Triton's Surface Research Articles

Triton Haze Analogs: The Role of Carbon Monoxide in Haze Formation

AbstractTriton is the largest moon of the Neptune system and possesses a thin nitrogen atmosphere with trace amounts of carbon monoxide and methane, making it of similar composition to that of the dwarf planet Pluto. Like Pluto and Saturn's moon Titan, Triton has a haze layer thought to be composed of organics formed through photochemistry. Here, we perform atmospheric chamber experiments of 0.5% CO and 0.2% CH4 in N2 at 90 K and 1 mbar to generate Triton haze analogs. We then characterize the physical and chemical properties of these particles. We measure their production rate, their bulk composition with combustion analysis, their molecular composition with very high resolution mass spectrometry, and their transmission and reflectance from the optical to the near‐infrared with Fourier Transform Infrared (FTIR) Spectroscopy. We compare these properties to existing measurements of Triton's tenuous atmosphere and surface, as well as contextualize these results in view of all the small, hazy, nitrogen‐rich worlds of our solar system. We find that carbon monoxide present at greater mixing ratios than methane in the atmosphere can lead to significantly oxygen‐ and nitrogen‐rich haze materials. These Triton haze analogs have clear observable signatures in their near‐infrared spectra, which may help us differentiate the mechanisms behind haze formation processes across diverse solar system bodies.

Heat flow in Triton: Implications for heat sources powering recent geologic activity

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 (H2O and NH3·2H2O). 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.

DIRECT DETECTION OF SEASONAL CHANGES ON TRITON WITH HUBBLE SPACE TELESCOPE

Triton is one of the few bodies in the solar system with observed cryo-volcanic activity, in the form of plumes at its south pole, which suggests large-scale surface volatile transport over time. Triton's large variations in obliquity have motivated prior predictions of changing atmospheric column densities of several orders of magnitude, driven by seasonal evaporation of surface volatiles. Using the Hubble Space Telescope, we directly imaged Triton's surface and have detected large-scale differences in increased and decreased reflectance when compared with Voyager data at UV, visual, and methane-band wavelengths. Our surface map shows regions of increased brightness at near-equatorial latitudes and near the Neptune-facing side, and darkened regions near longitudes of ±180°, indicating the presence of ongoing seasonal volatile transport.

A 0.4–2.5 μm spectroscopic investigation of Triton's two faces

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.

Triton surface evolution

Recent physical data of the Triton surface characteristics are given, with particular reference being made to the factors determining the mechanisms of that satellite's surface evolution. It is suggested that the photolysis, caused in Triton's ice surface by short-wave Solar radiation, should be considered as the basic mechanism responsible for the evolution of the surface of the satellite. Photodissociation of the CH 4 + h x M CH 3 +H-type is discussed. Ethane C 2 H 6 is shown to be the basic substance resulting from the above mentioned photodissociation and subsequent reaction involving CH 3 molecules. The time required for full photodestruction of CH 4 ice on Triton surface has been calculated, as well as the surface temperature of the satellite. Some other issues are touched upon.

HST/FOC Imaging of Triton

We present an analysis of HST/FOC images of Triton obtained on 20–24 September 1995. Images were taken in two wavelength bands using the F278M and F410M filters in order to obtain the first mid-UV map of Triton and to place further compositional constraints on Triton's surface by imaging farther into the UV than did Voyager. Our disk-resolved images provide full (if sparse) longitudinal coverage, with image sets taken at Triton central meridian longitudes of 100°, 210°, and 330°. We compare FOC-derived lightcurves and geometric albedo maps with 1989 Voyager imaging data and 1993 HST/FOS disk-averaged albedos. We find that Triton's disk-averaged albedo has decreased at both 2780 (12%) and 4100 Å (8%) and that an extensive region near longitude 340° has reddened significantly, with a 4100:2780 Å albedo ratio that is 15% lower than average. We conclude that these changes are evidence of recent, large-scale volatile transport on Triton.

