Fracturing the Martian crust

nakhla-meteorite-pyroxene-full-width

This PhD project will use neutron diffraction experiments to investigate the onset of fracturing in pyroxenes.

This project is funded for three and a half years as an STFC studentship.

Apply for this project

Read the eligibility criteria and application guidance below, then send your application to postgradoffice@nhm.ac.uk

Application deadline: Wednesday 28 February 2018.

The link between aqueous fluids and potential habitability has meant that assessing the distribution of liquid water at or near the surface of Mars throughout its history continues to be a strong focus of missions to Mars. Eons of meteorite bombardment coupled with limited tectonic resurfacing has left the martian crust in a highly fractured condition, and this fracture network must exert a profound effect, both on how fluids move through the near-surface environment, and on where they are stored in the sub-surface.

It follows that understanding the distribution of fluids in the near-surface of Mars requires a knowledge of the density and connectivity of fractures in the subsurface. Central to that is a knowledge of the fracture properties of those minerals that form a volumetrically significant part of the crust.

In terms of volume, pyroxenes are one of the most mineralogical constituents of the martian crust. Most of what we currently know about the mechanical properties of pyroxenes applies at the high temperature and pressure conditions that occur deep within planetary interiors; much less is known about those properties at the lower pressure conditions in which fracturing is prominent.

From a mechanical perspective, the behaviour of pyroxenes at these lower pressure conditions is intrinsically interesting because not only do they fracture, but they also undergo a range of crystallographically-controlled processes that initiate and modify those fractures. Some of these processes (notably mechanical twinning) are especially helpful because the orientation and magnitude of the stresses that cause the deformation may be established from measurements of the orientation and density of twins in the deformation microstructure.

martian-meteorite-element-map-two-column

Element map of part of Martian meteorite NWA 2046. (See also Irving et al., 2004, Lunar and Planetary Science 35, 1444). The blue is orthopyroxene which is surrounded (green) by pigeonite. Note the fractured nature of these pyroxenes.

The interplay between fracturing and crystallographically-controlled processes like twinning may be monitored during mechanical testing by measuring changes in the lattice spacings and unit cell dimensions of the minerals within a sample as it is being deformed. This may be achieved by performing the mechanical test within a beamline at a neutron or synchrotron facility and analysing the diffraction patterns collected at different applied stresses and/or temperatures. In this project mechanical tests will be performed on terrestrial pyroxene-bearing samples at the UK neutron source near Oxford (ISIS Facility). Primarily, these tests will:

a) monitor changes in stress within the samples during heating and cooling cycles, together with the onset and progress of non-elastic processes in response to those stresses, and

b) establish the stresses required for the onset of crystallographically-controlled deformation processes and fracturing during deformation tests performed at different temperatures and pressures.

August 2001

Results from a deformation experiment performed at the ISIS Facility on Carrara marble (Covey-Crump et al., 2017). Carrara marble is a mineral that also undergoes mechanical twinning. The onset of elastic twinning (twinning that reverses when the applied load is removed) is clearly visible from changes in the c lattice parameter (red data). In calcite, elastic twinning plays a part in initiating fracturing. There is probably a precursory elastic twinning phase in pyroxenes too but this has yet to be documented.

In addition a range of electron microscopy and optical microscopy techniques will be used to analyse the deformation microstructures produced in pyroxene-bearing samples that have been thermally cycled and deformed within the laboratory, and these will be compared with the microstructures observed in naturally deformed rocks, including those in Martian meteorites, in order to ground-truth the experiment findings.

Pyroxenes of different chemical composition and crystal structure will be investigated, with the long term goal of evaluating the mechanical properties of pigeonite, a commonly occurring pyroxene phase on Mars but one that is less abundantly preserved on Earth.

Since pyroxenes are a significant component of the crusts of other planetary bodies, eg, on Earth, the Moon and Venus, the findings will have implications for those bodies, but the primary focus will be on generating results that will be of direct relevance for ongoing planetary exploration programmes to Mars, including the InSight mission and the ExoMars2020 mission.

We seek an enthusiastic person for this project with a strong background in the physical sciences or material sciences or geology, and with an interest in applying their work in a planetary science context.

Suggested reading

Heap MJ, Byrne PK, Mikhail S, 2017, Low surface gravitational acceleration of Mars results in a thick and weak lithosphere: implications for topography, volcanism and hydrology. Icarus 281: 103-114

Taylor SR, McLennan SM, 2009, Planetary Crusts: Their Composition, Origin and Evolution, Cambridge University Press

McSween HY, 2015, Petrology on Mars. American Mineralogist 100: 2380-2395

Covey-Crump SJ, Schofield PF, Oliver EC, 2017, Using neutron diffraction to examine the onset of mechanical twinning in calcite rocks. Journal of Structural Geology 100: 77-97

For links to experimental research on the mechanical properties of pyroxenes see:

Eligibility

This project is funded for three and a half years as an STFC studentship, which will cover all fees and a student stipend if you are from the UK, or from the EU and meet residency requirements (settled status, or 3 years full-time residency in the UK). For full details on what is covered by the studentship please see the STFC guidance.

For informal enquiries or further information, please contact Dr Paul Schofield.

How to apply

Deadline: Wednesday 28 February 2018

Please send the following documents to Anna Hutson at the Postgraduate Office

  • Curriculum vitae
  • Covering letter outlining your interest in the PhD project, relevant skills training, experience and qualifications, and a statement of how this PhD project fits your career development plans.
  • Transcripts of undergraduate and master's degree results.
  • Two academic references including (if applicable) master's project supervisor.

Interview date: March 2018

Start date: October 2018

Further Information

For informal enquiries or further information, please contact Dr Paul Schofield.

Supervisors

The Natural History Museum

Paul Schofield

University of Manchester

Steve Covey-Crump

Katie Joy

 

Funded by 

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