Is there a possibility that there is amorphous ice on Mars?
From: Homero R. Gonzalez
Grade: 10-12
City: Santa Maria State/Prov.: Texas Country: USA
Area: Environment & Ecology
Message ID Number: 954277350.En
I been studying crystals, so I understand a little about the crystalization of substances. I recently read an article that stated that most of the ice in the universe may be in an amorphous state.
Homero R. Gonzalez
Mars of the Minds
Amorphous ice is a higher-energy form of ice that can form when water vapor encounters a very cold surface, so that it does not have time to find the lower energy crystalline configuration before it solidifies. It is possible that it could condense in cold regions far from stars, perhaps in interstellar molecular clouds. However, like super-cooled water, it is meta-stable. This means that it is never in equilibrium, never really in a stationary state of minimum energy. Instead, it is always slowly transforming into the lower energy crystalline form. But if it is cold enough, the transformation proceeds so slowly that it never gets there, even on an astronomical time scale. Notice, though, that since the rates of chemical changes (which the rearrangement of water molecules from the amorphous to the crystalline form really is) are generally very sensitive to temperature, we expect the process to go faster and faster as the temperature is raised. Conversion to the crystalline form is rapid above a temperature (Ref 1) of 120 K (that is, 120 Celsius degrees above absolute 0). Notice also that, since the crystalline form has lower energy, heat is released in the transition -- it is exothermic.
In general, the likelihood of its existing in large quantities in the cosmos is controversial. However, in the case of Mars, the answer to the question is pretty clear. The lowest temperature on Mars is about 150 K (Ref. 2), and even that temperature presumably occurs only transiently. Beneath the surface the minimum temperature should be higher still, (a time-average of the surface temperature, which ranges from 310 to 150 K, plus a possible contribution from any small residual heat flow up out of the planet's core). So Mars is not a place where amorphous ice is likely to exist.
Because of the exothermic nature of the transition, there are actually considerable grounds to doubt the existence of much amorphous ice even on bodies with temperatures near or below 120 K. The reason is that above a temperature near 120 K, the transition from the amorphous to the crystalline form goes so rapidly that the heat released does not have time to diffuse away, but warms the region significantly. Thus the ice becomes warmer still, and converts faster. The result would be a thermal runaway, a kind of "frozen conflagration", if you will. Notice that if the temperature warms above the critical level even for a short time, as might be caused by a volcanic eruption or meteor impact, the exothermic nature of the conversion should trigger the runaway and insure that any amorphous ice in the region will convert, and never revert to the amorphous form.
As a point of recent interest, older (Ref. 3) infrared spectra of Charon, Pluto's small close moon, revealed the presence of water ice on the surface. It had been expected that this ice might very well be in amorphous form, as the temperature of Charon never exceeds 80 K and should be more typically in the 35-50 K range. Also, ultraviolet rays from the Sun and cosmic radiation should slowly disrupt the organized pattern of hydrogen bonds that accounts for the structure of crystalline ice, producing the amorphous form near the surface. However, improved IR spectra of Charon presented in September 1999 at a meeting in at the Lowell Observatory (Ref. 4, 5) using the Hubble Space Telescope and the Keck 10 m telescope now show (Refs. 5, 6) that the ice on Charon is in fact predominantly crystalline, not amorphous. The reason for this is not entirely clear (Refs 5, 6.). Meteorite bombardment is one possibility, or some process that has not yet been considered. In any event, amorphous ice in the Solar System is as yet a theoretical suggestion, not yet established even for what seemed to be its most likely local.
I thank John S. Lewis of the University of Arizona at Tucson and Rodney A. Badger, Professor of Chemistry Emeritus at Southern Oregon University at Ashland, for helping to educate me on these matters; any errors are mine.
1. Jenniskens, P., Blake, D. F., and Kouchi, A., in Solar System Ices, B. Schmitt, C. de Berg, and M. Festou, Eds. (Kluwer Academic, Boston, 1998), pp. 139-151.
2. JPL summary information on Mars: http://pds.jpl.nasa.gov/planets/welcome/mars.htm
3. Buie, M.W., Cruikshank, D.P., Lebofsky, L.A., and Tedesco, E.F. Nature 329, 522 (1987).
4. "Pluto and Triton: Comparisons and Evolution over Time", workshop at Lowell Observatory, Flagstaff, AZ, 23-24 September 1999; http://www.lowell.edu/workshop/
5. Brown, M.E. and Calvin, W.M., Science 287, 107 (2000).
6. Young, E., Science 287, 53 (2000).