.P., Owen, T.C. [2006]. Bull. Am. Astron. Soc. 38, 518) find that the 2.406-μm feature on Triton could not be completely due to 13CO, our models show that it could not be accounted for entirely by ethane either. The multiple origin of this feature complicates constraints on the contribution of ethane for both bodies. [Copyright &y& Elsevier]

Abstract: To investigate the evolution of any processes on planetary surfaces in the outer Solar System, the rheological properties of non-water ices were studied by means of a sound velocity measurement system and a uniaxial deformation apparatus. A pulse transmission method was used to obtain longitudinal (V p) and transverse (V s) wave velocities through solid nitrogen and methane at temperatures ranging from 5 to 64K and from 5 to 90K, respectively. The measured velocities confirmed that the solid methane and solid nitrogen samples were non-porous polycrystalline samples without any cracks and bubbles inside. Compression tests at constant strain-rate were performed for solid nitrogen and methane at temperatures of 5–56K and 5–77K, respectively, at strain-rates of 10−4–10−2 s−1. Both brittle and ductile behavior was observed for solid nitrogen and methane under these conditions. The maximum strength of solid nitrogen was observed to be 9MPa in the brittle failure mode, and that of solid methane was 10MPa. These low strengths cannot support cantaloupe structures with the topographic undulation larger than several kilometers found on Triton’s surface, suggesting that other materials such as H2O ice could underlay solid methane and nitrogen and support these structures. [Copyright &y& Elsevier]

Abstract: This report arises from an ongoing program to monitor Neptune’s largest moon Triton spectroscopically in the 0.8 to 2.4μm range using IRTF/SpeX. Our objective is to search for changes on Triton’s surface as witnessed by changes in the infrared absorption bands of its surface ices , CO, and . We have recorded infrared spectra of Triton on 53 nights over the ten apparitions from 2000 to 2009. The data generally confirm our previously reported diurnal spectral variations of the ice absorption bands (Grundy and Young, 2004). Nitrogen ice shows a large amplitude variation, with much stronger absorption on Triton’s Neptune-facing hemisphere. We present evidence for seasonal evolution of Triton’s ice: the 2.15μm absorption band appears to be diminishing, especially on the Neptune-facing hemisphere. Although it is mostly dissolved in ice, Triton’s ice shows a very different longitudinal variation from the ice, challenging assumptions of how the two ices behave. Unlike Triton’s ice, the CO ice does exhibit longitudinal variation very similar to the ice, implying that CO and condense and sublimate together, maintaining a consistent mixing ratio. Absorptions by and ices show negligible variation as Triton rotates, implying very uniform and/or high latitude spatial distributions for those two non-volatile ices. [Copyright &y& Elsevier]

The evolution of the orbit of the second satellite of Neptune (Nereid) on time intervals up to 500 thousand years is investigated. The methods used by authors earlier are supplemented with the possibility of considering the influence of the attraction of the internal satellite Triton on the evolution of the orbit of the external satellite Nereid. [ABSTRACT FROM AUTHOR]

New Horizons 2015 flyby of the Pluto system has resulted in high-resolution topographic maps of Pluto and Charon, the most distant objects so mapped. DEM's over ~30% of each object were produced at 100-300 m vertical and 300-800 m spatial resolutions, in hemispheric maps and high-resolution linear mosaics. Both objects reveal more relief than was observed at Triton. The dominant 800-km wide informally named Sputnik Planum bright ice deposit on Pluto lies in a broad depression 3 km deep, flanked by dispersed mountains 3-5 km high. Impact craters reveal a wide variety of preservation states from pristine to eroded, and long fractures are several km deep with throw of 0-2 km. Topography of this magnitude suggests the icy shell of Pluto is relatively cold and rigid. Charon has global relief of at least 10 km, including ridges of 2-3 km and troughs of 3-5 km of relief. Impact craters are up to 6 km deep. Vulcan Planum consists of rolling plains and forms a topographic moat along its edge, suggesting viscous flow. [ABSTRACT FROM AUTHOR]

