Recent research from University College London (UCL) and the University of Cambridge has uncovered a fascinating new type of ice, potentially reshaping our understanding of water and its many complexities. Announced on February 2, 2023, this discovery pertains to what scientists have termed medium-density amorphous ice (MDA), a form of ice that exhibits a density matching that of liquid water, a unique characteristic among known types of ice.
Traditionally, ice is recognized for its crystalline structure, where molecules organize in a regular pattern. In contrast, amorphous ice is disorganized, lacking this structured form. While relatively rare on Earth, amorphous ice dominates in space settings, often arising in extreme cold where insufficient thermal energy prevents the formation of crystals.
The breakthrough was achieved through a process called ball milling, where researchers combined ordinary ice with steel balls in a jar that was cooled down to a frigid -200 degrees Celsius. Rather than tiny shards of typical ice, the researchers inadvertently created this novel type, MDA, which appears as a fine, white powder.
What makes MDA particularly intriguing is its implication within the icy moons of our solar system. The researchers hypothesize that the intense tidal forces exerted by gas giants like Jupiter and Saturn could generate shear forces similar to those produced by their experimental ball milling, thereby producing MDA in those extraterrestrial settings. Furthermore, when MDA is warmed, it has the capacity to release a considerable amount of heat, which could instigate tectonic activity and “icequakes” beneath the thick ice covering on moons such as Ganymede, one of Jupiter’s largest moons.
Professor Christoph Salzmann from UCL Chemistry emphasized the significance of discovering MDA, stating, "Water is the foundation of all life. Our existence depends on it, we launch space missions searching for it, yet from a scientific point of view it is poorly understood." His comments underline the complexity of water's properties and the implications this new form of ice could have.
Interestingly, the behavioral characteristics of water have perplexed scientists for decades. For example, water reaches maximum density at 4 degrees Celsius, becoming less dense upon freezing, which leads to ice buoyancy. The existence of MDA within the recognized density gap between previously known types of amorphous ice suggests that there might be two distinct liquid forms of water at lower temperatures—an idea supported by computer simulations but never empirically confirmed until now.
Salzmann urges a re-examination of existing models around water, asserting that the properties of MDA should prompt a nuanced understanding of liquid water, with empirical evidence potentially offering new insights into its structure. He remarked, “Existing models of water should be re-tested. They need to be able to explain the existence of medium-density amorphous ice. This could be the starting point for finally explaining liquid water.”
Going deeper, researchers suggest that MDA might represent the actual glassy state of liquid water—a solid state that mirrors liquid properties, akin to the way glass is simply the solid state of liquid silicon dioxide. Interestingly, the research team also considered an alternative scenario where MDA may exist in a highly sheared crystalline form.
The lead author of the study, Dr. Alexander Rosu-Finsen, elaborated on the surprising outcomes of their exploration, explaining, “We shook the ice like crazy for a long time and destroyed the crystal structure. Rather than ending up with smaller pieces of ice, we realized that we had come up with an entirely new kind of thing, with some remarkable properties.” The process allowed the team to create a computational model of MDA through repeated shearing of crystalline ice.
Moreover, the investigation into MDA extended beyond its formation. The scientists employed various techniques to scrutinize the structure and properties of this newfound ice, including X-ray diffraction, which examines patterns of reflected X-rays, and Raman spectroscopy, which evaluates how the material scatters light. Additionally, calorimetry was utilized to determine the substantial energy release that occurs when MDA recrystallizes upon warming, indicating that ice could potentially be a high-energy material that influences the geology of icy celestial bodies.
This breakthrough adds to the existing knowledge of the peculiar nature of water, particularly the mysteries surrounding its density and structure. While the discovery of liquid water is vital for understanding the requisite conditions for life, this finding of MDA opens a new chapter in the exploration of both terrestrial and extraterrestrial water.
In context, amorphous ice was first identified back in the 1930s, with more recent advancements leading to the discovery of its high-density variant in the 1980s through the compression of ordinary ice at near -200 degrees Celsius. Historically seen only in the high-altitude atmosphere of Earth, the potential presence of MDA in far-off celestial bodies could suggest that similar processes might be at play, offering a glimpse into how water behaves under exotic extraterrestrial conditions.
The research discussed is chronicled in the journal Science, signifying both the novelty of the discovery and its far-reaching implications for the fields of chemistry, astrophysics, and planetary science. As inquiries into MDA deepen, the possibility of a refined comprehension of water—and subsequently life as we know it—exists.
