Water below the central part of the ice sheet

Many years before the literature discussed in Chapter 1 was published there was related information on locations where liquid water should exist beneath ice sheets.

It was believed, at the end of the 19th century, that the temperature at the bottom of an ice sheet increased with increasing ice thickness. P. A. Kropotkin, a Russian Duke and co-founder of the world's anarchy movement was also a scientist. In a book on the Quaternary glaciation (Kropotkin, 1876), he postulated that the heat regime within thick, cold ice sheets, below the layer of ice exhibiting seasonal temperature changes, was at a steady state, with temperature increasing linearly with depth, allowing the geothermal heat flow to move toward the surface of the ice (assuming that the vertical movement within an ice sheet is negligible).

N. N. Zubov, a Russian "oceanologist", a rear admiral in the Soviet Navy, Professor at the Moscow State University, and a leading world specialist on Arctic sea ice, used this approach for analyses of thermal conditions at the bottom of cold, thick Antarctic-type glaciers (Zubov, 1955, 1956, 1959). If the temperature increases linearly with depth and the gradient is governed by a need to pass all the geothermal heat flow through the ice cover by thermal conductivity only, then there is a critical ice thickness, corresponding to the bottom temperature of the ice sheet, equal to an ice melting point (Figure 2.1(A)). Figure 2.1(B) corresponds to a thin glacier, in which the bottom temperature is below the freezing point of water. This case corresponds to the existence of permafrost below the ice.

This approach led Zubov to conclude that the ice sheet thickness cannot be greater than its critical thickness. A greater thickness in Zubov's model would correspond to the bottom temperature being higher than the melting point of ice. However, he was the first to publish the fact that the thickness of the Antarctic Ice Sheet in some locations, as determined from seismic soundings, was much greater than his critical thickness (Zubov, 1956). As a result, he introduced a bottom layer, with a thickness equal to the difference between the measured thickness and the calculated critical one (Figure 2.1(C)). He attributed this layer, with some surprise

Figure 2.1. A-A represents the upper surface of the ice; 1 - ice thickness; 2 - "some mixture of water and ice"; 3 - bedrock, as proposed by Zubov. (A) The thickness of the ice is equal to "the critical thickness" corresponding to an ice bottom temperature equal to the ice melting temperature. (B) "Thin" ice, in which the bottom temperature is less than the ice freezing point. This case represents the existence of a layer of permafrost below the ice. (C) "Thick" ice, with a thickness larger than the critical thickness. In Zubov's model (C), some bottom layer should exist, in which the thickness is equal to the difference between a measured thickness and a calculated critical one. Zubov thought that it "could be some mixture of water and ice ..." (Zubov, 1956, p. 26).

Figure 2.1. A-A represents the upper surface of the ice; 1 - ice thickness; 2 - "some mixture of water and ice"; 3 - bedrock, as proposed by Zubov. (A) The thickness of the ice is equal to "the critical thickness" corresponding to an ice bottom temperature equal to the ice melting temperature. (B) "Thin" ice, in which the bottom temperature is less than the ice freezing point. This case represents the existence of a layer of permafrost below the ice. (C) "Thick" ice, with a thickness larger than the critical thickness. In Zubov's model (C), some bottom layer should exist, in which the thickness is equal to the difference between a measured thickness and a calculated critical one. Zubov thought that it "could be some mixture of water and ice ..." (Zubov, 1956, p. 26).

to himself, to a mixture of water and ice (Zubov, 1956). Figure 2.2 shows Professor Zubov at about the time he wrote this paper about the critical thickness of Antarctic ice.

In 1959, A. P. Kapitsa, a young Russian geographer also from Moscow State University, used Zubov's approach to suggest the existence of liquid water lenses below the ice in places identifying with Figure 2.1(C) in central parts of the East Antarctic Ice Sheet (Kapitsa, 1961). However, soon after this, a new approach to an understanding of the temperature distribution in cold, Antarctic-type ice sheets appeared - one that showed more accuracy than Zubov's model.

