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NASA NTRS1990

Magnetospheric interaction with Triton's ionosphere

The large electron densities measured by the Voyager radio occultation experiment are attributed to the precipitation of magnetospheric electrons with energy above 10 keV. Because the ionospheric electric Pedersen conductivity of Triton is about 10,000-20,000 mho and the Alfven conductance is about 3.5 mho, direct convective flow of plasma into the essentially infinitely conducting ionosphere is negligible. Magnetospheric electrons are transported to Triton's ionopause by curvature drift as a result of weak magnetic field line draping in a sub-Alfvenic plasma interaction with Triton. At the ionopause energetic electrons have a high probability of elastic and inelastic scattering and precipitate into the upper atmosphere. The average power dissipation is estimated to be about (2 - 3) x 10 to the 8th W.

Strobel, Darrell F.↗

NASA NTRS1990

Nitrogen ion clusters in Triton's atmosphere

The nitrogen ion chemistry that controls Triton's ionospheric composition and may be responsible for the hazes detected by the Voyager spacecraft are discussed. In particular, it is shown that nitrogen cluster ion formation should readily occur in Triton's thin cold atmosphere. The very low temperatures of Triton's atmosphere imply that these clustered ions can nucleate into solid nitrogen particles, creating the extended visible hazes. A model based on the chemical kinetics of nitrogen ions predicts a dense ionosphere at 200-400 km, as detected by Voyager radio occultation measurements.

Delitsky, Mona L.↗

NASA NTRS1990

The photochemistry of methane in the atmosphere of Triton

The model of Summers and Strobel (1989) for photochemical reactions in the Uranus atmosphere was modified and used for quantitative calculations of methane in the atmosphere of Triton. The principal adjustable parameters in the new model are the surface CH4 concentrations and the vigor of vertical mixing in Triton's lower atmosphere. It is shown the rate of methane photolysis that was calculated is sufficient to generate a smog of condensed C2H2, C2H4, C2H6, and C4H2 particles in the lowest 30 km of Triton's atmosphere, with an optical depth consistent with the Voyager imaging results.

Strobel, Darrell F.↗

NASA NTRS1990

Spectral geometric albedo and bolometric Bond albedo of Neptune's satellite Triton from Voyager observations

The spectral geometric albedo and the bolometric Bond albedo of Triton are calculated using data from the Voyager spacecraft photopolarimeter and science experiments. The geometric albedo is not inconsistent with the presence of a weak absorption feature in Triton's spectrum near 0.75 micron. The bolometric Bond albedo (0.65) is consistent with the 38 K daytime surface temperature of Triton. The results are also in agreement with the 37.5 K temperature of nitrogen at an infrared basal pressure of 14 microbar.

Nelson, R. M.↗

NASA NTRS1991

Triton: A hot potato

The effect of sunlight on the surface of Triton was studied. Widely disparate models of the active geysers observed during Voyager 2 flyby were proposed, with a solar energy source almost their only feature. Yet Triton derives more of its heat from internal sources (energy released by the radioactive decay) than any other icy satellite. The effect of this relatively large internal heat on the observable behavior of volatiles on Triton's surface is investigated. The following subject areas are covered: the Global Energy Budget; insulation polar caps; effect on frost stability; mantle convection; and cryovolcanism.

Kirk, R. L.↗

NASA NTRS1990

Energy sources for Triton's geyser-like plumes

Four geyser-like plumes were discovered near Triton's south pole in areas now in permanent sunlight. Because Triton's southern hemisphere is nearing a maximum summer solstice, insolation as a driver or a trigger for Triton's geyser-like plumes is an attractive hypothesis. Trapping of solar radiation in a translucent, low-conductivity surface layer (in a solid-state greenhouse), which is subsequently released in the form of latent heat of sublimation, could provide the required energy. Both the classical solid-state greenhouse consisting of exponentially absorbed insolation in a gray, translucent layer of solid nitrogen, and the 'super' greenhouse consisting of a relatively transparent solid-nitrogen layer over an opaque, absorbing layer are plausible candidates. Geothermal heat may also play a part if assisted by the added energy input of seasonal cycles of insolation.

Brown, R. H.↗

NASA NTRS1990

Triton's plumes - The dust devil hypothesis

Triton's plumes are narrow columns 10 km in height, with tails extending horizontally for distances over 100 km. This structure suggests that the plumes are an atmospheric rather than a surface phenomenon. The closest terrestrial analogs may be dust devils, which are atmospheric vortices originating in the unstable layer close to the ground. Since Triton has such a low surface pressure, extremely unstable layers could develop during the day. Patches of unfrosted ground near the subsolar point could act as sites for dust devil formation because they heat up relative to the surrounding nitrogen frost. The resulting convection would warm the atmosphere to temperatures of 48 k or higher, as observed by the Voyager radio science team. Assuming that velocity scales as the square root of temperature difference times the height of the mixed layer, a velocity of 20 m/sec is derived for the strongest dust devils on Triton. Winds of this speed could raise particles provided they are a factor of 1000 to 10,000 less cohesive than those on earth.

