Pluto System Research Articles

Phase Curves of Nix and Hydra from the New Horizons Imaging Cameras

Abstract NASA’s New Horizons spacecraft’s voyage through the Pluto system centered on 2015 July 14 provided images of Pluto’s small satellites Nix and Hydra at viewing angles unattainable from Earth. Here, we present solar phase curves of the two largest of Pluto’s small moons, Nix and Hydra, observed by the New Horizons LOng Range Reconnaissance Imager and Multi-spectral Visible Imaging Camera, which reveal the scattering properties of their icy surfaces in visible light. Construction of these solar phase curves enables comparisons between the photometric properties of Pluto’s small moons and those of other icy satellites in the outer solar system. Nix and Hydra have higher visible albedos than those of other resonant Kuiper Belt objects and irregular satellites of the giant planets, but not as high as small satellites of Saturn interior to Titan. Both Nix and Hydra appear to scatter visible light preferentially in the forward direction, unlike most icy satellites in the outer solar system, which are typically backscattering.

Albedo matters: Understanding runaway albedo variations on Pluto

The data returned from NASA’s New Horizons reconnaissance of the Pluto system show striking albedo variations from polar to equatorial latitudes as well as sharp longitudinal boundaries. Pluto has a high obliquity (currently 119°) that varies by 23° over a period of less than 3 million years. This variation, combined with its regressing longitude of perihelion (360° over 3.7 million years), creates epochs of “Super Seasons” where one pole is pointed at the Sun at perihelion, thereby experiencing a short, relatively warm summer followed by its longest possible period of winter darkness. In contrast, the other pole experiences a much longer, less intense summer and a short winter season. We use a simple volatile sublimation and deposition model to explore the relationship between albedo variations, latitude, and volatile sublimation and deposition for the current epoch as well as historical epochs during which Pluto experienced these “Super Seasons.” Our investigation quantitatively shows that Pluto’s geometry creates the potential for runaway albedo and volatile variations, particularly in the equatorial region, which can sustain stark longitudinal contrasts like the ones we see between Tombaugh Regio and the informally named Cthulhu Regio.

A search for temporal changes on Pluto and Charon

A search for temporal changes on Pluto and Charon was motivated by (1) the discovery of young surfaces in the Pluto system that imply ongoing or recent geologic activity, (2) the detection of active plumes on Triton during the Voyager 2 flyby, and (3) the abundant and detailed information that observing geologic processes in action provides about the processes. A thorough search for temporal changes using New Horizons images was completed. Images that covered the same region were blinked and manually inspected for any differences in appearance. The search included full-disk images such that all illuminated regions of both bodies were investigated and higher resolution images such that parts of the encounter hemispheres were investigated at finer spatial scales. Changes of appearance between different images were observed but in all cases were attributed to variability of the imaging parameters (especially geometry) or artifacts. No differences of appearance that are strongly indicative of a temporal change were found on the surface or in the atmosphere of either Pluto or Charon. Limits on temporal changes as a function of spatial scale and temporal interval during the New Horizons encounter are determined. The longest time interval constraint is one Pluto/Charon rotation period (∼6.4 Earth days). Contrast reversal and high-phase bright features that change in appearance with solar phase angle are identified. The change of appearance of these features is most likely due to the change in phase angle rather than a temporal change. Had active plumes analogous to the plumes discovered on Triton been present on the encounter hemispheres of either Pluto or Charon, they would have been detected. The absence of active plumes may be due to temporal variability (i.e., plumes do occur but none were active on the encounter hemispheres during the epoch of the New Horizons encounter) or because plumes do not occur. Several dark streak features that may be deposits from past plumes are identified.