In 1955, Dr. Gordon de Q. Robin, a new name in glaciology at that time, a former first mate on a World War II submarine in the Australian Navy, and a graduate physicist of Melbourne University, published his first and famous article giving a theoretical explanation of the vertical temperature distribution through the Maudheim Ice Shelf, Antarctica, and the Greenland ice cap near Camp Century (Robin, 1955). This article resulted from his participation as a member of the Norwegian-British-Swedish Antarctic Expedition (1949-1952) based at Maudheim Station on the Maudheim Ice Shelf, a floating mass of ice some 200 m thick. He found that the ice temperature increased from a mean annual temperature near the surface to that of the freezing temperature of seawater at the bottom of the ice shelf. However, this change was not linear, as Zubov suggested. The temperature increased very slowly with depth in the upper part of the ice shelf, and then changed rapidly in the lower part as it approached the bottom of the ice shelf. Dr. Robin showed that a

Figure 2.2. Professor Zubov in 1955, when he wrote his papers on the critical thickness of Antarctic ice.

measured profile corresponded to a theoretical profile in a steady-state system, with temperature, surface accumulation, and ice thickness assumed to be constant. This temperature profile is valid if a thermal conductivity equation includes both thermal conductivity and vertical heat convection terms, due to the vertical movement of ice from the surface to the bottom of the ice due to continuous accumulation at the surface and melting of the ice shelf at the bottom.

This approach accurately explained an experimental temperature distribution for a 1,200 m thick ice sheet in Greenland near Camp Century. The article written by Dr. G. de Q. Robin remained for many years a classic work in glaciology. Figure 2.3 shows Dr. G. de Q. Robin at the time he wrote this paper.

This approach, as in those used by Kropotkin and Zubov, also used a steady-state approach to calculate temperature conditions at depth in ice sheets of Antarctica and Greenland. However, it also required that two terms of the energy equation be included, thermal conductivity and the vertical component of convection, resulting in a non-linear vertical temperature distribution in the ice sheet, making it possible to achieve a good correlation between calculated (measured) profiles and experimental data.

This meant that heat transfers vertically not only by heat conductivity, but by continuous vertical movement of cold particles of ice from the surface to the bottom of a glacier as a result of the deformation and spreading of ice particles due to

Figure 2.3. Dr. G. de Q. Robin at the XIII Scientific Committee on Antarctic Research (SCAR) meeting at Jackson Hole, Wyoming, U.S.A. in 1974, at the time of his radio-echo sounding discovery of subglacial lakes.

gravity, and a continuous replacement of ice by accumulation of snow at the surface, leading to a vertical convection of cold from the surface to internal, warmer parts of the ice sheet. Dr. Robin correctly explained the experimental temperature distribution for the 200 m thick Maudheim Ice Shelf and also for the 1,200 m thick ice sheet in Greenland near Camp Century. He suggested that the temperature distribution in these ice masses was formed under the influence of a geothermal heat flow from beneath the grounded ice sheet in Greenland, or because the bottom temperature was equal to the ice melting point in the case of a floating ice shelf.

All this heat flow for a steady-state approximation is transferred upward in the ice. Dr. Robin showed that the temperature at the bottom of the Greenland Ice Sheet near Camp Century, which was calculated using this method, was much higher than the constant temperature at the surface, but lower than the freezing temperature of the ice, illustrating a good correlation with measured temperature profiles.

It is interesting that if Robin applied his methodology to conditions in the central part of the Antarctic Ice Sheet, the calculated bottom temperature would be higher than the freezing temperature of water. Because this is physically imposs ible, a change in boundary conditions would be required for the ice-bedrock interface. He would then presumably include a term for a heat sink due to the permanent melting of ice at this boundary, and would make the melting rate sufficiently high so that the bottom temperature would be equal to the freezing point of water. However, Dr. Robin did not consider the case for a thick, grounded ice sheet, so did not see the need to include a term for bottom melting. Because of that omission, the phenomenon of permanent melting at the bottom of very thick ice sheets, and the possibility of subglacial lakes due to water accumulation was not apparent at that time.

