arXiv:1409.1261v1 [astro-ph.EP] 3 Sep 2014 rmohr Jri ta.20;Vlse l 06.Sc a iron not Such oxidized 2006). but from al. result data, may et present, Vilas some actually 2000; if from al. feature, 0.7 et reported a (Jarvis been others of from has near Detection band the 1 2004). tion to around Holman visible distinct center & Grav a the a show and with from family Himalia its infrared feature of of of absorption members part spectra broad other The be the to of family. thought most collisional Jovian all of single are group a prograde which the satellites, of irregular member largest the and from and far is satellites connection irregular genetic clear. implication the the the though between with asteroids, origin these objects, common these P/D-type a Holman or asteroids, referred of & be C-type to Grav to as analogy satellites 2003; to these By to lead al. 2006). near-infrared often et al. characteristics the Grav et in Vilas 2001; slopes 2004; in red al. slopes et slightly red (Rettig slightly and to through optical flat Visible the with gener- spectra show featureless sparse. spectroscopy ally is and we photometry compositions information ori- near-infrared the their the Unfortunately, understand on to bodies. have attempt these to of used gins been often have 2007, final al. vio- (Nesvorn´y the which et system 2014). instability solar during dynamical the system were rearranged planet lently giant solar satellites a outer irregular of stages the the or from that 1979) captured recent suggested al. more have et while (Pollack 1971), ideas Franklin drag & gas (Colombo work collisions by previously Early from orbits captured unclear. heliocentric were remains objects the – that orbits body suggested inclined parent and their eccentric, distant, about in satellites small – [email protected] [email protected] iai stelreto h oinirglrsatellites irregular Jovian the of largest the is Himalia satellites irregular the of compositions surface The planets giant the of satellites irregular the of origin The rpittpstuigL using 2018 typeset 8, Preprint October version Draft cl3 ical oeso h rgno h reua satellites. irregular the surpr is s of asteroids origin which le of the asteroids between group of this distant belt and models Europ more Himalia asteroid 52 between and outer match alteration Europa. spectral the 52 aqueous in asteroid for 0.7 situated type evidence a generally C/CF of asteroids the detection to of tentative similar the strikingly from trum suggested been had epeetamdu eouinsetu fJptrsirglrsate irregular Jupiter’s of spectrum resolution medium a present We µ iiino elgcladPaeaySine,California Sciences, Planetary and Geological of Division pcrlrgo.Tesetu hw oeiec o aqueously for evidence no shows spectrum The region. spectral m H 3 THE 1. A INTRODUCTION T E tl mltajv 5/2/11 v. emulateapj style X µ PCRMO UIE’ REUA AELT HIMALIA SATELLITE IRREGULAR JUPITER’S OF SPECTRUM M on okn nvriyApidPyisLbrtr,Laur Laboratory, Physics Applied University Hopkins Johns µ Ga ta.2003; al. et (Grav m µ absorp- m rf eso coe ,2018 8, October version Draft ..Rhoden A.R. ABSTRACT ..Brown M.E. eotie dnia pcrlo 13 2 star G2V a 91-3, minutes. G 150 of 36 of and spectral time with night integration identical first total obtained the coadds, a We spectra 150 for For such with second, 24 the emis- second obtained minutes. 1 thermal we 45 were higher and times of with each integration wavelengths, time spectra sion, longer integration such the 12 total shorter obtained the a seconds the we for For 7.5 and night were coadds, slit. 30 times wide with integration arcsecond setting 0.57 posi- wavelength two the between target on the tions nodded we setting, spectral iai pcr eeotie tarassbten1.0 between airmasses at obtained were spectra Himalia iie pcrlrne ot oe h ul2.2-3.8 - 2.25 full covering the one cover settings, a 3.10 spectral covers to two configuration so requires al. current region range, et its (McLean spectral in limited NIRSPEC resolution NIRSPEC moderate spectrograph 1998). facility infrared the near using 2013 November Himalia of body satellites. the parent irregular the the discuss of for we and origin Finally the for Himalia. implication of possible the composition discuss and 3 surface feature absorption at unambiguous the absorption model shows and an spectrum of The evidence Observatory. 