1. Formation
Are interstellar ices porous and how
do the pores collapse?
C. R. Hill1, C. Mitterdorfer2, T. G. A. Youngs3, D. T. Bowron3, N. Pascual1,
O. Auriacombe1,4, T. Loerting2, H. J. Fraser1
1 The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
2 Institute of Physical Chemistry, University of Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria
3 ISIS Facility, Rutherford Appleton Laboratory, Harwell Oxford, Didcot, Oxon, OX11 0QX, UK
4 RAL Space, Science & Technology Facilities Council, Rutherford Appleton Laboratory, Harwell Oxford, Didcot, Oxon, OX11 0QX, UK
Acknowledgements
We acknowledge ISIS for access to beam time under the director’s discretion procedure. We are indebted to
Chris Goodway and the pressure and furnace group at ISIS for their work on the cryostat for in situ growth.
C. R. Hill acknowledges The Open University for a studentship and the Royal Society of Chemistry, the IAU
and The Open University for financial support to attend this conference.
Summary
• Interstellar ices are likely to be porous.
• When the pores collapse, they transform from cylinders to platelets.
• Pore collapse is not auto-catalytic below 117 K and is not necessarily accompanied by pore clustering.
Background
Results
References
1. Leger et al., A&A, 79, 256 (1979)
2. Patashni et al., Nature, 250, 313 (1974)
3. Smith & Wright, MNRAS, 414, 3764 (2011)
4. Schereger & Wolf, A&A, 517, 87 (2010)
5. Ehrenfreund et al., Planetary and Space
Science, 51, 473 (2003)
6. Mitterdorfer et al. PCCP, 16, 16013 (2014)
7. Hill et al. in prep.
Amorphous solid water
• A form of water ice that has no long
range order.
• It has been detected on interstellar
dust grains1, comets2, star forming
regions3 and around young stellar
objects4.
• It is important for surface chemistry,
gas trapping and maybe even planet
formation5.
• It can be porous and the pores
collapse on heating6.
Neutron scattering
• Probes structure of a material.
• Gives information about pore size and
shape, and how these properties change as
a function of time and temperature.
Fig. 1. Formation of p-ASW (porous amorphous solid water)
and c-ASW (compact amorphous solid water) in space and
in the laboratory.
77 K ice heating 50 K ice deposition 50 K ice heating
Fig. 2. Example neutron scattering patterns of
ASW grown at 77 K taken between 117 and
125 K.
Fig. 3a. Periodic spacing (PS), pore radius (Rg) and pore
shape (s) between 78 and 144 K for ASW grown at 77 K.
Pore collapse begins at 121 K. The process is not
autocatalytic below 117 K and there is no evidence for
pore clustering7. b. Pore collapse from cylinders to
platelets.
Increasing
temperaturePore
collapse
Large distances Small distances
Fig. 4a. Periodic spacing (PS), pore radius (Rg) and pore
shape (s) for depositing ASW at 50 K. b. Pore expansion
and collapse while ice is growing.
Fig. 5a. Periodic spacing (PS), pore radius (Rg) and pore
shape (s) for heating ASW grown at 50 K. b. Pore collapse
and pore clustering.
a. a.
b. b.
Pore
shape
Pore
size
Pore
spacing
Pore
collapse
Slow pore
collapse
Pore clustering
p-ASW to c-ASW
phase change?
catherine.hill@open.ac.uk
a.
b.
1) Are interstellar ices porous?
2) If so, how do the pores collapse when the ice is heated?
Pore
shape
Pore
size
Pore
spacing
Pore
shape
Pore
size
Pore
spacing