Laboratory data on ices, minerals and organics for TNOs and Centaurs: what is missing ? C. de Bergh 1, B. Schmitt 2, D.P. Cruikshank 3, L. Moroz 4, E. Quirico 2 1 LESIA, Observ. Paris, France, 2 Laboratoire Planétologie Grenoble, France, 3 NASA/Ames, Moffet Field, Calif., USA, 4 Inst. For Planetary Res., DLR, Berlin and Inst. Planetology, Univ. Münster, Germany

What do we know about the surface composition of TNOs and Centaurs ?  From albedo measurements (available for only about 15 objects-mostly from Spitzer measurements): Generally low albedos, therefore the surfaces cannot be predominantly covered with pure ice(s). Some dark material must also be present.  From spectrophotometric measurements (available for about 130 objects): the colors are extremely diverse, from slightly blue to very red. Some TNOs are the reddest objects in the solar system. Different spectral shapes. Very red colors, probably due to organics. Color diversity very surprising, difficult to explain.  From spectroscopy (available for only about 30 objects in the visible, 20 in the near infrared, and a few objects in the thermal infrared with Spitzer): The visible spectra are generally featureless. Some near-infrared spectra are also featureless. Other near infrared spectra show clear absorption features. Some features are seen in thermal infrared spectra.

Summary of the species detected  Water ice on many objects  Methane ice on Pluto as well as on two (and maybe three) other large TNOs (near-IR and visible spectra)  CO ice on Pluto  Methanol ice (or a similar molecule) on Centaur Pholus and one TNO  N 2 ice on Pluto, and, maybe, TNO Sedna  maybe ammonia/ammonia hydrate on Charon and TNO Quaoar (a comparable absorption is seen in spectra of Miranda and not yet firmly identified)  maybe olivine (or another silicate) on Centaur Pholus (near-IR) and some fine-grained silicates on Centaur Asbolus (thermal-IR spectra)  maybe hydrated silicates on three TNOs (visible and near-IR spectra). New observations are needed.  In addition, there is an unidentifed continuum absorption beyond 2.2 micron for a number of objects

Presence of complex organics and other carbonaceous species ?  There is no detection of complex organics at the surface of these objects, but they are the best plausible compounds (based on what we know about other solar system objects - some asteroids, meteorites and IDPs, and based on irradiation or impact laboratory studies) to explain the very red color of some TNOs and Centaurs.  The spectral behavior of some complex organics allows to reproduce well the shape of the spectra. But many others could probably fit just as well.  Furthermore, some of them can account for the unidentifed continuum absorption beyond 2.2 micron that is observed for a number of objects.  Some are used to account for the very low measured albedos.

What laboratory data are needed for surface studies ? (I) Spectra : They are essential for identification, of course. These can be made in transmission or diffuse reflection For ices, they can be made with thin films (condensation on a cold window) or closed cryogenic cells (allow to study thick samples of good quality; reproducible but more difficult to make). Difficulties for ices: thin film spectra depend on the crystal quality, on the temperature of deposition, on the rate of deposition We need spectra at appropriate temperatures (T: from 20 to 65 K ?; lower than for outer planets’ satellites, higher than for the ISM), and different temperatures if we want to use them as thermometers In some cases, we need spectra of mixtures (ex: CH 4 ice diluted in N 2 ice). Spectra are sufficient for the study of spatial mixtures (but fixed grain size).

What laboratory data are needed for surface studies ? (II) Optical constants (real and imaginary parts of the complex refractive index m = n + ik) are needed to produce spectra for different grain sizes, to deal with intimate or molecular mixtures, to quantify the percentages of the various compounds present in the mixtures. These are difficult to obtain in the laboratory. The determination of accurate optical constants over a wide spectral range requires spectra for widely different thicknesses to deal with both weak and strong absorptions (and samples with high optical quality for the ices). There are different approaches to compute n and k from spectra (Kramers- Krönig, for example). Very few optical constants exist. In what follows, we consider essentially data up to about 5 micron (range covered by most existing spectra of TNOs and Centaurs) and mostly unirradiated species.

Water ice (I) Water ice has been detected as the surface of many Centaurs and TNOs. It is present in the crystalline state (an absorption characteristic of low T crystalline ice is detected at 1.65 micron) on some of the objects: TNOs Quaoar and 2003 EL 61 and its brighter satellite, and maybe also on TNO Orcus. It could be amorphous on a few objects (Centaur Chariklo, TNO 1996 TO 66 ) but the data are of insufficient quality to be sure. For other objects, we don’t know (too low S/ N). Crystalline ice versus amorphous ice at 50 K. Ice deposited at 50K (amorphous), heated to 160 K and then cooled to 50 K. (Mastrapa et al. Icarus, 183, 207, 2006).