2-4 Keck from Himalia of spectrum interpretation noise clear an 2004). prohibits of Brown & encounter resolution possibility (Chamberlain the distant spectral shows a 3 low spacecraft at from Cassini absorption spec- obtained the A Himalia of of phyllosilicates. trum in absorption 0.7 hydroxyl-group 3 clear they sharp that a have asteroids showed all which that found observed (2012) Emery & Takir aqueous bodies. by these caused on potentially processing minerals phyllosilicate in eosre iai ntengt f2 n 28 and 26 of nights the on Himalia observed We signal-to- moderate resolution high a present we Here belt outer the in asteroids dark of study a In nttt fTcnlg,Psdn,C 91125 CA Pasadena, Technology, of Institute µ V µ n h te oeig29 3.81 - 2.96 covering other the and m bopin u nta hw spec- a shows instead but absorption, m 7 = . itnea32dgesfo iai.All Himalia. from degrees 3.2 a distance a 4 sn n icl orcniewith reconcile to difficult and ising o vdnefrwtrie The ice. water for evidence how sdsatatriswihshow which asteroids distant ss l D20723 MD el, lt iai oeigtecrit- the covering Himalia llite µ µ 2. ,bttelwsga-onieand signal-to-noise low the but m, bopinfauecnitn with consistent feature absorption m stepooyeo class a of prototype the is a OBSERVATIONS lee hloiiae,as phyllosilicates, altered µ µ .Blww identify we Below m. µ bopinbands absorption m bandfo the from obtained m µ .I each In m. µ m 2 and 1.1, and the spectra of the calibrator star were ob- tained within 0.1 of the same airmass, and immediately 0.12 preceding or following the Himalia observation. 0.11 Himalia To obtain calibrated data from the raw spectra, we frost first subtract each temporally adjacent pair of images in 0.10 which the spectra are in two separate spatial positions. 0.09 This subtraction removes the bulk of the telescope and sky emission, leaving a positive and negative spectrum of 0.08 the source along with any small sky or telescope changes reflectance 0.07 that occurred between the two spectra. A point-source cronstedtite spectrum projects as a curve onto the detector, and sky 0.06 emission lines which fill the slit have variable curvature 2.5 3.0 3.5 4.0 as a function of wavelength. We use measurements of wavelength (µm) the positions of the bright calibrator star in the two slit positions and measurements of the positions of sky lines Fig. 1.— The spectrum of Himalia from 2.2 to 3.8 µm. The in the longer target exposures to create a template allow- shorter and longer wavelength NIRSPEC settings are shown as ing us to map the non-rectilinear geometry of the data black and red, respectively. The spectrum is compared to a CM onto a new rectilinear grid with the spatial dimension chondrite with a spectrum consistent with cronstedtite (blue) – a along one axis and the wavelength along the other. We commonly occurring meteoritic phyllosilicate – and to a model of fine grained water ice frost overlying dark grains (green). Neither then use this template to remap the spectral pairs onto material provides a satisfactory match to the spectrum of Himalia. a rectilinear grid. We find the spatial profile of the spec- trum by taking a median along the spectral dimension. The spectrum of Himalia shows a clear sign of absorp- Typical full-width at half-maximum (FWHM) in the spa- tion in the 3µm region (Fig. 1). Because of the re- tial dimension is 0.9 arcseconds. To obtain the flux at port of the 0.7 µm absorption feature and its associated each wavelength we fit the expected spatial profile plus with hydrated silicates, we first attempt to model the a linear offset to 50 pixels on either side of the maxi- 3 µm spectrum of Himalia with typical hydrated sili- mum of the spatial profile. Bad pixels are flagged and cate spectra. These spectra have peak absorption in the ignored in the fit. This procedure removes any residual unobserved 2.5 to 2.8 µm region and their reflectivities sky lines and also corrects for most of the thermal drift sharply rise longward of 2.8 µm (see the Takir & Emery of the telescope which can be important beyond about (2012) ”sharp” spectral group). In Figure 1 we compare 3.5 µm. Examining the final spectral images individu- Himalia to a spectrum of a CM chondrite measured un- ally, we found, however, that for about 20% of the longer der dry conditions (Takir et al. 2013) whose spectrum is wavelength spectra the pair subtracted poorly, leaving consistent with cronstedtite – a phyllosilicate which is a large residuals in the spatial dimension, which are most major constituent of CM chondrites – scaled to match likely caused by thermal changes in the