[meteorite-list] Reiner Gamma Swirl: Magnetic Effect of a Cometary Impact?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Apr 7 00:21:10 2006
Message-ID: <200604070106.k3716vX00419_at_zagami.jpl.nasa.gov>

http://www.esa.int/esaSC/SEM05FNFGLE_index_0.html

Reiner Gamma swirl: magnetic effect of a cometary impact?
SMART-1
European Space Agency
6 April 2006

This animation, made from images taken by the Advanced Moon Imaging
Experiment (AMIE) on board ESA's SMART-1 spacecraft, shows a feature
characterised by bright albedo, and called Reiner Gamma Formation.
 
The Reiner Gamma Formation, a totally flat area consisting of much
brighter material than the surrounding dark 'mare', is centred on an
area located at 57.8? West, 8.1? North, in the Oceanus Procellarum on
the near (visible) side of the Moon, and has an extension of
approximately 30 by 60 kilometres.

The AMIE camera obtained the images on 14 January 2006, from a distance
between 1599 and 1688 kilometres and with a ground resolution between
144 and 153 metres per pixel.

>From early ground-based observations, this feature was initially
misidentified as a crater. Only later detailed observations from orbit
(such as those performed by USSR's Zond-6, and NASA's Lunar Orbiter,
Apollo and Clementine missions) revealed its true nature: a very unusual
morphology, consisting of swirl-like patterns that do not correspond to
any topographic features.


[Anaglyph image of Reiner Gamma Formation]

Its main part consists of a bright pattern of elliptical shape, located
to the west of Reiner crater. Bright elongated patches extend to the
northeast in the Marius Hills region and small swirls extend to the
southwest. The origin of the Reiner Gamma Formation and other swirls
occurring on the lunar surface is still unclear.

Lunar swirls are associated with magnetic anomalies and some of these
swirls - such as Mare Ingenii and Mare Marginis - are "antipodal" to
large impact structures (that is they are located right into opposite
regions of the Moon globe).

So, it was suggested that the Reiner Gamma swirls correspond to
magnetised materials in the crust or iron-rich ejecta materials able to
deflect the solar wind (constant flow of charged particles coming from
the Sun). This would prevent surface materials to undergo maturation
processes, and so produce an optical anomaly.

However, Reiner Gamma Formation still stands as a particular case. In
fact, the magnetic anomaly does not correlate with the scale of the
lunar crust structure and large-scale anomalies seen on the far side.
Furthermore, the anomaly is not associated with any obvious antipodal
basin structure, and the surface material related to Reiner Gamma
appears optically very immature (the age for its emplacement could be
quite recent).

 
[Reiner Gamma Formation]

The analysis of NASA's Clementine imaging data showed that the optical
and spectroscopic properties of the local regolithic surface layer are
close to those of immature mare crater-like soils. This is consistent
with the properties of a shallow subsurface mare soil layer.

Considerations from works on impact cratering support the hypothesis
that the uppermost part of the regolith could have been modified through
an interaction with falling fragments of a low-density comet nucleus,
previously broken by tidal forces and having ploughed the regolith.

Then, the magnetic anomaly would not be the result of an antipodal
crustal field generated in the formation process of large impact basins.
It would rather arise from local effects during the interaction between
the lunar surface and cometary physical environment, with the
possibility that the solar wind is locally deflected and contributes to
the unusual optical properties.

So, the Reiner Gamma Formation could be an interesting site for future
human exploration because of the radiation deflected from the surface.
Further testing of this hypothesis requires access to the physical
properties of the surface to constrain the mechanisms of formation of
the lunar swirls. This is an ongoing task for the AMIE camera, aimed at
studying regolith photometric properties.

Note to editors
 
For considerations on the characteristics of the Reiner Gamma Formation,
this articles make reference to a paper by P. Pinet, V. Shevchenko, S.
Chevrel, Y. Daydou, C. Rosemberg , titled "Local and regional lunar
regolith characteristics at Reiner Gamma Formation: Optical and
spectroscopic properties from Clementine and earth-based data", and
published in 2000 in the JGR Planets scientific journal (105 (E4),
9457-9475).
 
For more information:
 
Jean-Luc Josset, SPACE-X Space Exploration Institute
E-mail: jean-luc.josset _at_ space-x.ch

Patrick Pinet, CNES/ Observatoire Midi Pyrenees
E-mail: patrick.pinet _at_ cnes.fr

Bernard H. Foing, ESA SMART-1 Project Scientist
E-mail: bernard.foing _at_ esa.int
Received on Thu 06 Apr 2006 09:06:56 PM PDT