The Pluto system after the New Horizons flyby

In July 2015, NASA’s New Horizons mission performed a flyby of Pluto, revealing details about the geology, surface composition and atmospheres of this world and its moons that are unobtainable from Earth. With a resolution as small as 80 metres per pixel, New Horizons’ images identified a large number of surface features, including a large basin filled with glacial ices that appear to be undergoing convection. Maps of surface composition show latitudinal banding, with non-volatile material dominating the equatorial region and volatile ices at mid- and polar latitudes. This pattern is driven by the seasonal cycle of solar insolation. New Horizons’ atmospheric investigation found the temperature of Pluto’s upper atmosphere to be much cooler than previously modelled. Images of forward-scattered sunlight revealed numerous haze layers extending up to 200 km from the surface. These discoveries have transformed our understanding of icy worlds in the outer Solar System, demonstrating that even at great distances from the Sun, worlds can have active geologic processes. This Review addresses our current understanding of the Pluto system and places it in context with previous investigations.  The New Horizons spacecraft performed a flyby of Pluto and its system in July 2015, providing more than 50 Gb of high-resolution images and data that transformed our view and understanding of the dwarf planet. This Review summarizes its main discoveries.

Ices on Charon: Distribution of H2O and NH3 from New Horizons LEISA observations

Charon, the largest moon of Pluto, appeared as a fairly homogeneous, gray, icy world to New Horizons during closest approach on July 14th, 2015. Charon's sub-Pluto hemisphere was scanned by the Ralph/LEISA near-IR spectrograph providing an unprecedented opportunity to measure its surface composition. We apply a statistical clustering tool to identify spectrally distinct terrains and a radiative transfer approach to study the variations of the 2.0-µm H2O ice band. We map the distribution of the ices previously reported to be present on Charon's surface, namely H2O and the products of NH3 in H2O. We find that H2O ice is mostly in the crystalline phase, confirming previous studies. The regions with the darkest albedos show the strongest signature of amorphous-phase ice, although the crystalline component is still strong. The brighter albedo regions, often corresponding to crater ejecta blankets, are characterized by larger H2O grains, possibly an indication of a younger age. We observe two different behaviors for the two absorption bands representing NH3 in H2O. The 2.21-µm band tends to cluster more in the northern areas compared to the ∼2.01-µm band. Both bands are present in the brighter crater rays, but not all craters show both bands. The 2.21-µm band is also clearly present on the smaller moons Hydra and Nix. These results hint that different physical conditions may determine the appearance or absence of these two different forms of NH3 in H2O ice in the Pluto system. We also investigate the blue slope affecting the spectrum at wavelengths longer than ∼1.8 µm previously reported by several authors. We find that the slope is common among the objects in the Pluto system, Charon, the smaller moons Nix and Hydra, and the darkest terrains on Pluto. It also characterizes the analog ice tholin obtained from irradiation of Pluto-specific materials (a mixture of N2, CH4, and CO ices) in the laboratory. Our modeling results show that Pluto ice tholins are widespread almost uniformly on Charon suggesting a common distribution possibly part of the original reservoir of materials that made up Charon. This was irradiated over the years to yield the gray color characteristic of Charon today. On top of the ‘primordial’ Pluto ice tholin there is the redder component produced by irradiation of the CH4 provided by Pluto's atmospheric contribution as illustrated by Grundy et al. (2016a).

Differentiation and cryovolcanism on Charon: A view before and after New Horizons

Before the arrival of the New Horizons probe at the Pluto-Charon system, we developed a series of models that predicted that Kuiper Belt Objects, even as small and as cold as Charon, have experienced internal ice-rock differentiation and possibly cryovolcanism. Confronting these predictions is a wide array of spectroscopy, imagery, and other data from New Horizons. In this article we compare the predictions against the new observations, and find that they largely support the expected history of the Pluto system and the evolution of Charon. Following the collision of two partially differentiated impactors with radii ≈1000 km, a disk of material formed around Pluto, from which Charon and Pluto’s other moons formed. Because the impactors did not completely differentiate, the disk contained rocky material from their crusts, explaining the moons’ different densities and compositions. Long-lived radionuclides in Charon, assisted by ammonia antifreeze in the ice, melted ice and created a subsurface ocean that eventually refroze ≈1.7−2.5 Gyr ago. The freezing of this ocean would have created extensional stresses that possibly created Serenity Chasma, and could have led to widespread resurfacing, explaining the apparently younger age of Vulcan Planum. Buildup of radiogenic heat then created a second, smaller ocean that refroze 0.5–1.7 Gyr ago. As it froze, cryovolcanism would have been enabled, possibly creating Kubrick Mons. Charon’s “moated mountains” such as Kubrick Mons have a natural explanation as cryovolcanoes depressing a thin lithosphere over a cryomagma chamber. We offer further predictions about other aspects of Charon’s surface. Our previous predictions that Charon is a world shaped by geological activity have been largely borne out by New Horizons observations.