telescope. We the continuum level at wavelengths shortward of 2.5 µm. discard those data affected by this problem, leaving a fi- While the match beyond 3.0 µm is adequate, shortward nal total integration time for the longer wavelength data of 3.0 µm the spectrum of Himalia turns upward rather of 122.5 minutes. The individual spectra are summed than continuing downward like a typical phyllosilicate. with 5σ and greater outliers from the mean at each wave- The spectrum of Himalia fits neither this particular min- length removed. An identical procedure is performed for eral nor the general characteristics of the Takir & Emery the stellar calibration spectra, and the target spectrum (2012) ”sharp” class of dark asteroids with 0.7 µm ab- is divided by the stellar spectrum to obtain a relatively sorptions and sharply rising spectra from 2.8 µm. calibrated spectrum. The shorter and longer wavelength The absorption centered at approximately 3.0 µm in spectra are scaled such that the median of the data in the spectrum of Himalia appears more similar to water the overlap region is identical. Wavelength calibration ice than to OH absorption. To examine this possibility is obtained by matching the spectrum of the calibrator we construct a Shkuratov model (Shkuratov et al. 1999) to a theoretical transmission spectrum of the terrestrial of dark grains covered with fine grained ice, following the atmosphere. prescription of Rivkin & Emery (2010). In our model the The original data has a spectral resolution of approx- dark grains have optical constants chosen to match the imately R/∆R∼2000. To increase the signal-to-noise overall albedo level of the continuum and the ice is a per resolution element, we convolve the spectrum with fine-grained frost with a volume fraction of 1.5% (chosen a gaussian filter with a FWHM of 8 pixels, for a fi- to match the depth of the absorption) coating the dark nal 2-pixel resolution of ∼500. To achieve the strongest grains. Water ice optical constants are obtained from possible constraint on any 3 µm absorption, we sepa- Mastrapa et al. (2009). This model provides an adequate rately calculate reflectances in windows centered at 2.828 match to the 2.8 to 3.1 µm range of the data, but Himalia and 2.857 µm, which present small transparent regions shows significantly more absorption out to 3.6 µm which through the atmosphere surrounded by strong telluric cannot be accounted for by water ice (Figure 1). absorption. The final calibrated spectrum is shown in A search of all available libraries of reflectance spectra Figure 1. While the spectrum is only calibrated in terms reveals no materials which can match the spectrum of of relative reflectance, we estimate the reflectance by as- Himalia. A closer match can be obtained, however, by suming a value of for the K-band albedo of 0.08 after comparing the spectrum of Himalia with those of dark Chamberlain & Brown (2004). main belt asteroids. The spectrum of asteroid 24 Themis, 3. for example, similarly contains a water ice-like feature RESULTS at ∼3.1 µm followed by continued absorption out to 3.6 3

semimajor axes inward of 3.1 AU had evidence for OH 0.12 bearing phyllosilicates suggestive of processing by liquid 0.11 Himalia water, while the majority of more distant asteroids had rounded 3 µm spectra which appear to be evidence for 0.10 the continued presence of water ice rather than hydrated silicates. The surface composition of the asteroids with 0.09 Europa-like spectra is unclear, though their semimajor 0.08 axes between the regions of the asteroid belt containing reflectance the other two types of spectra suggests that they might 0.07 52 Europa be some sort of transition objects. 0.06 In this context, the similarity of the spectrum of Hi- 2.5 3.0 3.5 4.0 malia with those of the Europa-like group is surprising. wavelength (µm) If Himalia is genetically related to the dark asteroids it might be expected to be like the most distant of those asteroids, which tend to have rounded, ice-like spectra. Fig. 2.