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List of Contents

The Mercury Mystery

Motivation

Regolith effects
Electron stimulated desorption

MIXS

Overview
FPA

MIXS Ground Reference Facility

Future Work

Slide 2

Mercury in false colour (Credit: NASA)

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The Mercury Mystery

The planets formed from a disk of gas and dust surrounding the early Sun.

Volatile material cannot condense at Mercury’s orbit, yet its surface is surprisingly volatile-rich.

Surprising low abundance of iron on the surface.

Mercury density = 5.43 g/cm³, Earth density = 5.51 g/cm³

Very large iron core

Slide 3

Artist’s interpretation of protoplanetary disk (Credit: NASA)

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The Mercury Mystery

A giant impact may have stripped away most of Mercury’s crust.

Explains the high density and skewed core.

However, large volatile inventory may dispute this theory.

Counter-argument: Mercury may not be as volatile-rich as MESSENGER data implies.

Thorium may sink.

More data needed to solve the ‘Mercury Mystery’.

Slide 4

Peplowski, Patrick N. et al. (2011). “Radioactive Elements on Mercury’s Surface from MESSENGER: Implications for the Planet’s Formation and Evolution”. In:Science333.6051, pp. 1850–1852.issn: 0036-8075.doi:10.1126/science.1211576. eprint: https://science.sciencemag.org/content/333/6051/1850.full.pdf.url:https://science.sciencemag.org/content/333/6051/1850.

Thorium against potassium abundances across different planetary bodies in the Solar System (Peplowski et al. 2011).

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Motivation

BepiColombo arrives at Mercury in December 2025.

Lots of research to do in that time – Maximise science return!

We have a computational model that uses fundamental parameters

This will include all physical effects that affect MIXS data

Validating the model empirically

Slide 5

BepiColombo trajectory (Credit: ESA)

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Regolith Effects

Need to quantify regolith effects

Regolith effects affect XRF intensity differently for different elements.

Regolith effects are caused by incidence angle, take-off angle, grain size, surface roughness, packing density.

Literature has conflicting results

So we want to hone in on each of these effects to find how much they affect XRF intensity.

Slide 6

SEM image of soda lime glass spheres

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Regolith Effects – Grain Size Effect

Regolith is heterogeneous granular matter ~ 100 μm at Mercury (Bunce et al. 2020).

Larger grain size distribution leads to a rougher surface.

Rough surfaces shadow incoming x-rays and shield outgoing fluorescing x-rays.

Smaller grains = higher XRF intensity

This relationship is energy-dependant.

Slide 7

SEM images of various regolith analogues (Weider et al. 2011)

Bunce, Emma J. et al. (Nov. 2020). “The BepiColombo Mercury Imaging X-Ray Spectrometer: Science Goals, Instrument Performance and Operations”. In: Space Science Reviews216.8, p. 126.issn: 1572-9672.doi:10.1007/s11214-020-00750-2.url:https://doi.org/10.1007/s11214-020-00750-2.

Weider, Shoshana Z. et al. (2011a). “Planetary X-ray fluorescence analogue laboratory experiments and an elemental abundance algorithm for C1XS”. In: Planetary and Space Science59.13. Exploring Phobos, pp. 1393–1407.issn: 0032-0633.doi:https://doi.org/10.1016/j.pss.2011.05.005.url:http://www.sciencedirect.com/science/article/pii/S0032063311001607.

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Regolith Effects – Incidence Angle Effect

As incidence angle increases, XRF occurs closer to the surface.

Path length is the same, depth of XRF different

Since x-rays are strongly attenuated by solid samples, x-rays emitted near the surface have a higher chance of escaping.

Energy-dependent

This results in a hardening of the spectra.

Diagram is a simple smooth sample, but this isn’t the case for regolith

Slide 8

Incoming x-rays

Penetration depth

Fluorescing x-rays

Path length

Variation in penetration depth with different incidence angles

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Electron Induced X-ray Emission

Starr et al. (2012) discovered electrons precipitating onto Mercury’s surface on the night side causing XRF.

Lindsay et al. (2016) mapped these events.

Strong dawn-dusk asymmetry.

Two clear latitudinal bands that match open-closed field line boundaries.

Precipitation caused by acceleration in magnetotail.