When Dr. Robin published his important work on this subject, I knew very little about the Antarctic Ice Sheet. As a graduate of the Moscow Aviation Institute, I worked on Ph.D. experiments that were involved in research connected with solving missile "re-entry problems". Using a variety of materials with low melting points, I manufactured models of nose cones and cylinders and installed them in the hot jet streams of huge rocket engines or in hot supersonic airstreams using specially designed apparatus. I studied how these supersonic shock waves melted manufactured/model "meteorites" of various materials and then compared my results with those effects felt by actual meteorites, which experienced similar conditions as they entered the Earth's atmosphere (Zotikov, 1959).

I was unaware at the time that my experiments from the point of view of mathematical physics were exactly similar to those of Dr. Robin. The heat transfer through pressurized, heated air behind a supersonic shock wave through the space between the wave and the solid boundary of the nose cone, was basically similar to the heat transfer through an ice thickness near an ice divide in the Antarctic from the surface to its bedrock as is shown at Figure 2.4.

Dr. Robin and I both assumed the process to be in steady state, and simplified heat transfer equations in both cases also happened to be equal, consisting of two terms, one for the heat conductivity and one for convection.

The first successful Soviet ballistic missile flight, including successful re-entry was in 1956. I gained my Ph.D. degree in 1957. An era of new and advanced ballistic missiles began, with numerous employment opportunities resulting from the growth in this prosperous industry. I became disenchanted, though, partly because of the inherent secrecy of my work and partly due to being part of a crowd of people all doing the same thing. After many years of hard work to receive a Ph.D. in such a demanding branch of science, I had time to look around and discovered that a major scientific event was being planned, the first Soviet Antarctic Expedition (SAE), part of a larger multinational program with an emphasis on the study of the Antarctic continent. The SAE represented the Soviet Union's role in the International Geophysical Year (IGY, 1957-1958). It was the first Russian expedition to Antarctica since 1821.

A friend of mine and co-worker, a mountain climber, was accepted for the expedition as the leader of a group assigned to construct a new scientific station in the middle of the Antarctic Ice Sheet (later named Vostok Station). I was aware that this station would be located at 3,500m above sea level on a plateau, which is why as a mountaineer my friend was given the job. I pleaded with him to take me along as a

Speed of the hot air behind the supersonic shock wave

Speed of the hot air behind the supersonic shock wave

Figure 2.4. An oriented meteorite melting and the Antarctic Ice Sheet bottom melting analogy. (A) Oriented meteorite ("Karakol"). Karakol is a particular name of one of the oriented meteorites, used in the studies of Zotikov (1959). There is a permanent new air supply through the supersonic shock wave surface to the space between the shock wave and the meteorite's surface, determined by the air layer thickness behind the shock wave (order of some parts of a millimeter). There is a permanent meteorite surface melting (evaporation, sublimation) near a front stagnant point (leading nose) area. (B) Antarctic Ice Sheet. There is a permanent new ice supply through the upper ice sheet surface to the space between the ice sheet's surface and its ice-bedrock boundary, determined by the ice sheet thickness (order of some thousands of meters). There is a permanent ice melting at the bottom of the central part of the Antarctic Ice Sheet.

Figure 2.4. An oriented meteorite melting and the Antarctic Ice Sheet bottom melting analogy. (A) Oriented meteorite ("Karakol"). Karakol is a particular name of one of the oriented meteorites, used in the studies of Zotikov (1959). There is a permanent new air supply through the supersonic shock wave surface to the space between the shock wave and the meteorite's surface, determined by the air layer thickness behind the shock wave (order of some parts of a millimeter). There is a permanent meteorite surface melting (evaporation, sublimation) near a front stagnant point (leading nose) area. (B) Antarctic Ice Sheet. There is a permanent new ice supply through the upper ice sheet surface to the space between the ice sheet's surface and its ice-bedrock boundary, determined by the ice sheet thickness (order of some thousands of meters). There is a permanent ice melting at the bottom of the central part of the Antarctic Ice Sheet.

