We have completed a new 1:15,000,000 global geologic map of Jupiter's volcanic moon, Io, based on a set of 1 km/pixel combined Galileo- Voyager mosaics produced by the U.S. Geological Survey. The map was produced over the last three years using ArcGIS(TM) software, and has undergone peer-review. Here we report some of the key results from our global mapping efforts, and how these results relate to questions regarding the volcano-tectonic evolution of Io

Crown, D. A.

Geissler, P. E.

Jaeger, W. L.

Keszthelyi, L. P.

Schenk, P. M.

Williams, David A.

Yff, Jessica

NASA Technical Reports Server

VOLCANISM ON IO: RESULTS FROM GLOBAL GEOLOGIC MAPPING.  D.A. Williams1, L.P. Keszthe-
lyi2, D.A. Crown3, P.E. Geissler2, P.M. Schenk4, Jessica Yff2, W.L. Jaeger2. 1School of Earth & Space Exploration, 
Arizona State University, Tempe, Arizona 85287 (David.Williams@asu.edu); 2Astrogeology Science Center, U.S. 
Geological Survey, Flagstaff, Arizona; 3Planetary Science Institute, Tucson, Arizona; 4Lunar and Planetary Insti-
tute, Houston, Texas. 
 
Introduction: We have completed a new 
1:15,000,000 global geologic map of Jupiter’s volcanic 
moon, Io, based on a set of 1 km/pixel combined Gali-
leo-Voyager mosaics produced by the U.S. Geological 
Survey [1]. The map was produced over the last three 
years using ArcGIS™ software, and has undergone 
peer-review. Here we report some of the key results 
from our global mapping efforts, and how these results 
relate to questions regarding the volcano-tectonic evo-
lution of Io. 
Previous Work: Previously we reported our tech-
niques for global mapping of Io [2] and on the devel-
opment of an Io database [3] that will include most Io 
data sets to address the surface changes due to Io’s 
active volcanism. Previously we also reported the per-
centage of Io covered by each of 14 process-related 
geologic material units and structures [4], and last year 
we presented a stratigraphic correlation of these map 
units [5]. Here we report results from visual, graphic, 
and statistical analyses of the map units and structures 
and discuss insights into the formation of plains, lava 
flow fields, paterae, mountains, and diffuse deposits. 
Results I (Plains): Plains units cover 66.6% of the 
surface, and (with the exception of a few outliers) are 
geographically distributed on Io. Red-brown plains 
dominantly occur >±30˚ latitude, and are thought to 
result from enhanced radiation-induced alteration of 
other plains units. White plains (typically enriched in 
SO2) occur mostly in the equatorial antijovian region 
(±30˚, 90˚-230˚W), possibly indicative of a regionally 
colder part of the satellite to preserve the SO2. Why is 
this one region colder such that SO2 concentrates here? 
The answer may be related to variations in crustal dis-
tribution of magma sources or delivery mechanisms, or 
perhaps crustal thickness, relative to other parts of Io. 
Outliers of white, bright, and red-brown plains occur-
ring in other regions likely result from long-term ac-
cumulation of white, yellow, and red diffuse deposits, 
respectively. 
Results II (Lava Flows): Lava flows cover 27.8% 
of the surface, the bulk of which (20.6%) are undi-
vided flows whose original composition (dark silicate 
or bright sulfur) cannot be determined. Bright flow 
fields outnumber dark flow fields by a ratio of ~1.5 to 
1; both of these are presumed to be the freshest and 
youngest lava flow types on Io. Only 16.8% of the 
bright flow fields are adjacent to dark flow fields. The 
association of adjacent bright and dark flows would be 
expected if sulfur flows are derived from secondary 
sulfur volcanism (i.e., melting of sulfur-rich country 
rock by heat from silicate magmas or lavas [6].) Thus, 
this result suggests that secondary sulfur volcanism 
may only have a minor role in Io’s current volcanic 
activity (although there may be a scale-dependence on 
these processes that requires further investigation). 
There is an unusual concentration of bright flows at 
~45˚-75˚N, ~60-120˚W, perhaps indicative of past, 
extensive primary sulfur volcanism in this region. 
However, this stands in stark contrast to the current 
correlation of active hot spots with surface materials, 
in which only 1.7% of hot spots correlate with bright 
flows, suggesting that at present primary sulfur volcan-
