Why Is There an Unfrozen Lake Under the Coldest Place on Earth?
― The Bottom of Antarctica's Ice Is Warmed by a Thick Blanket and a Heavy Lid
Antarctica has a spot that recorded −89 °C, the lowest air temperature ever measured at ground level. Right beneath it, under almost 4 km of ice, lies a "lake of liquid water" nearly 20 times the area of Lake Biwa. The very ice that should freeze it is, in fact, what protects it.
On farms in snowy regions, people say the soil freezes less in years with lots of snow. The air above is below freezing, yet the soil under the snow stays close to 0 °C.
At Vostok Station in Antarctica, −89.2 °C was recorded in 1983. It is regarded as the lowest air temperature ever recorded by ground-based weather observation on Earth.
Yet if you dig about 3,700 m down beneath that station, there is liquid water under the ice. It is called Lake Vostok, the largest subglacial lake in Antarctica. Why doesn't it freeze solid?
There Are Only Two Reasons It Doesn't Freeze
Heat from inside the Earth rises slowly through the ground. Almost 4 km of ice is a thick blanket that lets this heat escape only with difficulty. The heat builds up, so the deeper you go, the warmer it gets.
Ice squeezed hard melts even below 0 °C. About 330 atmospheres press on the bottom, and the melting point drops to around −2 °C.
In other words, the temperature at the bottom is pushed up, while the melting point is pushed down. Where the two meet, ice turns to water. Let's take them one at a time.
Earth's Heat Gets Trapped in the Ice Blanket
The inside of the Earth is still thousands of degrees. That heat leaks out through the ground at only about 0.05 watts per square metre. You could never feel it with your hand.
On ordinary ground, this heat escapes into the air at once. In Antarctica, though, almost 4 km of ice sits on top. A centimetre of ice is hardly good insulation. But pile up thousands of metres, and the heat struggles to reach the surface.
For heat to pass through, there must be a temperature difference between top and bottom. Even a small flow of heat needs a large difference to get through 4 km of ice. As a result, even if the surface is −55 °C, the bottom warms all the way to the melting point.
Look at the right side of Figure 1. If you extend the dotted line straight down, the bottom comes out at +34 °C. Of course, that doesn't actually happen. Once the melting point is reached, the surplus heat goes into melting ice. In a real ice sheet, cold snow falling on top carries coldness downward, so the temperature rises more gently than this dotted line. Even so, in many places the bottom does reach the melting point.
The Weight of the Ice Lowers the Melting Point
Most substances become more likely to turn solid when squeezed. Water is an oddball: ice takes up more room than liquid water. So when you press hard, it tips toward the more compact liquid. In other words, squeezed ice melts below 0 °C.
The effect is small. Each extra atmosphere lowers the melting point by only about 0.0075 °C. Pressing with a skate blade hardly does anything. But the weight of almost 4 km of ice comes to about 330 atmospheres. The melting point then drops to about −2.4 °C.
The water of Lake Vostok is estimated to be around −3 °C. That it stays liquid while colder than 0 °C is thanks to this heavy lid.
Subglacial lakes are found by sending radio waves down through the ice from the air. A lake's surface reflects radio waves strongly and looks flat. A 2012 inventory counted about 380 in Antarctica. Satellite measurements have also found cases where lakes are linked by rivers and drain or fill within a few years.
Summary
The bottom of Antarctica's ice doesn't freeze because the thick ice is a blanket that traps Earth's heat, and at the same time its weight is a lid that lowers the melting point. The bitter cold at the surface and the liquid lake below are two faces of the same ice.
The thicker ice gets, the more it melts its own base.
Beneath the coldest place on Earth, a lake is kept warm.
Water at the base of ice also plays a part in how glaciers move. See also Why does a glacier, though solid ice, flow like a river? For water that stays liquid below 0 °C, read Water that won't freeze even at −20 °C, and for ice that doesn't melt when pressed, Why are icy roads so slippery? For air from 800,000 years ago sleeping as bubbles in the same thick Antarctic ice, try Why does Antarctic ice hold air from 800,000 years ago?