Water Ice on Triton

We discuss the spectroscopic detection of H 2O ice on Triton, evidenced by the broad absorption bands in the near infrared at 1.55 and 2.04 μm. The detection of water ice on Triton reconfirms earlier preliminary studies (D. P. Cruikshank et al. 1984, Icarus 58, 293–305). Although crystalline H 2O ice has a distinctive spectral band at 1.65 μm, and our new models slightly favor the presence of this phase, we cannot unambiguously determine whether Triton's water ice is crystalline or amorphous. Both phases might be present, and special conditions in the surface microstructure may affect the spectroscopic signature of water ice in such a way that crystalline ice is present and its 1.65 μm spectral band is masked. Our spectra (1.87–2.5 μm) taken at an interval of nearly 3.5 years do not show any significant changes that might relate to reports of changes in Triton's spectral reflectance (B. Buratti et al. 1999, Nature 397, 219), or in Triton's surface pressure (J. L. Elliot et al. 1998, Nature 393, 765–767).

Triton’s Surface Age and Impactor Population Revisited in Light of Kuiper Belt Fluxes: Evidence for Small Kuiper Belt Objects and Recent Geological Activity

Neptune's largest satellite, Triton, is one of the most fascinating and enigmatic bodies in the solar system. Among its numerous interesting traits, Triton appears to have far fewer craters than would be expected if its surface were primordial. Here we combine the best available crater count data for Triton with improved estimates of impact rates by including the Kuiper belt as a source of impactors. We find that the population of impactors creating the smallest observed craters on Triton must be subkilometer in scale and that this small-impactor population can be best fitted by a differential power-law size index near -3. Such results provide interesting, indirect probes of the unseen small body population of the Kuiper belt. Based on the modern, Kuiper belt and Oort cloud impactor flux estimates, we also recalculate estimated ages for several regions of Triton's surface imaged by Voyager 2, and find that Triton was probably quite geologically active on a timescale no greater than 0.1–0.3 Gyr ago (indicating Triton was still active after some 90% to 98% of the age of the solar system), and perhaps even more recently. This activity must surpass that explainable by the surface geysers seen by Voyager 2 by many orders of magnitude. The time-averaged volumetric resurfacing rate on Triton implied by these results, 0.01 km3 yr-1 or more, is likely second only to Io and Europa in the outer solar system, and is within an order of magnitude of estimates for Venus and for the Earth's intraplate zones. This finding indicates that Triton likely remains a highly geologically active world at present, some 4.5 Gyr after its formation. We briefly speculate on how such a situation might obtain.

Composition, Physical State, and Distribution of Ices at the Surface of Triton

This paper presents the analysis of near-infrared observations of the icy surface of Triton, recorded on 1995 September 7, with the cooled grating spectrometer CGS4 at the United Kingdom Infrared Telescope (Mauna Kea, HI). This analysis was performed in two steps. The first step consisted of identifying the molecules composing Triton's surface by comparing the observations with laboratory transmission spectra (direct spectral analysis); this also gives information on the physical state of the components. Most of the bands in Triton's spectrum were assigned to specific vibration bands of the CH4, N2, CO, and CO2 molecules previously discovered. A detailed comparison of the frequencies of the CH4 bands confidently indicated that this molecule exists in a diluted state in solid β-N2. Three new bands peaking at 5717, 5943, and 6480 cm−1 (1.749, 1.683, and 1.543 μm, respectively) were also observed. Laboratory experiments have shown that C2H6 isolated in solid N2 fits well the second band, but this would imply the appearance of unobserved bands and thus rules out this assignment. However, C2H6 may exist in another physical state, and more experiments are necessary. No plausible candidate was found for these three bands when comparing with the spectra of nine molecules (C2H2, C2H4, C3H8, NH3, SO2, HC3N, CH3OH, NO, NO2).In view of the results of D. P. Cruikshank et al. (1993, Science261, 742; in preparation), the work presented here leads to two possible representations of the surface of Triton. First, a two-region surface composed of a N2:CH4:CO terrain, N2:CH4:CO consisting of a solid solution in which N2 is the dominant molecule, and of a H2O+CO2 terrain, composed of a mixture of pure crystalline H2O and CO2 grains. The second representation is a three-region surface composed of a N2:CH4:CO terrain and two geographically separated H2O and CO2 terrains.The second step of the analysis consisted of using a bidirectionnal reflectance model (S. Douté and B. Schmitt 1998, J. Geophys. Res. Planets103, 31367). The modeling first confirms the direct spectral analysis in that CH4 is diluted in solid β-N2, giving a high degree of confidence to the conclusion that the N2:CH4:CO terrain is in fact a solid solution. It also provides numerical information on this terrain, namely the size of the grains, the geographical abundance, and the CH4 and CO concentrations. The large grain size (around 10 cm) would mean that the texture of this terrain is a compact crystalline solid rather than granular, which is in agreement with calculations from J. Eluszkiewicz (1991, J. Geophys. Res.96, 19,217). In addition, an accurate modeling of the N2 band could suggest that the temperature is greater or equal to 35.6 K.Although undistinguishable in the spectra, a maximum of 10% surface area of pure CH4 ice can be present at the surface of Triton, thus explaining the high atmospheric CH4 abundance observed by Voyager 2.Finally, the modeling showed that none of the two- or three-region representations was able to fit simultaneously the K and H regions of the spectrum of Triton. The origin of this misfit is not yet elucidated, but an instrumental effect is suspected. Some questions about the physical state of the H2O and CO2 molecules are thus raised, but unfortunately observational constraints are missing. New near-infrared observations could partly provide these missing constraints, and would be important for detecting new molecules on Triton's surface. Such new data would be especially useful to identify the three bands at 5717, 5943, and 6480 cm−1 (1.749, 1.683, and 1.543 μm).

