Mason, N. J.

Miljkovic, K.

Zarnecki, J. C.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsEnvironmental effects on dust around EuropaConference or Workshop ItemHow to cite:Miljkovic, K.; Mason, N. J. and Zarnecki, J. C. (2010). Environmental effects on dust around Europa. In:41st Lunar and Planetary Science Conference, 1-5 Mar 2010, Houston, 1346-+.For guidance on citations see FAQs.c© 2010 The AuthorsVersion: Accepted ManuscriptLink(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2010/pdf/1346.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukENVIRONMENTAL EFFECTS ON DUST AROUND EUROPA.  K. Miljković1,2, N. J. Mason1 and J. C. Zar-necki1, 1Centre for Earth, Planetary, Space and Astronomical Research, The Open University, Walton Hall, Milton Keynes,  MK7 6AA,  UK,  2Department  of  Earth  Sciences, University  College  London,  Gower  Street,  London, WC1E 6BT, UK. (k.miljkovic@open.ac.uk).Introduction: A dust  model  has  been developed in order to explore the dust environment  around Eu-ropa. The dust is composed of five different dust popula-tions:  (i)  asteroidal  and  (ii)  halo dust  populations  as part of the interplanetary dust  particle  (IDP) popula-tion  [1];  (iii)  interstellar  dust  (ISD)  [2];  (iv)  the  Io stream and [3] (v) ring dust that originate from inside Jupiter’s system itself [4-5].  Critically for this study, we have also modelled the dust ejected from the sur-face  of  Europa  by  micrometeoroids  impact.  Should the ejected dust be a significant proportion of the total dust  environment  around  Europa,  a  dust  analyzer  in orbit  around  Europa  could  collect  some  of  this  dust and hence provide data on the surface composition of Europa.Method:  The impacting dust  flux (influx)  causes the  ejection  of  a  certain  amount  of  surface  material (outflux) and the relation between the influx and out-flux is determined through the mass  yield [6]. In im-pact  experiments  and  impact  modelling  in  Autodyn hydrocode, the size and velocity distributions of eject-ed  fragments  were  determined  [7].  Assuming  that these  distributions  are  independent  of  the  impactor size and velocity,  they can be used to model Europa ejecta, allowing the outflux dust to be analysed in de-tail.  The  outflux populates  the space  around Europa, where a small  part of the ejecta escapes  and the rest eventually  falls  back to the surface.  The  model  pre-dicts the dust densities of the ejecta outflux at any al-titude within Europa’s Hill sphere and the dust densi-ties at any altitude can be presented as a size distribu-tion of dust.  The  results  of the present  model  match the Galileo data (Fig.1-2) collected during several Eu-ropa flybys [8].Environmental  effects.  Physical  properties  at Europa: The uncertainties of the following dust mod-el  input  parameters  were  considered:  the  density  of the  impacting  micrometeoroids  [9],  the  micromete-oroid impact velocity, the hydrated salt content in the surface ice [10], the largest ejecta fragment, the slope of the cumulative size distribution of the ejected frag-ments,  the slope  of the ejecta  fragment  size-velocity distribution, the strength of Europa’s surface material and the maximum ejecta velocity.  Variations  in each parameter were tested in the dust model and their in-fluence on the spatial  density of the ejected dust de-termined.  In selecting  the input  parameters,  the sur-face  strength  introduces  the  largest  uncertainties [11-13]  (Fig.3).  Harder  surfaces  provide  ejecta  with faster speeds and vice versa [14]. This directly affects the spatial  density  of dust  around Europa  as  well  as the  fractionation  between  bound  and  escaping  dust. The other input parameters affect the spatial dust den-sity for up to an order of magnitude.Environmental  effects.  Non-gravitational forces present inside the Europa’s Hill sphere are: at-mospheric  drag [15-17] and ablation [18-20],  ion in-duced sputtering [21-23],  grain charging [24-26] and potential  (cryo)geyser-like activity.  