Slide 9

Nightside fluorescence events induced by electron precipitation (Lindsay et al. 2016)

Starr, Richard D., David Schriver, et al. (2012). “MESSENGER detection of electron-induced X-ray fluorescence from Mercury’s surface”. In: Journal of Geophysical Research: Planets117.E12.doi:https://doi.org/10.1029/2012JE004118. eprint: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2012JE004118.url:https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2012JE004118.

Lindsay, S.T. et al. (2016). “MESSENGER X-ray observations of magnetosphere–surface interaction on the nightside of Mercury”. In: Planetary and Space Science125, pp. 72–79.issn: 0032-0633.doi:https://doi.org/10.1016/j.pss.2016.03.005.url:http://www.sciencedirect.com/science/article/pii/S0032063315301501.

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Electron Stimulated Desorption

MESSENGER showed that the bulk composition of the exosphere are caused by impact vaporisation and ion-sputtering (Killen et al. 2018).

This electron-induced XRF may be a possible candidate that contributes to the dynamic exosphere.

Release of volatile material from surface

We can investigate this using our facility with the use of an electron gun and a mass spectrometer.

Additional goal: Can we deconvolve x-ray-induced XRF from electron-induced XRF?

Slide 10

Killen, Rosemary M. et al. (2018). “Understanding Mercury’s Exosphere: Models Derived from MESSENGER Observations”. In: Mercury: The View after MESSENGER. Ed. by Sean C. Solomon, Larry R. Nittler, and Brian J. Editors Anderson. Cambridge Planetary Science. Cambridge University Press, pp. 407–429.doi:10.1017/9781316650684.016�

Mercury’s exosphere (Credit: NASA)

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MIXS

The Mercury Imaging X-ray Spectrometer (MIXS)

Goal: Map the global elemental abundance

MIXS-C: Collimator with 10° FoV and 10° angular resolution. MIXS-C will provide global coverage (50 – 100 km spatial resolution).

MIXS-T: Telescope with 1.1° FoV and 9 arcminute angular resolution. MIXS-T can provide spatial resolution of up to 1 km.

Energy resolution ~ 138 eV @ Mn K

Slide 11

MIXS in the cleanroom

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FPA

FPA – Focal plane assembly

FPA on both MIXS-C and MIXS-T

Mechanical support of the detector and front-end electronics

Silicon drift detector (SDD) is a 64 x 64 pixel array.

Each pixel has its own readout node

0.5 - 7.5 keV

Slide 12

Top: FPA. Bottom: Detector (Bunce et al. 2020)

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The Ground Reference Facility

The MIXS Ground Reference Facility is a state-of-the-art x-ray fluorescence instrument

Custom built including spare parts from MIXS construction.

Modular – Parts can be swapped out.

In future will be fully automated to allow changes in viewing geometries.

A proxy for a real space environment

Mercury – Sample
Sun – X-ray source
Spacecraft – Detector

Slide 13

Exterior of MIXS Ground Reference Facility

Interior of MIXS Ground Reference Facility

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X-ray Source

An x-ray tube causes x-rays to be emitted, with a bremsstrahlung background.

The current and voltage may be set to any desired value, up to a maximum of 12 W to vary the spectrum.

Bremsstrahlung is not a good approximation of the Sun, but is a continuum.

Slide 14

X-ray source

Flay and Leach 2012

Flay, Nadia and Richard Leach (Jan. 2012). Application of the optical transfer function in X-ray computed tomography – a review

Weider, Shoshana Z., Larry R. Nittler, et al. (2015). “Evidence for geochemical terranes on Mercury: Global mapping of major elements with MESSENGER’s X-Ray Spectrometer”. In: Earth and Planetary ScienceLetters416, pp. 109–120.issn: 0012-821X.doi:https://doi.org/10.1016/j.epsl.2015.01.023.url:https://www.sciencedirect.com/science/article/pii/S0012821X15000448�

Weider, Nittler, et al. 2015

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Electron Gun

An electron gun is available to produce electrons of a similar energy to those observed at Mercury (Ho et al. 2011).

Can we deconvolve electron-induced XRF from x-ray-induced XRF?

Slide 15

Electron gun

Ho, George C. et al. (2011). “Observations of suprathermal electrons in Mercury’s magnetosphere during the three MESSENGER flybys”. In: Planetary and Space Science59.15. Mercury after the MESSENGER flybys, pp. 2016–2025.issn: 0032-0633.doi:https://doi.org/10.1016/j.pss.2011.01.011.url:https://www.sciencedirect.com/science/article/pii/S0032063311000390.