Triton's Surface Roughness Is Not Unusual

Previous estimates of Triton's macroscopic roughness have been hampered by the presence of atmospheric aerosols in Triton's atmosphere: only a loose constraint on the macroscopic roughness (<20°) could be obtained. This study determines the locations of the clouds on Triton and then examines those areas found to be relatively devoid of cloud cover to yield tighter constraints on the surface macroscopic roughness. The most prominent cloud feature is seen near 50°S, 160°W with smaller features appearing near 60°S, 270°W and 15°S, 280°W. The fact that the cloud cover is not largest toward the south pole argues against a sublimation origin for the clouds, though the results are not conclusive. The surface roughness ranges from ∼11° to ∼18° for Triton's various terrains. These are near the upper limit previously determined and, while relatively smooth, are not atypical of icy satellites. Roughness values near zero which would have been suggestive of an unusual surface texture and processes are clearly excluded. These results suggest that Triton's surface structure and the processes which govern its evolution are similar to those present on other icy satellites.

The spatial distribution of UV-absorbing regions on Triton

Using mosaic maps of Triton's surface made with images taken through the Voyager 2 UV and Clear filters, we have sought to: (1) distinguish between surface units containing UV-absorbing (UVA) materials and fresh volatile frosts; (2) place any such identified regions into a geological context; (3) suggest possible candidate materials for the UVAs; and (4) to interpret these regions in terms of seasonal volatile transport scenarios. In pursuit of these goals, we have identified both a region possibly containing involatile UVA materials and an adjoining region possibly covered by uncontaminated, fresh frosts. The UVA region correlates well with a distinct geological unit on Triton containing maculae and wind streaks. The location of these two regions suggests that they might be linked through the dynamical transport of volatiles between them. Hydrocarbon deposits may explain the UVA region. However, they cannot be ruled out in regions darker than average in the Clear filter. Also, we cannot rule out other UVA materials as a visible component of the UVA region, such as salts or sulfur-bearing compounds, but note that such material has not yet been detected and may be largely obscured by hydrocarbon deposits. Recent images of Triton taken by us with the post-COSTAR HST FOC in bandpasses centered near 4100 and 2780 Å will be analyzed to provide further information on the possible UVA and adjoining fresh frost regions. With resolution elements of ⋍400–700 km (depending on wavelength) at Triton's distance, the FOC images will provide the first mapping of these regions in the mid-UV (2780 Å). This wavelength corresponds to the extreme of the broad-band absorption feature seen in Triton's UV spectrum (Stern et al., 1995), suggesting that the contrast between the UVA region and frost regions may be enhanced in the HST images.