Ingersoll, Andrew P.↗

NASA NTRS1991

A control network of Triton

A control network for Triton has been computed using a bundle-type analytical triangulation program. The network contains 105 points that were measured on 57 Voyager-2 pictures. The adjustment contained 1010 observation equations and 382 normal equations and resulted in a standard measurement error of 13.36 microns. The coordinates of the control points, the camera orientation angles at the times when the pictures were taken, and Triton's mean radius were determined. A separate statistical analysis confirmed Triton's radius to be 1352.6 + or - 2.4 km. Attempts to tie the control network around the satellite were unsuccessful because discontinuities exist in high-resolution coverage between 66 deg and 289 deg longitude, north of 38 deg latitude, and south of 78 deg latitude.

Davies, Merton E.↗

NASA NTRS1991

On the lack of a magnetic signature of Triton's magnetospheric interaction on the Voyager 2 flyby trajectory

There is strong, albeit indirect, evidence for a sub-Alfvenic and transonic interaction between Triton and the Neptunian magnetosphere. A new inspection of magnetic field data around Triton encounter shows no evidence for the interaction. It is shown that this is due to the geometry of Triton's Alfvenic wings, which were probably too far from the spacecraft during the encounter. The situation is complicated by finite ion gyroradius effects, which tend to smear out the simple Alfven wings such as encountered at Io.

Neubauer, F. M.↗

NASA NTRS1992

Large quasi-circular features beneath frost on Triton

Specially processed Voyager 2 images of Neptune's largest moon, Triton, reveal three large quasi-circular features ranging in diameter from 280 to 935 km within Triton's equatorial region. The largest of these features contains a central irregularly shaped area of comparatively low albedo about 380 km in diameter, surrounded by crudely concentric annuli of higher albedo materials. None of the features exhibit significant topographic expression, and all appear to be primarily albedo markings. The features are located within a broad equatorial band of anomalously transparent frost that renders them nearly invisible at the large phase angles (alpha greater than 90 deg) at which Voyager obtained its highest resolution coverage of Triton. The features can be discerned at smaller phase angles (alpha = 66 deg) at which the frost only partially masks underlying albedo contrasts. The origin of the features is uncertain but may have involved regional cryovolcanic activity.

Helfenstein, Paul↗

NASA NTRS1992

Triton stellar occultation candidates - 1992-1994

A search for Triton stellar occultation candidates for the period 1992-1994 has been completed with CCD strip-scanning observations. The search reached an R magnitude of about 17.4 and found 129 candidates within 1.5 arcsec of Triton's ephemeris during this period. Of these events, around 30 occultations are expected to be visible from the earth, indicating that a number of Triton occultation events should be visible from major observatories. Even the faintest of the present candidate events could produce useful occultation data if observed with a large enough telescope. The present astrometric accuracy is inadequate to identify which of these appulse events will produce occultations on the earth; further astrometry is needed to refine the predictions for positive occultation identification. To aid in selecting candidates for additional astrometric and photometric studies, finder charts and earth-based visibility charts for each event are included.

Mcdonald, S. W.↗

NASA NTRS1992

Anomalous-scattering region on Triton

A photometric analysis of Voyager 2 images of a broad, 'anomalous scattering region' (ASR) on Triton shows its material to differ from the average Triton regolith in being only weakly backward scattering at all Voyager 2 camera wavelengths; the ASR also displays distinctive phase-dependent green/violet color ratios and clear-filter albedo. These characteristics are used to map the global distribution of the ASR areas for which photometric coverage is incomplete. The ASR may form an almost continuous band of material that runs parallel to the Triton equator, characterized by the presence of a transparent and optically thin, seasonally-controlled veneer of well-annealed solid N2.

Lee, Pascal↗

NASA NTRS1992

The influence of thermal inertia on temperatures and frost stability on Triton

It is presently argued, in view of (1) a thermal inertia model for the surface of Triton which (like previous ones) predicts a monotonic recession of permanent N2 deposits toward the poles and very little seasonal N2 frost in the southern hemisphere, and (2) new spectroscopic evidence for nonvolatile CO2 on Triton's bright southern hemisphere, that much of that bright southern material is not N2. Such bright southern hemisphere volatiles may allow the formation of seasonal frosts, thereby helping to explain the observed spectroscopic changes of Triton during the last decade.

