Astronomical Science

The Tenuous of and Triton Explored by CRIRES on the VLT

1 Emmanuel Lellouch Its orbit around the — retrograde brightest areas are attributed to N2 , Catherine de Bergh1 and out of ’s equatorial plane — while dark regions are thought to result Bruno Sicardy1 indicates that it is most likely a Kuiper from the irradiation of ice- Hans-Ulrich Käufl2 Belt object that has been captured by rich areas, giving birth to (hetero­ Alain Smette2 Neptune. Triton’s orbit and obliquity are polymer molecules formed by UV radia­ such that at some time during the 165- tion). The large surface variegation of year-long Neptune year, each of Triton’s Pluto is also responsible for a large heter­ 1 LESIA-Observatoire de Paris, CNRS, poles points almost directly toward the ogeneity in the surface , var­ UPMC Univ. Paris 06, Univ. Paris- — the obliquity of the current orbit is ying from about 40 K in the bright areas Diderot, France 50°, resulting in large seasonal effects in to 60 K in the darkest regions. 2 ESO the insolation.

Beyond their similar sizes and dynamical Tenuous and poorly understood The Pluto and Neptune’s kinship, Pluto and Triton have similar, ­atmospheres largest satellite, Triton, are two small albeit not identical, compositions. - icy bodies surrounded by tenuous and based near-infrared (NIR) observations Pluto and Triton both have tenuous poorly known atmospheres. The high indicate that both are covered by atmospheres formed by the sublimation spectral resolution and high sensitivity dominated by , plus some meth­ of the more volatile surface ices, whose of CRIRES on the VLT have permitted ane and . Ethane ice composition, to first order, must reflect a major step forward in the study of has also been detected on Pluto, while that of the surface and the relative vola­ these atmospheres, and especially of and ice are present tility of the various ices. In spite of their their composition. Absorptions due to on Triton. Detailed studies of ice band large distance from the Sun, these methane and carbon monoxide in these positions indicate that most of the ices atmospheres are strongly driven by solar atmospheres have been detected at are “diluted” by N2 ice, i.e. are mixed with heat. Given the large inclination of the 1.66 and 2.35 µm, providing an insight it in small proportions at the molecular rotation axes (and in Pluto’s case the into the way in which these atmos- level. The encounter with Triton ) they must also vary pheres are maintained, the surface– in 1989 unveiled a frigid (38 K) but re­ with time. Pluto’s is expected atmosphere interactions, their seasonal markably active world, with high surface to completely collapse at aphelion — evolution and thermal balance. diversity (see Figure 1). Until the New around 2113. Horizons spacecraft gets to Pluto — the encounter is scheduled for July 2015 — Triton’s atmosphere was discovered by

Pluto and Triton: Frigid twins in the outer and reveals how similar Pluto’s appear­ Voyager 2. Its main component, N2, ance is to Triton’s, Pluto will remain a was detected in the , with a crudely resolved 0.1-arcsecond dot. Yet ground pressure of 14 µbar, and gaseous

Pluto and Triton are twins in the outer even at the modest resolution of the CH4 could also be observed. On Pluto, Solar System. Once the ninth planet and ­Hubble Space Telescope (HST; see Fig­ N2 must also be the major gas, but can­ now classified as a dwarf planet, Pluto ure 2), Pluto exhibits large brightness not be observed from Earth, and only is a prominent member of the , contrasts, up to 6:1 in reflectance. The a low signal-to-noise (S/N) detection of the population of small primitive bodies orbiting the Sun beyond Neptune (e.g., Barucci et al., 2010). Its heliocentric orbit, Figure 1. Image of ­Triton’s South Polar fairly typical for a body in 3:2 resonance Caltech - cap, seen by Voyager 2 with Neptune, is elliptical (perihelion at in 1989. 29.5 Astronomical Units [AU] and aphe­ lion at 49.0 AU) and inclined (by 17° to the plane). Pluto’s axis of rotation is inclined by 120° to its orbital plane. NASA/JPL Courtesy Since its discovery in 1930, Pluto has covered only one third of its 248-year orbit and its most recent closest ap­proach to the Sun occurred in 1989. With a diam­ eter of about 2340 kilometres, Pluto is, along with , one of the two largest members of the trans-Neptunian popula­ tion.

At a mean distance of 30 AU from the Sun, Triton is the largest satellite of Nep­ tune, with a diameter of 2707 kilometres.