Water ice (II) Are existing laboratory data adequate ? For the near-IR part, good optical constants for crystalline water ice exist for temperatures between 270 and 20 K (most complete set of data from Grundy and Schmitt (1998) and covering the 1 to 2.7 micron range). When going to lower T, the data at short wavelengths become more uncertain. Spectra of crystalline water ice for different temperatures (Grundy and Schmitt, JGR, 103, 25809,1998)

Water ice (III) Lack of data on amorphous water ice (very difficult to obtain; cannot be made in closed cells). We need also more systematic studies on irradiated water ice at low T (see next talk by R. Mastrapa ?). This is very important for a full interpretation of TNOs data. In addition, we need more data on mixtures of water ice with other species (some work has been done on mixtures of H 2 O with components such as CH 4, CO 2, HCN and NH 3 ; see next talk by R. Mastrapa).

Methane ice (I) It has recently been detected on the two large TNOs: 2005 FY 9 and 2003 UB 313 and it may be present on Sedna. The methane bands in spectra of 2005 FY 9 and 2003 UB 313 are stronger than for Pluto. On Pluto, methane occurs as pure ice or diluted in nitrogen. On TNO 2003 UB 313, from the positions of the bands (shift in wavenumber between laboratory spectra of pure ice and of diluted ice), it appears to be present predominantly as pure methane ice (and no nitrogen ice is firmly detected, but a better fit with SINFONI spectra is obtained when nitrogen ice is added to the models; see poster by Dumas et al.). On 2005 FY 9, we don’t know (but no nitrogen ice is detected either). Are existing laboratory data adequate ? Good optical constants exist for pure methane ice in the range: microns for temperatures between 20 and 50 K (optical path-lengths up to 1 cm). (Grundy et al., Icarus, 155, 486, 2002). Some optical constants exist also for CH 4 diluted in N 2 (Quirico and Schmitt, Icarus, 127, 354, 1997).

Methane ice (II) But, even for Pluto, the very weak absorptions of methane ice are not properly taken into account. Spectrum of Pluto with model (Douté et al., Icarus, 142, 421, 1999) As the methane bands are even stronger in spectra of at least two other TNOs, more laboratory studies on pure methane ice using closed cells that allow to form thicker samples (more than 1 cm thick) than has been done so far are in order. But this is very difficult to achieve. Need for more spectra of N 2 :CH 4 mixtures: we need spectra with higher concentrations of methane than has been obtained so far. Also very difficult. PLUTO

Nitrogen ice Detected on Pluto (predominantly in the beta phase). May be present on Sedna. PLUTO (Douté et al. 1999) Existing laboratory data ? Good optical constants exist for pure nitrogen ice around 2.15 microns (2-0 band), both in the beta and alpha states (transition temperature: 35.6 K at zero pressure). They were obtained from transmission spectra with 1-cm thick samples (Grundy et al., Icarus, 105, 254, 1993). Both phases may be present on TNOs (but very narrow band for alpha-nitrogen).

Carbon monoxide ice Detected on Pluto at 2.3 micron (high spectral resolution is required to detect this narrow absorption). We do not know for sure if CO exists as pure ice or diluted in N 2 (there may be a predominantly CO spot - HST spectral images). Spectrum of Pluto with model ( Douté et al., Icarus, 1999). Are existing laboratory data adequate ? Optical constants exist for pure carbon monoxide ice. Some work has been done on CO ice diluted in nitrogen (alpha and beta phases) (Quirico and Schmitt, Icarus, 128, 181, 1997). Will be very important for New Horizons. PLUTO

Methanol (?) on Centaur Pholus and TNO UKIRT spectrum of Pholus (Cruikshank et al. 1998). VLT spectrum of (Barucci et al. 2006). The 2.27 micron band in Pholus spectrum had been attributed to frozen methanol or a photolytic product of methanol with small molecular weight (agreement with the data not perfect, however). It seems to be present also on TNO Are existing laboratory data adequate ? The spectra of methanol from different laboratories differ significantly. Need for more data on methanol, and particularly more optical constants (existing data obtained from a single transmission spectrum at 90 K; Cruikshank et al. 1998).