Spencer, John R.↗

NASA NTRS1992

A thermal model for the seasonal nitrogen cycle on Triton

The seasonal N2-cycle model presently used to characterize such observed phenomena on Triton as atmospheric pressure and surface albedo features at the time of the Voyager encounter incorporates diurnal and seasonal subsurface heat conduction, and can account for the heat capacity of N2 frost deposits. The results obtained by this model differ from those of previous studies in that they do not predict the seasonal freezing-out of the Triton atmosphere; even for a wide range of input parameters, the bright southern polar cap is seen as rather unlikely to be N2. The results support the microphysical arguments for the presence of either dark or smooth translucent N2 frosts on the Triton surface.

Hansen, Candice J.↗

NASA NTRS1993

Ices on the surface of Triton

The near-infrared spectrum of Triton reveals ices of nitrogen, methane, carbon monoxide, and carbon dioxide, of which nitrogen is the dominant component. Carbon dioxide ice may be spatially segregated from the other more volatile ices, covering about 10 percent of Triton's surface. The absence of ices of other hydrocarbons and nitriles challenges existing models of methane and nitrogen photochemistry on Triton.

Cruikshank, Dale P.↗

NASA NTRS1993

The phase composition of Triton's polar caps

Triton's polar caps are modeled as permanent nitrogen deposits hundreds of meters thick. Complex temperature variations on Triton's surface induce reversible transitions between the cubic and hexagonal phases of solid nitrogen, often with two coexisting propagating transition fronts. Subsurface temperature distributions are calculated using a two-dimensional thermal model with phase changes. The phase changes fracture the upper nitrogen layer, increasing its reflectivity and thus offering an explanation for the surprisingly high southern polar cap albedo (approximately 0.8) seen during the Voyager 2 flyby. The model has other implications for the phase transition phenomena on Triton, such as a plausible mechanism for the origin of geyser-like plume vent areas and a mechanism of energy transport toward them.

Duxbury, N. S.↗

NASA NTRS1994

Chemical investigation of Titan and Triton tholins

We report chromatographic and spectroscopic analyses of both Titan and Triton tholins, organic solids made from the plasma irradiation of 0.9:0.1 and 0.999:0.001 N2/CH4 gas mixtures, respectively. The lower CH4 mixing ratio leads to a nitrogen-richer tholin (N/C greater than 1), probably including nitrogen heterocyclic compounds. Unlike Titan tholin, bulk Triton tholin is poor in nitriles. From high-pressure liquid chromatography, ultraviolet and infrared spectroscopy, and molecular weight estimation by gel filtration chromatography, we conclude that (1) several H2O-soluble fractions, each with distinct UV and IR spectral signatures, are present, (2) these fractions are not identical in the two tholins, (3) the H2O-soluble fractions of Titan tholins do not contain significant amounts of nitriles, despite the major role of nitriles in bulk Titan tholin, and (4) the H2O-soluble fractions of both tholins are mainly molcules containing about 10 to 50 (C + N) atoms. We report yields of amino acids upon hydrolysis of Titan and Triton tholins. Titan tholin is largely insoluble in the putative hydrocarbon lakes or oceans on Titan, but can yield the H2O-soluble species investigated here upon contact with transient (e.g., impact-generated) liquid water.

Mcdonald, Gene D.↗

NASA NTRS1996

Astrometry of Single-Chord Occultations: Application to the 1993 Triton Event

This paper outlines a method for reducing astrometric data to derive the closest approach time and distance to the center of an occultation shadow for a single observer. The method applies to CCD frames, strip scans or photographic plates and uses a set of field stars of unknown positions to define a common coordinate system for all frames. The motion of the occulting body is used to establish the transformation between this common coordinate system and the celestial coordinate system of the body's ephemeris. This method is demonstrated by application to the Tr6O occultation by Triton on 1993 July 10 UT. Over an interval of four nights that included the occultation time, 80 frames of Triton and Tr6O were taken near the meridian with the U.S. Naval Observatory (USNO) 61-inch astrometric reflector. Application of the method presented here to these data yields a closest approach distance of 359 +/- 133 km (corresponding to 0.017 +/- 0.006 arcsec) for the occultation chord obtained with the Kuiper Airborne Observatory (KAO). Comparison of the astrometric closest approach time with the KAO light-curve midtime shows a difference of 2.2 +/- 4.1 s. Relative photometry of Triton and Tr6O, needed for photometric calibration of the occultation light curve, is also presented.

Olkin, Catherine B.↗