- Make a fairly large block of ice in the freezer and lay it across two cups like a bridge.
- Tie a water-filled plastic bottle to each end of a thin wire. Hang the wire over the middle of the ice.
- Wait in a cold place for tens of minutes to a few hours. The wire sinks into the ice. Even so, the ice it passed through stays joined together.
The ice right under the wire is pressed and melts, the water flows around to the top of the wire, and when the pressure is gone it freezes again. This is called "regelation". However, heat conducted by the wire from the surroundings also plays a part, so it is not an experiment of pure pressure alone. Be careful with blades and heavy objects, and do it together with an adult.
For Those Who Want to Know More ― Terms, Formulas, and Links to TextbooksShows which level each topic belongs to, from middle-school science to university courses
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school physics and earth science
- High school+Advanced high-school material, or textbook sidebar content
- UniversityUniversity-level specialist subjects (thermodynamics, glaciology) not taught in high school
- ResearchTopics not yet taught as settled fact even at university, still being studied by researchers
Middle schoolTerms: This phenomenon has names
- Subglacial lake: a lake of liquid water that collects at the bottom of a thick ice sheet in Antarctica or Greenland.
- Geothermal heat flux: the amount of heat leaking from the Earth's interior to the surface, expressed in watts per square metre.
- Pressure melting: the phenomenon in which the higher the pressure, the lower the melting point of ice. Melting under pressure and freezing again when released is called "regelation".
Middle schoolHigh schoolChecking with formulas: the melting point and temperature at the bottom of the ice
There are two main quantities: the temperature at which ice melts at the bottom (the melting point), and what temperature the bottom would reach if heat only conducted. The first comes from a pressure formula, the second from a heat-conduction formula.
| In symbols (pressure) | P ＝ ρ × g × h |
| In symbols (melting point) | Melting point drop ΔT ＝ β × P |
| In symbols (heat) | q ＝ k × ΔTheat ÷ h → temperature difference ΔTheat ＝ q × h ÷ k |
| In words | Pressure at the bottom = ice density × gravitational acceleration × thickness. Melting point drop = coefficient × pressure. Heat through ice = thermal conductivity × temperature difference top to bottom ÷ thickness |
| Where the formulas come from | The first is the weight of the ice above divided by area (balance of forces). The second treats the relation between temperature and pressure at which ice and water can coexist (the Clausius–Clapeyron equation) as a straight line over a narrow range. The third is the law of heat conduction (Fourier's law): heat flows from the hot side to the cold side in proportion to the temperature difference. |
| Symbol | Meaning and unit |
| ρ | Density of ice (kg/m³) |
| g | Gravitational acceleration (m/s²) |
| h | Ice thickness (m) |
| β | Melting point drop per megapascal of pressure (°C/megapascal) |
| q | Geothermal heat flux (watts/m²) |
| k | Thermal conductivity of ice (watts/m/°C) |
| Ice thickness (above Lake Vostok) | About 3,700 m |
| Pressure per metre of ice (density 917 kg/m³, gravitational acceleration 9.8) | About 0.009 megapascals |
| Melting point drop β | Said to be about 0.074 °C per megapascal |
| 1 atmosphere | About 0.1 megapascals |
| Geothermal heat flux q | Said to be about 0.05 watts per m² |
| Thermal conductivity of ice k | Said to be about 2.1 |
| Annual mean surface temperature | About −55 °C |
| Pressure at the bottom (megapascals) | 0.009 × 3700 ≒ 33 |
| In atmospheres | 33 ÷ 0.1 ＝ 330 |
| Melting point drop (°C) | 33 × 0.074 ≒ 2.4 |
| Temperature difference per metre needed to carry the heat (°C) | 0.05 ÷ 2.1 ≒ 0.024 |
| Temperature difference across the whole ice (°C) | 0.024 × 3700 ≒ 89 |
| Bottom temperature if heat only conducts (°C) | 89 − 55 ＝ 34 |
The melting point at the bottom is about −2.4 °C. Meanwhile, if heat only conducted, the bottom would reach +34 °C, far beyond the melting point. In reality, falling snow carries coldness downward, so the temperature difference is smaller, and in some places the bottom stays frozen solid. Even so, this rough estimate shows that "the thicker the ice, the more easily its bottom melts".
