The interstellar region contains several percent of the whole mass of the galaxy. New stars and planetary systems will be generated from this so-called interstellar matter, (Figure 53). It contains the basic ingredients of all known space objects including our own Earth. Samples of this unique extra-solar system matter were brought back to Earth during NASA’s stardust mission in order to study it in the best-suited laboratories around the world including the most powerful synchrotron sources.

Infrared image of the dust rich horsehead nebula

Fig. 53: Infrared image of the dust rich horsehead nebula, a star forming region (image credit NASA, ESA, HST).

The stardust spacecraft collected dust particles from the interstellar medium over several months. The micrometre to sub-micrometre sized, fast flying grains were collected within Aerogel tiles, a transparent, high purity solid silicon oxide glass foam, developed to slow down and capture these grains with minimum damage. Thin slices of Aerogel containing impact tracks were cut from the trays. The tiny particles were studied in situ, in the Aerogel, applying nano synchrotron X-ray fluorescence (XRF) and diffraction (XRD) techniques at the nanofocussing beamline ID13 [1], and the ID22NI nanoprobe (now at to ID16B) and ID22 microprobe (now closed) beamlines [2].

At ID13, analyses by synchrotron XRF microscopy of the elemental composition of eight candidate impact features extracted from the Stardust Interstellar Dust Collector (SIDC) were performed, coupled with scanning XRD [1,3]. Six of the features were unambiguous tracks, and two were crater-like features. Five of the tracks are so-called “midnight” tracks - that is, they had trajectories consistent with an origin either in the interstellar dust stream or as secondaries from impacts on the Sample Return Capsule (SRC). Synchrotron XRD analyses of these stardust interstellar preliminary examination (ISPE) candidates [3] revealed that two of these particles contain crystalline materials: the terminal particle of track 30 contains olivine and spinel, and the terminal particle of track 34 contains olivine. The terminal particle of track 30, Orion, shows elemental abundances, normalised to Fe, that are close to chondritic type CI values, and a complex, fine-grained structure (Figure 54). The terminal particle of track 34, Hylabrook, shows abundances that deviate strongly from CI, but shows little fine structure and is nearly homogenous. An additional track, with a trajectory consistent with secondary ejecta from an impact on one of the spacecraft solar panels, contains abundant Ce and Zn. This is consistent with the known composition of the glass covering the solar panel.

Tricolour RGB = (Fe, Ni, Ca) high-resolution (100 nm per pixel) map of Orion

Fig. 54: Tricolour RGB = (Fe, Ni, Ca) high-resolution (100 nm per pixel) map of Orion (reproduced with permission from [1]).

In addition eight interstellar candidate impact features were studied by hard X-ray, quantitative, XRF elemental imaging and XRD on the ID22NI nanoprobe and ID22 microprobe beamlines [2]. Three features were unambiguous impact tracks, and the other five were identified as possible, but not definite, impact features. Overall, an absolute quantification of elemental abundances in the 15 ≤ Z ≤ 30 range was produced by means of corrections of the beam parameters, reference materials, and fundamental atomic parameters. Seven features were ruled out as interstellar dust candidates (ISDC) based on compositional arguments. One of the three tracks, previously studied at ID13, contained two physically separated, micrometre-sized terminal particles, the most promising ISDCs, Orion (see above) and Sirius. We found that the Sirius particle was a fairly homogenous Ni-bearing particle and contained about 33 fg of distributed high-Z elements (Z > 12). The measurements at ID22NI show that Orion is a highly heterogeneous Fe-bearing particle and contains about 59 fg of heavy elements located in hundred nanometre phases, forming an irregular mantle that surrounds a low-Z core.

To date, only a small number of the particles captured by the Stardust Interstellar Dust Collector and returned to Earth have been extracted and analysed. Of these, seven have features consistent with an origin in the contemporary interstellar dust stream. The interstellar dust candidates are readily distinguished from debris impacts on the basis of elemental composition and/or impact trajectory. The seven candidate interstellar particles are diverse in elemental composition, crystal structure, and size. The presence of crystalline grains and multiple iron-bearing phases, including sulfides, in some particles, indicates that individual interstellar particles differ from any other representative model of interstellar dust inferred from astronomical observations and theory.

