In the dirty snowball model for cometary nuclei, comet-nucleus materials are regarded as mixtures of volatile ices and relatively non-volatile minerals or chemical compounds. Carbonaceous chondrite meteorites are regarded as useful analogs for the rocky component. To help elucidate the possible physical geochemistry of cometary nuclei, preliminary results are reported of calorimetric experiments with two-component systems involving carbonaceous chondrites and water ice. Based on collective knowledge of the physics of water ice, three general types of interactions can be expected between water and minerals at sub-freezing temperatures: (1) heterogeneous nucleation of ice by insoluble minerals; (2) adsorption of water vapor by hygroscopic phases; and (3) freezing- and melting-point depression of liquid water sustained by soluble minerals. The relative and absolute magnitude of all three effects are expected to vary with mineral composition

Allton, Judith H.

Gooding, James L.

English

NASA Technical Reports Server

!N92-10888
WATER/ROCK INTERACTIONS IN EXPERIMENTALLY SIMULATED
"DIRTY SNOWBALL" AND "DIRTY ICEBALL' COMETARY NUCLEI
James L. Gooding I and Judith H. Allton 2 ISN21/Planetary Science Branch, NASA/Johnson
Space Center, Houston, TX 77058 USA. 2C23/Lockheed Engineering and Sciences Co., Houston, "IX
77058 USA.
Introduction. In the "dirty snowball" model for cometary nuclei [1], comet-nucleus materials
are regarded as mixtures of volatile ices and relatively non-volatile minerals or chemical compounds.
Although results from the Giotto and Vega spacecraft flybys of comet P/Halley indicate a complex
chemistry for both the ices and dust in the nucleus [2], carbonaceous chondrite meteorites are still
regarded as useful analogs for the rocky component [3]. Previous considerations of the behavior of
water in cometary nuclei have focussed on theoretical evaluations of the effects of phase transitions on
heat balance [4] or the kinetics of ice retention [5]. Apparently, less attention has been paid to
water/mineral interactions despite the wealth of information about the mineralogical dependence of
water behavior in frozen soils [6]. To help elucidate the possible physical geochemistry of cometary
nuclei, we report preliminary results of calorimetric experiments with two-component systems involving
carbonaceous chondrites and water ice.
Based on collective knowledge of the physics of water ice [7,8], three general types of interactions
can be expected between water and minerals at sub-freezing temperatures: (a) heterogeneous
nucleation of ice by insoluble minerals; (b) adsorption of water vapor by hygroscopic phases; (c)
freezing- and melting-point depression of liquid water sustained by soluble minerals. The relative and
absolute magnitudes of all three effects are expected to vary with mineral composition.
Samples and Methods. Twoseries
of experiments were performed in a
differential scanning calorimeter (DSC)
with homogenized powders (silt-sized
and freer grains) of whole-rock
meteorites and comparison samples [9].
In Series 1, approJdmately equal masses
of mineral/rock powder and deionized
water were blended into mud at room
temperature (295-298 K) and crimp-
sealed in an aluminum container; a
physically separate droplet of deionized
water, overhanging the mud, served as
internal standard. Series 2 used the
same procedure except that a dry
mineral/rock sample was exposed only
to water vapor from the overhanging
droplet. Each sample container was
placed in a Perkin-Eimer DSC-2C
instrument and cooled at 10 K/rain to
< 200 K, followed by re-heating at 10
DEFINITION OF SIMPLE FREEZING/MELTING PARAMETERS
= = ! =
-'_ 600 _
E '
o_ 400 _
-J
It.
II
i_. _ Cl
-i- 200 -
It.
r'_
0
240
_ ATf = Tot - Tel
ATm= Tbl -- To1
J
i
26O
TEMPERATURE(K)
273 K
?
Freeze
Melt
L
!80
Figure 1. Simple parameters measured from DSC extrapolated-onset
temperatures of freezing and melting peaks for water in sample
(peaks a, b) and internal standard (peak c). The 273 K
reference marker is not part of the parameterization.
K/min, under a continuous gas purge of 20 cm 3Ar/min. (Experiments down to 100 K verified absence
of significant transitions below 200 K). DSC heat-flow data were acquired during multiple freeze/thaw
cycles. Certified mercury (A H(melt) = 11.469 J/g at 234.3 K; National Institute of Standards and