Pluto’s global surface composition through pixel-by-pixel Hapke modeling of New Horizons Ralph/LEISA data

On July 14th 2015, NASA’s New Horizons mission gave us an unprecedented detailed view of the Pluto system. The complex compositional diversity of Pluto’s encounter hemisphere was revealed by the Ralph/LEISA infrared spectrometer on board of New Horizons. We present compositional maps of Pluto defining the spatial distribution of the abundance and textural properties of the volatiles methane and nitrogen ices and non-volatiles water ice and tholin. These results are obtained by applying a pixel-by-pixel Hapke radiative transfer model to the LEISA scans. Our analysis focuses mainly on the large scale latitudinal variations of methane and nitrogen ices and aims at setting observational constraints to volatile transport models. Specifically, we find three latitudinal bands: the first, enriched in methane, extends from the pole to 55°N, the second dominated by nitrogen, continues south to 35°N, and the third, composed again mainly of methane, reaches 20°N. We demonstrate that the distribution of volatiles across these surface units can be explained by differences in insolation over the past few decades. The latitudinal pattern is broken by Sputnik Planitia, a large reservoir of volatiles, with nitrogen playing the most important role. The physical properties of methane and nitrogen in this region are suggestive of the presence of a cold trap or possible volatile stratification. Furthermore our modeling results point to a possible sublimation transport of nitrogen from the northwest edge of Sputnik Planitia toward the south.

Charon's light curves, as observed by New Horizons’ Ralph color camera (MVIC) on approach to the Pluto system

Light curves produced from color observations taken during New Horizons’ approach to the Pluto-system by its Multi-spectral Visible Imaging Camera (MVIC, part of the Ralph instrument) are analyzed. Fifty seven observations were analyzed, they were obtained between 9th April and 3rd July 2015, at a phase angle of 14.5° to 15.1°, sub-observer latitude of 51.2 °N to 51.5 °N, and a sub-solar latitude of 41.2°N. MVIC has four color channels; all are discussed for completeness but only two were found to produce reliable light curves: Blue (400–550nm) and Red (540–700nm). The other two channels, Near Infrared (780–975nm) and Methane-Band (860–910nm), were found to be potentially erroneous and too noisy respectively. The Blue and Red light curves show that Charon's surface is neutral in color, but slightly brighter on its Pluto-facing hemisphere. This is consistent with previous studies made with the Johnson B and V bands, which are at shorter wavelengths than that of the MVIC Blue and Red channel respectively.

Long-term surface temperature modeling of Pluto

NASA’s New Horizons’ reconnaissance of the Pluto system has revealed at high resolution the striking albedo contrasts from polar to equatorial latitudes on Pluto, as well as the sharpness of boundaries for longitudinal variations. These contrasts suggest that Pluto must undergo dynamic evolution that drives the redistribution of volatiles. Using the New Horizons results as a template, we explore the surface temperature variations driven seasonally on Pluto considering multiple timescales. These timescales include the current orbit (248 years) as well as the timescales for obliquity precession (peak-to-peak amplitude of 23° over 3 million years) and regression of the orbital longitude of perihelion (3.7 million years). These orbital variations create epochs of “Extreme Seasons” where one pole receives a short, relatively warm summer and long winter, while the other receives a much longer, but less intense summer and short winter. We use thermal modeling to build upon the long-term insolation history model described by Earle and Binzel (2015) and investigate how these seasons couple with Pluto’s albedo contrasts to create temperature effects. From this study we find that a bright region at the equator, once established, can become a site for net deposition. We see the region informally known as Sputnik Planitia as an example of this, and find it will be able to perpetuate itself as an “always available” cold trap, thus having the potential to survive on million year or substantially longer timescales. Meanwhile darker, low-albedo, regions near the equator will remain relative warm and generally not attract volatile deposition. We argue that the equatorial region is a “preservation zone” for whatever albedo is seeded there. This offers insight as to why the equatorial band of Pluto displays the planet’s greatest albedo contrasts.

