Tuesday, 16 July 2013

Masers on Ice

 Potentially, nuclear powered masers could send a probe many kilometres through ice to putative oceans on Europa and Callisto.



The recent re-development of masers at the National Physical Laboratory in the U.K. has an interesting implication for the exploration of subglacial lakes. 


The recent failure of the drilling expedition that was attempting to sample the waters of the subglacial Lake Ellsworth in the Antarctic illustrates the difficulty of such an operation. Despite huge efforts by the British Antarctic Survey team, the process of drilling through 1 km of ice was halted by technical problems. Although the project is in abeyance, there are strong intellectual incentives to make further efforts to penetrate the ice cover of the lake and explore what lies within.

Since the existence of subglacial lakes was first predicted by Peter Kropotkin at the end of the 19th Century almost 400 of these bodies of fresh water have been discovered underneath ice. It is a combination of geothermal heating and pressure from the overlying ice that is believed to enable these fresh water lakes to exist, even in Antarctica. A substantial international research programme has identified three particular subglacial lakes that are of special interest as the sequestered water they contain may have remained isolated from the outside environment for many thousands of years. 

Lake Ellsworth, Lake Vostok and Lake Whillans are the subject of drilling explorations as they could contain forms of life never seen before. Layered sediments on the lake beds should also provide an historical record of regional or global environmental changes and events. Whereas Lake Whillans is believed to be part of an ice stream flowing into the Ross ice shelf, Lake Ellsworth and Lake Vostok are believed to contain ecosystems that have remained isolated from the rest of the world for thousands of years. Studying a previously unknown ecosystem on Earth is interesting enough, but there is a another reason to explore these special places. Just as physics enabled Kropotkin and later A. Zotikov  to predict the existence of subglacial lakes, the possibility of oceans underneath the icy surfaces of Jupiter's moon Europa and Saturn's moon Callisto have been postulated. A study of life forms in terrestrial subglacial lakes could help in the search for life on these distant moons. 

Soon after the discovery of Lake Ellsworth in in 1996 British scientists started planning an expedition that would answer the question as to whether there was a lower limit to the amount of light, temperature and nutrient levels that would support life. Previously, the so called 'black smokers' on the mid ocean ridges were the only known sites where life existed without light. These mini-volcanoes, expelling super heated water laced with a rich soup of chemicals, are very different from the very cold and nutrient poor depths of subglacial lakes. Prompted by this question about the limits of life, a 16 year project evolved into an expedition equipped with a hot water drill designed to provide a borehole wide enough to insert a sampling probe into the lake. 

To help with sterilisation and to withstand high pressure the device was made with titanium. The torpedo like sampling probe was made at the National Oceanography Centre at Southampton under the aegis of the National Environmental Research Council.  The five-metre-long sampling probe was to enter the lake, take 24 separate water samples and retrieve a lake bed sediment core. The decision to use hot water to melt the ice was an obvious choice. The scheme was devised to prevent uncontrolled flows of water from the lake by creating a sump reservoir of water 300 meters below the ice. 

The pressure from the overburden of ice keeps the subglacial lake liquid, like the layer of water that forms under the blade of an ice skate. It was realised quite early that this same pressure could cause water to violently unwell from the drilled hole at the moment of penetration of the lake. If this happened it would have been similar to the oil 'gushers' that used to occur in Texas. To avoid this the plan was to create a hot water cavity and use a submerged pump to balance the pressure from the lake. Stopping any net movement of water between the lake and the surface would minimise the possibility of contamination by organisms from the outside biosphere. 

In the end the ambitious plan was too difficult to achieve. It was not possible to link the under-ice sump of melt water with the bore hole that was to deliver the probe. After extended attempts the team ran out of the gas supplies used to melt snow for pumping into the holes. The team have retreated back to Britain and are re-assessing the plan. Since the British attempt at Lake Ellsworth the Russian team at Lake Vostok have retrieved lake water samples by a simpler method. By partially withdrawing their drill bit away from the lake, water was allowed to enter the bore hole and freeze. Later, the drill extracted some of this frozen lake water, leaving a frozen section in place to act as a plug. The Lake Vostok team were effectively using the drill as a kind of syringe to extract lake water and hold it in a column. Controversially, the simplicity of the Russian system comes at at price. 

At Lake Vostok fluids, used to prevent the the borehole collapsing, present a substantial contamination risk. Large quantities of kerosene and CFC were needed to prevent the hole closing at the base. The biological material retrieved from the ice, formed from frozen lake water, may be genuine local organisms or it may be contamination from the drilling. The situation is similar to the process by which a mosquito accidentally introduces a malarial parasite to the bloodstream of a person while attempting to feed from a blood vessel. Rejecting criticism of their failure to use a hot water drilling system, the Russian team insisted that not enough energy could be supplied. In the case of the British Lake Ellsworth project this did indeed turn out to be true.