Technology SRM-2225) was used for DSC primary calibration. Separate experiments were performed
on the Allende (CV3), Murchison (CM2), Orgueil (CI), Holbrook (L6), and Pasamonte (eucrite)
meteorites as well as on peridotite PCC-1 (U. S. Geological Survey), saponite (Clay Minerals Society
SapCa-1), montmorillonite (CMS STx-1), and serpentine (Franciscan Formation, California).
412
c-7
https://ntrs.nasa.gov/search.jsp?R=19920001670 2020-03-19T16:33:07+00:00Z
<_
SERIES 1: UQUID WATER IN SYSTEM
--A
LU .... i .... , .... . .... , .... , .... , ....
Posamonte
15 Orgueil T_(eucrite)
(C'_i. SAPOIIllE i jr Murchis°n,,_ (_ (CM2)
,o .) 1Holbrook I
5 _NE _ Water
T Blank
0 .... I .... * .... l .... i .... i .... / ....
0 5 10 15 20 25 30 35
Avg. A Tm
Figure 2. Summary of freezing/melting parameters measured for Series
('dirty icebali') experiments. Points with error bars represent mean
(± one standard deviation of the mean) results from replicate (3-4)
analyses of individual samples. The overall negative trend of data
points indicates control by heterogeneous nucleation of ice.
Deviation of the Orgueil point from the trend reflects additional
freezing- and melting-point depression by water-_oluble salts.
SERIES 2: WATER-VAPOR CONDENSATION
15 ..... , .... , .... , .... , .... , ....
10 (S_l_ I) _ " Holbrook
T kl _?_ "o'--......Allende
....f?, ",,.
^ ., / \ Murchison
urgueu J. "_CM2o (c0 )
"" " 1
-5 S_PONm_
--10 .... ' .... ' .... ' .... ' .... ' ....
0 5 10 15 20 25 .30
AT
m
Figure3. Summary of freezing/melting parameters meuured for Series 2
('dirty snowball') experiments. Points with error bars represent
mean (± one standard deviation of the mean) results from replicate
(3-4) analyr, es of individual samples. Numbered points (1-5) denote
individual cycles for Orgueil. Absence of coherent trends (cf., Fig. 2)
indieates different control of water/rock interaction relative to those
involving liquid water. Here samplus are distingebhed mostly by
their relative mrptioa of water vapor. Orgueil e_ibits transition
from "mowbail" to "kebaB" with repeated thermal cycling.
Although the DSC data contain
o.ll._z.t _.t; ._l Mfinlvorr_r;nt" /'_ar, a|,_a7
pattern) information for each
sample [9], we wish to convey the
systematics of water/rock
interactions by reducing the
measurements to simple
parameters that describe the
freezing and melting behavior of
water. Accordingly, the parameters
A Tm and A Tf are dermed (Fig. 1) to
quantitatively describe how
mineralogy affects freezing and
melting relative to a pure water
internal standard. We emphasize
that much additional diagnostic
information could be derived by
analyses of DSC peak shapes and
enthalpic ratios; our use of A Tm
and A Tf here serves only to
conveniently summarize variations
in water/rock systems under
freezing conditions.
Results. SERIES 1. These
experiments can be considered
"dirty iceball" simulations in which
water is uniformly distributed in a
relatively dense mixture. Because
freezing of water depends heavily
on hetcrogcncotm nucleation [7,8],
and the aluminum container is a
relatively poor nucleator, liquid
water undercools substantially
before freezing in the absence of
minerals. The mineral/rock
substrates were distinguished by
their relative abilities to nucleate
ice from undercooled water [9]. If
heterogeneous nucleation was the
only (or dominant) control of
freezing, and melting-point
depression by dissolved material
was not important, a plot of A"lf vs.
A Tm should comprise a negatively
sloping data trend (i.e., A Tf and
A Tm should be inversely correlated
as defmed in Fig. 1). Indeed,
experimental results are consistent
with dominance of heterogeneous
nucleation (F'gg. 2).
413
The trend among mineral and rock samples can be understood in terms of the major minerals in
each material. As reviewed previously in another context [10], the relative ice-nucleation abilities of
igneous silicates are expected to be olivine - plagioclase < < pyroxene. Phyllosilicates are expected to
exhibit a wide range of nucleation abilities; non-expandable clays (e.g., chlorites) are generally expected
to be better nucleators than expandable clays (e.g., montmorillonite, saponite) [10]. The ice-nucleation
effectiveness of montmorillonite-type days, however, is expected to vary with degree of interlayer
expansion which, in turn, will depend on degree of hydration and identities of interlayer cations [10].
