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USGS2011

The Moon Mineralogy Mapper (M3) imaging spectrometer for lunar science: Instrument description, calibration, on‐orbit measurements, science data calibration and on‐orbit validation

The NASA Discovery Moon Mineralogy Mapper imaging spectrometer was selected to pursue a wide range of science objectives requiring measurement of composition at fine spatial scales over the full lunar surface. To pursue these objectives, a broad spectral range imaging spectrometer with high uniformity and high signal-to-noise ratio capable of measuring compositionally diagnostic spectral absorption features from a wide variety of known and possible lunar materials was required. For this purpose the Moon Mineralogy Mapper imaging spectrometer was designed and developed that measures the spectral range from 430 to 3000 nm with 10 nm spectral sampling through a 24 degree field of view with 0.7 milliradian spatial sampling. The instrument has a signal-to-noise ratio of greater than 400 for the specified equatorial reference radiance and greater than 100 for the polar reference radiance. The spectral cross-track uniformity is >90% and spectral instantaneous field-of-view uniformity is >90%. The Moon Mineralogy Mapper was launched on Chandrayaan-1 on the 22nd of October. On the 18th of November 2008 the Moon Mineralogy Mapper was turned on and collected a first light data set within 24 h. During this early checkout period and throughout the mission the spacecraft thermal environment and orbital parameters varied more than expected and placed operational and data quality constraints on the measurements. On the 29th of August 2009, spacecraft communication was lost. Over the course of the flight mission 1542 downlinked data sets were acquired that provide coverage of more than 95% of the lunar surface. An end-to-end science data calibration system was developed and all measurements have been passed through this system and delivered to the Planetary Data System (PDS.NASA.GOV). An extensive effort has been undertaken by the science team to validate the Moon Mineralogy Mapper science measurements in the context of the mission objectives. A focused spectral, radiometric, spatial, and uniformity validation effort has been pursued with selected data sets including an Earth-view data set. With this effort an initial validation of the on-orbit performance of the imaging spectrometer has been achieved, including validation of the cross-track spectral uniformity and spectral instantaneous field of view uniformity. The Moon Mineralogy Mapper is the first imaging spectrometer to measure a data set of this kind at the Moon. These calibrated science measurements are being used to address the full set of science goals and objectives for this mission.

Journal of Geophysical Research E: Planets↗

USGS2021

Improving the usability of Galileo and Voyager images of Jupiter’s moon, Europa

NASA's Voyager 1, Voyager 2, and Galileo spacecraft acquired hundreds of images of Jupiter's moon Europa. These images provide the only moderate- to high-resolution views of the moon's surface and are therefore a critical resource for scientific analysis and future mission planning. Unfortunately, uncertain knowledge of the spacecraft's position and pointing during image acquisition resulted in significant errors in the location of the images on the surface. The result is that adjacent images are poorly aligned, with some images displaced by more than 100 km from their correct location. These errors severely degrade the usability of the Voyager and Galileo imaging data sets. To improve the usability of these data sets, we used the U.S. Geological Survey Integrated Software for Imagers and Spectrometers to build a nearly global image tie-point network with more than 50,000 tie points and 135,000 image measurements on 481 Galileo and 221 Voyager images. A global least-squares bundle adjustment of our final Europa tie-point network calculated latitude, longitude, and radius values for each point by minimizing residuals globally, and resulted in root mean square (RMS) uncertainties of 246.6 m, 307.0 m, and 70.5 m in latitude, longitude, and radius, respectively. The total RMS uncertainty was 0.32 pixels. This work enables direct use of nearly the entire Galileo and Voyager image data sets for Europa. We are providing the community with updated NASA Navigation and Ancillary Information Facility Spacecraft, Planet, Instrument, C-matrix (pointing), and Events kernels, mosaics of Galileo images acquired during each observation sequence, and individual processed and projected level 2 images.

