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Peer-reviewedCC BY 4.0Astronomy & Astrophysics

Evidence of mafic silicates in the blue unit of Phobos: Vis-NIR analogs within the C and Xc-type asteroid taxonomy

  • G. Munaretto1
  • J. Beccarelli1,2
  • M. Pajola1
  • G. Cremonese1
  • P. Borin1
  • S. Fornasier3
  • A. Lucchetti1
  • F. Tusberti1
  • A. Tullo1
  • C. Re1
  • M. Almeida4
  • M. Read5
7 more authors
  • K. Mason-Piercy4
  • N. Thomas4
  • C. Rossi1
  • L. Penasa1
  • M. R. Patel5
  • A. Valantinas6
  • L. L. Tornabene7
7 affiliations
  1. 1INAF Astronomical Observatory of Padova
  2. 2CISAS, Università degli Studi di Padova
  3. 3LIRA, Université Paris Cité, Observatoire de Paris, Université PSL, Sorbonne Université, CNRS
  4. 4Physikalisches Institut
  5. 5School of Physical Sciences, The Open University
  6. 6Brown University, Department of Earth, Environmental and Planetary Sciences
  7. 7Institute for Earth and Space Exploration, University of Western Ontario, Dept. of Earth Sciences
Open at sourceDOICite

Abstract

Context. The composition of Phobos’ blue unit is poorly constrained due a lack of diagnostic spectral features. A ≈1 μm absorption was tentatively identified in past studies, but there has not been a conclusive detection. This situation has precluded the identification of spectroscopic analogs, thus preventing accurate compositional analyses.

Aims. The aim of this study is to validate the ≈1 μm absorption in the Phobos blue unit and identify novel asteroidal spectral analogs that fully reproduce its spectral shape.

Methods. We validated a Phobos blue unit spectrum from Mars Express/OMEGA with TGO/CaSSIS photometry, whose absolute accuracy is well established, and identified novel asteroidal analogs.

Results. The OMEGA and CaSSIS datasets show agreement within 1 σ , confirming the 1 μm band at ≥3 σ for the first time. We identified novel improved asteroidal analogs belonging to the C-complex and Xc types in the SMASSII taxonomy.

Conclusions. The 1 μm absorption, suggesting the presence of mafic silicates in the Phobos blue unit, and the identification of C and Xc-type analogs support an exogenous origin, providing key compositional information for the future JAXA/MMX mission.

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Cite

BibTeXRIS
BibTeX
@article{munaretto2026evidence,
  title = {{Evidence of mafic silicates in the blue unit of Phobos: Vis-NIR analogs within the C and Xc-type asteroid taxonomy}},
  author = {G. Munaretto and J. Beccarelli and M. Pajola and G. Cremonese and P. Borin and S. Fornasier and A. Lucchetti and F. Tusberti and A. Tullo and C. Re and M. Almeida and M. Read and K. Mason-Piercy and N. Thomas and C. Rossi and L. Penasa and M. R. Patel and A. Valantinas and L. L. Tornabene},
  journal = {Astronomy \& Astrophysics},
  year = {2026},
  month = oct,
  doi = {10.1051/0004-6361/202661396},
  url = {https://www.aanda.org/10.1051/0004-6361/202661396},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - Evidence of mafic silicates in the blue unit of Phobos: Vis-NIR analogs within the C and Xc-type asteroid taxonomy
AU  - G. Munaretto
AU  - J. Beccarelli
AU  - M. Pajola
AU  - G. Cremonese
AU  - P. Borin
AU  - S. Fornasier
AU  - A. Lucchetti
AU  - F. Tusberti
AU  - A. Tullo
AU  - C. Re
AU  - M. Almeida
AU  - M. Read
AU  - K. Mason-Piercy
AU  - N. Thomas
AU  - C. Rossi
AU  - L. Penasa
AU  - M. R. Patel
AU  - A. Valantinas
AU  - L. L. Tornabene
PY  - 2026
DA  - 2026/10/15
JO  - Astronomy & Astrophysics
DO  - 10.1051/0004-6361/202661396
UR  - https://www.aanda.org/10.1051/0004-6361/202661396
AB  - Context. The composition of Phobos’ blue unit is poorly constrained due a lack of diagnostic spectral features. A ≈1 μm absorption was tentatively identified in past studies, but there has not been a conclusive detection. This situation has precluded the identification of spectroscopic analogs, thus preventing accurate compositional analyses.  Aims. The aim of this study is to validate the ≈1 μm absorption in the Phobos blue unit and identify novel asteroidal spectral analogs that fully reproduce its spectral shape.  Methods. We validated a Phobos blue unit spectrum from Mars Express/OMEGA with TGO/CaSSIS photometry, whose absolute accuracy is well established, and identified novel asteroidal analogs.  Results. The OMEGA and CaSSIS datasets show agreement within 1 σ , confirming the 1 μm band at ≥3 σ for the first time. We identified novel improved asteroidal analogs belonging to the C-complex and Xc types in the SMASSII taxonomy.  Conclusions. The 1 μm absorption, suggesting the presence of mafic silicates in the Phobos blue unit, and the identification of C and Xc-type analogs support an exogenous origin, providing key compositional information for the future JAXA/MMX mission.
SN  - 0004-6361
SN  - 1432-0746
ER  -