The puzzling detection of x-rays from Pluto by Chandra

Using Chandra ACIS-S, we have obtained low-resolution imaging X-ray spectrophotometry of the Pluto system in support of the New Horizons flyby on 14 July 2015. Observations were obtained in a trial “seed” campaign conducted in one visit on 24 Feb 2014, and a follow-up campaign conducted soon after the New Horizons flyby that consisted of 3 visits spanning 26 Jul to 03 Aug 2015. In a total of 174 ksec of on-target time, in the 0.31 to 0.60keV passband, we measured 8 total photons in a co-moving 11×11 pixel2 box (the 90% flux aperture determined by observations of fixed background sources in the field) measuring ∼121,000×121,000 km2 (or ∼100×100 RPluto) at Pluto. No photons were detected from 0.60 to 1.0keV in this box during the same exposures. Allowing for background, we find a net signal of 6.8 counts and a statistical noise level of 1.2 counts, for a detection of Pluto in this passband at >99.95%confidence. The Pluto photons do not have the spectral shape of the background, are coincident with a 90% flux aperture co-moving with Pluto, and are not confused with any background source, so we consider them as sourced from the Pluto system. The mean 0.31 - 0.60keV X-ray power from Pluto is 200 +200/-100 MW, in the middle range of X-ray power levels seen for other known Solar System emission sources: auroral precipitation, solar X-ray scattering, and charge exchange (CXE) between solar wind (SW) ions and atmospheric neutrals. We eliminate auroral effects as a source, as Pluto has no known magnetic field and the New Horizons Alice UV spectrometer detected no airglow from Pluto during the flyby. Nano-scale atmospheric haze particles could lead to enhanced resonant scattering of solar X-rays from Pluto, but the energy signature of the detected photons does not match the solar spectrum and estimates of Pluto's scattered X-ray emission are 2 to 3 orders of magnitude below the 3.9±0.7×10−5cps found in our observations. Charge-exchange-driven emission from hydrogenic and heliogenic SW carbon, nitrogen, and oxygen (CNO) ions can produce the energy signature seen, and the 6×1025 neutral gas escape rate from Pluto deduced from New Horizons’ data (Gladstone et al. 2016) can support the ∼3.0 +3.0/-1.5 × 1024 X-ray photons/s emission rate required by our observations. Using the solar wind proton density and speed measured by the Solar Wind Around Pluto (SWAP) instrument in the vicinity of Pluto at the time of the photon emissions, we find a factor of 40 +40/-20 lower SW minor ions flowing planarly into an 11×11 pixel2, 90% flux box centered on Pluto than are needed to support the observed emission rate. Hence, the SW must be somehow significantly focused and enhanced within 60,000km (projected) of Pluto for this mechanism to work.

TOPOGRAPHIC MAPPING OF PLUTO AND CHARON USING NEW HORIZONS DATA

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.

TOPOGRAPHIC MAPPING OF PLUTO AND CHARON USING NEW HORIZONS DATA

Abstract. 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.