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Vacuum Pump and Cooler

Slide 16

Scroll pump

Turbo pump

Ethanol bath

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Rotating Arms

The detector and the x-ray/electron sources are positioned on motors that can be rotated via software.

Angles can be changed in 0.01° increments.

This allows the replication of spacecraft viewing geometry of the planet.

This will be useful in determining how the angle impacts x-ray fluorescence intensity.

Slide 17

Blue circles indicate motor that rotates arms

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Mass Spectrometer

A mass spectrometer has been installed to measure the material that is desorbed form the sample.

Investigate sensitivity to surface-magnetosphere-exosphere coupling.

How does ESD affect XRF spectrum?

Going to shine electrons onto analogue samples.

Can observe any correlation in peaks in mass spectrometer and changes in XRF intensity.

Slide 18

Mass spectrometer on exterior of chamber

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Sample Stage

Allows multiple samples to be held within the chamber.

We can test multiple analogues.

Slide 19

Sample stage

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XRS vs. MIXS

Slide 20

XRS Spectrum

FWHM = 880 eV @ 5.90 keV

(Schlemm et al., 2007)

MIXS Spectrum

FWHM = 190 eV @ 5.90 keV

Schlemm C.E. et al. (2007) The X-ray Spectrometer on the MESSENGER Spacecraft.

In: Domingue D.L., Russell C.T. (eds) The Messenger Mission to Mercury. Springer, New York, NY.

https://doi.org/10.1007/978-0-387-77214-1_11

Energy/keV

Counts

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Experiment Plan

Get samples well characterised

Mie scattering
SEM
Laser profile analysis
ICP-MS

Prepare samples

Fusion bead
Pressed pellet
Thin section
Resin
Vacuum safe powder

Slide 21

SEM image of soda lime glass 30 µm sample

Mie scattering results of soda lime glass 30 µm sample

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Experiment

Experiment:

Test XRF on different sample preparation methods.
Test XRF intensity with grain size, viewing geometries, surface roughness
Deconvolve physical effects from XRF effects (matrix effects)

Trying to improve on literature to understand how regolith effects work in more detail (e.g. Näränen et al. 2009, Weider et al. 2011).

Slide 22

Weider, Shoshana Z. et al. (2011). “Planetary X-ray fluorescence analogue laboratory experiments and an elemental abundance algorithm for C1XS”. In: Planetary and Space Science59.13. Exploring Phobos, pp. 1393–1407.issn: 0032-0633.doi:https://doi.org/10.1016/j.pss.2011.05.005.url:http://www.sciencedirect.com/science/article/pii/S0032063311001607

Näränen, Jyri et al. (2009). “Regolith effects in planetary X-ray fluorescence spectroscopy: Laboratory studies at 1.7–6.4keV”. In: Advances in Space Research44.3, pp. 313–322.issn: 0273-1177.doi:https://doi.org/10.1016/j.asr.2009.03.023.url:https://www.sciencedirect.com/science/article/pii/S0273117709002105

Exterior of MIXS Ground Reference Facility

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Future Steps

Define a suite of suitable analogues to represent various terranes of Mercury. These will be used to test our model.

Compare published data from other instrument teams, e.g. MERTIS (Morlok et al. 2019), PSL (Maturilli, Helbert, and Arnold 2019).

The facility is now ready and we look forward to pressing ahead with our experimental work.

Slide 23

Morlok, Andreas et al. (2019). “Mid-infrared spectroscopy of planetary analogues: A database for planetary remote sensing”. In:Icarus324, pp. 86–103.issn: 0019-1035.doi:https://doi.org/10.1016/j.icarus.2019.02.010.url:https://www.sciencedirect.com/science/article/pii/S0019103518306754

Maturilli, A., J. Helbert, and G. Arnold (2019). “The newly improved set-up at the Planetary Spectroscopy Laboratory (PSL)”. In: Infrared Remote Sensing and Instrumentation XXVII. Ed. by Marija Strojnik and Gabriele E. Arnold. Vol. 11128. International Society for Optics and Photonics. SPIE, pp. 187–196.doi:10.1117/12.2529266.url:https://doi.org/10.1117/12.2529266.

BepiColombo Mercury flyby (Credit: ESA)

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