Historical Photometric Evidence for Volatile Migration on Triton

Analysis of CCD images of Triton obtained with the 1.5-m telescope on Palomar Mountain shows that in the time period surrounding the Voyager 2 encounter with the satellite (1985-1990), no changes in the satellite's visual albedo or color occurred. The published observations of Triton in the 0.35- to 0.60-μm spectral region obtained between 1950 and 1990 were reanalyzed to detect historical variability in both its albedo and visual color. Analysis of the photometry indicates that there is little, if any, change in Triton's visual geometric albedo. This result is consistent with the albedo pattern observed by Voyager and the change in sub-Earth latitude. Two distinct types of color changes are evident: a significant secular increase in the blue region of the visual spectrum since at least the 1950s, and the reported dramatic reddening of Triton's spectrum in the late 1970s. The latter change can be explained only by a short-lived geological phenomenon. Triton's changing pole orientation with respect to a terrestrial observer cannot explain the secular color changes. These changes imply volatile transport on a global scale on Triton's surface during the past 4 decades. We present two models which show that either removal of a red volatile from Triton's polar cap or deposition of a blue volatile in the equatorial regions can explain the secular color changes. A third possibility is that the changes are the result of the α-β phase transition of nitrogen and subsequent fracturing of the polar cap region (N. S. Duxbury and R. H. Brown, 1993, Science 261,748).

Coupling of volatile transport and internal heat flow on Triton

Recently Brown et al. (1991) showed that Triton's internal heat source could amount to 5–20% of the absorbed insolation on Triton, thus significantly affecting volatile transport and atmospheric pressure. Subsequently, Kirk and Brown (1991a) used simple analytical models of the effect of internal heat on the distribution of volatiles on Triton's surface, confirming the speculation of Brown et al. that Triton's internal heat flow could strongly couple to the surface volatile distribution. To further explore this idea, we present numerical models of the permanent distribution of nitrogen ice on Triton that include the effects of sunlight, the two‐dimensional distribution of internal heat flow, the coupling of internal heat flow to the surface distribution of nitrogen ice, and the finite viscosity of nitrogen ice. From these models we conclude that: (1) The strong vertical thermal gradient induced in Triton's polar caps by internal heat‐flow facilitates viscous spreading to lower latitudes, thus opposing the poleward transport of volatiles by sunlight, and, for plausible viscosities and nitrogen inventories, producing permanent caps of considerable latitudinal extent; (2) It is probable that there is a strong coupling between the surface distribution of nitrogen ice on Triton and internal heat flow; (3) Asymmetries in the spatial distribution of Triton's heat flow, possibly driven by large‐scale, volcanic activity or convection in Triton's interior, can result in permanent polar caps of unequal latitudinal extent, including the case of only one permanent polar cap; (4) Melting at the base of a permanent polar cap on Triton caused by internal heat flow can significantly enhance viscous spreading, and, as an alternative to the solid‐state greenhouse mechanism proposed by Brown et al. (1990), could provide the necessary energy, fluids, and/or gases to drive Triton's geyser‐like plumes; (5) The atmospheric collapse predicted to occur on Triton in the next 20 years (Spencer, 1990) may be plausibly avoided because of the large latitudinal extent expected for permanent polar caps on Triton.

Spectroscopic Determination of the Phase Composition and Temperature of Nitrogen Ice on Triton

Laboratory spectra of the first overtone band (2.1480 micrometers, 4655.4 reciprocal centimeters) of solid nitrogen show additional structure at 2.1618 micrometers (4625.8 reciprocal centimeters) over a limited temperature range. The spectrum of Neptune's satellite Triton shows the nitrogen overtone band as well as the temperature-sensitive component. The temperature dependence of this band may be used in conjunction with ground-based observations of Triton as an independent means of determining the temperature of surface deposits of nitrogen ice. The surface temperature of Triton is found to be 38.0(+2.0)(-1.0) K, in agreement with previous temperature estimates and measurements. There is no spectral evidenceforthe presence of alpha-nitrogen on Triton's surface, indicating thatthere is less than 10 percent carbon monoxide in solid solution with the nitrogen on the surface.

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.

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.

A massive early atmosphere on Triton

We present models of a massive early atmosphere on Triton powered by tidal evolution of an early eccentric orbit toward the present-day, circular retrograde orbit. For this first suite of models we use the currently present volatiles on Triton's surface—methane, nitrogen, and carbon monoxide—together with a trace amount of molecular hydrogen which could have been present either as a photochemical product or as a residuum of the solar or circum-Neptunian nebula. These species are assumed to be outgassed onto the surface and to create an atmosphere in vapor pressure equilibrium as the crust is warmed by tidal heating. Atmospheric opacity sources are binary collisions among nitrogen (or carbon monoxide), methane, and hydrogen, using the collision-induced absorption coefficients of Courtin (1988, Icarus 75, 245–254). These are frequency averaged to construct gray-atmosphere profiles as a function of tidal heating rate, hydrogen abundance, and thermodynamic behavior of the volatile species. For zero hydrogen abundance, the atmosphere raised by tidal heating remains optically thin for all values of the tidal heating flux. For (constant) hydrogen atmospheric fractions in excess of 0.01% a massive atmosphere is raised by modest tidal heating fluxes, the amount of gas ultimately limited by the supply from the interior. The lifetime of such an atmosphere is limited by the rate of escape of infrared-active gases and could have exceeded a billion years. Atmospheric escape powered by solar extreme ultraviolet heating, and enhanced by the massive nature of the atmosphere, could have removed the equivalent of a layer of methane and nitrogen hundreds of meters deep. The thick atmosphere would have directly impeded impactors up to a kilometer in size, as well as encouraged volcanism and resurfacing, thereby contributing to the youthfulness of Triton's surface.