Figure 2.5. Academician Treshnikov was instrumental in constructing Vostok Station just above Lake Vostok.

member of his group, stating that I would be willing to do everything including shoveling snow, washing dishes, any role at all, but he said "No".

His reply in some respect was an act of providence, for later events would provide me with the opportunity to work in Antarctica as a scientist instead of one of many anonymous people who opened Vostok Station, as well as giving me the opportunity to be one of the first to reach the surface of the ice cover of Lake Vostok. As it turned out, my mountaineering friend and his group never reached their destination. They were prevented by many unfortunate circumstances, leaving them about 300 km short of their destination. Instead, a small station (Komsomols-kaia) was established, cargo was left there, and the team were flown back to Mirny. Months later, before the end of the expedition, its leader, Dr. A. F. Treshnikov, famous Russian polar explorer and later academician of the U.S.S.R. Academy of Sciences, organized and took command of a new convoy of large tractors. His team drove to the place where Vostok Station was erected at the end of summer 1957. Since then, the base has been occupied nearly every summer and winter. Figure 2.5 shows academician Dr. A. F. Treshnikov.

Deep in my heart I dreamed for the opportunity of being accepted on an Antarctic expedition. I realized now that I must find a different approach, so I applied directly to the very top of the organization in charge of expeditions, and convinced the scientific leader of the new Antarctic expedition that I could apply my knowledge of solving heat transfer problems to the study of the heat regime of the Antarctic Ice Sheet. I was accepted, and at the end of 1958 sailed away to spend a winter at Mirny Station as "glaciologist-thermophysicist" of the 4th SAE.

My meteorite-melting study thus became important for my glaciology applications when I went to Antarctica to study the heat regime of the Antarctic Ice Sheet. The mechanisms and non-dimensional equations of vertical heat transfer through the thick Antarctic Ice Sheet, when applied to a subglacial bedrock surface in its central part near an ice divide, are similar to the equations of heat transfer from a hot supersonic shock wave to a solid surface of a leading nose (a stagnant point) of an oriented meteorite. Especially interesting and unexpected was the situation where non-dimensional Peclet numbers for both cases were more than 1. A Peclet number is a multiplication of a "characteristic" size for a "characteristic" speed divided by a temperature conductivity coefficient. A Peclet number represents the ratio of heat transport by convection to heat transport by conductivity.

The boundary equation of heat transfer between the solid body of my "meteorites" and a liquid (a layer of air between the solid body and shock wave) was also important in the study of the heat regime of the ice sheet. It included the heat flow through the solid to the solid/liquid boundary, and heat flow through the layer. For steady-state conditions, the difference between these flow rates was used for solid surface melting of the leading nose of my "meteorites". This gave me the idea to develop a method to calculate thermal conditions at the bottom of the ice sheet in order to determine whether melting is occurring, using Zubov's "critical thickness'' concept, but on a more accurate level, calculating a temperature profile across the ice sheet from a thermal conductivity equation with an ice convection term. I also wanted to calculate the speed of melting, if it existed. The vertical velocity of descending ice compaction for this term for the surface of the ice sheet was assumed to be equal to the surface accumulation ratio (a steady-state approach).

Analyses of the data collected on temperature, ice thickness, and annual snow accumulation collected on the traverse of the ice sheet from Mirny Station to Vostok Station, and on to the South Pole using a thermal conductivity equation with a convection term, increased the critical thickness of the ice (Figure 2.6). Nevertheless, the actual thickness of the ice sheet in all its central regions is much more than the critical thickness (Zotikov, 1961). This means that in this area the bottom temperature of the ice is equal to its melting point, and heat flow from the bottom of the ice sheet to its surface is less than the geothermal heat flow from beneath. It also means that heat flow cannot leave, or be removed from, the ice/rock interface because of the high heat resistance of very thick ice. The difference between these rates of heat flow is used to melt the ice at the bottom of the ice sheet. The melted water fills depressions in the subglacial bed and forms subglacial lakes: part of the water moved to the edges of the ice sheet becoming refrozen. Vostok Station is located in just such an area (Zotikov, 1962).

