Overview of Shortwave: An icy mystery — What are lake stars?
This episode of NPR’s Shortwave (host Regina Barber) investigates “lake stars”: dark, branching, star-like melt patterns that appear in snow-covered ice over lakes. Geophysicist Victor Tsai (Brown University, visiting student at Woods Hole) explains how they form, how he recreated them in the lab, what they imply about ice safety on Earth, and why similar “spider” features on Jupiter’s moon Europa might point to liquid water near the surface.
Key points and main takeaways
- Lake stars form when slightly warm lake water seeps up through a small hole in lake ice covered by a slushy snow layer, melting radial channels through the slush and producing star-like branching patterns.
- The branching occurs because flowing water “channelizes” (prefers pathways) rather than spreading uniformly; heat diffusion vs. flow sets the pattern.
- Tsai recreated lake stars in a cold lab using blender-made slush and a slow, steady drip of near-freezing water; patterns form within minutes.
- Similar star/spider patterns seen on Europa (1–2 km across) could be formed by the same physics—if so, they imply liquid water was (or is) much closer to Europa’s surface than some models suggest.
- Ice-safety implication: a recently formed lake star indicates thin ice and is unsafe to walk on; if the lake star has been preserved under subsequently thickened ice it may be safe, but caution is advised.
How lake stars form (concise physics)
- Conditions required: thin layer of ice, snow/slushy snow on top, and a small hole in the ice bottom allowing relatively warm lake water to seep up.
- Process: warm water melts through the slushy snow, first forming a circular melt pool. As flow continues, water channels into arms and branches because it preferentially follows paths of least resistance (channelization vs. diffusion), creating the star pattern.
- Important parameter: flow rate — it must be slow and steady (like a leaky faucet) so the slush isn’t catastrophically destroyed and branching can develop.
Lab experiment — how Tsai recreated lake stars
- Setup: cold lab, flat cold surface, blender-made slush as snow layer, and a slow drip of slightly-warm (near-freezing) water through the slush.
- Tips for DIY replication: make slush in a blender; place it on a flat, cold surface; drip water slowly (better if water is below room temperature so melting is gradual). Patterns appear within minutes.
- Observations: starts circular at the drip point, then branches into star-like arms as channelization takes over.
Europa connection and implications
- Feature: Europa shows spider/star-like surface patterns ~1–2 km across (much larger than Earth lake stars, which are typically up to ~30 feet).
- Interpretation: Modeling and lab analogs suggest the same melting-and-channelizing physics could produce Europa’s patterns, implying episodic liquid water near the surface when the features formed.
- Why it matters: proximity of liquid water to the surface affects the feasibility of future missions aiming to sample subsurface water (drilling or probing). If water was close to the surface historically, that increases the chance of accessible habitability signatures—but timing of formation is uncertain, so it doesn’t prove present-day shallow water.
Safety guidance (practical takeaways)
- If a lake star appears recently (snow and slush still visibly disturbed), the ice is likely thin and unsafe to walk on.
- If the lake star is preserved under a later-formed thicker ice layer, it may be safe—still exercise normal ice-safety precautions.
- General rule: when in doubt, wait several days and follow local ice-safety guidelines before venturing onto frozen lakes.
Notable quotes / memorable lines
- “If there were no diffusion of heat, it would basically just channelize into one single channel… it would be a single snake.”
- Lab anecdote: Tsai used a kitchen blender to make slush for his cold-lab experiments.
Why this is interesting
- Lake stars are a neat example of relatively simple physical processes producing complex natural patterns.
- Studying them links small-scale terrestrial phenomena to planetary science questions about where liquid water exists on icy worlds—and therefore to the search for habitable environments beyond Earth.
Actionable ideas / further reading
- If curious and cautious: try the lab demo (blender slush + slow drip) in a cold setting to see star patterns form in minutes.
- For planetary interest: read Tsai et al.’s paper (mentioned in the episode) on lake-star-like features and Europa for details on modeling and implications for subsurface water.