Bruce Moomaw
Fri, 01 Nov 2002 08:21:13 -0800
Since this article, on closer examination, turns out to consist of only about 6 pages of type, I think I'll simply reprint it in three successive E-mails (with descriptions of the relevant points in the substantial amount of accompaying pictorial data): CRYOBOT: AN ICE PENETRATING ROBOTIC VEHICLE FOR MARS AND EUROPA from the 2001 IEEE Aerospace Conference Authors (all from JPL): (1) Wayne Zimmerman Cryobot Lead Engineer (2) Robert Bonitz Cryobot Controls Engineer (3) Jason Feldman Cryobot Mechanical Engineer 1. INTRODUCTION The primary focus of NASA's current planetary program revolves around Mars and understanding more about the planet's climate history -- in particular, what happened to the water that once existed on the surface, and is there still water in any abundance? Although the Mars Polar Lander crashed, it would have been the first polar sortie to the Mars South Pole to understand the polar climate history and dynamics, and to look for the presence of water. The north polar cap is also of interest because of Mars Gloval Surveyor images which show a signficant residual ice pack composed of circumferential dunes, layered deposits, and overlying ice deposits. MGS images show hundreds of layers of 1 to 100 meters thickness. There appear to be layers with large-scale (100 meter) albedo bands as well. The bands extend and are individually identifiable for hundreds of kilometers. This offers a very rich look at planet climatic history, as well as the presence of water. Similarly, Europa's fractured crust as imaged by Galileo shows signs of a rich mineral content in areas of upwelling. Again, the Europan ice crust contains a rich history of both sub-ice and inter-ice geophysical and chemical evolution. In both cases, access to the deep ice environments to unlock the history, evolutionary processes, and potential mystery of life must employ revolutionary technologies which meet the highly constrained mass, volume, and power budgets of spacecraft today. The cryobot is one of the revolutionary technologies being developed to enable these missions. 2. DESCRIPTION OF MARS AND EUROPA ICE ENVIRONMENTS 2.1 -- Mars The climate and role of water on Mars have been receiving increasing attention in the last decade as evidenced by Mars Global Surveyor (which recently imaged evidence of large-scale erosion) and follow-up planet visits by Mars Climate Orbiter and Mars Polar Lander, both of which unfortunately failed. Despite previous Mars successes like Viking, Pathfinder, and MGS, the evolution of climate and whether or not life exists or existed at one time, still remain elusive questions in the minds of scientists. Polar misions have great interest because the climate history is likely preserved in the reservoirs of ice and water. Further, the likely presence of water provides an essential ingredient for life. Ice flow, sublimation coupled with global wind patterns, sediment deposition, and wind erosion are believed to be the most important processes the shape the polar caps. However, little is known about the composition, porosity, density, and stratigraphy of polar ice. These fundemental questions about planet history and possible life are best examined using a cryobot mole penetrator, which can deliver both imaging and sampling instruments to the ice cap subsurface environment. 2.2 -- Europa Recent magnetometry data received from Galileo confirmed that there is indeed a salty, maneral-rich ocean beneath the Europan ice crust. This recent finding is extremely important in its implications relative to containing the ingredients for extant life. We know, from deep ocean and polar cap empirical data collected here on Earth, that deep hydrothermal vents and gas bubbles trapped at the ice-water interface provide a source of nutrients for microorganisms. If recent data are accurate about the ocean on Europa, then the possibility of a liquid ocean and subsequent ice-liquid interface presents an opportunity for the presence of past or extant life. Again, these fundamental questions about the preservation of Europa's evolutionary history in the ice, ice/ocean chemical makeup, and possibility of finding life are best examined using a cryobot vehicle which can penetrate the ice layer and perform in-situ analysis of both the ice and ocean. 