2011

U.S. Department of the Interior

U.S. Geological Survey

Prepared for the National Aeronautics and Space Administration

Geologic Map of Io

By David A. Williams, Laszlo P. Keszthelyi, David A. Crown, Jessica A. Yff, Windy L. Jaeger, 

Paul M. Schenk, Paul E. Geissler, and Tammy L. Becker

Pamphlet to accompany

Scientific Investigations Map 3168
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Contents

Introduction ....................................................................................................................................................1

Base Map Data Products ............................................................................................................................1

Color Mosaic .........................................................................................................................................1

Galileo Monochrome Mosaic .............................................................................................................2

Combined Mosaic ................................................................................................................................2

Merged Mosaic ....................................................................................................................................2

Mapping Techniques ....................................................................................................................................2

General Geology ............................................................................................................................................4

Material Units .......................................................................................................................................5

Mountain Units ............................................................................................................................5

Plains Units ..................................................................................................................................6

Patera Floor Units .......................................................................................................................6

Flow Units .....................................................................................................................................7

Tholus Units .................................................................................................................................8

Diffuse Deposits ..........................................................................................................................8

Structure .........................................................................................................................................................8

Stratigraphy ...................................................................................................................................................8

Geologic History ............................................................................................................................................9

Acknowledgments ......................................................................................................................................10

Appendix .......................................................................................................................................................10

References Cited .........................................................................................................................................10

Tables

1. Latitude-longitude coordinates of type examples of map units shown in figure 1. ....................14

2. Distribution of geologic material units as percentage of Io’s surface area. ................................15

3. Locations, heights, and areal extents of the ten largest mountains, ten lineated mountains, 

and nine mottled mountains on Io. ...........................................................................................16

4. Locations and areal extents of the ten largest patera floors, ten dark patera floors, and ten 

bright patera floors on Io. ..........................................................................................................17

5. Locations and areal extents of the ten largest lava flow fields, ten largest bright lava flow 

fields, and ten largest dark lava flow fields on Io. .................................................................18

6. Stratigraphic justification of Io map units as displayed in the Correlation of Map Units. .........19

7. Imaging coverage of Io from the Voyager ISS, Galileo SSI, and New Horizons instruments. ..22

8. List of the new Io global mosaics produced by the USGS, combining Galileo and Voyager 

images. ..........................................................................................................................................23

9. Image numbers of Galileo and Voyager images used to make the Combined Mosaic. .............24

10. Image numbers of Galileo images used to make the SSI-only global Color Mosaic. ..............25
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1

Introduction

Io, discovered by Galileo Galilei on January 7–13, 1610, 

is the innermost of the four Galilean satellites of the planet 

Jupiter (Galilei, 1610). It is the most volcanically active object 

in the Solar System, as recognized by observations from six 

National Aeronautics and Space Administration (NASA) 

spacecraft: Voyager 1 (March 1979), Voyager 2 (July 1979), 

Hubble Space Telescope (1990–present), Galileo (1996–2001), 

Cassini (December 2000), and New Horizons (February 2007). 

The lack of impact craters on Io in any spacecraft images at any 

resolution attests to the high resurfacing rate (1 cm/yr) and the 

dominant role of active volcanism in shaping its surface (Johnson and others, 1979). High-temperature hot spots detected by 

the Galileo Solid-State Imager (SSI), Near-Infrared Mapping 

Spectrometer (NIMS), and Photopolarimeter-Radiometer (PPR) 

usually correlate with darkest materials on the surface, suggesting active volcanism (McEwen and others, 1998a; Lopes and 

others, 2001; Williams and others, 2002). The Voyager flybys 

obtained complete coverage of Io’s subjovian hemisphere at 

500 m/pixel to 2 km/pixel, and most of the rest of the satellite 

at 5–20 km/pixel (Smith and others, 1979a,b). Repeated Galileo 

flybys obtained complementary coverage of Io’s antijovian 

hemisphere at 5 m/pixel to 1.4 km/pixel (McEwen and others, 

2000b; Keszthelyi and others, 2001; Turtle and others, 2004). 

Thus, the Voyager and Galileo data sets were merged to enable 

the characterization of the whole surface of the satellite at a 

consistent resolution. The United States Geological Survey 

(USGS) produced a set of four global mosaics of Io in visible 

wavelengths at a spatial resolution of 1 km/pixel (Becker and 

Geissler, 2005), released in February 2006, which we have 

used as base maps for this new global geologic map. Much has 

been learned about Io’s volcanism, tectonics, degradation, and 

interior since the Voyager flybys, primarily during and following the Galileo Mission at Jupiter (December 1995–September 

2003), and the results have been summarized in books published 

after the end of the Galileo Mission (Bagenal and others, 2004; 

Davies, 2007; Lopes and Spencer, 2007). Our mapping incorporates this new understanding to assist in map unit definition and 

to provide a global synthesis of Io’s geology.

Base Map Data Products

Four distinct image products were created by the USGS 

and were used in this mapping project: (1) a global mosaic of 

SSI color images (hereafter referred to as “color mosaic”); (2) 

a global mosaic of the best resolution Galileo SSI monochrome 

images (hereafter referred to as “Galileo monochrome mosaic”); 

(3) a global mosaic of the best quality and highest resolution Voyager Imaging Science Subsystem (ISS) and Galileo 

SSI monochrome images (hereafter referred to as “combined 

monochrome mosaic”); and (4) a merged product combining 

Galileo color information with the higher resolution combined 

monochrome mosaic (hereafter referred to as “merged mosaic”). 

There are important technical details to consider regarding the 

production of the new global Io mosaics (fig. 1A–1D). Each of 

these products is described more fully below. An extensive set 

of ancillary data was developed for each mosaic to help users 

understand the various combinations of data from different 

sensors, filters, dates, and illumination and viewing geometries. 

These include footprint plots that show the identity of each of 

the component images (see appendix) and diagrams that show 

the incidence, emission, and phase angles, as well as the spatial 

resolutions of the individual frames used.

Color Mosaic

By the conclusion of the Galileo Mission, higher resolution color data and consistent phase-angle coverage by the 

SSI prompted a revision of the global color mosaics (fig. 1A) 

produced earlier in the mission (Geissler and others, 1999). The 

Galileo SSI GLOCOL01 sequence recorded during orbit I31 

(August 2001) completed longitudinal coverage at an optimum 

phase angle near 4°, low enough to reveal subtle color variations with minimal topographic influences but high enough to 

exclude the opposition surge. The antijovian hemisphere was 

imaged in color during orbit C21 (June 1999) at a resolution of 

1.3 km/pixel, revealing unprecedented detail (please see appendix for explanation of Galileo mission abbreviations). Archinal 

and others (2001) established a planet-wide control net, ensuring a much higher level of geometric fidelity than that achieved 

by Geissler and others (1999).

The global color mosaic that we used was constructed in 

two steps. First, the consistent phase angle (3.5–4.5°) 756 nm 

(NIR), 563 nm (GRN), and 411 nm (VIO) filter images from 

orbits G2 (September 1996), C9 (June 1997), C21 (June 1999), 

and I31 (August 2001) were calibrated using the best end-ofmission calibration information, coregistered to subpixel precision, and map-projected using the Archinal and others (2001) 

camera-pointing corrections. A Lunar-Lambert limb-darkening 

correction was next performed, after having first determined 

that a coefficient of 0.7 was adequate for all three colors. 

Finally, the images were mosaicked together to form a baseline 

estimate of Io’s global color that was free of photometric effects 

caused by variations in phase angle but subject to spatial-resolution limitations and geometric distortions, especially in areas 

imaged near the limb.

The second step improved on the global color mosaic by 

including higher resolution images from orbit E6 (14° phase, 

February 1997), adjusted to match the color and contrast of the 

4° phase angle data. The selected color images were processed 

as above, hand-edited to remove topographic shadows and 

bright or dark pixels too near the limb, and mosaicked using a 

numerical procedure (Soderblom and others, 1978) that reduces 

the mismatch at the seams. This mosaic represents our best 

understanding of Io’s appearance during the Galileo Mission 

and is the only product of the current set that can be relied upon 

for photometric interpretations (such as deciding whether any 

given feature is brighter or darker than another). Users should 

keep in mind several limitations: the use of false color (NIRGRN-VIO), required because of the limited wavelengths of the 

SSI filters; variations in incidence and emission angles; uncertainties in the limb-darkening correction, especially at the poles; 
2

and mixed resolutions, ranging from 1.3 to 21 km/pixel at the 

equator, with the poorest resolution on the subjovian hemisphere of Io.

Galileo Monochrome Mosaic

A monochrome mosaic (fig. 1B) suited to studies of local 

and regional morphology was created by mosaicking the best 

quality global monitoring images taken by the Galileo SSI at a 

spatial resolution of 1 km/pixel. This mosaic is made up of 32 

monochrome images taken at various phase angles and local 

times of day, so care must be taken to note the solar illumination direction when deciding whether topographic features 

display positive or negative relief. In general, the illumination 

is from the west over longitudes from 0° to 270° W. and from 

the east over longitudes from 270° W. to 360° W. The images 

were empirically adjusted in brightness and contrast to match 

one another in areas of overlap. Most of the images were taken 

using the clear filter (CLR, 663 nm), but GRN and NIR filter 

images were substituted when they had a larger digital number 

(DN) contrast than other available images. Image resolutions 

range from 1.3 to 10 km/pixel along the equator, and the poorest 

coverage is on Io’s subjovian hemisphere.

Combined Mosaic

Although the subjovian hemisphere of Io was poorly seen 

by Galileo, superbly detailed Voyager 1 images cover longitudes 

from 240° W. to 40° W. and the nearby southern latitudes. A 

global combined mosaic (fig. 1C) of the best resolution images 

from both Galileo and Voyager 1 was assembled that includes 

50 Voyager 1 images with spatial resolutions locally exceeding 

the 1 km/pixel scale of the final mosaic. These Voyager images 

were calibrated with an additional correction for dark current 

build-up during passage through the plasma torus, as described 

by McEwen (1988). Although most of these images neatly overlie the Galileo coverage, the Voyager 1 images of regions west 

of 0° longitude appear displaced from the positions predicted 

from the control net derived from the solution of the combined 

Galileo and Voyager data set. We suspect that the discrepancy 

arises from regional topography west of the subjovian point 

and have adjusted the positions of the Voyager 1 images (all 

acquired near the limb of Io) to match the best-fit geometry. 

Solar illumination of these longitudes was from the east during 

the Voyager 1 flyby. The one area not well imaged by either 

Galileo or Voyager is the north pole.