MAPSEC

The recent re-development of the maser at the National Physical Laboratory could circumvent these difficulties. Until recently the maser was a complicated device needing high magnetic fields and cryogenic cooling. The new device at the NPL is a relatively simple crystal that accepts electricity and emits a maser beam.  A much larger version could project maser beams deep into an ice field. Because radio beams of different wavelengths can constructively interfere to create a third wavelength it should be possible to use scaled-up versions of the new masers to project beams of radio energy through ice and combine them at any depth to create the wavelength used in microwave cookers to boil water. 

If this zone of water-specific heating were to be directed in front of a drill tip, it should be possible to ease it through the ice by melting it in advance. If the ice were to not just melt but also boil, a circular column of steam could sterilise the outside of the drill as it moved down through the ice. Rather than using two overlapping masers it could be possible to use the drill core as a wave guide to send a non-maser microwave beam 'down the pipe' and then combine it with a maser beam aimed at the tip. The radio source for the piped beam could be a conventional magnetron or klystron.

If the purpose of the drilling is to deliver a capsule to a subglacial lake then the next stage of the process might be to dispense with the drill altogether and replace the mass of metal with a device on its own. The zone of melted water could be directed to the base of a capsule so that it sinks into the ice under gravity. Alternatively the capsule could actively propel itself through the ice/water mixture using stored on-board energy or even by harvesting radio energy from the beams that envelope it. The shell of the capsule would need to be in two counter rotating sections so that their torque would cancel out. Each half of the capsule would have opposing 'threads', left handed and right handed. The spiral protrusions on the surface could double as dipoles for received the harmonic wavelengths of radio energy not involved in melting the ice. Dispensing with a drill to deliver the capsule means that its width is not constrained by the maximum diameter of a drill. The project would be a Maser Assisted Polar Subglacial Exploration Capsule (MAPSEC).

Crucially, as he zone of melting would be limited to an area immediately around the capsule the water would re-freeze after its passage. An upwelling of water would be stopped by this plug of refrozen ice. Once floating in the lake water the MAPSEC capsule/submarine would undertake either a pre-programmed sequence of manoeuvres or it could be guided by radio. The maser beams could be switched off or reduced in power to serve as radar. Boiling of the water would be avoided as biology is of primary interest, but on its way down the lake the MAPSEC would have been effectively sterilised by the maser beams. The screw thread on the exterior surface would be less effective in water so some water jet propulsion would be needed. Ports for sampling of lake water are required anyway. 

After completing its mission the MAPSEC would rise to the top of the subglacial lake and make contact with the bottom of the ice. The zone of maser-melting would be restarted and the MAPSEC would ascend through the ice for retrieval on the surface. The zone of melting would be directly above the capsule while ascending, and again the temperature on its surface could be temporarily raised to above boiling to sterilise the surface if required. Once again, the melted ice would refreeze after the passage of the MAPSEC, preventing any uncontrolled upwelling of water.  An upward pressure from the lake water might initially help to propel the MAPSEC upwards for a short distance, but the column of water would meet with the intense cold of the surrounding ice. 


Main features of the MAPSEC proposal

1) Refreezing of water after the passage of the MAPSEC capsule would seal the melted channel.

2) Elimination of a drill would remove constraints on the size and diameter of the capsule.

3) The whole MAPSEC concept could be tested at very little cost using small scale models. Later, small capsules could be delivered to specific water channels under glaciers in Europe. The test site could be a short distance from a road.



MAPSEC

Rather than passively sinking into the melted ice the MAPSEC probe could actively rotate sections to propel itself, using either on-board batteries or harvested radio energy from harmonic wavelengths not interacting with the ice.


The probe descends through the ice towards the subglacial lake. The melt water re-freezes behind the melting zone. 




The probe breaks through into the water and exploration begins.


The water exploration over, the probe is directed towards the ice. The masers combine to melt the ice ahead of it.

The probe ascends back to the top of the ice with samples and data. The melted channel re-freezes behind it, preventing an upward flow of water.

In the more distant future this technique could be developed for use on the ice moons of Jupiter and Saturn. As the wavelengths from the two masers is not that which interacts with the water dipole, there should be little attenuation of the beams by the ice. The water-melting wavelength would only be derived from their constructive interference where they overlap. Potentially, nuclear powered masers could send a probe many kilometres through ice to putative oceans on Europa and Callisto. Three spacecraft would be required to land in a rough triangle. Two outer maser spacecraft would illuminate the probe released from a third spacecraft.


FAR FUTURE

Another application for the NPL design of masers could be to vaporise ice on the surface of a comet that was approaching a collision with Earth. A suitably equipped spacecraft orbiting the comet could excavate a channel of boiling water within it's ice and the resulting jet of steam might be enough to alter the trajectory. For this purpose a single maser would suffice.

John Stockton.

14th July 2013

Lake Ellsworth
http://www.ellsworthlive.org.uk/

NERC
http://www.nerc.ac.uk/press/releases/2011/24-ellsworth.asp

NPL video
http://www.youtube.com/watch?v=M4EQN6-EXHY

BBC
http://www.bbc.co.uk/news/science-environment-19281566

Nature
http://www.nature.com/nature/journal/v488/n7411/full/nature11339.html


Wikipedia
http://en.wikipedia.org/wiki/Subglacial_lake