2.3 -- Earth Our own polar caps here on Earth are currently of great interest to terrestrial glaciologists because of recent concern over global warming. Additionally, the Antarctic polar cap contains possible treasures of early Earth history in the form of deep-ice lakes which have been sealed off from the surface for perhaps more than a million years. One such lake is Lake Vostok, which is located approximately in the center of the Antarctic continent and is roughly half the size of Lake Ontario. It lies at a depth of about 3 km beneath the ice pack. The lake remains above freezing as a result of pressure and heat from the Earth's interior. Possible finds include micro-organisms which have not existed for one million years on our planet's surface; possible prehistoric fossils embedded in the lake bottom material; understanding of how the Antarctic continent formed based on lake bottom stratigraphy/material analysis; and whether there are any remnants of extraterrestrial particles deposited by the flow from the ice pack over the evolution of the planet. One of the most important challenges to be faced in penetrating this lake is how to maintain its pristine character. Again, a cryobot vehicle, which can penetrate the ice pack and yet allow the hole to freeze up behind it to prevent forward contamination, appears to be the best solution for meeting the contaminant-free challenge. 3. BRIEF HISTORY OF EARTH ICE MELTING SYSTEMS Work on polar ice-pack penetrators started in the early 1960s with the Philberth probe developed for the Expedition Glaciology International at Greenland. The Philberth probe was a pasive heating penetrator with heaters mounted in the nose. The probe was powered from the surface via a cable (tether), and was able to reach depths of 90 meters, 260 meters, 218 meters, and 1 km. The primary scientific motivation for launching the probe was to understand subsurface ice-cap motion as a function of internal temperature fluctuations. H. Aamot of the Army Cold Regions Research and Engineering Laboratory furthered the Philberth work and developed probes using a pendulum stabilization approach (i.e., the probe hangs by either a tether or a protrusion into the ice at the top of the probe, and freely seeks the downward gravity vector as a means of steering). This probe reached depths slightly over 100 meters. The University of Nebraska Polar Ice Coring Office (PICO) also built a pendulum-stabilized melting probe, which included a telemetry link in the tether. This thermal probe also used passive heating as the primary ice phase-change mechanism for mobility, and allwoed constant temperature/ice flow measurements to be taken during descent. The PICO probe was further modified to enable melt-water conductivity and micro-particulate measurements to be made. This probe reached depths of 100 to 200 meters. The PICO probe configuration represents the current state of the art in passive melt probe design. The thermal probes described above are typically 2.5 to 3.5 meters in length, with shell diameters on the order of 10 to 12 cm, and mass on the order of hundreds of kilograms. Power required is on the order of 3 to 5 kilowatts. Later models, which pay the tether out from a cavity in the probe to allow the cable to refreeze behind the vehicle as it descends, have incorporated differential heating (i.e., the ability to switch heaters on and off) to enable steering. All of these passive heating probes penetrate at rates of about 2 meters per hour. Ice penetration has also been accomplished mechanically using rotary cutting and coring bits (by the Russian Academy of Mining and Drilling, Antarctica) as well as hot-water drilling. The Russian ice corers have reached depths of 2 to 3 km, while the hot water drilling systems have penetrated approximately 1 km. While hot-water drilling is very efficient (i.e., drill rates on the order of tens of meters per hour), the energy expended in recirculating borehole meltwater back to the surface is considerable (on the order of megawatts). With the considerably larger energy input, mechanical and water-jet probes are able to penetrate at rates significantly higher than thermal probes with heaters. There are some significant differences between the Cryobot and existing terrestrial thermal probes. Since the Cryobot is being designed for Mars polar cap and Europa ice pack penetration, it must be considerably smaler in geometry as well as mass and power. The JPL Cryobot is 1 to 1.25 meters long, 12 cm diameter, and weighs on the order of 40 kg. The actual flight version of the mole will be betwen 0.8 and 1 meter in length, and will weigh 20 to 25 kg. As a much smaller vehicle, the Cryobot melts with 1 kilowatt thermal energy, versus the 3 to 5 kW used by terrestrial probes. In modeling the heat transfer and fluid dynamics of the phase- change process, JPL engineers found that terrestrial probes are actually inefficient in how they transfer heat for melting. Tight constraints on launch mass and volume drove JPL engineers to define the minimum amount of energy required to sustain the melt process. Additionally, while terrestrial thermal probes are manually controlled from the surface by an operator, the Cryobot will be a fully self-contained robotic vehicle with sufficient on-board state sensors, imaging, and computing to enable autonomous control and fault recovery. This advanced vehicle design is discussed in detail in the following sections. (TO BE CONTINUED) == You are subscribed to the Europa Icepick mailing list: [EMAIL PROTECTED] Project information and list (un)subscribe info: http://klx.com/europa/