High schoolHigh school+Why water is an "oddball"
High schoolWater molecules hold hands through hydrogen bonds, and in ice they build a hexagonal framework with lots of gaps. So ice takes up about 9% more space than water and floats. When pressed, the change proceeds toward the more compact liquid side. In a "phase diagram" this appears as the boundary between solid and liquid leaning to the left.
High school+Above Lake Vostok, the bottom of the ice is thought to melt on the north side of the lake, while lake water freezes back on in the south. This is because the ice thickness varies from place to place, and so does the melting point. The "refrozen ice" stuck to the underside was collected by drilling and studied as a clue to the lake's water.
UniversityThe heat budget of an ice sheet, and the effect of advection
The relation between the melting point of ice and pressure is derived from the latent heat of fusion and the volume change, using the Clausius–Clapeyron equation. The temperature distribution in an ice sheet is found from a heat-budget equation that adds, to Fourier conduction, advection by the snowfall rate (the effect of cold ice carried downward) and frictional heat produced as the ice flows. The more snow falls, the colder the bottom; the thicker the ice in the interior, the more readily the bottom melts.
📖 Derivations and further reading: Clausius–Clapeyron equation (Wikipedia) ／ Lake Vostok (Wikipedia)
ResearchWhat is still not clear
- Is there life in the lake? At Lake Whillans in West Antarctica, a 2013 drilling found microbes in the water. What lives in the deeper, more isolated Lake Vostok is unclear, partly because of the problem of contamination during drilling.
- Where, and how much, geothermal heat comes out. There are hardly any places in Antarctica where heat flux can be measured directly, and values vary widely from place to place. Predictions of where the bottom is melting depend on it.
- How does water at the bottom change the flow of ice? Water under the ice is thought to act as a lubricant, making it easier for ice to slide toward the sea. How sudden drainage of lake water affects the speed of ice flow is still being studied.
So the content of this article, too, is "an explanation of what is known so far". Values for lake temperature and geothermal heat change depending on the method of estimation.
Links to Textbooks (by Level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: "changes of state", "pressure" | Ice and water, pressure from weight |
| High school | Physics: "heat"; Earth science: "Earth's interior" | How heat travels, geothermal heat flux |
| High school+ | Chemistry: "phase diagrams" | Water's property of melting when pressed |
| University | Thermodynamics, glaciology | Clausius–Clapeyron equation, heat budget of ice sheets |
| Research | Polar science, astrobiology | Life under the ice, distribution of geothermal heat |
| ― | Links to daily life | Soil freezing less in snowy years is also because a thick layer keeps ground heat from escaping |
- Wright, A. and Siegert, M. (2012) A fourth inventory of Antarctic subglacial lakes. Antarctic Science
- Wikipedia, "Lake Vostok" (ボストーク湖, Japanese edition)
- Kapitsa, A. P. et al. (1996) A large deep freshwater lake beneath the ice of central East Antarctica. Nature 381
- Christner, B. C. et al. (2014) A microbial ecosystem beneath the West Antarctic ice sheet. Nature 512
- Cuffey, K. M. and Paterson, W. S. B. (2010) The Physics of Glaciers, 4th ed. Elsevier
※This article is a popular-science explanation for general readers. The figures given are rough estimates to help you understand the mechanism. Antarctic observations are reported with different values depending on the measurement method and the period.