Astronomical Science

Surface Spectroscopy of , and Triton

Silvia Protopapa1 acterise the ice content of the surfaces the signal of Pluto and Triton in M-band Tom Herbst 2 and to search for similarities and differ- and of Charon in L-band in three nights. Hermann Böhnhardt1 ences.

Observing procedure 1 Max-Planck Institute for Pluto, Charon and Triton with NACO at Research, Lindau, Germany the VLT The Pluto-Charon binary and Triton 2 Max-Planck Institute for , were used in the visible as the reference Heidelberg, Germany Scientific aim sources for the adaptive optics correc- tions. For Pluto-Charon the NACO slit The observations of the Pluto-Charon (width 172 mas) was set along the orbi- We present new reflectance spectra of ­binary and of Triton were obtained with tal position angle of the binary, to acquire Pluto and Triton taken with the ESO the adaptive optics instrument NACO them both simultaneously. For Triton the adaptive optics instrument NACO at the at the ESO VLT during 3–7 August 2005. slit was placed at the parallactic angle. VLT and covering the wavelength range For Pluto-Charon the aim was to resolve The acquisition of the targets was per- 1–5 µm. Apart from known and ex- the binary system and to measure spec- formed in K-band and a suitable offset pected absorption bands from tra of the two objects, for the first time correction, depending on the zenith dis- ice, our data reveal new absorption ­individually. So far, such type of spectra tance, was applied to achieve the opti- bands centred around 4.0 µm and could only be obtained from disentan- mum slit position of the objects in L- and 4.6 µm never detected before. The latter gling unresolved and (Charon M-band (though sacrificing slightly the absorption could be related to the pres- occulted by Pluto) measurements of the JHK signal – at least for higher airmasses ence of CO ice at the body surfaces. system. Moreover, and even more impor- – by slit losses due to wavelength-de- Charon’s spectrum is also measured in tant, we intended to extend the wave- pendent atmospheric refraction). The the wavelength range 1–4 µm, for the length coverage of the surface spectros- usual A-B-B-A nodding (with some jitter) first time simultaneously with, but iso- copy beyond K-band, i.e. we were aim­- was applied for the observations. In order lated from, that of Pluto. The non-detec- ing for Pluto spectra up to 5 µm and for to remove at once the telluric and solar tion of Pluto’s moonlets (unknown at the Charon at least up to 4 µm, with the goal features from Pluto/Charon and Triton’s time of observation) in acquisition im- to detect further surface ice absorption spectra, observations of the nearby solar ages of Pluto-Charon provides a lower bands predicted from models of the avail- analogue star HD 162341, recorded ap- limit of 18.8 mag for the K-band bright- able JHK spectra and to search for sig- proximately once per 1.5 hour, were per- ness of Hydra and Nix. natures of yet unknown . Triton, vis­ formed. In this way every Pluto-Charon ible from the VLT at the same time, was a and Triton observation had an associated welcome object for comparison, since its ‘before’ and ‘after’ set of calibration star Dwarf and New Horizons JHK spectrum is similar to that of Pluto observations, made with similar sky con- (but different from Charon), and at least ditions and identical instrument and AO Things are changing in the outer Solar as well known as the former. Like Pluto, settings. System: Pluto got ‘degraded’ by the the 3–5 µm region of Triton is still unex- IAU from a ‘real’ to a dwarf one. plored for this object, considered to be a Shortly before this terrestrial decision, object captured by . Data reduction NASA launched the New Horizons space- craft to approach Pluto and Charon in Besides the usual data reduction for IR 2015 – and possibly one or two, yet un- Telescope and instrument set-up spectroscopy, special attention was paid detected, Kuiper Belt objects thereafter. to the wavelength calibration and the Also, recently, two new small , NACO at the VLT Unit Telescope 4 spectrum curvature correction of the Nix and Hydra, were discovered around (Yepun) was our first choice for this pro- NACO data. The former applies because Pluto. Despite all these changes, Pluto gramme since it combines the high spa- no arc lamp spectra for L- and M-band and Charon, remain of high scientific tial resolution of adaptive optics, needed are available in NACO; hence, atmos- ­interest, in particular since they can be to resolve Pluto-Charon (variable along pheric emission and absorption features con­sidered – together with Neptune’s the orbit from 0.5–0.9?), and at the same were used as wavelength reference in- Triton – as the best prototypes for time benefits from considerable signal- stead. The latter results from differential the ice worlds in the outer Solar Sys- to-noise improvements for the spectros- atmospheric refraction over the large tem and the Kuiper Belt. Best, because copy. Moreover, NACO allowed to cover wavelength range and was corrected by these three objects are bright and thus the full wavelength range from 1–5 µm pixel shifts of the spectra applying the accessible for -based observations at once with the intended spectral resolu- ­atmospheric refraction formula. Thereaf- not easily possible for other bodies of tion using the prism L27_P1. Neverthe- ter, we used optimum extraction to im- that kind. Here, we present new IR ob- less, the NACO exposure time calculator prove signal-to-noise ratio over aperture servations of Pluto, Charon and Triton indicated that, even with the great advan- extraction. In order to recover from the performed with the VLT in order to char- tages of adaptive optics and low-disper- unavoidable slit losses in the short wave- sion prism, it would be difficult to measure length region, we combined the extracted