Temperature profiles from the surface to the bottom of the ice sheet were calculated for Vostok Station and Komsomolskaia Station, and showed that the bottom ice temperature in both cases was equal to the melting point of ice (Figure 2.7).

Critical thickness

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5Í0 20 Distance from I

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* ■ " ^—The area of continuous lea malting at jM contact with tha bad f 8 40 MM

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Si

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Distance from Mirny, km mmtyea/(watar} 125,

Distance from Mirny, km

Pi 0

1500 2000 2500 3000 Distance from Mimy, km "The a roa of continuous lea maitinfl at tha contad wilh the bed

Figure 2.6. Determination of an area of permanent bottom ice melting in the central part of the Antarctic Ice Sheet (Zotikov, 1962). Open circles represent data obtained by the second (1957-1958) and third (1958-1959) SAEs, filled circles represent observations of the fourth (1959-1960) expedition.

1500 2000 2500 3000 Distance from Mimy, km "The a roa of continuous lea maitinfl at tha contad wilh the bed

Figure 2.6. Determination of an area of permanent bottom ice melting in the central part of the Antarctic Ice Sheet (Zotikov, 1962). Open circles represent data obtained by the second (1957-1958) and third (1958-1959) SAEs, filled circles represent observations of the fourth (1959-1960) expedition.

These results were quickly recognized by the press. A leading Moscow newspaper Izvestia published an interview with me in 1963, the article was entitled "Melts or Does Not Melt". The article stated "In analyzing heat transfer processes within the Antarctic Ice Sheet, our group made an interesting conclusion. The bottom layers of the ice sheet were melting permanently ... ," furthermore, "It is possible to suppose that there is a sea of freshwater there ... that contains oxygen,

Figure 2.7. Temperature profiles for an ice sheet at Vostok Station and Komsomolskaia Station (Zotikov, 1962). The "0" temperature is the ice melting temperature at the bottom of the ice sheet.

which is supplied to the water from the surface by ice descending from the surface by compaction. It is also possible that there is some unusual type of life there. What kind of life? We do not know..." (Izvestiya, 1963).

Our calculations have shown (Zotikov, 1962) that the rate of permanent melting at the bottom was about 1-4 mm of ice per year. This ice formed originally at the surface of the ice sheet as a result of snow precipitation and accumulation. Snowfall at the surface formed a layer that was transformed first into firn and later, by compaction, into dense glacier ice. The measured density of ice at the ice/firn boundary was about 0.70-0.75 x103 kg m , relatively low because the ice has many unconnected air bubbles. The density of pure ice without bubbles is about 0.90-0.95 x 103 kg m . This means that each volume of ice that melted at the bottom brought with it at least 15% air under conditions at the surface of the ice sheet. It was easy to calculate how much of the air, under pressure beneath the ice sheet (about 300 bars), was derived from the surface, carried to the bottom, and released into the subglacial water. In this sense the ice sheet works as a huge, high-pressure air compressor! (It was not known at the time that gases at pressures and temperatures present at the bottom of the ice sheet form solid cryohy-drates and gas hydrates.) So, I thought (Zotikov, 1977), that this air would be partly dissolved in the water. The time required for this "compressor cycle" was of the order of a million years. This means that the height of a water column brought to the bottom for that time period would be of the order of 0.001 m per year x 1,000,000 years = 1,000 m, and the height of an air column brought to the bottom for this period would be 15% of the height of water, or 150 m for pressure conditions at the surface, or about 0.5 m compressed to 300 bars representing the bottom pressure. This air layer is essentially insignificant and it would be dissolved in the water if we did not consider the following complication to the process.