Ammonia and/or ammonia hydrate on Charon and Quaoar ? An absorption feature present at 2.2 micron in spectra of Charon (also detected in spectra of Miranda) and observed by several groups has been tentatively assigned (Brown and Calvin 2000) to a combination of ammonia and ammonia hydrate. A comparable feature may be present in spectra of TNO Quaoar (Jewitt and Luu, 2004, Subaru telescope). But higher quality data are required to be sure. Charon Quaoar

Ammonia and/or ammonia hydrate (II) ? Data on pure ammonia are incomplete. Some problem with slopes in the visible. For ammonia hydrates, spectra recorded by different teams differ. The optical constants used so far for ammonia hydrates are very limited. They have been determined only from spectra with 1 and 3 per cent of ammonia and at a temperature of 77 K (data from Clark; optical constants from Roush). Model spectra of pure water ice at 70 K and NH 3.H 2 O ice (1%) at 77 K (Verbiscer et al. 2006). Grain sizes (from top to bottom): 50, 100, 250, 500, and 1000 microns. There is a need for more laboratory data on ammonia hydrates. Difficult…

Minerals An absorption around 1 micron in spectra of Centaur Pholus has been attributed to olivine. Pholus spectrum and model from Cruikshank et al. (1998) Spitzer spectra (thermal IR) also show the presence of fine-grained silicates on Centaur Asbolus. Irradiation of silicates can redden spectra. Need for more laboratory data on: crystalline silicates at low T, amorphous olivines and pyroxenes, Fe-free silicates, fine-grained silicates,... PHOLUS

The organic compounds that have been introduced in models so far are:  Titan and Triton tholins (red slopes)  Ice tholins and kerogen-type organics (red slopes and some flattening in IR)  Some type of amorphous carbon (spectrally neutral, low albedo)  HCN polymers (to account for absorption beyond 2.2 micron) Complex organics and other carbonaceous compounds that have been included in models

Why these choices ?  Mostly because they are compounds for which we have optical constants.... We lack strong constraints. Slopes are not sufficient (higher quality spectra of TNOs/Centaurs are needed to search for possible near-IR absorptions). More about tholins: Different types of tholins have been studied. They were obtained by irradiating either gaseous mixtures (Titan tholins: 90% N 2 -10%CH 4 ; Triton tholins: 99.9%N %CH 4 ), or ice mixtures (Ice tholins I: 86%H 2 O-14%C 2 H 6 ; Ice tholins II: 80%H 2 O-16%CH 3 OH-3.2%CO %C 2 H 6 ). Titan tholins: spectra obtained in different laboratories differ (depend on pressure for gaseous tholins, Imanaka et al. 2004). Very few tholins have had their optical constants measured. It is very difficult to derive optical constants for spectra with important slopes in the visible.

More about kerogens: Kerogen (essential constituent of sedimentary rock on Earth) is a family of highly disordered macromolecular organic materials made of C,H,O, and traces of N and S. The dominant constituent of organic matter in carbonaceous chondrites is similar to kerogen. There are different types of kerogens. Even for kerogen of a given type, the spectra are different from each other (depends on, e.g., maturity). In only one case (Khare et al. 1991) have optical constants been measured (type II kerogen: of marine origin). Spectra derived from optical constants of type II kerogen (Khare et al. 1991) Spectrum from Clark et al. (1993)

Other complex organics and carbonaceous compounds that have been considered Natural complex hydrocarbon material like solid bitumens (asphaltite and kerite) (red slopes and low albedo). But non-irradiated asphaltite is too red. Irradiated bitumens (more neutral slopes and low albedo) Irradiation products of simple hydrocarbons ices (methane, methanol, benzene) (see, e.g., Brunetto et al.)

Conclusion As higher quality spectra are becoming available for TNOs, and with the prospect of the New Horizons mission, it is very important to do more laboratory studies in the near infrared and visible ranges on already identified species as well as on other plausible constituents (such as ices of CO 2, H 2 CO, non-methane hydrocarbons, some nitriles,.. many other silicates and many other natural and synthetic carbonaceous compounds). In particular, we need data on: Ices and silicates at low (but not too low) temperatures Ice mixtures, and also mixtures of ices, minerals and organics Ices with thicker samples (as for CH 4 ) More measurements of optical constants are badly needed. (all this will be presented in a chapter in preparation for the TNOs book) And more irradiation experiments must be carried out.