Merged Mosaic

To present the most information in a single view of Io, the 

global color, derived from the Galileo color images, was superimposed on the higher resolution combined mosaic to create a 

merged mosaic (fig. 1D). We calculated color ratio images from 

the Galileo data and applied them to the monochrome mosaic, 

which required that the color ratios of the composite images 

match the color ratios of the Galileo data. That is, the red brightness was computed as the product of the monochrome mosaic 

multiplied by the ratio of the Galileo NIR/GRN filters and the 

blue brightness as the monochrome mosaic times the Galileo 

VIO/GRN filter ratio. The Galileo SSI’s silicon charge-coupled 

device (CCD) was sensitive to longer wavelengths than the vidicon cameras of Voyager, so distinctions between red and yellow 

hues can be more easily discerned. However, this approach 

suffers from an obvious disadvantage in areas where the surface 

appearance changed dramatically during the >20 year interval 

between the Voyager and Galileo eras, for example near Kanehekili (lat 18° S., long 34° W.).

Full-resolution TIFF and JPEG versions of the merged 

mosaic, as well as an MPEG (Quicktime compatible) animation, 

are available for free download from the NASA Planetary Photojournal (http://photojournal.jpl.nasa.gov), under PIA09257.

Mapping Techniques

Geologic mapping of Io has focused on specific types of 

geologic material units and structural features, as first recognized by Schaber (1980, 1982), because of (1) the quantity of 

concurrent volcanic activity on Io and (2) the variable quality of the global spacecraft data. These constraints mean that 

it is not practical to separate map units based on both age and 

lithology. The types of material units and structures that were 

defined served as a template for mapping four specific regions 

of Io’s subjovian hemisphere, and the following maps were 

produced and published by the USGS during the Voyager era: 

Maasaw Patera, 1:1,000,000 (Moore, 1987); Ra Patera quadrangle, 1:2,000,000 (Greeley and others, 1988); Ruwa Patera 

quadrangle, 1:5,000,000 (Schaber and others, 1989); and Lerna 

region, 1:5,000,000 (Whitford-Stark and others, 1991). These 

maps were then synthesized to produce the first global geologic 

map at 1:15,000,000 (Crown and others, 1992). Although the 

relatively low resolution of the antijovian hemisphere imaged 

by Voyager inhibited the completeness of the global map, 

Schaber’s (1980, 1982) early work and the Voyager-era maps 

continue to serve as templates of material units and structures 

on Io that are refined using later spacecraft data.

The material units described in the previously published 

USGS maps required only minor modification when applied 

to regional geologic mapping of Io using Galileo imaging data 

in the early 2000s. During the Galileo era, three regional maps 

(image spatial resolutions ~190–570 m/pixel) of sections of 

the antijovian hemisphere were produced and published in 

scientific journals: Chaac-Camaxtli Paterae region (Williams 

and others, 2002); Culann-Tohil Paterae region (Williams and 

others, 2004); and Zamama-Thor region (Williams and others, 

2005). Since the end of the Galileo Mission, four additional 

Galileo-based regional maps have been produced: Amirani-Gish 

Bar region (Williams and others, 2007a); Zal region (Bunte 

and others, 2008); Hi’iaka-Shamshu region (Bunte and others, 

2010); and Prometheus Eruptive Center (Leone and others, 

2009).

Using the experience we gained from our past work on 

various Voyager- and Galileo-era maps, particularly the Ami-
3

rani-Gish Bar map (Williams and others, 2007a), we developed 

the following five-step strategy for producing a new global map 

using the USGS mosaics (see also Williams and others, 2007a):

Step 1. Map diffuse deposits separately using color data 

only (color mosaic)

Step 2. Map mountains, plateaus, and layered plains 

(combined mosaic, supplemented with low-sun 

images)

Step 3. Map vents and paterae (combined and color 

mosaic)

Step 4. Map lava flows: first map those around each vent, 

then add more detail within flucti (combined and 

color mosaic)

Step 5. Map plains last (combined and color mosaic)

Map units were defined on the basis of their color characteristics, albedo, morphology, and surface texture, using primarily the global mosaics but with knowledge of characteristics 

revealed in individual Voyager and Galileo images. Descriptions 

of subunits or regions covered by high-resolution data have 

been included. The mapping strategy that we developed for the 

global map was initially tested through mapping of the polar 

regions with ArcGIS 9.3 software, which was then applied to the 

equatorial region. This was a stringent test of our strategy, given 

the difficulty in polar mapping, because of the widely varying 

resolution in the polar regions. Insights gained from the test for 

each of the steps of our mapping strategy are presented below:

Step 1. Map diffuse deposits using color mosaic. The SSI 

color data alone are sufficient to identify and map 

the five different colors of diffuse deposits on Io. 

In some cases, the context of the diffuse deposits 

is not clear, and we found it beneficial to switch 

from views of the color mosaic to the combined 

and merged mosaics to see the underlying morphology. This is especially true around positive- or 

negative-relief topographic features. With the 

exception of the green diffuse deposit centered 

on Culann Patera, all other green diffuse deposits 

observed in higher resolution Galileo SSI images 

are too small to be mapped on the global mosaics.

Step 2. Map mountains, surrounding plateaus, and structural features using the combined mosaic. Many 

of these features are easily visible in the combined 

and merged mosaics, where the mosaics include 

images that were obtained at low-sun angle to 

show the shadows associated with scarps, ridges, 

and other local relief. For areas of the mosaics 

where such images were not included, other data 

sources were consulted (for example, original 

Voyager or Galileo image products, Voyager-based 

shaded-relief maps, or other products). Additional 

stretching was also found to maximize viewing 

of the combined mosaic. The merged mosaic is 

also useful to map mountains, layered plateaus, 

scarps, and other structural features. Lineated, 

mottled, and undivided mountain materials were 

all mapped using these mosaics.

Step 3. Map vents and paterae. Bright, dark, and undivided patera floor materials were identified using 

all of the mosaics. However, we found that the 

merged mosaic was the best tool to identify these 

features.

Step 4. Map lava flow fields. Bright, dark, and undivided 

flow materials were identified using all of these 

mosaics. The merged mosaic was the ideal product to map these features (in terms of color and 

context), although switching views to the Galileo 

monochrome and combined mosaics was useful 

for robust mapping.

Step 5. Map plains. Plains include the regions that did 

not fit into the previous categories. The primary 

inter-patera plains materials have four subunits 

based on color and morphology (yellow, white, red 

brown, and layered). Lower resolution coverage 

and color variations in the polar mosaics make 

some identifications difficult, although all subunits 

are present. All four mosaics were reviewed to 

map these features. One region of the north polar 

plains is covered at particularly poor resolution in 

all mosaics, and, although we think it is probably 

red-brown plains material, we conservatively left 

it mapped as a zone of poor resolution.

The mosaics revealed various linear and point features that 

we were able to map. The linear features include scarps, ridges, 

grooves, graben, one sinuous channel, and lineaments (linear 

structures whose form cannot be interpreted). The point features 

include small positive-relief edifices (interpreted to be shields 

or cones), small dark spots (interpreted to be volcanic vents, 

either very small paterae or sources for outgasing), or small pits. 

Any feature <20 km diameter was mapped as a point feature, 

and the feature name in the map symbol explanation includes 

the word “small”. Most of these features have a width of a 

few pixels across in the mosaics when viewed at 100 percent 

zoom and, thus, are most appropriately mapped using lines and 

points. Linear features range in length from several pixels at 

100 percent zoom to tens of kilometers in length. Point features 

and geomorphologic structures (linear features) were mapped as 

separate layers in the ArcGIS project. Note that not all detail is 

visible in the printed map at the print map scale. Please refer to 

the electronic version of the Io map for full detail.

Because the primary focus of the mapping was to determine the areal extents of the major material units, we produced 

material-unit polygons from which contact lines were generated 

later. Because of the distinctive colors of Io’s volcanic materials, most of the mapped material units have distinct contact 

relations that are shown as certain contacts. Mountains and 

patera floors have structural contacts, which are positive-relief 

edifices and closed topographic depressions, respectively. Thus 

their structural boundaries serve as contacts. Layered plains, by 

definition, are revealed by their bounding scarps. Flow materials are typically darker or brighter than the surrounding plains 

and have definite or gradational contacts. The three plains units 

have three different colors, and typically certain or gradational 

contacts can be identified. Some mountains grade into layered 

plains, and some undivided flow materials from specific volcanoes may grade into those from its neighbors. In these cases, 

gradational or approximate contacts are shown on the map.
4

General Geology

Io was recognized to have unusual spectroscopic characteristics in the 1970s, prior to the 1979 NASA Voyager flybys. 

Also prior to the Voyager flybys, Io’s 4:2:1 Laplace orbital 

resonance with Europa and Ganymede was predicted to cause 

severe tidal heating, resulting in active volcanism (Peale and 

others, 1979). The Voyager flybys confirmed this prediction; 

not only was Io volcanically active, it is the most volcanically 

active body in the Solar System. Voyager showed that Io has an 

intensely colored surface, which suggests the presence of sulfur 

compounds, and a lack of impact craters, which is interpreted 

as evidence for a high resurfacing rate (1 cm/yr, Johnson and 

others, 1979) and a very young surface (Smith and others, 

1979a,b). When navigation images showed a 260-km-high 

plume on Io’s limb (Morabito and others, 1979), searches of 

other images revealed additional active plumes. The detection 

of SO2 gas from the Loki plume and of surface hot spots by 

the Infrared Interferometer Spectrometer (IRIS) demonstrated 

that rampant volcanism was occurring on Io (Pearl and others, 

1979).

Perhaps the greatest debate ignited by the Voyager data 

revolved around the composition of the active volcanism—

sulfur or silicates? Advocates of the sulfur hypothesis pointed 

to a predominantly sulfur surface composition in Earth-based 

spectral data (Wamsteker and others, 1974) and the presence 

of SO2 frost on the surface in Voyager data (Fanale and others, 

1979; Smythe and others, 1979). Sagan (1979) and Pieri and 

others (1984) suggested that the observed surface colors indicated different allotropes of sulfur. However, the validity of 

color arguments was questioned, because the Voyager ISS did 

not accurately reproduce true color images (Young, 1984; Nash, 

1987). Advocates of the silicate hypothesis pointed to Io’s bulk 

density (~3,500 kg m-3) and the topography of Io’s mountains 

and volcanoes: slopes on Io’s mountains and paterae required 

mechanically strong silicates (Clow and Carr, 1980; Moore and 

others, 1986). Although the temperatures measured by the IRIS 

suggested sulfur—molten sulfur pools and flows were expected 

to be in the range of 400–600K (sulfur vaporizes at ~718K at 

Io’s surface), as opposed to temperatures of ~1,500K for molten 

basalts—cooling silicates pass through temperatures in the same 

range as sulfur. Thus, the temperatures of hot spots detected by 

IRIS (Pearl and others, 1979), which were below 650K, could 

be interpreted as either composition. The first strong evidence 

for silicate volcanism came from post-Voyager Earth-based telescopic observations that showed eruption temperatures >800K, 

which were definitely too high for sulfur (Johnson and others, 

1986; Veeder and others, 1994). Galileo observations confirmed 

the presence of mafic silicate volcanism (Carr and others, 1998; 

McEwen and others, 1998b) and suggested that Io could erupt 

lavas with temperatures consistent with ultramafic silicates 

(McEwen and others, 1998b; Davies and others, 2001; Williams and others, 2001a). More recent reevaluation of Galileo 

temperature data, however, suggests that ultramafic compositions are not required to explain Ionian eruption temperatures 

(Keszthelyi and others, 2007). Although Io appears to be 

dominated by silicate volcanism at most active hot spots, there 

is some evidence, such as bright, lightly colored lava flows and 

low eruption temperatures, for active sulfur volcanism as well 

(Williams and others, 2001b, 2004).