Volatile processes in Triton's atmosphere and surface

The August, 1989 Voyager encounter with the Neptune system provided the fundamental information required to understand the volatile budget and transport processes on Neptune's large moon Triton. The discovery of nitrogen in Triton's atmosphere at a surface pressure of roughly 15–20 microbars, and surface temperature of 35–40K, combined with earlier ground-based spectra. strongly indicate that the atmosphere is supported by extensive deposits of surface nitrogen frosts, which are moved seasonally through the atmosphere in response to varying insolation. Likewise, Voyager and ground-based data support the notion that methane frost exists on the surface and is resupplying the atmosphere. The simplest model of global volatile transport, in which the albedo of the frost is constant, illustrates heuristically some interesting aspects of this system but does not reproduce the frost distribution seen by Voyager. Instead, models in which the nitrogen deposits have variable albedo, caused by grain size changes or contaminants, must be invoked. The grain growth properties and thermodynamics of pure nitrogen may also determine the radiating properties of the nitrogen frosts, which must be accounted for in the transport models. Finally, simple sublimation of nitrogen from the lit pole is not the only means of transport through the atmosphere: the detection of plumes above the surface may indicate the operation of moist convective processes which affect the transfer of both mass and energy from summer to winter regions.

A thermal model for the seasonal nitrogen cycle on Triton

A Mars thermal model has been adapted to study the seasonal nitrogen cycle on Triton. Unlike other models published to date, it incorporates diurnal and seasonal subsurface heat conduction, and accounts for the heat capacity of N 2 frost deposits. Key observables that this model attempts to reproduce include Triton's atmospheric pressure and surface albedo features at the time of the Voyager encounter, as well as changes in Triton's disk-integrated spectral properties between 1977 and 1989. Subsurface heat conduction is found to have an important effect on the behavior of Triton's polar caps and atmospheric pressure fluctuations. The results of this model differ from those of previous studies in that they generally do not predict the catastrophic freeze-out of Triton's atmosphere on a seasonal basis. The model results indicate that even for a wide range of possible input parameters, it is unlikely that Triton's bright southern polar cap is a seasonal nitrogen deposit, because it would have largely sublimated before the Voyager encounter. However, these results do not rule out the possibility of a large bright permanent polar cap in Triton's southern hemisphere. The best agreement between the model results and the available observations is obtained when Triton's seasonal nitrogen frost deposits are assumed to be darker than the undelying substrate. Assuming a constant nitrogen frost albedo of 0.62, a frost emissivity of 0.5, a substrate albedo of 0.8, and a substrate thermal inertia of 7 × 10 −3 cal cm −2 K −1 sec −1 2 yields good agreement with the surface pressure and global albedo patterns observed during the Voyager encounter, as well as a rapid change in disk-integrated albedo from 1977 to 1989 due to the retreat and disappearance of a dark south seasonal polar cap. These results lend support to microphysical arguments for the presence of dark, or smooth, translucent nitrogen frost on the surface of Triton.

The Effect of Surface Roughness on Triton's Volatile Distribution

Calculations of radiative equilibrium temperatures on Triton's rough surface suggest that significant condensation of N(2) may be occurring in the northern equatorial regions, despite their relatively dark appearance. The bright frost is not apparent in the Voyager images because it tends to be concentrated in relatively unilluminated facets of the surface. This patchwork of bright frost-covered regions and darker bare ground may be distributed on scales smaller than that of the Voyager resolution; as a result the northern equatorial regions may appear relatively dark. This hypothesis also accounts for the observed wind direction in the southern hemisphere because it implies that the equatorial regions are warmer than the south polar regions.