ism has a minor role in Io’s current activity. 20.3% of 
hot spots detected by telescopic and spacecraft obser-
vations correlate with dark flow fields and another 
9.3% correlate with undivided flow fields. Thus, lava 
flows make up less than one-third of Io’s heat sources. 
Results III (Paterae): Paterae are circular to ir-
regular volcano-tectonic depressions on Io, thought to 
be similar to terrestrial calderas [7]. Evidence suggests 
some of these contain periodically foundering lava 
lakes [8], whereas others are resurfaced by bright or 
dark lava flows. We have mapped a total of 425 pate-
rae on Io, an increase from the 417 previously identi-
fied by [7]. Yet even though paterae cover only 2.5% 
of Io’s surface (and dark patera floor material covers 
only 0.5%), Io’s hot spots dominantly occur within 
paterae (63.9% of all hot spots, with 45.3% correlated 
with dark patera floor material). The fact that 93.5% of 
Ionian hot spots correlate with either dark (younger) or 
undivided (older) patera floors or lava flows suggests 
that silicate materials are the dominant component of 
Io’s recent volcanism. 
Results IV (Mountains): Mountains cover only 
3.1% of Io’s surface, yet are some of the most dra-
matic features observed in spacecraft images. A major-
ity of Io’s mountains, 37.9%, were mapped as undi-
vided mountain materials, in contrast to mapping 
27.1% as Lineated mountains, 22.1% as Layered 
plains, 3.6% as Tholi (volcanic mountains), and 2.9% 
as Mottled mountains. These results demonstrate that 
variable imaging coverage of Io’s mountains inhibits 
more accurate mapping of undivided mountains into 
the other units. As expected, lineated mountains, 
thought to be tectonically uplifted crustal blocks [9], 
are generally taller than the more degraded mottled 
54
https://ntrs.nasa.gov/search.jsp?R=20110002769 2019-08-29T18:27:46+00:00Z
 mountains and layered plains. Volcanic tholi are less 
than 2km tall, indicative of mafic to ultramafic materi-
als that tend not to build tall volcanic edifices. We are 
working on additional correlations, including the loca-
tions and areas of mountains with heights and image 
resolution, to further identify relationships that can 
provide insights on the genesis of these features. 
Results V (Diffuse Deposits): Diffuse deposits 
(DD) that mantle the other units cover ~18.2% of Io’s 
surface, and are distributed as follows: red (47.2% of 
all DD), white (37.9%), yellow (11.5%), black (3.3%), 
and green (~0.1%). Red DD are thought to be derived 
by condensation of S2 gas and recrystallization to 
short-chain sulfur (S3-S4 ± chlorides) from volcanic 
vents. These are dominated by ring-like units resulting 
from outbursts, except for that found surrounding vol-
canoes such as Pele that are continuously replenished 
by ongoing activity. White DD, mostly irregular in 
shape and often surrounding lava flow boundaries, 
result from vaporization, condensation, and reaccumu-
lation of SO2 frosts around warm flow margins. The 
relative lack of pyroclast-bearing DD, i.e., dark (black) 
DD (thought to be derived from silicate pyroclasts) 
and yellow DD (thought to be derived from sulfur py-
roclasts) compared to gas-derived White and Red DD 
(14.8% vs. 85.1%) may suggest that sulfur and SO2 do 
not have a major role as volatiles that disrupt or inter-
act with sulfur or silicate magmas. 
References: [1] Becker, T. and P. Geissler (2005), 
LPS XXXVI, Abstract #1862. [2] Williams, D.A. et al. 
(2007), Icarus, 186, 204-217. [3] Rathbun, J.A. and 
S.E. Barrett (2007), LPS XXXVIII, Abstract #2123. [4] 
Williams, D.A. et al. (2008), LPS XXXIX, Abstract 
#1003. [5] Williams, D.A. et al. (2009), LPS XL, Ab-
stract #1403. [6] Greeley, R., et al. (1984), Icarus 60, 
189-199. [7] Radebaugh, J., et al. (2001), JGR 106, 
E12, 33,005-33,020. [8] Lopes, R.M.C., et al. (2004), 
Icarus 169, 140-174. [9] Schenk, P. and Bulmer, M. 
(1998), Science, 279, 1514-1517. 
 
Figure 1. The global geologic map of Io, produced using ArcGIS™ software based on the combined Galileo-Voyager mosaics 
produced by the U.S. Geological Survey [1]. For additional information on mapping strategy, areal coverage of map units, and 
stratigraphic correlation of map units, see [2, 4, 5]. 
55


Volcanism on Io: Results from Global Geologic Mapping

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