Frost flowers in the news, continued…

Many thanks to Robert Krulwich, co host of NPR’s RadioLab, for a great article on frost flowers.  Reading through the comments I can see that there is a lot of public interest in this phenomenon.  I want to take the opportunity to clarify a couple of details that Robert didn’t have the time to go into in his article.

One comment to the article notes that frost flowers probably melt to 1-2 mililiters, not milimeters.  Mililiters is correct… the average frost flower is 2-4 cm tall and has a similar diameter.  They are mostly air however, and melt to a much smaller volume.  If you want to know more about the geometry of frost flowers check out these two papers:

Frost flower surface area and chemistry as a function of salinity and temperature

Specific surface area, density, and microstructure of frost flowers

There were a number of comments on the blog regarding how salt gets into frost flowers.  Frost flowers can be so salty that they are bitter to taste (like bitterns at a solar salt harvesting pond).  This seems counter-intuitive, frost flowers are derived from atmospheric moisture that has gone through a distillation process via evaporation at the ice surface (one comment noted that a small amount of salt can evaporate with the water vapor, this is true, but accounts only for a very small quantity).  The salt comes from the surface of newly formed sea ice, which is very salty due to the process of brine rejection during ice formation.

This brine at the ice surface is wicked upwards into the frost flowers by capillary action, as one reader correctly guessed (or at least this is our best understanding at the moment).  This process stops when all the available brine has been used up, or the frost flower melts under its own over-accumulation of salt (often as the day warms slightly) destroying the capillary flow.  Here’s a very brief video created from a slide in a presentation I made a while back that illustrates this:

One reader of the blog raised a very important issue regarding all this salt.  Its presence in the frost flowers (as in all sea ice) means that these structure are not solid.  Unless it is extremely cold (-54 C for sea ice) all saline ice is a porous matrix containing liquid brine and interspaced among solid, almost entirely salt-free crystals.  The more salt, the larger the volume of brine in the matrix.  This is what makes sea ice so interesting as an ecosystem, all that liquid brine at even very cold temperatures offers a habitat for a diverse array of microscopic life.  The following images of Arctic winter time sea ice (from Krembs et al. 2002) illustrate this occupation of sea ice pore spaces by microbial life.