58 The Messenger 129 – September 2007 L- and M-band spectra, combined over 1.0 Figure 1: Pluto’s spectrum in the range all nights, with selected object spectra in of wavelengths 1–5 µm. The species responsible for the absorption bands the JHK wavelength region and taken detected in our spectrum are marked 0.8 CH4 at low airmasses. The extracted spectra in the figure. No object flux is meas- ? were normalised to published values for ured in the atmospheric absorption bands at 2.5–2.8 and 4.1–4.4 µm. the at fixed wavelengths and 0.6 thereafter median averaged. This way we ? were able to reconstruct global spectra of 0.4 CH4 CH4 the three targets – flux permitting – over Geometric Albedo 2ν4 ν +ν the full wavelength range from 1–5 µm. 2 4 CH4

0.2 ν3

Pluto and Triton spectra from 1 to 5 µm 0.0 12354 In Figures 1 and 2 we present the reflect- Wavelength (microns) ance spectra of Pluto and Triton, ob- tained with NACO in the 1–5 µm wave- 1.0 Figure 2: Triton’s spectrum in the CH range of wavelengths 1–5 µm. The length range at a S/N in J-K and L- band 4 species responsible for the absorption equal to 50 and 11, respectively. For 0.8 ? bands detected in our spectrum are Pluto, it is the first time that the L-band is marked in the figure. No object flux is measured without contamination by light measured in the atmospheric absorp- tion bands at 2.6–2.8 and 4.1–4.4 µm. from Charon, and for both objects 0.6 CH M-band spectra were never measured 4 ? 2ν CH4 4 before. + 0.4 ν2 ν4 Geometric Albedo


ν3 Known compounds 0.2

The published spectra of the two objects 0.0 have established that there is solid CH4 12354 on Pluto and Triton’s surface (Douté et Wavelength (microns) al. 1999; Quirico et al. 1999; Grundy et al. 2002), easily noticed from various absorp­ tion features in the JHKL bands. This is ­Pluto’s spectrum from 2.8 to 3.1 µm, con- ­ (Olkin et al. 2007), it is not possible confirmed – in particular also for L-band – sidered to be a ­diagnostic for the ratio to reproduce the absorption band ­centred by our new NACO results. Indeed, in the of areas with pure CH4 ice and CH4 ice around 4.0 µm in our NACO data and range of wavelength 1.0–2.5 µm, the most ­diluted in N2 (Olkin et al. 2007), is differ- ­indicated in Figure 1 by a question mark. prominent features evident in Pluto and ent from the one of Triton. Since the This absorption band is also present

Triton’s spec­tra are strong CH4 absorp- spectrum of pure methane has a steeper in Triton’s spectrum (Figure 2). Another tion bands near 1.16, 1.38, 1.66, 1.79, slope in this region, this finding suggests question mark can be found in the figures 2.20, 2.31 and 2.37 µm (Figures 1 and 2). that the percentage of diluted methane is at an absorption band centred around Because of the low spectral resolution higher in Triton than in Pluto. Another dif- 4.6 µm, visible in both Pluto and Triton’s of the NACO prism, it is not possible to ference between Pluto and Triton’s spec- spectra. This signature was found unex- detect in both object spectra the absorp- tra is the albedo around the ν3 band of pectedly. A first interpretation ­assigns it to tion bands of N2 at 2.148 µm and CO CH4 from 3.1 to 3.6 µm. As observed by CO ice. In fact, CO ice has a strong ab- at 2.35 µm already found by Owen et al. Olkin et al. (2007), this constrains the sorption band at 4.67 µm ­(Palumbo and