INTERPLANETARY MAGNETIC FIELD SECTOR FROM SOLAR WIND AROUND PLUTO (SWAP) MEASUREMENTS OF HEAVY ION PICKUP NEAR PLUTO

ABSTRACT On 2015 July 14, the New Horizons spacecraft flew by the Pluto system. The Solar Wind Around Pluto (SWAP) instrument on board New Horizons, which detects ions in the energy per charge range ∼0.035 to 7.5 keV/q, measured the unique interaction between the solar wind and Pluto's atmosphere. Immediately after the closest approach, SWAP detected a burst of heavy ion counts when the instrument's field of view (FOV) was aligned north and south of the Sun–Pluto line and approximately normal to the solar wind flow direction, suggesting their origin as heavy neutral atoms from Pluto that were ionized and being picked up by the solar wind. The trajectories of heavy pickup ions depend on the interplanetary magnetic field (IMF). New Horizons is not equipped with a magnetometer, and we cannot directly measure the IMF. However, we can utilize SWAP's measurements and instrument FOV during this brief period of time to determine the most likely sector of the IMF that could reproduce SWAP's observations of heavy ion pickup. We find that the IMF was most likely in an outward sector, or retrograde to the planets’ motion, during the Pluto encounter, and that the heavy ions detected by SWAP are more likely CH 4 + than N 2 + . This supports the existence of a methane exosphere at Pluto.

The atmosphere of Pluto as observed by New Horizons

Observations made during the New Horizons flyby provide a detailed snapshot of the current state of Pluto's atmosphere. Whereas the lower atmosphere (at altitudes of less than 200 kilometers) is consistent with ground-based stellar occultations, the upper atmosphere is much colder and more compact than indicated by pre-encounter models. Molecular nitrogen (N2) dominates the atmosphere (at altitudes of less than 1800 kilometers or so), whereas methane (CH4), acetylene (C2H2), ethylene (C2H4), and ethane (C2H6) are abundant minor species and likely feed the production of an extensive haze that encompasses Pluto. The cold upper atmosphere shuts off the anticipated enhanced-Jeans, hydrodynamic-like escape of Pluto's atmosphere to space. It is unclear whether the current state of Pluto's atmosphere is representative of its average state--over seasonal or geologic time scales.

The geology of Pluto and Charon through the eyes of New Horizons.

NASA's New Horizons spacecraft has revealed the complex geology of Pluto and Charon. Pluto's encounter hemisphere shows ongoing surface geological activity centered on a vast basin containing a thick layer of volatile ices that appears to be involved in convection and advection, with a crater retention age no greater than ~10 million years. Surrounding terrains show active glacial flow, apparent transport and rotation of large buoyant water-ice crustal blocks, and pitting, the latter likely caused by sublimation erosion and/or collapse. More enigmatic features include tall mounds with central depressions that are conceivably cryovolcanic and ridges with complex bladed textures. Pluto also has ancient cratered terrains up to ~4 billion years old that are extensionally faulted and extensively mantled and perhaps eroded by glacial or other processes. Charon does not appear to be currently active, but experienced major extensional tectonism and resurfacing (probably cryovolcanic) nearly 4 billion years ago. Impact crater populations on Pluto and Charon are not consistent with the steepest impactor size-frequency distributions proposed for the Kuiper belt.

Pluto's interaction with its space environment: Solar wind, energetic particles, and dust.

The New Horizons spacecraft carried three instruments that measured the space environment near Pluto as it flew by on 14 July 2015. The Solar Wind Around Pluto (SWAP) instrument revealed an interaction region confined sunward of Pluto to within about 6 Pluto radii. The region's surprisingly small size is consistent with a reduced atmospheric escape rate, as well as a particularly high solar wind flux. Observations from the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument suggest that ions are accelerated and/or deflected around Pluto. In the wake of the interaction region, PEPSSI observed suprathermal particle fluxes equal to about 1/10 of the flux in the interplanetary medium and increasing with distance downstream. The Venetia Burney Student Dust Counter, which measures grains with radii larger than 1.4 micrometers, detected one candidate impact in ±5 days around New Horizons' closest approach, indicating an upper limit of <4.6 kilometers(-3) for the dust density in the Pluto system.