Voyager detected a high heat flow from Io’s surface, 

resulting from the many active volcanic hot spots that radiate 

Io’s internal heat to space. The combination of Voyager IRIS 

measurements of thermal emission from hot spots and diskintegrated Earth-based telescopic observations enabled preGalileo estimates of Io’s heat flow of ~1.5 W m-2 (Morrison and 

Telesco, 1980; Johnson and others, 1984; McEwen and others, 

1985) to at least 2.5 W m-2 (Veeder and others, 1994). Additional Galileo observations, including the polar regions, suggest 

that current best estimates of Io’s heat flow are ~3±1 W m-2 and 

~1.3±0.4 x 1014 W (Veeder and others, 2004).

Io’s extensive color palette is thought to result from a variety of sulfur-bearing compounds that mantle the surface and are 

produced by active volcanism (Geissler and others, 1999). Spectral detection of various sulfur species extends back to the 1970s 

(Johnson and McCord, 1971; Wamsteker and others, 1974). 

Voyager positively identified SO2 on the surface (Smythe and 

others, 1979), and McCauley and others (1979) suggested that 

the agent responsible for the many erosional scarps observed on 

the surface was the explosive escape of sulfur dioxide from subterranean SO2 aquifers. The idea of subterranean SO2 reservoirs 

gained some support from the analysis of Galileo data of the 

movement of the Prometheus plume (Kieffer and others, 2000). 

Today it is well understood that sulfur compounds mantle the 

surface, although the exact composition of these sulfur compounds remains ambiguous. SO2 is ubiquitous on Io, except 

at locations of hot spots (Douté and others, 2001). The yellow 

color observed in Galileo SSI images is thought to be either 

cyclo-octal sulfur (S8) with or without a covering of SO2 frosts 

deposited by plumes or, alternatively, polysulfur oxide and S2O 

without large quantities of elemental sulfur (Hapke, 1989). The 

red materials are interpreted to be composed of short-chain 

sulfur molecules (S3, S4) resulting from either breakdown of 

cyclo-S8 by charged-particle irradiation (in polar regions) or by 

condensation of S2-rich volcanic gases in the plumes of active 

vents (in the equatorial region, Spencer and others, 2000). 

Greenish-yellow patches exist in or near several active vents 

and are thought to be an alteration product of silicate and sulfur 

that is perhaps composed of either sulfur compounds contaminated by iron from silicates or silicates such as olivine or pyroxene with or without sulfur-bearing contaminants (Geissler and 

others, 1999). White to gray materials are interpreted to be composed of concentrated, coarse- to medium-grained SO2 resulting 

from plume fallout and subsequent recrystallization (Carlson 

and others, 1997; Douté and others, 2001, 2002). Black materials mostly correlate with active hotspots and occur as patera 

floors, as lava flow fields, or as dark diffuse materials near or 

surrounding active vents. These materials are most consistent 

with Mg-rich orthopyroxene (enstatite or bronzitehypersthene) 

and indicate silicate lava flows or lava lakes (within paterae) 

or diffuse silicate pyroclastic deposits of mafic to ultramafic 

composition (Geissler and others, 1999).

Mountains are the dominant structural landforms that are 

visible on Io and are recognized as steep-sided edifices rising 

more than ~1 km above the plains (Schenk and others, 2001). 

Approximately 150 mountains were identified and mapped by 
5

Carr and others (1998), Schenk and others (2001), Jaeger and 

others (2003), McEwen and others (2004), and Turtle and others 

(2007). Io’s mountains typically rise approximately 6 km in 

height; the highest (Boösaule Montes) rises >17 km above the 

surrounding plains. Galileo images show that many mountains 

are partly or completely surrounded by plateaus, layered plains, 

and debris aprons (Turtle and others, 2001). Various models 

have been proposed to explain the origin of the mountains 

(Schenk and Bulmer, 1998; Turtle and others, 2001; McKinnon 

and others, 2001; Jaeger and others, 2003). Their asymmetrical 

shapes suggest uplift along thrust faults, implying that compressional uplift is probably the dominant mechanism. McEwen and 

others (2000a) and Jaeger and others (2003) showed statistical 

evidence that far more paterae and mountains were in direct 

contact with each other than random chance would allow (probability ~0.1%). This result suggests a possible genetic link; 

perhaps the magma exploits the weakness in the lithosphere 

created by deep orogenic faults. Interestingly, Schenk and 

others (2001) showed a global anticorrelation between the areas 

with somewhat higher concentrations of observed mountains 

and the areas with somewhat higher concentrations of observed 

volcanic centers. However, the areas of low mountain concentration also correlate with areas with poor imaging for detecting 

topographic features (only medium resolution images and (or) 

high sun). Therefore, it is not yet confirmed that this anticorrelation is not an observational bias. Radebaugh and others (2001) 

found evidence that the concentration of observed paterae is 

slightly affected by observational bias.

Material Units

The original geologic mapping of Io using Voyager images 

by Schaber (1980, 1982) divided Io into geologic units of four 

principal classes: plains material, mountain material, patera 

floor material, and volcanic flows. Ephemeral diffuse deposits, 

an additional unit class, was also recognized in the Voyager data 

and mapped using stippled patterns superposed upon the other 

units (Crown and others, 1992). These five classes of material 

units continue to serve as useful designators for mapping Ionian 

materials even as higher spatial resolution and more accurate 

color data were obtained by later missions, particularly Galileo. 

These new data enabled additional subunits of these five classes 

to be defined and characterized, and spectral analysis of Galileo 

SSI and NIMS data allowed compositional information to be 

correlated with color and, to a lesser extent, morphological 

information. These data also enabled new studies of degradational features and the recognition of volcano-tectonic relations. 

Major results from the study of Galileo data of Io in terms of 

surface geology, composition, and volcanic activity can be 

found in McEwen and others (1998a,b) and Lopes-Gautier and 

others (1999) and in special issues of Science (Volume 288, 19 

May 2000), the Journal of Geophysical Research (Volume 106, 

25 December 2001), and Icarus (Volume 169, May 2004), as 

well as in review papers by Geissler (2003) and Lopes and Williams (2005) and in books by Bagenal and others (2004), Lopes 

and Spencer (2007), and Davies (2007). The distribution of 

material units as percentage of Io’s surface area is given in the 

Results section (table 2). Below, we include brief descriptions 

of the major categories of Ionian material units to supplement 

the information provided in the Description of Map Units and 

figure 2, including initial descriptions from previous Voyagerbased image analysis and mapping.

Mountain Units

Initial study of Voyager images first revealed the presence 

of mountains on Io. Schaber (1980) described mountains as isolated high massifs covering 1.9 percent of the total area of the 

first preliminary geologic map of Io (that map covered 26.5% 

of the surface at a spatial resolution of 1–10 km/pixel). At this 

time, mountains were interpreted to be the oldest geologic unit 

exposed on the surface, based on stratigraphic relations (Schaber, 1980). Their relief was estimated as at least 9±1 km high, 

and the mountains had a tectonically disrupted, topographically 

rugged surface (Schaber, 1980), probably composed of silicate 

volcanic material to support this significant relief (Carr and 

others, 1979; Masursky and others, 1979; Sagan, 1979; Clow 

and Carr, 1980). Mountains were associated with layered plains 

and paterae, and Schaber’s description of some mountains 

containing pit craters and vents suggests that features we now 

recognize as tholus material (volcanic mountains) were initially 

lumped together with tectonic mountains. At this time, it was 

recognized that mountains were locally mantled by diffuse 

deposits of various colors, and in some cases high-albedo aureoles were identified surrounding the bases of some mountains, 

such as Haemus Montes. Lobate scarps just north of a mountain 

near Creidne Patera were recognized as having a morphology 

similar to landslides or lava flows (Schaber, 1980).

During the subsequent geologic mapping of Voyager-based 

quadrangles and specific areas, additional subdivisions of mountain units were made. Both Greeley and others (1988) in the Ra 

Patera quadrangle and Whitford-Stark and others (1991) in the 

Lerna Regio region separated mountain material from either 

dome material or cone material (respectively) to distinguish 

apparent volcanic mountains from tectonic ones. Schaber and 

others (1989) subdivided mountain materials in the Ruwa Patera 

quadrangle into three types: grooved, smooth, and undivided. 

The descriptions and interpretations of these units, including the 

presence of ridges and grooves on the grooved material and the 

evidence of down-slope movement in the smooth material, indicate that our units are the direct descendants of these mountain 

materials used in the Voyager-era maps.

As outlined earlier, there are two dominant theories on the 

formation and distribution of Io’s mountains, and a good review 

is presented in Jaeger and others (2003). Mountain genesis is 

still not well understood. Schenk and Bulmer (1998) suggested 

that a horizontal lithospheric compressive stress is generated 

because of Io’s rapid resurfacing rate, which results in uplift 

of crustal blocks via thrust faulting. Alternatively, McKinnon 

and others (2001) suggested that sustained reduction in Io’s 

volcanic activity at local, regional, or global scales results in 

lithospheric heating that causes a large compressive stress at the 

base of the lithosphere. This stress causes, over time, fluctuating thermally induced stress as resurfacing rates wax and wane, 
6

leading to alternating episodes of tensile and compressive 

faulting. Such repeated episodes of normal and reverse faulting 

might produce coherent crustal blocks (mountains) that float in 

a matrix of highly disrupted material, similar to the chaos terrain of Europa (McKinnon and others, 2001). Through study of 

additional Galileo images, Jaeger and others (2003) cataloged 

143 mountains and mountain-like features; of 92 mountains of 

sufficient resolution to infer a tectonic origin, 38 abut paterae. 

They proposed that orogenic faults associated with mountains 

act as conduits that feed magma to the surface, enabling patera 

formation near mountains. Lithospheric swells, produced by 

thermal diapirs impinging on the base of the lithosphere, may, 

in some cases, focus compressive stress and cause the formation 

of isolated mountains (Jaeger and others, 2003).

