Detailed studies have been done of ice creep as related to the icy satellites, Ganymede and Callisto. Included were: (1) the flow of high-pressure water ices II, III, and V, and (2) frictional sliding of ice I sub h. Work was also begun on the study of the effects of impurities on the flow of ice. Test results are summarized

Durham, W. B.

Heard, H. C.

Kirby, S. H.

English

NASA Technical Reports Server

CREEP OF ICE: FURTHER STUDIES 
H.C. ~eardl, W.B. ~urham', and S.H. ~ i r b ~ ~  
1 ~ ~ ~ ~ ~ ,  Livermore, CA 94550: 'USGS, Menlo Park, CA 94025 
In the past year we have completed detailed studies of 
(1) the flow of high-pressure water ices 11, 111, and V, and 
(2) frictional sliding of ice I . Preprints describing both 
studies are now being circulate8 among interested colleagues 
before submittal for publication. In addition, we began 
work on the'second phase of the current proposal--the study 
of the effect of impurities on the flow of ice. We 
summarize the results of these three projects below. 
Flow of hish pressure ice. Over the past two and one- 
half years we have tested 36 different samples of high 
pressure ice to pressures as high as 550 MPa and 
temperatures as low as 158 K. The appropriate portion of 
the H20 phase diagram is shown in Figure 1. From our tests 
we have derived 77 independent measurements of strength 
(flow stress as a function of temperature, confining 
pressure, and strain rate) and a 
host of other data related to 
phase transformation kinetics and 
phase metastability in ice. The 
flow results for ices 11, 111, 270 
and V are summarized in Figure 2. 
Ice I1 is the strongest of the 260 
phases, having a strength 
slightly higher than that of ice 2 50 
, and ice I11 is the weakest. 2LO 
T If: e low strength of ice I11 is 
- 
striking: under a fixed set of 230 
conditions ice I11 will flow - 
approximately 100-1000 times 2 220 
faster than ice I1 or ice V. 3 4- 
This is important to the 2 210 al 
evolution of a large icy moon a E 200 
such as Ganymede or Callisto. An Q, 
ice I11 layer, situated between a I- 190 
layer of ice V and a layer of ice 
Ih or 11, will behave over long 180 
periods essentially as a liquid, 
unable to transmit meaningful 170 
levels of shear stress between 
the layers bounding it. 160L ' I ' I I I 1 100 200 300 LOO 500 600 
We have found that on the Pressure ( M  Pa) laboratory time scale of a few 
days, important metastable 
relationships exist between the Fig. 1. H20 phase 
phases of ice. The strongest of diagram. 
https://ntrs.nasa.gov/search.jsp?R=19870014107 2020-03-20T10:30:08+00:00Z
these is the 
metastable existence 
of ice I11 in the 
warmer portion of the 
ice I1 stability 
field, a phenomenon 50 - 
known for many 
decades. What we 
have found is that - CI 
these metastabilities - 
- 
endure evenunder 
large amounts of 
deformation, where 
local structural 
rearrangements might ICE 111 (P=2SO MPa) 
be expected to seed ICE V (P=500 MPa) 
the transformation 
from ice I11 to ice 
11. Ice I11 deformed 
at temperatures as 
low as 210 K does not 
revert to I1 during 
1 
LI Q.~IIIf;II- ; 
deformation, The 3.8 L.0 4.2 L.L 4.6 I L.8 I 5.0 I 5.2 I 5.L I 5.6 I 
transformation from 
ice I11 to V and back 10001 T (K)  
(dashed line in 
Figure 1 at P=350 Fig. 2. Flow of ices II,III,and V 
MPa) can also occur 
within the ice I1 
field without producing any sign of ice 11, even when the 
transformation is accompanied by deformation of the sample. 
Although it seems unlikely that such metastabilities endure 
over geologic time scales, the point is interesting because 
of the low strength of ice I11 and the probability that 
present and past temperature profiles within the icy moons 
are very close to the ice 11-111 phase boundary. 
Frictional slidins in ice Ih. We can simulate sliding 
along existing large-scale faults in the crust of an icy 
moon by means of triaxial mechanical testing of cylinders of 
ice composed of two pieces that slide with respect to one 
another. The Voyager photos suggest such faults exist on 
the moons' surfaces, so our laboratory data should further 
constrain evolutionary models of the moons. We performed 
tests at temperatures from 77 to 115 K, near the expected 
surface temperatures of the moons, and at confining 
pressures from 0.3 to 250 MPa. The results from tests at 
77 K are shown in Figure 3. Temperature turns out to play 
no noticeable role in the frictional characteristics of ice, 
nor does rate of sliding, which varied from to 
mm/s. The variable of overriding importance is the normal 
I l l l ( r l ' l l l ' l l l l l l l I 1 I I I  . 
, 
/ 
, 
H,O Ice . , , 
- / - 
.
T = 7 7 K  
45" to 0 ,  
- Ground Surface - 
= 2.9 x lw3 mmls 0 /'
: !$ - - 
>fi0 - 
, r e 8 .  
1 1 1 1  I I I I  1 1 1 1  I 1 1 1  I I I I  
0 50 100 150 200 
Normal Stress, on (MPa) 
Fig. 3. Frictional strength of ice. 
stress across the fault: the frictional strength rises 
2 MPa for every 10 MPa increase in the normal stress (Figure 
3). This strength is considerably less than that for most 
rocks, ~pplying these results to icy moons, weaker fault 
strength means increased tendency for deeper faults, that 
is, the depth at which plastic flow becomes favored over 
frictional sliding becomes deeper. Under extensional 
stresses of the order of 10 MPa, sliding on existing faults 
would be expected to extend to a depth of approximately 10 
km in ice. 
Effect of im~urities on the flow of ice. This phase of 
the project began very recently. The first series of tests 
on samples of ice I from the important mintermediate" 
temperature regime 243 K to 195 K) has shown unambiguously 
that small amounts (less than 1%) of particulate impurities 
have no effect on the plastic flow properties of ice. The 
work will be extended to different temperature regimes, 
different amounts of particulate impurities, and perhaps 
different phases of ice. The study of dissolved impurities 
in water ice, especially methane and ammonia, is the subject 
of a current proposal to NASA. 


Creep of ice: Further studies

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