New Horizons unveils the Pluto system

Planetary Science In July 2015, the New Horizons spacecraft flew through the Pluto system at high speed, humanity's first close look at this enigmatic system on the outskirts of our solar system. In a series of papers, the New Horizons team present their analysis of the encounter data downloaded so

PLUTO’S ATMOSPHERE FROM THE 2015 JUNE 29 GROUND-BASED STELLAR OCCULTATION AT THE TIME OF THE NEW HORIZONS FLYBY*

ABSTRACT We present results from a multi-chord Pluto stellar occultation observed on 2015 June 29 from New Zealand and Australia. This occurred only two weeks before the NASA New Horizons flyby of the Pluto system and serves as a useful comparison between ground-based and space results. We find that Pluto's atmosphere is still expanding, with a significant pressure increase of 5 ± 2% since 2013 and a factor of almost three since 1988. This trend rules out, as of today, an atmospheric collapse associated with Pluto's recession from the Sun. A central flash, a rare occurrence, was observed from several sites in New Zealand. The flash shape and amplitude are compatible with a spherical and transparent atmospheric layer of roughly 3 km in thickness whose base lies at about 4 km above Pluto's surface, and where an average thermal gradient of about 5 K km−1 prevails. We discuss the possibility that small departures between the observed and modeled flash are caused by local topographic features (mountains) along Pluto's limb that block the stellar light. Finally, using two possible temperature profiles, and extrapolating our pressure profile from our deepest accessible level down to the surface, we obtain a possible range of 11.9–13.7 μbar for the surface pressure.

The long-wavelength thermal emission of the Pluto-Charon system fromHerschelobservations. Evidence for emissivity effects

Thermal observations of the Pluto-Charon system acquired by the Herschel Space Observatory in February 2012 are presented. They consist of photometric measurements with the PACS and SPIRE instruments (nine visits to the Pluto system each), covering six wavelengths from 70 to 500 $\mu$m altogether. The thermal light curve of Pluto-Charon is observed in all filters, albeit more marginally at 160 and especially 500 $\mu$m. Putting these data into the context of older ISO, Spitzer and ground-based observations indicates that the brightness temperature (T$_B$) of the system (rescaled to a common heliocentric distance) drastically decreases with increasing wavelength, from $\sim$53 K at 20 $\mu$m to $\sim$35 K at 500 $\mu$m, and perhaps ever less at longer wavelengths. Considering a variety of diurnal and/or seasonal thermophysical models, we show that T$_B$ values of 35 K are lower than any expected temperature for the dayside surface or subsurface of Pluto and Charon, implying a low surface emissivity. Based on multiterrain modeling, we infer a spectral emissivity that decreases steadily from 1 at 20-25 $\mu$m to $\sim$0.7 at 500~$\mu$m. This kind of behavior is usually not observed in asteroids (when proper allowance is made for subsurface sounding), but is found in several icy surfaces of the solar system. We tentatively identify that a combination of a strong dielectric constant and a considerable surface material transparency (typical penetration depth $\sim$1 cm) is responsible for the effect. Our results have implications for the interpretation of the temperature measurements by REX/New Horizons at 4.2 cm wavelength.

Isotopic constraints on the source of Pluto׳s nitrogen and the history of atmospheric escape

The origin and evolution of nitrogen in solar system bodies is an important question for understanding processes that took place during the formation of the planets and solar system bodies. Pluto has an atmosphere that is 99% molecular nitrogen, but it is unclear if this nitrogen is primordial or derived from ammonia in the protosolar nebula. The nitrogen isotope ratio is an important tracer of the origin of nitrogen on solar system bodies, and can be used at Pluto to determine the origin of its nitrogen. After evaluating the potential impact of escape and photochemistry on Pluto׳s nitrogen isotope ratio (14N/15N), we find that if Pluto׳s nitrogen originated as N2 the current ratio in Pluto׳s atmosphere would be greater than 324 while it would be less than 157 if the source of Pluto׳s nitrogen were NH3. The New Horizons spacecraft successfully visited the Pluto system in July 2015 providing a potential opportunity to measure 14N/15N in N2.