Abstract: New mapping reveals 100 probable impact craters on Triton wider than 5 km diameter. All of the probable craters are within 90° of the apex of Triton''s orbital motion (i.e., all are on the leading hemisphere) and have a cosine density distribution with respect to the apex. This spatial distribution is difficult to reconcile with a heliocentric (Sun-orbiting) source of impactors, be it ecliptic comets, the Kuiper Belt, the scattered disk, or tidally-disrupted temporary satellites in the style of Shoemaker–Levy 9, but it is consistent with head-on collisions, as would be produced if a prograde population of planetocentric (Neptune-orbiting) debris were swept up by retrograde Triton. Plausible sources include ejecta from impact on or disruption of inner/outer moons of Neptune. If Triton''s small craters are mostly of planetocentric origin, Triton offers no evidence for or against the existence of small comets in the Kuiper Belt, and New Horizons observations of Pluto must fill this role. The possibility that the distribution of impact craters is an artifact caused by difficulty in identifying impact craters on the cantaloupe terrain is considered and rejected. The possibility that capricious resurfacing has mimicked the effect of head-on collisions is considered and shown to be unlikely given current geologic constraints, and is no more probable than planetocentrogenesis. The estimated cratering rate on Triton by ecliptic comets is used to put an upper limit of ∼50 Myr on the age of the more heavily cratered terrains, and of ∼6 Myr for the Neptune-facing cantaloupe terrain. If the vast majority of cratering is by planetocentric debris, as we propose, then the surface everywhere is probably less than 10 Myr old. Although the uncertainty in these cratering ages is at least a factor ten, it seems likely that Triton''s is among the youngest surfaces in the Solar System, a candidate ocean moon, and an important target for future exploration. [Copyright &y& Elsevier]

Astrometric positions of the Neptunian Satellite Triton are given for the opposition of Neptune for the years 1996, 2003, 2005 and 2006. The 943 observed positions were obtained at the Cassegrain focus of a 156-cm reflector. In our reduction, the up-to-date catalogue of stars UCAC2 was chosen to ensure a proper astrometric calibration. Our observed positions are compared to theoretical positions provided from JPL and IMCCE ephemerides. The observed minus calculated residuals have s.d. values of the order of 0.04 arcsec. [ABSTRACT FROM AUTHOR]

Abstract: BVI photometry of Triton and Proteus was derived from HST images taken in 1997. The VEGAMAG photometric technique was used. Triton was found to be brighter by a few percent than observations of the 1970''s and 1980''s, as expected due to the increasingly greater exposure of the bright south polar region. The leading side was also found to be brighter than the trailing side by 0.09 mag in all filters—50% larger than reported by Franz [Franz, O.G., 1981. Icarus 45, 602–606]. Contrary to our previous results [Pascu, D., et al., 1998. Bull. Am. Astron. Soc. 30, 1101], we found no episodic reddening. Our previous conclusions were based on an inaccurate early version of the Charge Transfer Efficiency (CTE) correction. The present result limits the start of the reddening event reported by Hicks and Buratti [Hicks, M.D., Buratti, B.J., 2004. Icarus 171, 210–218]. Our (B–V) result of supports the global blueing described by Buratti et al. [Buratti, B.J., Goguen, J.D., Gibson, J., Mosher, J., 1994. Icarus 110, 303–314]. Our observations of July 1997 agree with the Voyager results and are among the bluest colors seen. We found Proteus somewhat brighter than earlier studies, but in good agreement with the recent value given by Karkoschka [Karkoschka, E., 2003. Icarus 162, 400–407]. A leading/trailing brightness asymmetry was detected for Proteus, with the leading side 0.1 mag brighter. The unique differences in action of the endogenic and exogenic processes on Triton and Proteus provides an opportunity to separate the endogenic and exogenic effects on Triton. [Copyright &y& Elsevier]