Our mapping shows that mountain material makes up ~3 

percent of Io’s surface, generally has colors and albedo similar 

to bright plains material, and is usually only visible in lowsun images where shadows highlight scarps, ridges, grooves, 

and mountain peaks. Layered plains, which include mesas and 

plateaus, make up another 4.4 percent of the surface and are 

often related to degrading mountains. We mapped 240 mountains and layered plains units on Io (121 mountains and 119 

layered plains units), compared to the 143 mountains previously 

identified by Jaeger and others (2003). Additional mountains, 

interpreted from limb fits and stereo image analyses by Schenk, 

were included in our map. Note that, at the resolution of these 

mosaics, mountains typically grade into layered plains, which 

allows different mappers to interpret this boundary differently. 

All mountains identified on Io have heights between 1 and >17 

km (see Schenk and Bulmer, 1998; McKinnon and others, 2001; 

Schenk and others, 2001; Turtle and others, 2001, 2007; Jaeger 

and others, 2003). We characterized three types of mountain 

materials (fig. 3) on Io: lineated (unit ml

, ~1.5%, containing 

well-defined ridges and grooves, interpreted to be tectonically 

modified, uplifted, autochthonous crustal blocks), mottled (unit 

mm, ~0.2%, containing lobes and dome-like hills, interpreted 

to include materials displaced by mass-wasting processes), and 

undivided (unit mu, ~1.3%, in which limited resolution prohibits classification as either lineated or mottled). The ten largest 

mountains (of all types) on Io, the ten largest lineated mountains, and the nine mottled mountains and all mountain areas 

are reported in table 3. In general, the lineated mountains (as a 

group) are higher than mottled mountains, which is consistent 

with the interpretation that mottled mountains have undergone 

degradation by mass wasting (figs. 4, 5). About 40 percent 

of mountains occur in close proximity to paterae (Jaeger and 

others, 2003), suggesting that at least some paterae and mountains have a genetic link. In several cases, volcanic vents are 

seen to preferentially form along the faults that bound mountains (McEwen and others, 2000a; Jaeger and others, 2003).

Plains Units

The initial geologic mapping of best-resolution Voyager 

images described three types of plains materials on Io’s surface: inter-vent plains, layered plains, and eroded layered plains 

(Schaber, 1980, 1982). Plains units comprised approximately 

one-half of the 34.8 percent of the Ionian surface that was 

mapped, and inter-vent plains represent the dominant type 

(39.6% of the mapped surface). Plains are also recognized as 

the material type that has the largest areal extent in our global 

characterization, although the proportion of the surface identified 

as plains is higher globally than in the initial Voyager mapping. 

The plains were interpreted to be stratified materials from plume 

fallout interbedded with flow and fumarole deposits, and the 

different units were distinguished by the presence of bounding 

scarps (layered plains) and clusters of small erosional remnants 

of a once more continuous surface (eroded layered plains). 

The plains units in Voyager images showed a variety of colors 

attributed to the presence of sulfur. Mapping of 1:5M-, 1:2M-, 

and 1:1M-scale quadrangles all included intervent or interpatera 

plains units, as well as recognized layered plains (or plateaus) 

that have a higher concentration of structural features (Moore, 

1987; Greeley and others, 1988; Schaber and others, 1989), similar to the 1:15M Voyager global map (Crown and others, 1992).

Our mapping shows that plains material covers ~67 percent 

of Io’s surface and is subdivided into yellow (~18%), white 

(~9%), and red-brown (~35%) units. A fourth unit, layered plains 

(~4%), consists of any of the other types that are morphologically isolated into distinct layers separated from underlying 

plains by distinct bounding scarps indicative of erosional processes or mass wasting from mountains. In general, plains material is thought to consist of various combinations of silicate and 

sulfur-rich material in the form of buried lava flows, pyroclastic 

deposits, and frosts from condensed volcanic gases that make up 

the upper crust of Io, further mantled by sulfur-rich (yellow) or 

SO2-bearing + contaminants (white) deposits traceable to one or 

more of Io’s many active volcanoes (McEwen and others, 2000b; 

Williams and others, 2002). The color differences in the plains 

are due to variations in composition or alteration. White plains 

(unit pbw) are dominated by coarse- to medium-sized grains of 

SO2 snow and frost + contaminants (Carlson and others, 1997; 

Douté and others, 2001, 2002, 2004) due, in some cases, to the 

long-term accumulation of white diffuse deposits. Yellow plains 

(unit pby) may be dominated by other sulfur-bearing compounds 

(S8, SnO and S2O) and (or) SO2 frosts mixed with these compounds (Hapke, 1989; Geissler and others, 1999) due, in some 

cases, to the long-term accumulation of yellow diffuse deposits. 

Red-brown plains (unit prb) in equatorial regions (<±30°) are 

colored by the long-term accumulation of red diffuse plume 

deposits containing short-chain sulfur (S3, S4) recrystallized 

from condensed S2 gas (Spencer and others, 2000), some of 

which contain sulfur chlorides (Schmitt and Rodriguez, 2003). In 

middle to high latitudes (>±30°), extensive red-brown plains are 

thought to result from alteration of sulfur compounds in yellow 

plains by radiation exposure (Johnson, 1997; Geissler and others, 

1999). Figures 6–8 include examples of white plains material at 

the highest resolution obtained by the Galileo SSI, in particular 

showing the poorly understood hummocky texture.

Patera Floor Units

Schaber (1980, 1982) mapped a vent wall and floor unit 

that included the wall and floor materials of pit craters, shield 
7

craters, and fissures. These materials were interpreted to be volcanic in origin and consist of lava flows and pyroclastic deposits. In Voyager data, extreme variations in color were noted, and 

complex wall and floor morphologies were described. The unit 

comprised 4 percent of the mapped region, which was slightly 

higher than, but comparable to, our new global characterization. Patera floor materials were divided into three types (bright, 

dark, and undivided) in subsequent mapping of the Ra Patera 

(Greeley and others, 1988) and Ruwa Patera (Schaber and 

others, 1989) quadrangles; these were subsequently grouped 

together in the Voyager global map (Crown and others, 1992). 

The detailed mapping of Maasaw Patera by Moore (1987) 

distinguished patera wall, rim, and floor materials and described 

slumping of the patera walls.

We mapped 425 paterae on Io, and our mapping shows that 

patera floor materials make up ~2.5 percent of Io’s surface. By 

definition, this unit is restricted to the interiors of these calderalike, volcano-tectonic depressions. At the resolution of our global 

mosaics, we do not map a patera wall unit, but we mapped the 

location of the patera walls with a bounding scarp symbol. Patera 

floors span the full range of colors from bright white to yelloworange to black, often with multiple-colored deposits within 

one patera. This suggests that the compositions of patera floor 

materials include mixes of silicates and various sulfur-bearing 

compounds, including relatively pure sulfur dioxide deposits 

(either frost accumulations or flows: Williams and others, 2002, 

2004). The processes that create this variety of materials during 

paterae formation and evolution are necessarily complex and are 

not well understood. Hypotheses for patera formation include 

caldera-forming collapses, tectonic basins filled with lava, and 

the exhumation of sills (Radebaugh and others, 2001; Williams 

and others, 2002; Keszthelyi and others, 2004). The spatial and 

spectral resolution of Galileo SSI images is generally sufficient 

to enable characterization of only three subunits of patera floor 

materials: bright (unit pfb, 0.4%, possibly sulfur-dominated), 

dark (unit pfd, 0.5%, presumably silicate-dominated), and undivided (unit pfu, 1.6%, uncertain composition). The ten largest 

patera floor units and their areas derived from our mapping are 

reported in table 4. Although there is evidence of distinct lava 

flows in ~14 percent of patera floors, there is sufficient ambiguity regarding emplacement mechanisms in available imagery 

(ponded lava flows vs. lava lakes) to justify treating patera floor 

material as a separate material unit from flow material (Williams 

and others, 2004), which is consistent also with the Voyager-era 

mapping precedent. However, high-resolution SSI images (fig. 8) 

contain features morphologically similar to those found in terrestrial lava flow fields, and the compositions of patera floor materials and flow materials are expected to span the same range.

Flow Units

Early studies of best-resolution Voyager images of Io that 

cover 34.8 percent of the mappable surface identified four 

volcanic flow units associated with volcanic vents (paterae) 

(Schaber, 1980, 1982). These flow units composed 31.2 percent 

of the mapped region, compared to the ~28 percent determined 

in our global characterization. One, the crater cone unit, corresponds to what is now referred to as tholus material, and the 

other three (pit crater, shield crater, and fissure flows) were 

distinguished based on the associated vent geometry and setting. Pit crater flows included broad flow fields extending from 

large (~>80–100 km diameter) paterae (for example, Amaterasu 

Patera) in contrast to the typically more narrow and sinuous 

flow shapes within the shield crater flow unit (for example, Ra 

Patera). Formal 1:5M quadrangle mapping built upon this theme 

but also divided the patera and shield materials into light, dark, 

and undivided categories to emphasize albedo patterns and 

allow portrayal of individual lobes (Schaber and others, 1989; 

Whitford-Stark and others, 1991). Mapping in the Lerna Regio 

region followed this scheme but also subdivided flow materials 

into fissure, patera, young, old, undivided, and interpatera flows 

(no obvious source), and interpretations considered these to be 

either lava flows or pyroclastic flows (Whitford-Stark and others, 

1991). Mapping at 1:1M scale of Maasaw Patera and 1:2M scale 

of Ra Patera included division of flow units by both albedo and 

geography, because flow materials were grouped and associated 

with a particular source vent/volcanic center. The Voyager global 

compilation applied these earlier themes to the 1:15M-scale 

mapping by defining a series of different flow materials, based 

primarily on morphology, linked to specific source vents (Crown 

and others, 1992). Separation of different flow units or subunits 

based on albedo alone was abandoned, because of the map scale 

and to better emphasize individual volcanic centers and the range 

of observed morphologies. The most abundant mapped flow 

units were lobate, plains-forming, patera, and undivided materials. The patera material generally corresponds to the shield crater 

material mapped earlier by Schaber (1980, 1982). A common 

stratigraphic relation was noted between younger lobate flows 

and older plains-forming flows, where they were in contact.

Our mapping shows that flow materials make up ~28 percent of Io’s surface and are typified by their generally elongated 

planform shape (lengths >> widths) and sharp contacts with 

the other units (Williams and others, 2002, 2004). Like the 

patera floor materials, flow materials are generally characterized 

using color and albedo as bright (unit fb, ~4%, possibly sulfur 

dominated), dark (unit fd, ~3%, presumably silicate dominated), 

and undivided (unit fu, ~21%, uncertain composition). The ten 

largest flow fields and their areas, derived from our mapping, 

are reported in table 5. Flow materials are interpreted to be 

the result of one or more outpourings of lava onto the surface. 

