X-ray diffraction yields clues to Martian cryosphere

Clathrate hydrates are crystalline solids made from water – they resemble ice, but they have a cage-like structure which contains trapped gases. These clathrates are likely to have played a key role in forming the geology of the planet Mars, including its signature chaotic terrains.

Mars has both liquid and solid ice, but this water is not pure, and is likely to contain dissolved salts such as MgCl2 and CaCl2. However, no study has studied how clathrate hydrates form in the kind of briny solutions found on Mars, and previous research has instead been restricted to the behaviour of these complex molecules in pure water.

So a group of researchers led by Professor Aneurin Evans at Keele University and Dr Stephen Thompson at Diamond Light Source used the specialised instrumentation available on the I11 beamline to track X-ray diffraction data from clathrate hydrates as they formed and dissociated under a range of temperatures that mimicked conditions on Mars, and in the presence of a variety of salts.

The results reveal that CO2 clathrate hydrates are less stable in saline solutions, dissociating at temperatures 10-20 K less than those for pure water solutions. However, the results also confirm that these structures are still likely to exist in the conditions found within the Martian cryosphere, and their activity may have generated some of the most striking features on that planet.


Water on Mars

Crystal structure of CO2 clathrate hydrate. Image shows the view along the crystallographic (111) direction of a cube (outlined in blue) made up of 3x3 unit cells. The cage structure is formed by the interaction of CO2 gas with water molecules in ice. Carbon atoms are shown in black, oxygen atoms in red and hydrogens in yellow.
Crystal structure of CO2 clathrate hydrate. Image shows the view along the crystallographic (111) direction of a cube (outlined in blue) made up of 3x3 unit cells. The cage structure is formed by the interaction of CO2 gas with water molecules in ice. Carbon atoms are shown in black, oxygen atoms in red and hydrogens in yellow.
The cryosphere is the part of the planet which contains solid water, and the Martian cryosphere (which extends up to 22 km beneath the surface at the poles and up to 9 km at the equator) is of particular interest: it is likely that it has played a significant part in the planet’s geological history, and Mars's north polar ice cap is one of its most prominent features. Liquid water is also thought to exist deeper within the planet’s crust.
Clathrate hydrates are crystalline, water-based solids physically resembling ice, but with a cage-like atomic structure where water molecules, bound together with hydrogen bonds,  encase a ‘guest’ molecule, such as CO2. Clathrates are only formed at high pressures and low temperatures – conditions that exist within the cryosphere on Mars. Clathrates may have been even more common in the planets’ past, when atmospheric pressure was higher, which means that clathrates would have been stable at shallower depths than today.
However, previous research suggests that Martian water reservoirs are unlikely to be pure water. Instead, the water on Mars is more likely to be a brine, containing dissolved chloride salts, which significantly change the chemical properties and behaviour of the liquid: clathrates form at lower temperatures in saline solutions, and the resulting changes in clathrate stability could alter the conditions in which gas molecules occupy their cages. While previous research has explored the behaviour of clathrate hydrates in pure water, the Keele-Diamond team is the first perform in-situ studies of the stability of clathrate hydrates in water containing various kinds of salts.  Analysis of the data from these experiments also formed part of the PhD project work of the study’s lead author Emmal Safi, whose work was jointly funded by Diamond Light Source and Keele University.
Clathrate hydrate activity is especially interesting to planetologists, because its cage like structure can open up and suddenly release the gases trapped within.
Dr Stephen Thompson, one of the Diamond Light Source scientists who co-authored the study, noted: 

Plumes of methane gas have been observed coming off the Martian surface on occasion. The origin of these plumes is still a matter of debate, but one explanation is that they are caused by clathrate hydrate activity.

Martian chaos terrain, a distinctive Martian geographical feature, is likely to have been caused by the removal of surface material including clathrates: this results in the distinctive appearance of depressions hundreds of metres deep across the Martian surface, formed of tilted, flat-topped blocks. The dissociation of CO2 clathrates is also thought to be one of the causes of past floods on the Martian surface.
Other geographical features on Mars include rivulets of what is likely to be brine, and this too may be liquid which was formerly trapped in clathrate structures: the lattice structure of clathrate hydrates collapses into liquid water or normal water ice without the support of the gas molecule trapped within. Clathrate hydrate activity also has the potential to affect any possibly microbial life which could potentially exist beneath the surface where it would be sheltered from the harsh surface conditions: the sudden degassing of clathrate hydrate could catastrophically disrupt such habitats.