Abstract: Technological advances over the next 10 – 15 years promise to enable a number of smaller, more capable science missions to the outer planets. With the inception of miniaturized spacecraft for a wide range of applications, both in large clusters around Earth, and for deep space missions, NASA is currently in the process of redefining the way science is being gathered. Technologies such as 3-dimensional multi-chip modules, micro-machined electromechanical devices, multi functional structures, miniaturized transponders, miniaturized propulsion systems, variable emissivity thermal coatings, and artificial intelligence systems are currently in research and development, and are scheduled to fly (or have flown) in a number of missions. This study will leverage on these and other technologies in the design of a lightweight Neptune orbiter unlike any other that has been proposed to date. The Neptune/Triton Explorer (NExTEP) spacecraft uses solar electric earth gravity assist and aero capture maneuvers to achieve its intended target orbit. Either a Taurus or Delta-class launch vehicle may be used to accomplish the mission. [Copyright &y& Elsevier]

Abstract: In 2001, NASA began assembling the Aerocapture Systems Analysis Team, a team of scientists and engineers from multiple NASA centers. Their charter is to perform high-fidelity analyses of delivering scientifically compelling orbital missions that use aerocapture for orbit insertion at their destinations. After establishing scientific credibility, studies focus on aerocapture systems design and performance, including approach navigation, flight mechanics, aerothermodynamics, and thermal protection. The team''s October 2001–September 2002 study examined a mission to explore the organic environment of Titan and its chemical, geological, and dynamical context. Its architecture includes a Titan polar orbiter that would complete and extend Cassini''s soon-to-begin global mapping, aiding global extrapolation of findings from a mobile in situ element (rover, blimp, etc.). The in situ element would perform remote sensing and in situ investigations, for analysis and characterization of Titan''s surface, shallow subsurface, atmosphere, processes occurring there, and energy sources driving it all. The study concentrated on the orbiter and orbit insertion, largely treating the in situ element as a black box with data relay requirements. October 2002–September 2003 the team studied a mission to perform Cassini/Huygens-level exploration of the Neptune system. Before aerocapture this mission would deploy and support multiple Neptune atmospheric entry probes. After aerocapture the orbiter uses Triton as a “tour engine”, in much the same manner as Cassini uses Titan, to provide many Triton flybys and orbit evolution for detailed investigation of Neptune''s interior, atmosphere, magnetosphere, rings, and satellites. This presentation summarizes the missions’ science objectives, instrumentation, and data requirements that served as the foundations for the studies, and describes mission design requirements and constraints that affect the science investigations. [Copyright &y& Elsevier]

The diverse populations of icy bodies of the outer Solar System (OSS) give critical information on the composition and structure of the solar nebula and the early phases of planet formation. The two principal repositories of icy bodies are the Kuiper belt or disk, and the Oort Cloud, both of which are the source regions of the comets. Nearly 1000 individual Kuiper belt objects have been discovered; their dynamical distribution is a clue to the early outward migration and gravitational scattering power of Neptune. Pluto is perhaps the largest Kuiper belt object. Pluto is distinguished by its large satellite, a variable atmosphere, and a surface composed of several ices and probable organic solid materials that give it color. Triton is probably a former member of the Kuiper belt population, suggested by its retrograde orbit as a satellite of Neptune. Like Pluto, Triton has a variable atmosphere, compositionally diverse icy surface, and an organic atmospheric haze. Centaur objects appear to come from the Kuiper belt and occupy temporary orbits in the planetary zone; the compositional similarity of one well studied Centaur (5145 Pholus) to comets is notable. New discoveries continue apace, as observational surveys reveal new objects and refined observing techniques yield more physical information about specific bodies. [ABSTRACT FROM AUTHOR]