Albedo variations in the flows are generally thought to indicate 

age on the surface: the freshest dark flows are the darkest; the 

freshest bright flows are the brightest (McEwen and others 

1998a; Williams and others, 2002). In other words, dark flows 

lighten with age and light flows darken with age toward an 

intermediate albedo with time. Correlation between NIMS and 

SSI data also shows that the darkest materials appear to correlate with active hot spots (Lopes-Gautier and others, 1999; 

Lopes and others, 2001, 2004), and only rarely does a hot spot 

correlate with a bright flow (Williams and others, 2004). Older 

flows have intermediate albedo and colors and ill-defined internal unit boundaries and are defined as undivided flow materials. 

These are likely either sulfur or silicate compositions mantled 

by various pyroclastic materials, which tends to homogenize 

the appearance of flows. This explains the relatively large 
8

abundance of undivided flow materials. Because of the generally lower resolution of the global mosaics, we did not attempt 

to subdivide flows based on fissure or point sources, although 

many undivided flow materials can be related to their likely 

source vent. The highest resolution SSI images of flow materials (fig. 6) suggest that they are morphologically and texturally reminiscent of terrestrial compound pahoehoe flow fields 

(for example, Kilauea, Hawaii) or platy ridged lava flows (for 

example, Laki, Iceland) (Keszthelyi and others, 2001).

Tholus Unit

We mapped bright tholus materials (unit tb, ~0.1%), which 

are positive-relief domical edifices that are interpreted to be 

volcanic constructs, primarily shield volcanoes, or cones, 

depending upon morphology and flank slopes of the edifices. 

These features are relatively rare on Io at current spatial resolution, suggesting that the low-viscosity and (or) large volumes of 

Ionian silicate and sulfur lavas being erupted favor production 

of flow fields over constructs.

Diffuse Deposits

Diffuse deposits (figs. 5–8) mantle underlying topography 

in a manner characteristic of fine-grained fragmental material 

and typically occur on or near active volcanic centers. They 

cover ~18 percent of our map area, are superposed on all other 

units, and occur in five distinct colors (Williams and others, 

2004): yellow (~2.1%), white (~6.9%), black (~0.6%), red 

(~8.6%), and green (~0.01%). These colors are interpreted to 

indicate the dominant chemical constituents: sulfur compounds, 

sulfur dioxide + contaminants, silicates, short-chain sulfur 

and (or) sulfur chlorides, and products of silicate-sulfur alteration, respectively. Although diffuse deposits can be ephemeral, 

long-term accumulation of red diffuse, yellow diffuse, and 

white diffuse deposits may lead to the formation of red-brown 

plains, yellow bright plains, and white bright plains materials, 

respectively. Whereas red diffuse deposits occur either as rings 

or asymmetric deposits around active volcanic centers, white 

diffuse deposits typically occur as irregularly shaped units 

surrounding lava flows. We map diffuse deposits only where 

they are distinct enough to discern their boundaries and, thus, 

constrain their sources. Note that this is only true if the gas vent 

was active during the Galileo era, because we use the Galileo 

SSI color mosaic to identify and map these deposits.

Structure

A wide range of structural features, including scarps, 

ridges, grooves, pits, mesas, and lineaments, are visible in 

the base-map mosaics, mostly in regions where images were 

obtained at low-sun angle. Scarps delineate layered plains, 

patera rims, and margins of mountains. Scarps in the plains 

appear to be largely degradational features. They are commonly 

associated with scarp-parallel fractures that are likely to be tension cracks that indicate incipient slope failure. There are also 

enigmatic grooves in the plains that suggest narrow grabens. 

These are likely to be shallow features, because compressional 

stresses rapidly increase with depth in Io’s lithosphere due to 

the weight of overlying material (Jaeger and others, 2003). In 

general, more structural features can be seen in higher resolution images than can be mapped from our base-map mosaics. 

Many structural features, particularly in the plains, are buried 

by extensive volcanic deposits. Schenk and Williams (2004) 

recognized the only sinuous lava channel, Tawhaki Vallis (east 

of Hi’iaka Montes, mapped as a sinuous lineament), identified to date on Io; it is interpreted to have formed by erosion of 

sulfur-rich country materials by flowing lava.

Shadow lengths and directions in low-sun images reveal 

that most paterae have depressed interiors, similar to terrestrial 

calderas, and that mountains are indeed positive relief edifices. 

As in all previous Io studies, no impact craters were detected or 

mapped on the surface, supporting the contention that Io’s average surface is very young and no substantial areas are less than 

a few millions of years old (Johnson and others, 1979; McEwen 

and others, 2000a). A suspiciously circular feature occurs within 

Heno Patera (lat 57.1° S., long 311.5° W.). This 71.1-km-diameter patera contains a dark floor and dark flows that surround a 

bright circular rim approximately centered on the patera floor. 

Although this feature was mapped by Whitford-Stark and others 

(1991), they did not comment on it. We propose that it could be 

a result of lava flooding in and around an impact crater (Williams and others, 2007b), but the existing data are ambiguous.

Stratigraphy

Stratigraphy on Io is difficult to determine for several reasons: (1) there are no impact craters visible in any image at any 

resolution, which prohibits the application of crater statistical 

techniques to determine crater retention ages or cratering model 

ages of surfaces; (2) the photometric properties and spatial 

resolution of images are quite variable at all scales (compare 

with orbiter images) and high-resolution images are limited 

in surface coverage; and (3) the ongoing active volcanism, 

particularly emplacement of condensed gases and tephra from 

plumes, leads to rapid resurfacing rates (Johnson and others, 

1979), such that fresh volcanic deposits can mask older features 

or alter them beyond recognition. Nevertheless, our mapping 

has demonstrated that each of our map units has attributes that 

enable them to fit into a general stratigraphic correlation:

(1) Lava flow units tend to have distinctive albedos, 

colors, and lobate contacts with their surroundings 

and can, therefore, be easily distinguished from plains, 

patera floors, and mountains.

(2) Patera floor units by definition occur within paterae, 

which are assumed to be topographic depressions, and 

(because they are composed of similar material as lava 

flow units) have distinctive albedos and colors, as well 

as circular, elliptical, or partly curvilinear shapes that 

easily distinguish them from plains, lava flows, and 

mountains.
9

(3) Mountain and layered-plains units tend to have albedos 

and colors similar to plains materials, such that they 

can only be differentiated from plains if low-sun-angle 

images exist that produce shadows on scarps, ridges, 

grooves, or mass-wasting lobes; however, where these 

shadowed features exist, mountains can be easily 

mapped, and their albedos and colors are generally 

distinctive from lava flows and patera floors.

(4) Plains units can be easily distinguished from lava flows 

and patera floors, from mountains if low-sun images 

exist showing the mountains’ scarps, and from one 

another if there is a sharp color difference; however, in 

some cases, distinct plains-plains contacts are gradational or difficult to detect.

(5) In many cases, diffuse deposits are the easiest to put in 

a local stratigraphic column, because they are clearly 

recently emplaced features (like some lava flows and 

patera floor units); in other cases, the stratigraphy is 

unclear, because diffuse deposits fade in time. Red 

diffuse deposits in the form of rings (except at Pele) 

and white diffuse deposits surrounding the perimeters 

of lava flow fields mark occurrences associated with 

distinctive eruptions; in contrast, many circular or ringlike yellow diffuse deposits (for example, Prometheus), 

red diffuse deposits at Pele, and white diffuse deposits 

represent ongoing, periodic to continuous eruptions. 

Additional analyses of discrete diffuse deposits at a 

regional scale could provide the potential for correlation across the surface.

Often at specific volcanic centers, it is possible to recognize 

the multiple styles of eruptions that are occurring (for example, 

explosive and effusive, or variation in types of effusive and 

explosive deposits, based on albedo-color differences), but it is 

not possible to recognize the order of eruptive products (without 

higher-resolution imaging) in ongoing eruptions. Where higher 

resolution (<1 km/pixel) images exist, regional mapping has 

demonstrated that a stratigraphic sequence of volcanic eruptions can be identified, for example, in the western Culann-Tohil 

region (Williams and others, 2004), from oldest to youngest: 

(1) presence of bright plains; (2) formation of a small shield, 

Tsũi Goab (TG) Tholus; (3) emplacement of a dark diffuse 

deposit on TG Tholus and surrounding plains; (4) emplacement 

of bright lava flows (TG Fluctus) overlying the dark diffuse 

deposit and plains.

Nevertheless, despite these challenges, a general stratigraphy of Io’s surface materials can be recognized that offers 

insight into the role of various geologic processes as described 

in the Correlation of Map Units (map sheet). Figures 9–11 give 

three examples from the global geologic map that show the 

various stratigraphic relations that led to the development of the 

Correlation of Map Units (see also table 6).

Geologic History

How long has Io’s rampant volcanism been occurring? This 

is an important question that has implications for Io’s overall 

geologic evolution. Research by Keszthelyi and McEwen (1997) 

suggests that if Io has maintained its current level of activity 

since the formation of the Solar System ~4.6 Ga, then it should 

have completely melted its interior 40+ times, resulting in highly 

differentiated, highly silicic volcanic products. Yet, studies of the 

Galileo data over the last decade suggest that active volcanism is 

centered around eruptions of high-temperature, relatively undifferentiated, mafic to ultramafic material (McEwen and others, 

1998b; Davies and others, 2001; Williams and others, 2001a; 

Keszthelyi and others, 2007), and no evidence of more evolved, 

more silicic material has been found. One possible explanation of these results is that Io only relatively recently (from a 

geologic perspective) entered into its Laplace resonance with 

Europa and Ganymede to trigger its tidally based volcanic activity. Alternatively, Io must have some mechanism to efficiently 

mix the crust back into the mantle. Further examination of the 

volcanism and tectonics of Io is required to more fully investigate hypotheses regarding the formation and evolution of Io, and 

further study of this new global map will aid in these studies.

With the completion of the global mapping, what insights 

can be gained about Io’s geologic evolution? As indicated in 

the Correlation of Map Units (map sheet), the oldest materials 

exposed at the surface of Io are crustal materials that have been 

uplifted to form the various mountain units. Tectonic and geophysical arguments suggest that the upper crust of Io is ~20–30 

km thick (Ross and others, 1990; Jaeger and others, 2003). Our 

mapping and other studies support the hypothesis of Schenk and 

Bulmer (1998) that the accumulation of volcanic materials on the 

surface causes compression of the upper crust, eventually leading to tectonic fracturing, faulting, and uplift of crustal blocks 

that form mountains (our lineated mountain material). Over time 

these materials are mantled and undergo gravitational collapse, 

forming mottled mountain materials that grade into layered 

plains. Surrounding the mountains are the various plains materials whose upper surfaces must be very young, based on the lack 

of impact craters, but whose lower layers are likely the same 

crustal materials that make up the mountains. The degradational 

scarps and other observations point to a significant thickness (up 

to a few kilometers) of volatile-rich materials mantling the plains 

(Moore and others, 2001). This volatile-rich material is probably 

largely confined to the uppermost crust as compression collapses 

pore space (Jaeger and others, 2003; Jaeger and Davies, 2006).