(1993) and Cruikshank et al. (1993) for fraction of pure N2 on the surface. Hence, Strazzulla 1993). Pluto and Triton, respectively. The low concluding from our NACO spectra, the resolution in JHK bands also does not presence of pure N2 is greater in Triton allow us to detect CO2 in Triton’s spec- than in Pluto. Charon’s spectrum from 1 to 4 µm trum. Our NACO spectrum of Charon in the In the range of wavelengths 2.8–4.1 µm, Unknown features wavelength range between 1 and 4 µm is our spectra of Pluto and Triton show shown in Figure 3. Charon’s spectrum strong absorptions near 3.3, 3.5 and By modelling Pluto’s surface spectrum by has previously been studied in some de-

3.8 µm, corresponding to methane’s ν3, geographical mixtures of pure methane, tail in the JHK wavelength region (Buie ν2 + ν4 and 2 ν4 vibrational transitions methane diluted in and pure and Grundy 2000), but was never meas- (Grundy et al. 2002; Olkin et al. 2007). It ­nitrogen (Douté et al. 1999), or pure meth- ured above 2.5 µm. is important to note that the slope of ane, methane diluted in nitrogen and

The Messenger 129 – September 2007 59 Astronomical Science Protopapa S. et al., Surface Ice Spectroscopy of Pluto, Charon and Triton

As expected, our Charon spectrum is 0.5 Figure 3: Charon’s spectrum in the dominated by the 1.5 and 2.0 µm absorp- range of wavelengths 1–4 µm. The species responsible for the absorption tion bands of ice. The spectrum 0.4 bands detected in our NACO spec- shows the 1.65 µm spectral feature char- trum are marked in the figure. No ob- H 0 acteristic of crystalline water ice, for the 2 ject flux is measured in the atmos- pheric band between 2.5 and 2.8 µm. first time identified by Brown and Calvin 0.3 H 0 ? (2000). Because of the low prism resolu- 2 tion, it is not possible to detect the ab- H20 0.2

sorption band at 2.21 µm related to the Geometric Albedo presence of hydrate NH3H2O, that should exist uniformly distributed on 0.1 Charon’s surface.

0.0 A narrow absorption band is found 1 234 around 3.7 µm, indicated by a question Wavelength (microns) mark in Figure 3. It cannot be repro- duced by models of Charon’s spectrum with pure H2O ice darkened by a spec- trally neutral continuum absorber (Olkin et al. 2007). Hence, it remains unidenti- fied for the time being.

A brief search for Pluto’s moonlets

At the time of our NACO observations, Pluto’s satellites, Hydra and Nix, had yet to be discovered (Weaver et al. 2006). Although the direct images of the Pluto- Charon system, taken with NACO for the slit acquisition of the binary, were never meant to be searched for the very faint new moonlets, we made an attempt to detect them in our few short (3 sec) ex- posures (see Figure 4). Unfortunately, we Hydra didn’t find Pluto’s new moons in our frames. Given the limiting magnitude of Nix about 18.8 mag determined for our ac­ quisition images, this detection is not sur- prising since both objects should have K-band magnitudes well beyond 21 mag. Longer integrations would have certainly displayed the moons in the NACO im- ages, but would have resulted in a reduc- tion of exposure time for our prime – and only – programme at that time, the spectral analysis of the surface ices on Pluto, Charon, and Triton. References Figure 4: Median average of the slit acquisition ­images of Pluto and Charon in the night 4 August Brown M. E. and Calvin W. M. 2000, Science 287, 2005. The expected positions of the moons Hydra 107 and Nix are indicated by crosses. Buie M. W. and Grundy W. M. 2000, Icarus 148, 324 Cruikshank D. P. et al. 1993, Science 261, 742 Douté S. et al. 1999, Icarus 142, 421 Grundy W. M., Buie M. W. and Spencer J. R. 2002, AJ 124, 2273 Olkin C. B. et al. 2007, AJ 133, 420 Owen T. C. et al. 1993, Science 261, 745 Palumbo M. E. and Strazzulla G. 1993, A&A 269, 568 Quirico E. et al. 1999, Icarus 139, 159 Weaver H. A. et al. 2006, 439, 943

60 The Messenger 129 – September 2007 (OSF) in July 2007. July in (OSF) Facility Support tions Opera- the at tenna an ALMA Japanese first the of Arrival

- Astronomical News Photo: F. Mac Auliffe, ESO