It was found that none of these processes can effect the motion, popula-tion and destruction of dust ejecta fragments because the ejected fragments only spend a little time in flight, regardless of whether they are bound (and fall back to the surface) or escape (and leave Europa’s Hill sphere upon ejection) by overcoming the gravitational  force. The largest effect has the Lorentz force on the small-est ejecta fragments, providing the grains become suf-ficiently charged and ejected at high speeds. Conclusions: Europa, as a representative of an at-mosphereless  body with a low gravity, suffers a con-stant  micrometeoroid  bombardment  and  ejection  of the surface  material  into the surrounding  space.  The fastest ejected fragments  (up to a few % of the total ejected mass)  should be capable of leaving the satel-lite’s  surface  (and  contribute  to the Jovian  dust  ring population),  while  the rest (~97 %) falls  back to the surface after reaching no higher than a few thousand km above the surface.  The time the micron size ejec-ta fragments  spend in flight  is  not  sufficient  for  the environment to substantially alter the dust fragments. This can be used to support the installation of a dust detector  on board  of the proposed  EJSM mission  in orbit around Europa [27]. It is likely that dust, collect-ed  at  predicted  100  –  200  km altitudes  [28],  could contain compositional  information about the surface. It is also calculated that over the three-month orbital campaign  period  [28]  the  dust  detector  (with  5  cm2 sensor area) should be exposed to about 104 dust frag-ments ejected from the surface of Europa [7]. References: [1] Divine, N. (1993),  JGR, 98, 17029-17048. [2] Landgraf,  M. et al.  (1999)  JGR,  105,  10343–10352. [3]  Krüger,  H.  and  Grün,  E.  (2002)  COSPAR,  15,  Proc. IAU Coll. Proc. 181, Oxford, UK, 144–159. [4] Thiessenhusen,  K-U et al. (2000) Icarus, 144, 89–98. [5]  Krivov,  V.  et  al.  (2002)  JGR,  107,  5002.  [6] Koschny, D and Grün, E (2001) Icarus 154, 391–401. [7] Miljkovic, K. (2009) PhD thesis,  UK. [8] Krüger, H. et al. (2003) Icarus, 164, 170–187. [9] Love, S.G. 1346.pdf41st Lunar and Planetary Science Conference (2010)et al. (1994) Icarus, 111, 227–236. [10] McCord et al. (1998)  Science, 280,  1242–1245.  [11]  Petrović,  J.  J. (2003), J. Mater. Sci., 38, 1–6. [12] Hiraoka, K. et al. (2008)  JGR, 113, E02013.  [13]  Stempel,  M.M. et al. (2005) Icarus, 177, 297-304. [14] Melosh, H.J (1984) Icarus,  59,  234–260.  [15]  Hall,  D.T.  et  al.  (1995) Nature,  373,  677–679.  [16]  Hall,  D.T.  et  al.  (1998) Astrophys.  J.,  449,  475–481.  [17]  Shematovich,  V.I. et  al.  (2005)  Icarus,  173,  480–498.  [18]  Öpik,  E.J. (1958)  Physics of meteor flight in atmosphere, Wiley Interscience  Publishers  Inc,  New York,  60-79.  [19] Chyba, C.F. et al. (1990) Science, 249, 366–373. [20] Campbell-Brown,  M.D.  and  Koschny,  D.  (2004) A&A, 418, 715–758. [21] Evans, A. (1994) The dusty  universe, John Wiley & Sons,  Praxis publ.,  England. [22]  Johnson,  R.E.  et al  (1998)  Geophys.  Res.  Lett.,  25, 17, 3257–3260. [23] Cooper, J.F et al. (2001) Ica-rus,  149,  133–159.  [24]  Kliore,  A.J.  et  al.  (1997) Science, 277, 355–358. [25] Saur, J. (1998) JGR, 103, 19947–19962.  [26]  Kivelson,  M.G.  et  al.  (1997) Science,  276,  1239–1241.  [27]  http://sci.esa.int/  [28] Clark,  K. et al.  (2009) Int. workshop on Europa lan-der. [29]  Krivov et al.  (2003)  Planet.  Space Sci. 51, 251–269.Additional  Information: The  authors  thank  As-trium  for  co-funding  the  PhD  studies  of  Katarina Miljković,  without which this research would not be possible. Fig. 1. Radial spatial density of ejecta from Eu-ropa, for different mass thresholds compared with Galileo data. Fig 2. Cumulative spatial density of dust at Europa, for dust masses larger than m. 1.E-081.E-071.E-061.E-051.E-041.E-031.E-021.E-011.E+001.E-15 1.E-13 1.E-11 1.E-09 1.E-07ejecta mass, m [g]Cumulative size distribution, N [m-3]bound (1.3 MPa)bound (0.01 MPa)bound (10 MPa)unbound (1.3 MPa)unbound (0.01 MPa)unbound (10 MPa)Fig. 3. The change in bound and unbound fragments around Europa due to change in surface tensile strength.1346.pdf41st Lunar and Planetary Science Conference (2010)

Environmental effects on dust around Europa

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Environmental effects on dust around Europa

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