We suggest that volcanism on Io has been happening for at 

least the last few million years to build the stress necessary to 

form the mountains. We think volcanism has been going on for a 

much longer period of time, although the rapid reworking of the 

crust has obliterated any evidence of older activity. The currently 

visible paterae and lava flow fields (fluctus, pl. fluctüs) probably 

formed simultaneously or subsequently to the currently visible 

mountains. The oldest volcanic surfaces are related to centers 

that became inactive in the last decades to millennia and are 

mapped as undivided patera floor materials and undivided flow 

materials. More distinct volcanic constructs, including the bright 

tholus material, probably formed more recently. The visible surface of the plains is formed from the coalescence and homogenization of older eruptive products (pyroclastics and lavas).

The time frame of decades to years marks the period of 

spacecraft observations, from Voyager (1979) to Galileo (1996-
10

2001) to New Horizons (2007). While no new mountains or 

paterae have formed in this time interval, we have witnessed 

the formation of the Zamama flow field and possibly its conelike constructs (Davies and others, 1997; Schenk and others, 

2004; Williams and others, 2005) between Voyager and Galileo 

observations and ongoing emplacement of new flow fields at 

Pillan Patera and the Prometheus and Amirani Eruptive Centers, among others (McEwen and others, 1998a); lavas in patera 

floors at Pillan, Pele, Loki, and Gish Bar Paterae, among others 

(McEwen and others, 1998a; Keszthelyi and others, 2001; Rathbun and others, 2002; Lopes and others, 2004; Turtle and others, 

2004); and various diffuse deposits at Pele, Pillan, and Thor 

Eruptive Centers, among many others (McEwen and others, 

1998a; Geissler and others, 1999, 2004b; Keszthelyi and others, 

2001; Turtle and others, 2004). These formations are the sources 

for the other geologic units, including bright and dark patera 

floor materials, bright and dark flow materials, and all five types 

of diffuse materials. The most recent spacecraft flyby (New 

Horizons, Feb. 28, 2007) recorded evidence of surface changes 

in both lava flow fields and diffuse deposits at several locations on Io (Spencer and others, 2007). Through the analysis of 

Galileo observations, we identified three types of eruptive styles 

on Io—flow-dominated or Promethean, explosion-dominated or 

Pillanian (Keszthelyi and others, 2001; Williams and Howell, 

2007), and intra-patera or Lokian (Lopes and others, 2004; Williams and Howell, 2007)—that informed our interpretations of 

the mapped material units.

This global mapping project, despite use of the excellent 

quality 2006 USGS combined mosaics, was hampered by the 

nonuniformity of spatial and spectral resolution coverage of 

Io from the Voyager and Galileo missions. It is clear that the 

next phase of Io observation must include (1) long-term distant 

monitoring of Io to assess the variation in Io’s volcanism at 

a range of time scales (minutes, hours, days, months, years) 

and (or) (2) collection of global data from the ultraviolet to the 

thermal-infrared at consistent resolutions and phase angles (for 

example, global visible color imaging at ~250 m/pixel or better, 

with multiple high-resolution spots at ~5–50 m/pixel). Because 

the harsh radiation environment at Io and propulsion requirements prohibit the use of an Io orbiter, we suggest that an Iodedicated Jupiter orbiter, hardened against radiation and capable 

of ~50+ close Io flybys covering all longitudes in daylight, is an 

ideal choice for a future Io mission. This suggestion is consistent with the 2003 National Research Council (NRC) Planetary 

Decadal Survey and its supporting white paper (Spencer and 

others, 2002) and the Io exploration white paper submitted for 

the 2011 NRC Planetary Decadal Survey (Williams and others, 

2009). Future mission concepts for the Jupiter system are also 

consistent with all or parts of this approach (Kwok and others, 

2007). Only with long-term distant monitoring and (or) globally 

consistent high-resolution imaging at multiple wavelengths will 

Io reveal more of her hidden secrets.

Acknowledgments

The authors would like to thank Tracy K.P. Gregg and 

Rosaly M.C. Lopes for their reviews that improved the quality 

of this map. The authors would like to thank Trent Hare, Rose 

Hayward, Ken Tanaka, and the staff of the U.S. Geological Survey’s Astrogeology Science Center for their assistance in producing the base map products and for editing and support using 

ArcGIS software for producing this map. We also thank the 

USGS publication staff for editing and production of this map. 

The authors would like to acknowledge all of the engineers, 

scientists, and managers who helped bring NASA’s Voyager and 

Galileo missions to fruition, without which we would not have 

had the data to produce this map. In particular, we would like 

to thank Ken Klaasen, Herb Brenneman, Greg Louvanis, and 

colleagues at NASA’s Jet Propulsion Laboratory for their hard 

work obtaining and processing the Io data obtained during the 

Galileo mission. We would like to thank all of our colleagues in 

the Galileo Io Working Group and in the outer planet satellites 

community who have provided years of useful discussion and 

insights, particularly Alfred McEwen, Rosaly Lopes, and John 

Spencer for lead roles in acquiring Io data from the Galileo 

SSI, NIMS, and PPR instruments (respectively). The lead 

author would like to thank Charles Bradbury for his support in 

maintaining the mapping computer at Arizona State University 

and Ronald Greeley for the opportunity to rejoin the Galileo 

SSI Team as an associate and work on the project through the 

Galileo extended missions. This work was supported by the 

National Aeronautics and Space Administration through the 

Outer Planets Research Program grants NNG05GH43G and 

NNX08AQ66G.

Appendix

The appendix for this map consists of four tables, tables 

7–10. Table 7 contains a summary of the imaging coverage of Io 

from the Voyager, Galileo, Cassini, and New Horizons missions. 

Table 8 is a list of the Io global mosaics produced by the USGS 

in 2005, including the filters used and data sources of Galileo 

and Voyager images. Table 9 lists the image numbers of Galileo 

images used to make the SSI-only global Color Mosaic.

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14

Table 1. Latitude-longitude coordinates of type examples of map 

units shown in figure 2.

Unit label Unit name Number

of outcrops1

Coordinates

(lat, long) 

Diffuse deposits

dby yellow bright diffuse material 16 19°, 141°

dbw white bright diffuse material 233 –37°, 163°

dd dark diffuse material 18 –40°, 272°

dr red diffuse material 70 –22°, 161°

dg green diffuse material 2 –20°, 160°

Flow deposits

fd dark flow material 203 23°, 115°

fb bright flow material 143 13°, 149°

fu undivided flow material 223 –37°, 165°

Patera floor deposits

pfd dark patera floor material 235 –28°, 160°

pfb bright patera floor material 84 13°, 142°

pfu undivided patera floor material 263 –26°, 158°

Tholus deposits

tb bright tholus material 7 0°, 163°

Plains deposits

pby yellow bright plains material 26 –25°, 175°

pbw white bright plains material 29 15°, 195°

prb red-brown plains material 38 –55°, 135°

pl layered plains material 129 –7°, 165°

Flow deposits

ml lineated mountains material 39 –26°, 162°

mm mottled mountains material 9 –28°, 165°

mu undivided mountain material 73 –49°, 336°

1For some units, particularly the plains units, individually mapped outcrops have 

been merged in ArcGIS to larger, continuous outcrops.
15

Table 2. Distribution of geologic material units as percentage of Io’s surface 

area.

[Diffuse deposits are superposed on all other materials and cover 18.2% of Io’s surface. 

Io’s surface area = 4.17 x 107 km2]

Unit label Unit name Area (km2) Area (%)

Plains deposits

prb red-brown plains material 1.41 x 107 33.4

pby yellow bright plains material 7.68 x 106 18.4

pbw white bright plains material 3.75 x 106 8.9

pl layered plains material 1.84 x 106 4.4

region of poor resolution (likely red-brown 

plains material)

7.20 x 105 1.7

Total plains 2.81 x 107 66.6

Mountain deposits

ml lineated mountains material 6.40 x 105 1.5

mm mottled mountains material 8.05 x 104 0.2

mu undivided mountain material 5.54 x 105 1.3

tb bright tholus material 5.25 x 104 0.1

Total mountain material 1.33 x 106 3.1

Patera floor deposits

pfb bright patera floor material 1.84 x 105 0.4

pfd dark patera floor material 1.93 x 105 0.5

pfu undivided patera floor material 6.75 x 105 1.6

Total patera floor material 1.05 x 106 2.5

Flow deposits

fb bright flow material 1.80 x 106 4.3

fd dark flow material 1.23 x 106 2.9

pfu undivided flow material 8.70 x 106 20.6

Total flow material 1.17 x 107 27.8

Diffuse deposits

dby yellow bright diffuse material 8.76 x 105 2.1

dbw white bright diffuse material 2.90 x 106 6.9

dr red diffuse material 3.61 x 106 8.6

dd dark diffuse material 2.68 x 105 0.6

dg green diffuse material 4.09 x 103 0.01
16

Table 3. Locations, heights, and areal extents of the ten largest mountains, 

ten lineated mountains, and nine mottled mountains on Io.

[-, no data]

Feature name

(dominant map unit)

Latitude 

( ̊)

Longitude 

( ̊ W.)

Height

(km)

Area

(km2)

Mountains

Carancho (lineated) –1.37 318.07 8.5 42,064

Capaneus Mensa (undivided) –16.45 121.11 9.5 38,102

Pan Mensa (lineated) –51.83 31.16 5 33,540

Ethiopia Planum (lineated) –45.38 24.79 4.5 31,331

Euboea Montes (undivided) –47.85 335.83 13.4 30,729

Monan Mons (lineated) 15.22 104.55 6.5 30,200

Danube Planum (lineated) –22.34 257.76 5.5 29,857

Unnamed (undivided) –17.40 61.31 5.1 29,025

Unnamed (lineated) –4.36 63.13 2.9 28,085

Euxine Mons (undivided) 26.24 126.44 7.7 27,444

Lineated mountains

Carancho –1.37 318.07 8.5 42,064

Pan Mensa –51.83 31.16 5 33,540

Ethiopia Planum –45.38 24.79 4.5 31,331

Monan Mons 15.22 104.55 6.5 30,200

Danube Planum –22.34 257.76 5.5 29,857

Unnamed –4.36 63.13 2.9 28,085

Skythia Mons 26.15 98.77 6 27,396

Haemus Mons –69.95 43.19 10.8 24,712

Unnamed –20.74 109.36 4.5 23,219

N. Nemea Planum –65.65 248.78 6 22,982

Mottled mountains

S. Tohil Mons –28.08 163.49 <9 25,601

SE. Shamshu Mons –12.07 71.21 2.9 17,732

Unnamed 8.34 261.64 4 14,021

Unnamed -43.87 124.88 - 12,519

NW. Pan Mensa -48.97 34.80 <5 4,929

Unnamed 7.07 284.38 4.5 3,173

NW. Shamshu Mons –10.80 73.31 <3 1,632

Unnamed –66.3 243.79 - 625

Unnamed –7.08 278.16 - 263
17

Table 4. Locations and areal extents of the ten largest patera floors, ten 

dark patera floors, and ten bright patera floors on Io.