We mentioned earlier that in our early model the central area of the bottom melting of the ice sheet was surrounded by a belt, where the thickness of the ice was less than the critical thickness. We mentioned that the water from the central area moves to this belt of thinner ice and slowly refreezes (this ice is different from ice formed from accumulated and compacted snowfall). It is known that the process of slow freezing of water is a good means of separating out any inclusions within it. So, it is possible that the water would go to the edges of the ice sheet in the form of a layer of clear ice, free of gases. These layers of clear ice derived from refreezing are commonly observed in icebergs near the coastal perimeter that have become inverted in the water. If this mechanism of refreezing of meltwater occurs, it is reasonable to expect to find a sufficient layer of compressed air above the subglacial lakes of Antarctica, at least in some areas. This possibility presents a useful scenario for writers of science fiction (Figure 1.3).

When I presented these calculations and hypotheses at the meeting of the Soviet Antarctic Committee in 1963, the leader of the third SAE, who led the Russian traverse tractor-sledge party to the Pole of Inaccessibility (Sovetskaia Station), remarked excitedly:

Completing the traverse there we decided to blow-up all the unused remaining explosives that we carried for seismic work, about a ton of it. When we fired the shot, we became alarmed because the surface of the ice moved back and forth wildly, and dangerously sounding noises came from beneath. We felt that we were on the top of the roof of a dome, empty inside, and this roof and the dome were strongly damaged by this blow. We felt that we would fall down through to the empty interior . . .

In contrast to many of the old sailors' stories from remote parts of the world, we can propose a reasonable scientific explanation for this event.

For the moment, though, we considered that the empty dome filled with air under high pressure existed above the subglacial lake. Figure 2.8 shows a photo of Dr. Zotikov when he worked on his subglacial melting ideas.

The next step in developing an understanding of subglacial melting in central Antarctica was presented in an article and a map at the International Symposium of the Scientific Hydrology Association (Zotikov, 1963), which showed that an area of permanent melting at the bottom of the central part of the Antarctic Ice Sheet, with a melting rate of a few millimeters of ice per year, was a large one, covering hundreds of thousands of square kilometers, incorporating the sites of Vostok, Byrd and Amundsen-Scott Stations (Zotikov, 1963). A map of the area is shown in Figure 2.9.

The probability of life existing below the ice sheet was of less interest to people at that time than other aspects of the study (e.g., the existence of highly pressurized air or the influence of bottom melting on the mass balance of the Antarctic Ice Sheet).

Figure 2.8. Dr. Zotikov in 1965 in the old South Pole Station library (at about the time he published his articles about subglacial melting in the central part of the Antarctic Ice Sheet).

Figure 2.9. Map of Antarctica showing regions of permanent ice melting at the bottom of the central part of the ice sheet (adapted from Zotikov, 1963). 1 - melting region, calculated with an assumption that only geothermal heat flow, averaged for the Earth's surface at 52mWm~2, comes to the ice/bedrock interface from below; 2 - a heat flow twice as large comes to the ice/ bedrock interface (104 mWm~2).

Figure 2.9. Map of Antarctica showing regions of permanent ice melting at the bottom of the central part of the ice sheet (adapted from Zotikov, 1963). 1 - melting region, calculated with an assumption that only geothermal heat flow, averaged for the Earth's surface at 52mWm~2, comes to the ice/bedrock interface from below; 2 - a heat flow twice as large comes to the ice/ bedrock interface (104 mWm~2).

The same approach to the probability of the existence of life below the ice sheet was repeated by American scientists when they penetrated the ice sheet and reached the bottom at Byrd Station, West Antarctica, and similarly when the bottom of the ice cover was reached above fast moving ice streams and the presence of liquid water was found in West Antarctica. All these situations are comparable, and can be explained by the same mechanisms.

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