20 The Messenger 145 – September 2011 90 offered by CRIRES makes it a very pow­ erful instrument to study tenuous and distant atmospheres. During a first ser­ vice run in August 2008, we obtained a

45 high S/N detection of the 2ν3 band of

) methane at 1.67 μm in Pluto’s spectrum. es As Pluto’s is re

eg ­typically only ~ 15 μbar, lines are purely

(d 0 ­Doppler-broadened and the highest de spectral resolution is desirable. We used tu ti a slit of 0.4 arcseconds, combined with La the adaptive optics module MACAO, –45 obtaining an effective spectral resolution of ~ 60 000. This high resolution was also instrumental in separating the meth­ ane lines of Pluto from their telluric –90 counter­parts, so that methane lines from 2700 90 180270 East longitude (degrees) Pluto, interlinked within the Q-branch of the 2ν3 band of telluric methane (see Figure 2. A map of Pluto from HST observations and Pluto, and how they may change Figure 3), could be observed. The quality (from Buie et al., 2010). The brightest region near with time, remain very poorly character­ of the spectrum (17 individual lines, 180° longitude is suspected to be CO ice-rich. ised. Atmospheric methane was not re- including high-J lines) made it possible to observed and the search for additional separate abundance and temperature atmospheric methane was achieved, compounds failed. In the last three years, effects in Pluto’s spectrum (Lellouch et using observations from the NASA Infra­ however, CRIRES observations at the al., 2009). Pluto’s mean methane temper­ red Telescope at 1.67 µm (Young et al., VLT have brought about very significant ature is 90 K, which implies that Pluto 1997). Pluto’s atmosphere, on the other progress. cannot have a deep and tropo­ hand, has been much more studied from sphere as had been proposed (otherwise stellar . Although these ob­ the bulk of the methane would be at servations cannot identify a particular CRIRES spectra of the atmospheres of ~ 40 K). By linking this to a re-analysis of species and are also unable to probe the Pluto and Triton measurements, new con­ atmosphere down to the surface itself, straints on the structure of Pluto’s lower occultations are an extremely powerful The unprecedented combination of sen­ atmosphere and surface pressure could way of revealing the atmosphere’s ther­ sitivity and spectral resolution in the NIR also be derived, indicating that Pluto’s mal structure. Pluto’s upper atmosphere appears to be “warm” ~ 100 K, as op­­ 0.4 posed to ~ 60 K for Triton at the same R7 Observed, 1 August 2008 0.35 pressure level), a likely effect of heating 90 K, 0.75 cm-am

s) R6 by methane (similar to ozone heating 0.3

in the Earth’s ). In contrast, unit . 0.25 R5

Pluto’s lower atmosphere and the way rb R3 R2 0.2 the upper atmosphere connects to x (a the 40–60 K surface are poorly under­ Flu 0.15 stood. Remarkably, repeated occultations showed that Pluto’s atmosphere has 0.1 expanded, roughly doubling in surface 1642 1644 1646 1648 1650 1652 1654 1656 1658 pressure, from 1988 to 2002, even though Wavelength (nanometres) Pluto has started to recede from the Sun. Similarly, Triton’s surface pressure 0.35 Observed, 1 August 2008 90 K, 0.75 cm-am has been shown to increase through the 0.3

1990s. s) 0.25 unit . R0 Q branch (Q1-Q8) P1 Despite these remarkable findings, the rb 0.2

composition of the atmospheres of Triton x (a 0.15 Flu P4 0.1 P3

Figure 3. The spectrum of Pluto’s atmosphere show­ 0.05 1664 1666 1668 1670 1672 1674 1676 1678 1680 ing the fit to the 2ν3 band of methane, observed by CRIRES (from Lellouch et al., 2009). Wavelength (nanometres)

The Messenger 145 – September 2011 21 Astronomical Science Lellouch E. et al., The Tenuous Atmospheres of Pluto and Triton Explored by CRIRES

Figure 4. A section of Triton’s spectrum in the 0.6 2.35 μm range, as seen by CRIRES, showing the CO = 0.30 cm-am detection of CO in Triton’s atmosphere (from 0.55 No CO ­Lellouch et al., 2010). s) 0.5 unit . current pressure is between 6.5 and rb 0.45

24 μbar (i.e. 40 000–150 000 less than on x (a R5 R4 R3 Flu 0.4 Earth), and that the CH4 atmospheric abundance (CH /N mixing ratio) is 0.5%. 4 2 0.35 Encouraged by the instrument’s capa­ 2335 2336 2337 2338 2339 2340 bilities, we decided to conduct a search Wavelength (nanometres) for atmospheric CO on Triton and Pluto.

