Elastic Constants of Artificial and Natural Ice Samples by Brillouin Spectroscopy

Abstract

ABSTRACT. The method of Brillouin spectroscopy has been used to measure the dynamic elastic moduli of local homogeneous regions in ice samples representing four different environments of formation . These included artificial ice frozen from distilled water, clear monocrystalline glacial ice, bubbly lake ice, and sea ice. The samples studied were found to have identical local elastic properties. Accordingly the elastic properties of homogeneo us monocrystalline ice have been found not to vary with sa mple age, with impurities present at the time of freezing , or with crystal quality. The bulk elastic properties of ice remain , of course, subject to modification by different crystal grain textures and the presence of inclusions of various sorts. Because the elastic constants obtained in the present work are subject to smaller overall uncertainty than values meas ured previously, it is believed that they are the most reliable obtained to date. The values at -16°C were determined to be c II = 139.29 ± 0.41 , Cl2 = 70.82 ± 0.39, Cl3 = 57.65 ± 0.23 , C33 = 150.10 ± 0.46, C44 = 30.14 ± 0. 11 (units of 10 8 N m -2 or kbar). A full range of derived elastic parameters for monocrystalline ice and for homogeneous isotropic polycrystalline ice has been calculated. RESUME . Conslanles idasliques d'echanlillons de glace arlificielle el nalurelle par speclrographie deBril/ouin. On a utilise la methode de la spectrographie de Brillouin pour mesurer les modules dynamiques elastiques de region s localement homogenes dans des echantillons de glace representant quatre diffe re ntes circonstances de formation. [\ s'agissa it d'une glace artificielle obtenue a partir d'eau distillee, d' une glace c laire monocri stalline, d' une glace bulleuse de lac et d' une glace de mer. On a trouve que les ec hantillons etudies avaient des proprietes elastiques locales identiques. Logiquement, les proprietes elastiques d ' une glace homogene mo nocri stalline ont ete trouvees independantes de I'age de I'echantillon, des impuretes pfl!Sentes au moment de la congelation ou de la qualite du crista!' Les proprietes elastiques globales de la glace demeurent, naturellement, s ujettes a modificatio ns selon les differentes textures cri stallines des grain s et la presence d'in c lu sion de differentes sortes. Comme les constantes eIastiques obtenues dan s la presente etude son t affectees d ' un e incertitude globa le inferieure aux va leurs precedemment mesurees on pense qu'elles sont les plu s dignes de con fiance obtenues jusqu 'ici. Les valcurs obtenues a -16°C ont ete: cll = 139, 29 ± 0 ,41 , CI2 =70,82± 0,39, cl3 =S 7.6S ± 0,23, c33= ISO, 10 ± 0,46 , c44 = 30,14 ± 0, 11 (unites: 10 8 Nm -2 ou kbars). Toute une serie de para metres eIas tiques derives ont ete calcules pour de la glace monocristalline et pour une glace polycristalline homogene et isotrope. ZUSAMMENFASSUNG. Elasliche Konslanlen von kiinsllichen und Ilalurlichen E isp roben, gel\lonnen mil Brillollin-speklroskopie.Das Verfahren der Brillouin-Spektroskopie wurde zur Mess ung der dyn...

Deriving micro- to macro-scale seismic velocities from ice-core <i>c</i> axis orientations

“…The resulting elasticity tensor in Voigt notation C ij (i, j = 1, 2, 3, 4, 5, 6) is a symmetric second-order tensor. In the case of monocrystalline ice I h , the components of the elasticity tensor were measured in the laboratory by means of Brillouin spectroscopy (Gammon et al, 1983). There are five independent components due to the hexagonal crystal symmetry.…”

“…Several sets of values for the elastic moduli have been found by different authors, as summarised in . Here, the monocrystal elasticity tensor C m by Gammon et al (1983), as measured on samples of artificial ice at −16 • C, is used for all calculations: …”

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“…Based on these results and the values obtained in Section 3, we will assume s f ¼1 MPa, a conservatively low value. The shear modulus of intact ice is 4 GPa (Gammon et al, 1983), but fractures or pores may reduce this value. Diurnal tidal stresses on Europa are w0.1 MPa, while stresses due to non-synchronous rotation or polar wander (Leith and McKinnon, 1996;Greenberg et al, 2003) may be up to a few MPa, and stresses due to ice-shell freezing may exceed 10 MPa (Nimmo 2004b).…”

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