As a kaolinite-type clay, serpentine is expected to be no more than a moderately good nucleator.
Among our test samples, the poorest ice nucleators contain high abundances of olivine (e.g., peridofite,
Allende) whereas the best ice nucleators are composed mostly of pyroxene (e.g., Pasamonte). The
phyllosilicate-rich meteorites, Murchison and Orgueil, show intermediate ice-nucleation abilities that
probably reflect a complex interplay of water with igneous silicates, clay minerals, and possibly organic
matter. A second minor "melting" peak in frozen Orgneil, and a systematic decrease in the onset of
melting in both Orgueil and Murchison [9], further suggest interactions of water with hygroscopic
minerals. In fact, the deviation of Orgueil from the overall inverse trend in Fig. 2 can be understood as
effects of freezing- and melting-point depressions created by dissolved salt minerals (e.g., epsomite).
SERIES 2. These experiments can be considered "dirtysnowball" simulations in which water is
progressively condensed within a porous substrate; in contrast with the "iceball"simulation, liquid water
is less important and uniformity may be established only after significant elapsed time. Whereas
freeze/thaw cycling of Series 1 samples revealed little, if any, systematic change with time, data for
Series 2 samples showed pronounced changes with successive thermal cycles. Freezing and melting
peaks controlled by the mineral/rock samples grow during successive freeze/thaw cycles, presumably
as water vapor is progressively adsorbed and condensed on the initially dry samples. Characteristic
signatures are well established after 3-5 cycles. Effects are seen in all three of the carbonaceous
chondrites but are most pronounced for Orgneil, probably as a consequence of its abundant
hygroscopic phyllosilicates and sulfates. For the complete suite of samples, absence of a coherent trend
among the parameterized data (Fig. 3) indicates that, in contrast with Series 1, heterogeneous
nucleation was not the dominant effect. The apparent formlessness of the simple ATf/ATm
parameterization belies the fact that better inter-sample discrimination is achieved by comparison of
the full DSC curves [9]. For Orgneil, in particular, average values of the A T parameters fail to convey
time-dependent changes. Successive "snowball" cycles for Orgneil gave parameterized results that
showed a systematic trend toward "iceball"behavior (points 1-5 moving toward Series I point in Fig. 3).
Implications for In-Situ Cometary Analyzers. Besides identification of volatile ices (not
addressed in this paper), DSC-type experiments could help diagnose the rocky component of a comet
nucleus. In addition to conventional high-temperature DSC, in which minerals are identified by phase-
transition or decrepitation reactions, low-temperature DSC could sense the freezing interactions of
minerals with water. Based on our freezing/melting data, we would expect (at the minimum) that
eucrites could be distinguished from ordinary and carbonaceous chondrites and that Orgueil-type
chondrites could be distinguished from other chondrites. Additional experimental work would be
necessary, however, to establish the possible effects of other volatile ices, minerals, and organic matter.
References: [1] Whipple F. L. (1978) Moon and Planets, 19, 305-315. [2] Kissel J. et al (1986) Nature,
321, 280-282. [3] McSween H. Y. Jr. and Weissman P. R. (1989) Geochim. Cosmochirn. Actoo 53,
3263-3271. [4] Herman G. and Podolak M. (1985) Icarus, 61, 252-266. [5] Fanale F. P. and Salvail J. R.
(1987) Icarus, 7_ 535-554. [6] Anderson D. M. and Morgenstern N. R. (1973) In Permafrost: The
NotCh American Contribution to the Second International Conference, National Academy of Sciences,
Washington, DC, 257-288. [7] Hobbs P. V. (1974) Ice Physics, Clarendon, 837 pp. [8] Pruppacher H.
R. and Klett J. D. (1978) Microphysics of Clouds and Precipitation, D. Reidel, 714 pp. [9] Gooding J. L.
and AUton J. H. (1991) In Lunar Planet. Sci. XXI/, Lunar and Planetary Institute, Houston, Texas, 459-
460. [10] Gooding J. L. (1986) Icarus, 6(_ 56-74.
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Water/rock interactions in experimentally simulated dirty snowball and dirty iceball cometary nuclei

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