The best spectral region to look for CO et al. (2011) from James Clerk Maxwell and Triton are dissolved in N2 to form a is the 2.35 μm (2–0) band. In July 2009, Telescope observations, but their results solid solution, one would expect from after four hours of integration with CRIRES are hard to understand and inconsistent simple thermodynamics (Raoult’s law) under excellent sky conditions, we de­­ with the CRIRES ones. that their atmospheric mixing ratio would tected a strong signal of this molecule in be equal to the ratio of their vapour pres­

Triton’s atmosphere (shown in Figure 4), sures to that of N2, multiplied by their indicating a CO/N2 abundance of about Surface–atmosphere interactions ice phase abundance. Because CO and –4 6 × 10 (Lellouch et al., 2010). As a especially CH4 are less volatile than N2, bonus, the spectrum, which has a S/N of Our results on the abundances of CO and this would lead to atmospheric abun­

~ 80, showed many lines from the ν3 + ν4 CH4 in Pluto and Triton’s atmospheres dances much lower than observed (by a band of gaseous methane. This was the are summarised in Table 1, assuming factor of about seven for CO and over first time that methane had been observed current total pressures of 15 and 40 μbar, three orders of magnitude for CH4). at Triton since Voyager, and the data respectively. They are compared to the Therefore, surface–atmosphere interac­ showed that the column of meth­ abundances of these species measured tions or exchanges must proceed in a ane had increased by a factor of about in the surface. Note that the amounts ­different way. Essentially there are two four since 1989. This is in line with the of atmospheric CH4 and CO correspond means to explain the enhanced atmos­ occultation results, and certainly results to a few tens of nanobars only at 60–90 pheric CH4 and CO abundances. from a seasonal effect, with increased K. With hindsight, it is fascinating to think insolation on Triton’s ice-covered South­ that CRIRES is able to measure these The first explanation is that the enhanced ern polar cap. quantities on 0.1-arcsecond-sized objects atmospheric abundances result from

4.5 billion kilometres away! the sublimation of pure patches of CH4 Obtaining a spectrum of similar quality for and CO ices. This is probably what

Pluto is very challenging, as Pluto is not Pluto’s atmosphere is thus much (~ 20 occurs for CH4, whose much lower vola­ only somewhat smaller than Triton, but times) richer in CH4 than Triton’s — this tility compared to N2 is expected to lead, very dark in K-band (geometric explains the warmer stratosphere on over the course of seasonal sublimation– ~ 0.2 vs. 0.6 for Triton, due to strong Pluto; in contrast the two atmospheres condensation cycles, to geographically methane ice absorption; see, for exam­ have similar CO abundances. The same ­segregated (warmer than the N2 ice) ple, Figure 3 in Barucci et al., 2010). This is true for the surface abundances, and CH4 ice patches. Evidence for pure meth­ warranted another dedicated run in July the striking result in Table 1 is that within ane ice, co-existing with diluted methane, 2010, during part of which we experi­ the measurement uncertainties (which is in fact present in the surface spectra mented with the fast (“windowed”) read­ are about 20–40% for CH4, but as much of Pluto (Douté et al., 1999). On Triton the out mode of CRIRES. In this mode, only as a factor 2–3 for CO, due to the diffi­ case for pure methane is not as clear, detectors 2 and 3 of CRIRES are read culty of determining abundances from but the longitudinal distribution of meth­ and windowed, resulting in a noticeable unresolved, saturated lines) for both spe­ ane ice is known to be different from gain in sensitivity for faint targets. Overall, cies, the atmospheric and surface abun­ that of N2. This suggests that, in a similar after 7 hours 20 minutes on source, we dances are the same. way to Pluto, a region, or regions, of obtained a spectrum with a S/N of 15–20. enhanced CH4 possibly controls the CH4 This spectrum clearly showed the spec­ This result looks remarkably simple, yet atmospheric abundance. Calculations tral signatures of methane, and hints was not anticipated at all. Since for the indicate that pure or enhanced CH4 for CO at the position of nine lines. When most part ices on the surface of Pluto patches covering typically a percent of these lines are co-added, evidence for CO is obtained at 6σ confidence ­Lellouch( Pluto Triton Table 1. Comparison et al., 2011), providing a likely detection Atmosphere Surface Atmosphere Surface of surface and atmos­ — but not as obvious as on Triton. For pheric molecular abun­ CH /N 5 × 10–3 4 × 10–3 (a) 2.4 × 10–4 5 × 10–4 completeness, let us add that the detec­ 4 2 dances on Pluto and CO/N 5 × 10–4 1 × 10–4 6 × 10–4 1 × 10–3 tion of CO in Pluto’s atmosphere has 2 ­Triton. been claimed independently by Greaves (a) Pure methane is also present