— The spectrum of Himalia compared to the spectrum of the 52 Europa (blue), the prototype of the Takir & Emery (2012) Alternatively, if the Jovian irregular satellites are instead ”Europa-like” spectrum. The match is excellent, as is the match derived from the same source as the Kuiper belt objects, to the shorter wavelength portions of the spectrum. The surface the surface spectral similarity with objects in the mid- composition of the Europa-like group is unknown, but the other section of the asteroid belt points to either compositional members are all situated in the middle of the asteroid belt. or surface similarities between these populations or both. Further insight into the origin of this spectral similarity µm (Campins et al. 2010; Rivkin & Emery 2010), but a will come from the discovery of additional objects with close comparison of the two spectra shows that the peak Europa-like spectra in the asteroid belt. With only 3 ob- of absorption in the spectrum of Himalia is shifted by jects currently known the true extent of their distribu- ∼0.05 µm compared to that of 24 Themis. An excel- tion is unclear. Additional insight could also come from lent match to the spectrum of Himalia comes, however, similar observations of Jupiter trojan asteroids. In the from the spectrum of the CF type asteroid 52 Europa context of the Nice model, these objects are also thought obtained by Takir & Emery (2012). In Figure 2 we com- to derive from the Kuiper belt source region, and they pare the spectrum of Himalia to a model where we have are currently situated at the same semi-major axis and taken the spectrum of 52 Europa and allowed the spec- thus have the same thermal and irradiation environment tral slope and depth of absorption to be free parameters as the Jovian irregular satellites. An extensive visible in a fit to the data. The best fit gives a spectrum with a to near-infrared survey of these objects found none with ∼ 2.4 to 3.7 µm slope 23% redder than that of 52 Europa the broad 1 µm absorption seen on Himalia, so it is and an absorption 70% deeper. The resulting spectrum clear that these objects are not spectrally identical to Hi- is nearly indistinguishable from that of Himalia. More- malia. Nonetheless, examining their spectral morphology over, the overall shape of the visible through and 2 µm at 3 µm should provide important insight into the genetic spectrum with a broad absorption centered at ∼1.2 µm links and the surface processing of the varied populations appears very similar to the available broadband photom- of the middle part of the solar system. etry of Himalia (Grav et al. 2003; Grav & Holman 2004). However, the spectra of the Europa-like group of aster- We thank Andy Rivkin and Josh Emery for illuminat- oids lack any 0.7 µm absorption feature. Based on the ing conversations about the nature of 3 µm absorptions otherwise strong match between Himalia and 52 Europa in dark asteroids and for providing the spectra of Themis and the strong correlation between 0.7 µm absorption and Europa. Driss Takir made excellent suggestions and the appearance of the sharp OH-like absorption at which improved the paper. This research was supported not 3 µm seen on Himalia, we suspect that no actual by Grant NNX09AB49G from the NASA Planetary As- 0.7 µm absorption on Himalia is present. This suspi- tronomy Program. A. Rhoden was partially supported cion requires observational confirmation. We conclude through an appointment to the NASA Postdoctoral Pro- that had Himalia been a dark asteroid in the survey of gram at the NASA Goddard Space Flight Center, admin- Takir & Emery (2012) it would have clearly fit into the istered by Oak Ridge Associated Universities. The data group of Europa-type spectra that includes 52 Europa, presented herein were obtained at the W.M. Keck Ob- 31 Euphrosyne, and 451 Patientia. servatory, which is operated as a scientific partnership 4. among the California Institute of Technology, the Uni- DISCUSSION versity of California and the National Aeronautics and The Jupiter irregular satellite Himalia has a 0.5 to 3.8 Space Administration. The Observatory was made pos- µm spectrum which fits into one of the four spectral cat- sible by the generous financial support of the W.M. Keck egories identified by Takir & Emery (2012) for dark as- Foundation. The authors wish to recognize and acknowl- teroids in the outer asteroid belt. Himalia appears to edge the very significant cultural role and reverence that have a ”52 Europa-like” spectrum. The 3 known aster- the summit of Mauna Kea has always had within the in- oids with Europa-like spectra are all in the middle part digenous Hawaiian community. We are most fortunate to of the asteroid belt, with semimajor axes between 3.1 have the opportunity to conduct observations from this and 3.2 AU. The majority of the asteroids surveyed with mountain.

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