From: LARRY KLAES (ljk4@msn.com)
Date: Fri Nov 01 2002 - 08:52:53 PST
----- Original Message -----
From: Bruce Moomaw
Sent: Friday, November 01, 2002 11:19 AM
To: Europa Icepick
Subject: The Zimmerman Cryobot article (Part 1)
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: europa@klx.com
Project information and list (un)subscribe info: http://klx.com/europa/
This archive was generated by hypermail 2.1.2 : Sat Nov 02 2002 - 22:11:20 PST