Feature name

(dominant map unit)

Latitude

( ̊)

Longitude

( ̊ W.)

Area

(km2)

Patera floors

Unnamed (undivided) –37.1 244.7 23,769

Loki Patera (dark) 12.8 308.4 21,204

Kinich Ahau Patera (undivided) 51.0 310.6 21,084

NE. of Loki (undivided) 15.2 305.1 19,082

Unnamed (undivided) 1.1 254.8 17,985

Unnamed (undivided) 31.5 132.3 12,653

Unnamed (undivided) 38.5 24.0 12,440

Outside Amaterasu Patera (undivided) 36.3 306.3 12,135

Horus Patera (bright) –9.9 337.4 11,920

Itzamna Patera (undivided) –15.8 99.2 10,581

Dark patera floors

Loki Patera 12.8 308.4 21,204

Dazhbog Patera 55.1 301.5 8,968

Babbar Patera –39.8 271.6 7,021

W. Gish Bar Patera 16.2 90.3 6,263

Unnamed 44.1 232.7 5,708

E. Hi’iaka Patera –3.8 78.8 5,040

Tiermes Patera 22.4 350.2 4,854

Center Amaterasu Patera 38.1 306.5 4,658

N. Creidne Patera –52.2 342.2 4,374

Emakong Patera –3.3 119.9 3,675

Bright patera floors

Horus Patera –9.9 337.4 11,920

Tvashtar Catena 61.1 119.5 8,190

S. of Loki Patera 9.8 310.5 7,615

Cataquil Patera –24.2 16.7 7,304

Unnamed 19.4 334.1 7,297

Asha Patera –8.8 225.6 7,171

Unnamed –7.6 85.0 6,418

Unnamed 25.3 281.2 6,220

Huo Shen Patera –16.1 329.9 5,832

Mentu Patera 7.0 139.4 5,514
18

Table 5. Locations and areal extents of the ten largest lava flow fields, 

ten largest bright lava flow fields, and ten largest dark lava flow fields 

on Io.

Feature name

(dominant map unit)

Latitude

( ̊)

Longitude

( ̊ W.)

Area

(km2)

Lava flow fields

W. Girru Patera (undivided flows) 19.9 244.8 369,275

N. Mama Patera (undivided flows) –10.1 353.9 276,264

S. Ukko Patera (undivided flows) 16.6 11.1 274,836

W. Pele (undivided flows) –21.5 282.6 257,178

Ra Patera (undivided flows) –8.5 325.4 232,819

Lei-zi Fluctus (undivided flows) 15.1 40.3 205,479

W. Isum Patera (undivided flows) 32.2 225.1 202,407

W. Emakong Patera (undivided flows) –1.7 128.8 189,958

NW. Thor (undivided flows) 47.5 149.7 179,054

N. Ekhi Patera (undivided flows) –19.4 84.4 175,180

E. Savitr Patera (bright flows) 45.0 108.9 173,987

Bright lava flow fields

(surrounding undivided flow fields not included)

E. Savitr Patera 45.0 108.9 173,987

NW. of Heiseb Patera 37.2 257.7 136,326

S. of Shakuru Patera 20.5 264.5 91,090

Bulicame Regio 34.8 191.0 81,484

SW. of Arusha Patera –40.8 106.5 73,248

W. Chalybes Regio 49.0 96.9 57,446

N. Emakong Patera 3.1 118.5 53,911

North Polar 1 59.2 142.6 46,425

N. of Tvashtar Paterae 72.1 131.7 44,267

N. Tarsus Regio –22.7 57.6 42,843

E. Illyrikon Regio 69.2 187.9 38,597

Dark lava flow fields

(surrounding undivided flow fields not included)

Isum Patera 28.6 208.6 85,162

S. of Mycenae Regio –48.3 157.8 46,858

Lei-Kung Fluctus 40.3 206.3 45,610

S. of Gibil Patera –24.8 292.6 40,915

SW. of Girru Patera 20.0 246.6 39,901

Daedalus Patera 19.6 274.5 39,276

Amirani 23.4 116.0 34,077

N. of Arinna Fluctus 42.1 142.6 31,069

N. of Wayland Patera –25.2 223.5 30,476

NW. of Arinna Fluctus 40.8 149.1 26,832

W. of Wayland Patera –31.9 230.3 26,759
19

Table 6. Stratigraphic justification of Io map units as displayed in the Correlation of Map Units.

[s/c, spacecraft]

Unit name (unit label) Comment or justification

Yellow bright diffuse deposits (dby) Top boundary: Present in Voyager and Galileo images, formation ongoing. May be locally older th

dg.

Bottom boundary: Likely started forming when Io became volcanically active. Possibly younger th

neous with dbw, dd, dr, dg, pfd, pfb, fd, fb. Younger than pfu, fu, ml, mm, mu and younger than or 

with tb, pby, pbw, prb, pl. Long-term (decades) accumulation of dby may transform to pby.

White bright diffuse deposits (dbw) Top boundary: Present in Voyager and Galileo images, formation ongoing. May be locally older th

Bottom boundary: Likely started forming when Io became volcanically active. Possibly younger thneous with dby, dd, dr, dg, pfd, pfb, fd, fb. Younger than pfu, fu, ml, mm, mu and younger than or 

with tb, pby, pbw, prb, pl. Long-term (decades) accumulation of dbw may transform to pbw.

Dark diffuse deposits (dd) Top boundary: Present in Voyager and Galileo images, formation ongoing. May be locally older th

dg.

Bottom boundary: Likely started forming when Io became volcanically active. Possibly younger th

raneous with dby, dbw, dr, dg, pfd fd, pfb, fd, fb. Younger than pfu, fu, ml, mm, mu and younger th

neous with tb, pby, pbw, prb, pl. Deposits eventually buried to become part of plains.

Red diffuse deposits (dr) Top boundary: Present in Voyager and Galileo images, formation ongoing. May be locally older th

dg.

Bottom boundary: Likely started forming when Io became volcanically active. Possibly younger th

neous with dby, dbw, dd, dg, pfd, pfb, fd, fb. Younger than pfu, fu, ml, mm, mu and younger than o

ous with tb, pby, pbw, prb, pl. Long-term (decades) accumulation of dr may transform to prb.

Green diffuse deposits (dg) Top boundary: Present in Galileo but not Voyager images (likely due to imaging limitations); form

of dr superposed on fd, so is locally younger than dr. Formation is likely ongoing. May also be loc

dby, dbw, dd.

Bottom boundary: Likely started forming when Io became volcanically active. Possibly younger thraneous with dby, dbw, dd, dr, pfd, pfb, fd, fb. Younger than pfu, fu, ml, mm, mu and younger than 

ous with tb, pby, pbw, prb, pl. Deposits eventually transform to fu or pfu.

Dark flow material (fd) Top boundary: Present in Galileo images, among youngest flows, may not be present in Voyager im

formation ongoing). Observed forming in Galileo images. Older than or contemporaneous with fb, 

dbw, dd, dr, dg.

Bottom boundary: Likely started forming when Io became volcanically active. Younger than fu, pf

and younger than or contemporaneous with tb, pby, pbw, prb, pl. Accumulation of superposed diffu

tually transforms this unit into fu.

Bright flow material (fb) Top boundary: Present in Galileo images, among youngest flows, may not be present in Voyager im

formation ongoing). None observed forming in Galileo images. Older than or contemporaneous wi

dby, dbw, dd, dr, dg.

Bottom boundary: Likely started forming when Io became volcanically active. Younger than fu, pf

and younger than or contemporaneous with tb, pby, pbw, prb, pl. Accumulation of superposed diffu

tually transforms this unit into fu.
20

Table 6. Stratigraphic justification of Io map units as displayed in the Correlation of Map Units.—Continued

[s/c, spacecraft]

Unit name (unit label) Comment or justification

Undivided flow material (fu) Top boundary: None observed forming in Galileo or Voyager images. Must be older than fd, fb, pfd

dd, dr, dg, contemporaneous with pfu, and older than or contemporaneous with tb.

Bottom boundary: Likely started forming when Io became volcanically active. Younger than or conwith pby, pbw, prb, pl (any older fu buried by ongoing plains formation) and younger than or conte

ml, mm, mu. Long-term accumulation of superposed diffuse deposits eventually buries this unit, tr

plains.

Dark patera floor material (pfd) Top boundary: Present and observed to form in Galileo images, among youngest patera floors, may 

Voyager images (although formation ongoing). Older than or contemporaneous with pfb, fd, fb, db

Bottom boundary: Likely started forming when Io became volcanically active. Younger than pfu, f

and younger than or contemporaneous with tb, pby, pbw, prb, pl. Accumulation of superposed diffu

tually transforms this unit into pfu.

Bright patera floor material (pfb) Top boundary: Present in Galileo images, among youngest patera floors, may not be present in Voy

(although formation likely ongoing). None observed forming in Galileo images. Older than or contpfd, fd, fb, dby, dbw, dd, dr, dg.

Bottom boundary: Likely started forming when Io became volcanically active. Younger than pfu, f

and younger than or contemporaneous with tb, pby, pbw, prb, pl. Accumulation of superposed diffu

tually transforms this unit into pfu.

Undivided patera floor material (pfu) Top boundary: None observed forming in Galileo or Voyager images. Must be older than pfd, pfb, 

dd, dr, dg, contemporaneous with fu, and older than or contemporaneous with tb.

Bottom boundary: Likely started forming when Io became volcanically active. Younger than or conwith pby, pbw, prb, pl (any older pfu buried by ongoing plains formation) and younger than or con

with ml, mm, mu. Long-term accumulation of superposed diffuse deposits eventually buries this un

into plains.

Bright tholus material (tb) Top boundary: Present in Voyager and Galileo images. Older than or contemporaneous with dby, d

pfd, pfb, fd, fb. None observed forming during Voyager or Galileo missions, but formation may be 

Bottom boundary: Likely started forming when Io became volcanically active. Younger than or conwith pfu, fu, pby, pbw, prb, pl, ml, mm, mu. Long-term accumulation of superposed diffuse deposi

ies this unit, transforming it into plains.

Yellow bright plains material (pby) Top boundary: Plains formation is ongoing, but visible surface typically underlies units pfd, pfb, p

dby, dbw, dd, dr, dg (these units usually superposed on plains).

Bottom boundary: Likely started forming when Io became volcanically active. Younger than or conwith pl, pbw, prb, ml, mm, mu.

White bright plains material (pbw) Top boundary: Plains formation is ongoing, but visible surface typically underlies units pfd, pfb, p

dby, dbw, dd, dr, dg (these units usually superposed on plains).

