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
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.
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.
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.
Going further afield
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