page_title: Ice drilling
section_title: Autonomous probes
hierarchical_section_title: Ice drilling / Autonomous probes
caption_attribution_description: English: Design for a cryobot; it was never fully built to this design but a partial prototype was completed in 2001.
context_page_description: Ice drilling allows scientists studying glaciers and ice sheets to gain access to what is beneath the ice, to take measurements along the interior of the ice, and to retrieve samples. Instruments can be placed in the drilled holes to record temperature, pressure, speed, direction of movement, and for other scientific research, such as neutrino detection.
  Many different methods have been used since 1840, when the first scientific ice drilling expedition attempted to drill through the Unteraargletscher in the Alps. Two early methods were percussion, in which the ice is fractured and pulverized, and rotary drilling, a method often used in mineral exploration for rock drilling. In the 1940s, thermal drills began to be used; these drills melt the ice by heating the drill. Drills that use jets of hot water or steam to bore through ice soon followed. A growing interest in ice cores, used for palaeoclimatological research, led to ice coring drills being developed in the 1950s and 1960s, and there are now many different coring drills in use.
context_section_description: If the goal is to obtain instrument readings from within the ice, and there is no need to retrieve either the ice or the drill system, then a probe containing a long spool of cable and a hotpoint can be used.  The hotpoint allows the probe to melt its way through the ice, unreeling the cable behind it.  The meltwater will refreeze, so the probe cannot be recovered, but it can continue to penetrate the ice until it reaches the limit of the cable it carries, and send instrument readings back up through the cable to the surface.  Known as Philberth probes, these devices were designed by Karl and Bernhard Philberth in the 1960s as a way to store nuclear waste in the Antarctic, but were never used for that purpose.  Instead, they were adapted to use for glaciological research, reaching a depth of 1005 metres and sending temperature information back to the surface when tested in 1968 as part of the Expédition Glaciologique Internationale au Groenland (EGIG).
  Because thermal probes support their weight on the ice at the bottom of the borehole, they lean slightly out of the vertical, and this means they have a natural tendency to stray away from a vertical borehole towards the horizontal.  Various methods have been proposed to address this.  A cone-shaped tip, with a layer of mercury above the tip, will cause additional heat transfer to the lower side of a slanting borehole, increasing the speed of melting on that side, and returning the borehole to the vertical.  Alternatively the probe can be constructed to be supported by ice above its centre of gravity, by providing two heating rings, one of which is towards the top of the probe, and has a greater diameter than the rest of the probe.  Giving this upper ring a slightly lower heating power will cause the probe to have more bearing pressure on the upper ring, which will give it a natural tendency to swing back to vertical if the borehole starts to deviate.  The effect is called pendulum steering, by analogy with the tendency of a pendulum always to swing back towards a vertical position.
  In the 1990s NASA combined the Philberth probe design with ideas drawn from hot-water drills, to design a cryobot probe that had hot water jets in addition to a hotpoint nose.  Once the probe was submerged in a thin layer of meltwater, the water was drawn in and reheated, emerging at the nose as a jet.  This design was intended to help move particulate matter away from the nose, as a hot-water drill tends to.  A version with no analytical tools on board was built and field tested in Svalbard, Norway, in 2001.  It penetrated to 23 m, successfully passing through layers of particulates.
  Cryobots remain in good thermal contact with the surrounding ice throughout their descent, and in very cold ice this can drain a substantial fraction of their power budget, which is finite since they must carry their power source with them.  This makes them unsuitable for investigating the Martian polar ice cap.  Instead, NASA added a pump to the cryobot design, to raise meltwater to the surface, so that the probe, known as the SIPR (for Subsurface Ice Probe) descends in a dry hole.  The lower gravity on Mars means that the overburden pressure on the ice cap is much less, and an open borehole is expected to be stable to a depth of 3 km, the expected depth of the ice cap.  The meltwater can then be analyzed at the surface.  Pumping through a vertical tube will cause mixing, so to ensure discrete samples for analysis at the surface, a large bore and a small bore tube are used; the small bore tube is used for sampling, and then its contents are allowed to return to the probe and are pumped back up the large bore tube for use in experiments that do not depend on stratigraphy, such as searches for living organisms.  Leaving the analytical instruments on the surface reduces the necessary size of the probe, which helps make this design more efficient.
  Along with the water transport tubes, a heated wire ensures that the water stays liquid all the way to the surface, and power and telemetry is also carried from the surface
caption: Cryobot design for NASA Martian ice cap probe, partly built in 2001
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