 

Principal publication and authors
A.J. Westphal (a), R.M. Stroud (b),  H.A. Bechtel (c), F.E. Brenker (d),  A.L. Butterworth (a), G.J. Flynn (e),  D.R. Frank (f), Z. Gainsforth (a),  J.K. Hillier (g), Frank Postberg (g),  A.S. Simionovici (h), V.J. Sterken (i,j,k,l), L.R. Nittler (m), C. Allen (n),  D. Anderson (a), A. Ansari (o),  S. Bajt (p), Ron K. Bastien (f),  N. Bassim (b), J. Bridges (q),  D.E. Brownlee (r), M. Burchell (s),  M. Burghammer (v), H. Changela (u),  P. Cloetens (v), A.M. Davis (w),  R. Doll (x), C. Floss (x), E. Grün (y),  P.R. Heck (o), P. Hoppe (z),  B. Hudson (aa), J. Huth (z),  A. Kearsley (bb), A.J. King (w),  B. Lai (cc), J. Leitner (z), L. Lemelle (dd), A. Leonard (x), H. Leroux (ee),  R. Lettieri (a), W. Marchant (a),  R. Ogliore (ff), W.J. Ong (x),  M.C. Price (s), S.A. Sandford (gg),  J.-A. Sans Tresseras(v), S. Schmitz (d),  T. Schoonjans (t), K. Schreiber (x),  G. Silversmit (t), V.A. Solé (v),  R. Srama (hh), F. Stadermann (x),†,  T. Stephan (w), J. Stodolna (a),  S. Sutton (cc), M. Trieloff (g),  P. Tsou (ii), T. Tyliszczak (c),  B. Vekemans (t), L. Vincze (t),  J. Von Korff (a), N. Wordsworth (jj),  D. Zevin (a) and M.E. Zolensky (n), 30714 Stardust@home dusters (kk), Science 15, 786-791 (2014).
(a) Space Sciences Laboratory, University of California at Berkeley (USA)
(b) Materials Science and Technology Division, Naval Research Laboratory, Washington (USA)
(c) Advanced Light Source, Lawrence Berkeley Laboratory, Berkeley (USA)
(d) Geoscience Institute, Goethe University Frankfurt, (Germany)
(e) State University of New York at Plattsburgh (USA)
(f) Jacobs Technology/ESCG, NASA Johnson Space Center (JSC), Houston (USA)
(g) Institut für Geowissenschaften, University of Heidelberg (Germany)
(h) Institut des Sciences de la Terre, Observatoire des Sciences de l’Univers de Grenoble (France)
(i) Institut für Raumfahrtsysteme (IRS), University of Stuttgart (Germany)
(j) IGEP, TU Braunschweig (Germany)
(k) Max Planck Institut für Kernphysik, Heidelberg (Germany)
(l) International Space Sciences Institute, Bern (Switzerland)
(m) Carnegie Institution of Washington (USA)
(n) Astromaterials Research and Exploration Science, NASA JSC, Houston (USA)
(o) Field Museum of Natural History, Chicago (USA)
(p) Deutsches Elektronen-Synchrotron, Hamburg (Germany)
(q) Space Research Centre, University of Leicester (UK)
(r) Department of Astronomy, University of Washington, Seattle (USA)
(s) University of Kent, Canterbury (UK)
(t) University of Ghent (Belgium)
(u) University of New Mexico, Albuquerque (USA)
(v) ESRF
(w) University of Chicago (USA)
(x) Washington University, St. Louis (USA)
(y) Max-Planck-Institut für Kernphysik, Heidelberg (Germany)
(z) Max-Planck-Institut für Chemie, Mainz (Germany)
(aa) 615 William Street, Apt 405, Midland, Ontario (Canada)
(bb) Natural History Museum, London (UK)
(cc) Advanced Photon Source, Argonne National Laboratory, Lemont (USA)
(dd) LGL-TPE, Ecole Normale Superieure de Lyon (France)
(ee) University Lille 1 (France)
(ff) University of Hawai’i at Manoa, Honolulu (USA)
(gg) NASA Ames Research Center, Moffett Field (USA)
(hh) IRS, University Stuttgart (Germany)
(ii) Jet Propulsion Laboratory, Pasadena (USA)
(jj) Wexbury, Farthing Green Lane, Stoke Poges, South Buckinghamshire (UK)
(kk) Worldwide. List of individual dusters is at http://stardustathome.ssl.berkeley.edu/sciencedusters

 

References
[1] F.E. Brenker, A. Westphal, L. Vincze et al., Meteoritics & Planetary Science 49, 1594–1611 (2014).
[2] A.S. Simionovici, L. Lemelle et al., Meteoritics & Planetary Science 49, 1612–1625 (2014).
[3] Z. Gainsforth et al., Meteoritics & Planetary Science 49, 1645–1665 (2014).