22 The Messenger 145 – September 2011 the surface are sufficient to maintain the resolution on the discs of Pluto and Triton Solar System (Schaller & Brown, 2007) high methane abundance in both objects. is not possible, yet repeated observa­ and be able to develop atmospheres near tions, taking advantage of their rotation, perihelion. While such atmospheres This “pure ice” scenario is probably would allow one to search for possible have not yet been detected, searches are not valid for CO, which is thermodynami­ longitudinal variability of the CH4 and CO underway, especially from stellar occul­ cally not expected to segregate from atmospheric content. A correlation with tations. Should they be discovered, spec­

N2 and can stay mixed with it in all pro­ the appearance of the surface features troscopy with the Extremely Large Tele­ portions, and an alternative situation is and the longitudinal distribution of ices scope (ELT) could play a pivotal role in likely to occur. For this second explana­ would strongly advocate for the “pure ice” characterising them. tion, it is thought that seasonal cycles scenario. Monitoring the evolution of the form a very thin (a few molecules) surface minor compounds in the atmosphere upper layer enriched in CO (Trafton et al., over the upcoming years will also further References 1998) that inhibits the sublimation of the reveal the of atmosphere–surface underlying main ice layer. Exchanges interactions. In the future, spatially- Barucci, M. A. et al. 2010, The Messenger, 141, 15 between this thin layer (termed “detailed resolved observations with an instrument Buie, M. W. et al. 2010, AJ, 139, 1128 balancing layer”) and the atmosphere such as SIMPLE on the ELT (Origlia & Douté, S. et al. 1999, Icarus, 142, 421 lead to an atmospheric composition Oliva, 2009; Origlia et al., 2010) will permit Greaves, J. S., Helling, C. & Friberg, P. 2011, MNRAS, 414, L36 reflecting that of the volatile reservoir, as direct imaging, constraining, for example, Lellouch, E. et al. 2009, A & A, 495, L17. See also observed. Yet a mystery remains, as it the latitudinal distribution of these atmos­ ESO PR-0809. seems that at least one very bright area pheres. Lellouch, E. et al. 2010, A & A, 512, L8. See also on Pluto’s surface (Buie et al., 2010; and ESO PR-1015. Lellouch, E. et al. 2011, A & A, 511, L4 Figure 2) is associated with a CO ice- Studying Pluto and Triton’s atmospheres Origlia, L. & Oliva, E. 2009, Earth, & , rich region. It is hoped that data from is not only interesting in itself, but it 105, 123 New Horizons in 2015 will provide more opens a window on this class of tenuous Origlia, L., Oliva, E. & Maiolino, R. 2010, The information on this bright spot and its atmospheres dominated by sublimation Messenger, 140, 38 Schaller, E. L. & Brown, M. E. 2007, ApJ, 659, L61 possible connection with CO’s atmos­ equilibrium (of which is another exam­ Trafton, L. M., Matson, D.L. & Stansberry, J. A. 1998. pheric abundance. ple). Other ice-covered trans-Neptunian In Solar System Ices, Kluwer Academic Publish­ objects, especially the largest of them ers, 773 Validating these scenarios will require (Quaoar, Eris, ), may have Young, L. et al. 1997, Icarus, 153, 148 additional observations. Direct spatial retained their over the age of the

The , as observed with the VLT NACO adaptive optics instrument on 8 December 2001 when the planet was at a distance of 1209 million kilometres. The image is a ­composite of exposures in the near-infrared H- and K-bands and dis­ plays well the intricate, banded struc­ ture of the planetary atmosphere and the details of the . The dark spot close to the south pole is approximately 300 km across and is a polar vortex. More details can be found in Release eso0204.

The Messenger 145 – September 2011 23