Bottom boundary: Likely started forming when Io became volcanically active. Younger than or conwith pby, pl, prb, ml, mm, mu.

Red-brown plains material (prb) Top boundary: Plains formation is ongoing, but visible surface typically underlies units pfd, pfb, p

dby, dbw, dd, dr, dg (these units usually superposed on plains).

Bottom boundary: Likely started forming when Io became volcanically active. Younger than or conwith pby, pbw, pl, ml, mm, mu.
21

Table 6. Stratigraphic justification of Io map units as displayed in the Correlation of Map Units.—Continued

[s/c, spacecraft]

Unit name (unit label) Comment or justification

Layered plains material (pl) Top boundary: Layered plains degradation is ongoing. Typically older than or contemporaneous w

pfu, fd, fb, fu, tb, dby, dbw, dd, dr, dg (these units usually superposed on layered plains). 

Bottom boundary: Likely started forming when Io became volcanically active. Younger than or conwith pby, pbw, prb, ml, mm, mu.

Lineated mountain material (ml) Top boundary: No evidence of ml formation since s/c observations began, but likely is ongoing. Un

form mm (and possibly pl), therefore must be locally older than mm.

Bottom boundary: Likely started forming when Io became volcanically active. Older than or contetb, pfu, fu, pby, pbw, prb, pl.

Mottled mountain material (mm) Top boundary: No evidence of mm formation since s/c observations began, but likely is ongoing. D

therefore must be locally younger than ml.

Bottom boundary: Likely started forming when Io became volcanically active. Older than or contetb, pfu, fu, pby, pbw, prb, pl.

Undivided mountain material (mu) Top boundary: No evidence of mu formation since s/c observations began, but could be ongoing. C

units ml or mm, but limited by s/c resolution constraints.

Bottom boundary: Likely started forming when Io became volcanically active. Older than or contetb, pfu, fu, pby, pbw, prb, pl.
22

Table 7. Imaging coverage of Io from the Voyager ISS, Galileo SSI, and New Horizons instruments. 

From Smith and others (1979b), Galileo Team records (unpub.), and Spencer and others (2007).

Mission1 Flyby/orbit2 Date Coverage/resolution (km/px)

Voyager Voyager 1 March 5, 1979 Regional grayscale & color, ~35% of moon @ 0.5–2; 

global color @ 10–125 

Voyager Voyager 2 July 9, 1979 Global color: 80% of moon @ 10–45

Gal-NOM G1 June 29, 1996 Global color @ >30

Gal-NOM G2 Sept. 6, 1996 Plume monitoring @ 10–31

Gal-NOM C3 Nov. 6, 1996 Grayscale global @ 2.5–4

Gal-NOM E4 Dec. 18, 1996 Eclipse observations @ 18–35; Global color @ 6–12

Gal-NOM E6 Feb. 20, 1997 Regional color @16–21

Gal-NOM G7 April 3, 1997 Eclipse @ 33; Grayscale @ 6; Global color @ 12

Gal-NOM G8 May 7, 1997 Eclipse @19; Regional color @ 10–23

Gal-NOM C9 June 27, 1997 Eclipse @ 13–15; Regional color @ 6–17

Gal-NOM C10 Sept. 18, 1997 Eclipse @ 13; Global color @ 5–13

Gal-NOM E11 Nov. 7, 1997 Regional color @ 8–19

Gal-GEM E12 Dec. 16, 1997 Eclipse @ 12

Gal-GEM E14 Mar. 29, 1998 Regional color @ 3

Gal-GEM E15 May 31, 1998 Eclipse @ 11–13

Gal-GEM C21 June 30, 1999 Global color @ 1.4

Gal-GEM C22 Aug. 14, 1999 Plume monitoring @ 10–17

Gal-GEM I24 Oct. 11, 1999 High-res grayscale @ 0.01–0.63; Regional color @ 0.22; 

Global color @ 6–13

Gal-GEM I25 Nov. 26, 1999 Regional grayscale @ 0.14–0.2; Regional color @ 0.2

Gal-GEM E26 Jan. 4, 2000 Regional color @ 3–4

Gal-GMM I27 Feb. 22, 2000 High-res grayscale @ 0.01–0.02; Regional grayscale @ 

0.2–0.9; Global color @ 3

Gal-GMM G29 Dec. 28, 2000 Global color @ 10–17

Gal-GMM I31 Aug. 6, 2001 Global color @ 19–20

Gal-GMM I32 Oct. 16, 2001 High-res grayscale @ 0.013–0.4; Regional color @ 0.22; 

Global color @ 5–13

Cassini Jupiter Flyby Jan. 1–2, 2001 Color & grayscale timelapse movies @ 60–120

New Horizons Jupiter Flyby Feb. 28, 2007 190 grayscale images @ 14–22; 17 color nighttime 

& eclipse images @ 50–60; 7 near-IR image cubes @ 

140–170

1Gal-NOM, Galileo Nominal Mission (1996–1997); Gal-GEM, Galileo Europa Mission (1998–1999); Gal-GMM, Galileo 

Millennium Mission (2000–2002).

2For the Galileo mission, the orbit letter designates primary remote sensing target: I, Io; E, Europa; G, Ganymede; C, Callisto.
23

Table 8. List of the new Io global mosaics produced by the USGS, combining Galileo and Voyager images. All mosaics have a spatial resolution of 1 km/pixel. The versions shown in this report (fig. 1) are simple cylindrical projections centered on the antijovian

point (long 180 ̊ W.).

Mosaic Filter-wavelengths

used

Phase angle

( ̊)

Data sources Comments

Galileo SSI-only lowphase color765nm-GRN-VIO 4 57 images from Galileo orbits 

G2, E6, C9, C21, I31

Use for mapping colorful diffuse deposits

Galileo SSI-only 

monochrome

CLR, GRN, 756nm Various 32 images from Galileo orbits 

G1, G2, C3, E6, G7, C9, C10, 

E11, C21, C22, I24

Use for mapping surface morphology—

antijovian hemisphere

Combined GalileoVoyager monochromeGLL: CLR, GRN, 

756nm;

VGR: CLR, BLU

Various 50 Voyager 1 images, 32 Galileo 

images (see above)

Use for mapping surface morphology—

subjovian hemisphere

Merged Galileo-Voyager monochrome and 

SSI low-phase color

See rows 1 and 3 

above

Various, 

Color = 4

See rows 1 and 3 above Use for correlating mapped features with 

color—compositional relations
24

Table 9. Image numbers of Galileo and Voyager images used to make the Combined Mosaic.

Io Global Mosaic: Galileo SSI and Voyager 1 Combined for Best Resolution, December 2005.

Galileo SSI

(all SSI images are Clear filter unless otherwise specified)

Voyager 1

(all Voyager 1 images are Clear filter & Narrow Angle (NA)

camera unless otherwise specified)

OBSERVATION IMAGE_NUMBER NOTE v-# IMAGE_NUMBER v-# IMAGE_NUMBER

1 – G1ISGLOMON05 0349746300 Green v-2 1639133 (WA) v-30 1639152

2 – C22ISIOGEOD01 0512336700 Green v-3 1638227 (Blue) v-31 1639144

3 – E11ISTOPO__01 0420669000 v-4 1638858 v-32 1639158

4 – G2ISSRFMON01 0359986578 Green v-5 1638906 v-33 1639156

5 – C9ISSRFMON01 0401785378 v-6 1638954 v-34 1639150

6 – C10ISIOTOPO02 0413659700 v-7 1638946 v-35 1639154

7 – G7ISTOPMAP03 0389771978 v-8 1639239 (WA) v-36 1639148

8 – G7ISTOPMAP02 0389752400 v-9 1639104 v-37 1639146

9 – C3ISTOPMAP05 0368641300 v-10 1638918 (Blue) v-38 1639110

10 – E6ISTOPMAP01 0383694100 v-11 1638914 v-39 1639108

11 – C3ISTOPMAP01 0368558239 v-12 1639024 v-40 1639130

12 – C21ISALBEDO01 0506406726 Green v-13 1639026 v-41 1639124

13 – C3ISTOPMAP03 0368599800 v-14 1639042 v-42 1639112

14 – I24ISSTEREO01 0520821326 v-15 1639028 v-43 1639114

15 – C3ISTOPMAP04 0368620000 v-16 1639018 (Blue) v-44 1639132

16 - C3ISTOPMAP03 0368599813 v-17 1639012 (Blue) v-45 1639126

17 - C3ISTOPMAP01 0368558252 v-18 1639006 (Blue) v-46 1639116

18 – C21ISALBEDO01 0506406839 Green v-19 1639044 v-47 1639122

19 – C21ISCOLOR_01 0506406007 7560 v-20 1639002 v-48 1639120

20 – C21ISALBEDO01 0506406739 Green v-21 1639034 v-49 1639118

21 – C21ISSTEREO01 0506431239 v-22 1639036 v-50 1639128

22 – I24ISSTEREO01 0520821213 v-23 1639038 v-51 1638938

23 - I24ISSTEREO01 0520821200 v-24 1639056

24 - I24ISSTEREO01 0520821226 v-25 1639134

25 - I24ISSTEREO01 0520821265 v-26 1639136

26 - I24ISSTEREO01 0520821278 v-27 1639138

27 - C21ISSTEREO01 0506431313 v-28 1639140

28 - C21ISSTEREO01 0506431300 v-29 1639142

29 - C21ISSTEREO01 0506431339

30 - I24ISSTEREO01 0520821326

31 - I24ISSTEREO01 0520821339

32 - I24ISSTEREO01 0520821352
25

Table 10. Image numbers of Galileo images used to make 

the SSI-only global Color Mosaic.

OBSERVATION IMAGE_NUMBER FILTER

1 – C9ISPHOTOM01 0401740711 7560

0401740700 GREEN

0401740707 VIOLET

2 – G2ISSRFMON01 0359986607 7560

0359986578 GREEN

0359986604 VIOLET

3 – E6ISSRFMON02 0383758511 7560

0383758500 GREEN

0383758507 VIOLET

4 – C21ISCOLOR_01 0506405700 7560

0506405735

0506405770

0506405814

0506405849

0506405884

0506405928

0506405963

0506406007

0506406042

0506406077

0506406121

0506406156

0506406200

0506406235

0506406484

4 – C21ISALBEDO01 0506406626 GREEN

0506406639

0506406652

0506406665

0506406678

0506406700

0506406713

0506406726

0506406639

0506406752

0506406765

0506406778

0506406800

0506406813

0506406826

0506406839

4 – C21ISCOLOR_01 0506405688 VIOLET

0506405732

0506405767

0506405811

0506405846

0506405881

0506405925

0506405960

0506406004

0506406039

0506406074

0506406118

0506406153

0506406188

0506406232

0506406481

Table 10. Image numbers of Galileo images used to make 

the SSI-only global Color Mosaic.—Continued

OBSERVATION IMAGE_NUMBER FILTER