My First Ice

At Goddard, I work on the Sublimation Laboratory Ice Millimeter/submillimeter Experiment, or SubLIME. Since “Ice” is a key part of SubLIME, I needed to learn how to make cosmic ice analogues so I can study their chemistry. I’ll talk about the chemistry I’m looking at in a later post, but for now, I’m focusing on how we make our ice samples.

I am happy to report that, a couple of weeks ago, I successfully made my first ice! It wasn’t what you typically think of as ice for a couple of reasons. First, I made a methanol ice sample rather than water ice. Second, (water) ice is typically has a crystalline molecular structure in which the (water) molecules are highly ordered and form lattice structures (like sugar particles or grains of table salt). The ice in our experiment is formed at extremely low temperatures (10 K, which is a bit colder than -440 °F) and is formed as an amorphous, or highly unordered, solid (like cotton candy).

Crystalline solids (left) have an ordered molecular structure, whereas amorphous solids (right) — like the ices made in the SubLIME lab — are unordered or unstructured.

Our ices are made on a gold substrate attached to a cryostat (i.e., a thermostat that allows for very, very, very cold temperatures). This substrate is centered in a stainless steel vacuum chamber, which looks like a sphere with many ports coming off of it (a bit like a coronavirus). When our chamber is under high vacuum, we turn on the cryostat to cool the gas from 300 K (room temperature, K = Kelvin) to about 10 K (the temperatures in dense interstellar gas that eventually gives birth to a star).

The view through one of the window-equipped ports of our vacuum chamber. The gold substrate where we form our ice is the rectangular plate in the center of the photo. To the right is the end of the cold arm of the cryostat. The wire coming off the bottom of the substrate is a temperature diode so we can keep track of the temperature.

Once the vacuum chamber is cold, we leak some methanol from a vial attached by some metal tubes to the chamber with some valves in between. We slowly open the valves and turn off certain vacuum lines in the right order to allow the liquid methanol to be pulled out of the vial as a gas (which is possible because we are operating at such low pressures and temperatures that the methanol — a volatile compound — easily evaporates) which then freezes out onto the gold substrate, forming our solid, or ice, layer.

After letting the methanol slowly leak into our chamber and freeze onto the substrate for about 20 minutes, we close all our valves and pump out certain tubes before moving on to the next step: photolysis.

Photolysis is the breakdown or separation (-lysis) of molecules by light (photo-). To photolyze our ices, we use a hydrogen discharge lamp. We flood a glass cell with hydrogen gas and throw microwaves at it before using a Tesla coil to zap it. The zapped gas emits mostly ultraviolet (UV) light, but some visible light emitted from the glass cell gives off a pretty purple-y pink glow. The UV light funnels into the chamber and hits the ice, where it breaks molecules apart. The molecular fragments then recombine to form new molecules.

The hydrogen discharge lamp with its purple glow.

Using infrared (IR) spectroscopy, we watch how the ice changes over time. In the beginning, we have all methanol ice, but after an hour of photolysis, we have a slew of compounds like formaldehyde, methyl formate, dimethyl ether, and methane, as well as some other simple compounds. Looking to the future, we plan to make ice mixtures incorporating methanol with other compounds, meaning we will get even more complicated ice signatures (especially as we add different elements like nitrogen).

After photolyzing my first ice, I practiced gradually warming up the vacuum chamber to 100 K (30 K at a time) then at a rate of 1 K (a little under 2 °F) per minute. As I warmed up the chamber (and the ice on the substrate), different molecules began to sublime, or transition from a solid state to the gas phase. Along the way, I used IR spectroscopy to observe how features in my ice profile began to disappear as different compounds were whisked away into the gas. By about 250 K, my IR spectrum was looking pretty flat as my processed ices had all sublimed.

Making my first ice was so exciting and gave my confidence a significant boost. I’m getting closer to running my first complete experiments, and I now know how to do everything except the M (millimeter/submillimeter [spectroscopy]) in SubLIME. Fortunately, this is the type of spectroscopy I did as a Fulbright Research Fellow in Cologne Germany in 2015-2016 and during my Ph.D. (just not in the lab, but with radio telescopes), meaning I should be able to pick it up quickly. Hopefully, I’ll have new science to share with you soon!