— Andy Smedley | 23 March 2020
A little over two weeks ago the latest paper from the project was published. Given what’s happening in the world right now (it’s late March 2020), that seems a long time ago, but perhaps this short blogpost will provide some useful distraction whilst we all practise social distancing.
So our new study looks at how sunlight interacts with the blue ice where the vast majority of Antarctica’s meteorites are found. In the original paper that kicked off the project we used a simple mathematical model of how sunlight penetrates into the ice and how much is absorbed by the meteorite. How much sunlight is absorbed by a meteorite trapped in the ice determines how much it will heat up, and if it reaches the melting point of ice, how much the meteorite will then sink dow. In the original paper we used a fairly simple model of how sunlight is attenuated by blue ice and treated the system as one dimensional. In reality though it’s a 3D system and things are more complicated. One of the major ways in that it’s more complicated is that sunlight is made up of many different wavelengths (think the colours you see when it passes through a prism, or when a rainbow is visible, but extending beyond the range your eyes can see). Each of these wavelengths is affected by the ice properties slightly differently which means that the total attenuation is more subtle that we first assumed. When sunlight at infrared wavelengths hits the surface of the ice and passes into it, it’s rapidly absorbed by the ice. In contrast the part of sunlight corresponding to blue wavelengths is absorbed much less readily, and so is repeatedly scattered by the tiny bubbles within the ice. (These bubbles are actually tiny pockets of air trapped when snowflakes fell many thousands of years ago and their chemistry can help us understand how climate has changed on geological timescales.) As it so difficult for light at these blue wavelengths to be absorbed by the ice they continue to be scattered around inside the ice enhancing the amount of energy available to be absorbed by any dark meteorites present. As a result a meteorite sitting within the ice can act as a sink for nearby solar radiation, but, as well as absorbing more, because we now treat them 3-dimensionally, more energy is dissipated. To figure out how these different contributions balance out, we took the results of our sunlight modelling and added in the other things that might cause heating or cooling of the meteorite: the temperature of the air above the ice, the wind blowing over the surface, the motion of the ice, whether the meteorite gets warm enough to melt the ice and how far it then sinks and how these factors might vary over several years, plus the 3D nature of the problem.
All in, rather than iron meteorites being predicted to lie ~30 cm below the surface of the ice whilst their stony counterparts rise to the surface, this study suggests that the difference is much less, with iron meteorites being only 5-10 cm deeper than the stony ones. This isn’t a huge distance of course, and the blue ice is slightly translucent, but when you’re scanning from your skidoo for a speck of dark rock surrounded by the immensity of Antarctica, it’s enough to make spotting them virtually impossible. Interestingly though this new modelling shows the meteorite sinking mechanism is more nuanced than we first thought, with iron meteorites reaching the surface over the winter (when it is dark) before sinking into the ice early in the summer period after the sun rises. Though some questions remain, this seems more in line with what has been found in Antarctica as it gives the potential to find some iron meteorites if the conditions are right, and if the field expedition is during the early part of the summer.
If you fancy reading some of the technicalities of the paper, it can be found here.