Abstract: We present eight new 0.8 to 2.4 μm spectral observations of Neptune''s satellite Triton, obtained at IRTF/SpeX during 2002 July 15–22 UT. Our objective was to determine how Triton''s near-infrared spectrum varies as Triton rotates, and to establish an accurate baseline for comparison with past and future observations. The most striking spectral change detected was in Triton''s nitrogen ice absorption band at 2.15 μm; its strength varies by about a factor of two as Triton rotates. Maximum N2 absorption approximately coincides with Triton''s Neptune-facing hemisphere, which is also the longitude where the polar cap extends nearest Triton''s equator. More subtle rotational variations are reported for Triton''s CH4 and H2O ice absorption bands. Unlike the other ices, Triton''s CO2 ice absorption bands remain nearly constant as Triton rotates. Triton''s H2O ice is shown to be crystalline, rather than amorphous. Triton''s N2 ice is confirmed to be the warmer, hexagonal, β N2 phase, and its CH4 is confirmed to be highly diluted in N2 ice. [Copyright &y& Elsevier]

In this paper, we shall analyse a promising way to explain the huge crater asymmetry observed on Triton, the largest of Neptune''s satellites. Triton shows, as well as many other satellites in the Solar System, a non-symmetric crater distribution on its surface. This fact is principally due to the synchronous rotation of these satellites, as shown by many theoretical works (see Shoemaker and Wolfe, Satellites of Jupiter, University of Arizona Press, Tucson, 1992, p. 277; Horedt and Neukum, Icarus 60 (1984) 710; Zahnle et al., Icarus 136 (1998) 202; Zahnle et al., Icarus 153 (2001) 111). However, on Triton the asymmetry is much more pronounced than on other satellites, and it exceeds what the models, in which the source of the craters are bodies in heliocentric orbits, can account for. For this reason, many authors (Croft et al., Icarus 99 (1992) 94; Schenk and Sobieszczyk, American Astronomical Society, DPS Meeting, Vol. 31, 1999; Zahnle et al., Icarus 153 (2001) 111) proposed that the origin for Triton''s asymmetry has to be found in a swarm of bodies having planetocentric orbits, instead of heliocentric ones. Here, we analyse from a dynamical point of view the possibility that such swarm of fragments was generated by a collision between an inner satellite and a third object (a process we call ‘mass transfer’). Moreover, we discuss the possibility that the observed crater distribution on Triton comes from two populations: heliocentric bodies responsible for a few big craters, plus planetocentric bodies responsible for the big asymmetry.Finally, we discuss some implications for ground observations. [Copyright &y& Elsevier]

The thin atmosphere of Neptune''s moon Triton is dense enough to ablate micrometeoroids as they pass through. A combination of Triton''s orbital velocity around Neptune and its orbital velocity around the Sun gives a maximum meteoroid impact velocity of approximately 19 km s-1, sufficient to heat the micrometeoroids to visibility as they enter. The ablation profiles of icy and stony micrometeoroids were calculated, along with the estimated brightness of the meteors. In contrast to the terrestrial case, visible meteors would extend very close to the surface of Triton. In addition, the variation in the meteoroid impact velocity as Triton orbits Neptune produces a large variation in the brightness of meteors with orbital phase, a unique Solar System phenomenon. [Copyright &y& Elsevier]

We have obtained new observations of Triton with the ESO New Technology Telescope (La Silla, Chile) in October 2002. Using the high quality of NTT instrumentation, we were able to cover the entire 0.4–2.5 μm spectral range in a single night. We applied this procedure for two nights, well selected along the orbit of Triton, in order to cover essentially the trailing side one night, and the leading one the other night, obtaining the first face-resolved 0.4–2.4 μm spectra of Triton. We discuss here the spectra and the differences between the two faces, and the implications of these new results for a better understanding of the surface composition of Triton. In particular we found possible clues for the presence of rocky materials on Triton''s surface. [Copyright &y& Elsevier]