A team of scientists led by Dr Stephen Thompson, senior beamline scientist for the III beamline, therefore worked with the Keele University group to quantify and the effects of varying pressure and temperature dynamics on clathrate dynamics, using aqueous solutions of MgCl2, CaCl2 and NaCl2 to form an ice and CO2 clathrate system. 

Custom Kit

Over many years, I11 beamline scientists at Diamond Light Source have developed a customised gas delivery system which enables researchers to study the effects of a variety of different temperature and pressure conditions on samples.

Dr Thompson said:

The III beamline’s combination of a fast detector, a well-tested infrastructure for pumping gas and controlling temperature, and an intense, highly collimated X-ray beam is what we needed for this study.

His team collected Synchrotron X-ray Powder Diffraction (SXRPD) data using the fast position sensitive detection on beam I11, from a sample consisting of a tiny, single-crystal capillary (held within a specialised gas cell) filled with salt solution, which was mounted onto the beamline’s diffractometer, and cooled using a liquid nitrogen cryostream. Once the solution had frozen, high pressure gas admitted to the sample cell allowed clathrate hydrates to form.
This arrangement allowed the team to study the effects of a variety of different temperature and pressure cycles, with high precision: the temperature changes can be as quick as 6°K per minute, and the temporal resolution of the X-ray diffraction data can be as little as a few seconds.
This allowed the team to very closely track what was happening to clathrate hydrate structures in response to temperatures ranging from 90 to 250 Kelvin, and a pressure of 10 or 20 bars. 
Dr Thompson noted:

 X-ray diffraction data is rather like a fingerprint. It allows us to identify exact structures, as well as when they dissociate, and events like salt precipitation.

Using this data, the researchers found that clathrates do form in conditions that mimic the Martian cryosphere, even in the presence of salts likely to be present on Mars. However, the dissociation temperate for clathrates formed under these conditions was consistently about 20 K lower than that for clathrates in pure water.

However, the researcher found that clathrate dissociation was most affected not by the salt concentration in the aqueous solution, but by the kind of salt: magnesium is a stronger clathrate inhibitor compared to calcium.

The ice that formed in this briny solution also had interesting properties, said Dr Thompson.

It appeared to be a hybrid form of ice. We found both hexagonal and cubic structures, suggesting that ice on Mars may also be a hybrid form, containing both hexagonal and cubic features within a single stacking disordered phase. 

Compared to the CaCl2 solution, ice with a higher proportion of cubic structure was more common in the MgCl2 solution, and since the cubic structure allows gas molecules to escape more easily, this could explain why magnesium is a stronger clathrate inhibitor, with clathrates dissociating more easily in the MgCl2 solution.
The researchers speculate that if chloride salts are not homogenously distributed on the Martian surface, this could explain puzzling Martian features such as the methane plumes seen over the northern hemisphere in spring and summer. Since both methane and CO2 form cubic clathrate structures, these plumes could be caused by clathrates formed in areas with varying chloride content dissociating as they meet higher spring/summer temperatures. 

Going further afield

The research team are currently working on a similar experiment to track the dynamics of ammonium sulphate and methane clathrate hydrate activity, to mimic conditions on the sub-surface ocean on Saturn’s moon Titan. The gas cell and gas delivery system developed by the scientists on the I11 beamline also finds wider application among the research community. A prime example is the development of new and novel materials, such as metal-organic framework materials, with potential applications ranging from the capture and storage of gases and pollutants, to targeted drug delivery.
To find out more about the I11 beamline, or to discuss potential applications, please contact Principal Beamline Scientist Chiu Tang: chiu.tang@diamond.ac.uk

Related Publication:

Safi, E., Thompson, S. P. et al. (2019). X-ray powder diffraction study of the stability of clathrate hydrates in the presence of salts with relevance to the Martian cryosphere. Geochimica et Cosmochimica Acta, 245, 304–315. https://doi.org/10.1016/j.gca.2018.10.034