The Raz Fossae, a pair of &ap;15-km wide trough en echelon interpreted as grabens, can be used to propose an estimation of the depth to the brittle–ductile transition on Triton. This estimation may in turn give an idea of the thermal state of Triton''s icy lithosphere when these features formed. Given the young age of its surface, the conclusions obtained could be roughly applicable to the present state of this satellite of Neptune. Considering water or ammonia dihydrate as possible components of the lithosphere and a feasible range of strain rates, it was estimated that surface heat flow is greater than that inferred from radiogenic heating, especially for a lithosphere dominated by water. Also, an internal ocean could lie at a depth of only ∼20 km beneath the surface. The presence over the surface of an insulating layer of ice of low thermal conductivity (e.g., nitrogen) or of regolith would only substantially alter these estimates if the effective surface temperature were considerably higher than the observed value of 38 K. [Copyright &y& Elsevier]

A new model of albedo and emissivity of planetary surfaces covered by volatile ices in the form of porous slab-like deposits is described. In the model, a radiative transfer model is coupled with a microphysical model in order to link changes in albedo and emissivity to changes in porosity caused by ice metamorphism. Preliminary results for Triton, Pluto, and Io are presented (the martian CO2 caps will be the subject of a separate publication). The coupled model will aid in the interpretation of ground-based and spacecraft observations and should lead to advances in surface and atmospheric modeling. [Copyright &y& Elsevier]

The unbound hydrogen isotopes 4,5H have been studied in the one-proton knockout channel of 6He (240 MeV/u) impinging on a carbon target. The triton fragments originating from this channel were detected in coincidence with neutrons. Relative energy spectra as well as energy and angular correlations have been studied for the t+n and t+n+n systems. The analysis of the energy and angular correlations by the method of hyperspherical harmonic expansion allows to determine the relative weights of the most relevant partial waves in the three-body t+n+n final state. It is shown that the neutrons to a large extent occupy the p-shell and that the Iπ=1/2+ state is strongly populated as expected for the 5H ground state. No evidence for a narrow resonance in the t+n+n system is obtained, instead a broad structure peaked at 3 MeV above the threshold with about 6 MeV as a full width at half maximum is observed. The two-body t+n system reveals a resonance compatible with earlier results for 4H. [Copyright &y& Elsevier]

Polar wander may occur on Triton and Pluto because of volatile migration. Triton, with its low obliquity, can theoretically sublimate volatiles (mostly nitrogen) at the rate of ∼1013 kg year−1 from the equatorial regions and deposit them at the poles. Assuming Triton to be rigid on the sublimation timescale, after ∼105 years the polar caps would become large enough to cancel the rotational flattening, with a total mass equivalent to a global layer ∼120–250 m in depth. At this point the pole wanders about the tidal bulge axis, which is the line joining Triton and Neptune. Rotation about the bulge axis might be expected to disturb the leading side/trailing side cratering statistics. Because no such disturbance is observed, it may be that Triton’s surface volatile inventory is too low to permit wander. On the other hand, its mantle viscosity might be low, so that any uncompensated cap load might be expected to wander toward the tidal bulge axis. In this case, the axis of wander passes through the equator from the leading side to the trailing side; rotation about this wander axis would not disturb the cratering statistics. Low-viscosity polar wander may explain the bright southern hemisphere: this is the pole which is wandering toward the sub-Neptune point. In any case the “permanent” polar caps may be geologically very young. Polar wander may possibly take place on Pluto, due to its obliquity oscillations and perihelion-pole geometry. However, Pluto is probably not experiencing any wander at present. The Sun has been shining strongly on the poles over the last half of the obliquity cycle, so that volatiles should migrate to the equator, stabilizing the planet against wander. Spacecraft missions to Triton and Pluto which measure the dynamical flattening could give information about the accumulation of volatiles at the poles. Such information is best obtained by measuring gravity and topography from orbiters, as was done for Mars with the highly successful Mars Global Surveyor. [Copyright &y& Elsevier]