Earth and Space Science↗

USGS2012

Compositional variations on the Moon: Recalibration of Galileo solid‐state imaging data for the Orientale region and farside

Updated radiometric calibration and systematic processing procedures for Galileo solid‐state imaging (SSI) data from the first (1990) Earth‐Moon encounter are presented. These procedures were applied to a whole‐disk imaging sequence of the Moon centered near Mare Orientale, called Lunmap 14 (L14). Processing of L14 data included radiometric calibration, subpixel coregistration, scattered light removal, geometric control and reprojection, photometric normalization, and calibration to Earth‐based spectra. Coregistration and scattered‐light removal procedures are improvements over the initial calibration of the SSI mosaics. The effects of scattered light correction are best seen using a whole‐Moon view such as L14; resolution of the debated amounts of light scattering from within or outside the camera field of view is not necessary. Scattered light removal particularly affects the 1‐μm spectral region and has implications for interpretation of mafic mineral signatures in mare deposits of the lunar limb and farside. Recalibrated spectra indicate that mare ponds of the limb show moderately deep 1‐μm absorptions, and thus mafic mineral contents, comparable to those of other nearside basalts. Mafic mineral contents of Schiller‐Shickard cryptomaria are higher than previously thought and are similar to some low‐Ti nearside basalts. Many of the recalibrated spectra from South Pole/Aitken are similar to those of Schiller‐Schickard cryptomaria, suggesting that many of these soils represent a mixed mare/highland lithology. The hypothesis that there is an olivine enrichment in southern South Pole/Aitken basin is not supported strongly by spectra shown here.

Journal of Geophysical Research: Planets↗

USGS2001

Imaging of volcanic activity on Jupiter's moon Io by Galileo during the Galileo Europa Mission and the Galileo Millennium Mission

The Solid-State Imaging (SSI) instrument provided the first high- and medium-resolution views of Io as the Galileo spacecraft closed in on the volcanic body in late 1999 and early 2000. While each volcanic center has many unique features, the majority can be placed into one of two broad categories. The “Promethean” eruptions, typified by the volcanic center Prometheus, are characterized by long-lived steady eruptions producing a compound flow field emplaced in an insulating manner over a period of years to decades. In contrast, “Pillanian” eruptions are characterized by large pyroclastic deposits and short-lived but high effusion rate eruptions from fissures feeding open-channel or open-sheet flows. Both types of eruptions commonly have ∼100-km-tall, bright, SO 2 -rich plumes forming near the flow fronts and smaller deposits of red material that mark the vent for the silicate lavas.

Journal of Geophysical Research E: Planets↗

USGS2023

Relationship between explosive and effusive volcanism in the Montes Apenninus region of the Moon

Lunar Pyroclastic Deposits (LPDs) are sites of explosive volcanism and often occur in areas of effusive volcanism on the Moon. On Earth, it has been observed that most volcanism has both effusive and explosive phases, whereas on the Moon, these two types of volcanism have typically been considered separately. We hypothesize that the relationship between explosive and effusive volcanism on the Moon is similar to what is observed on the Earth, where individual eruptions can experience multiple phases rather than one type of volcanism always preceding another or occurring separately. We present observations from the Moon Mineralogy Mapper detailing compositional relationships between volcanic features in the lunar Montes Apenninus region. We evaluated whether co-located LPDs and effusive features (e.g., rilles, mare) could have erupted from the same volcanic vent or even at the same time based on their compositional similarities and stratigraphic relationships. We found that the LPDs have varied stratigraphic relationships with co-located effusive features. We identified LPDs near sinuous rilles that may be related to the formation of the rille, where explosive and effusive volcanism occurred at the same vent (e.g., Mozart Rille), and LPDs that may be unrelated to the rille (e.g., Rimae Bode and Rima Bode LPD). Our results suggest that lunar volcanism can mirror terrestrial volcanism, with explosive and effusive eruptions demonstrating more complex dynamics and relationships than previously thought. This variability suggests that the relationship between LPDs and nearby volcanic features cannot be generalized for studies on their resource potential, eruption styles, or deposit volume.

Journal of Geophysical Research: Planets↗

USGS2022

Fundamental science and engineering questions in planetary cave exploration

Nearly half a century ago, two papers postulated the likelihood of lunar lava tube caves using mathematical models. Today, armed with an array of orbiting and fly-by satellites and survey instrumentation, we have now acquired cave data across our solar system—including the identification of potential cave entrances on the Moon, Mars, and at least six other planetary bodies. These discoveries gave rise to the study of planetary caves. To help advance this field, we leveraged the expertise of an interdisciplinary group to identify a strategy to explore caves beyond Earth. Focusing primarily on astrobiology, the cave environment, geology, robotics, instrumentation, and human exploration, our goal was to produce a framework to guide this subdiscipline through at least the next decade. To do this, we first assembled a list of 198 science and engineering questions. Then, through a series of social surveys, 114 scientists and engineers winnowed down the list to the top 53 highest priority questions. This exercise resulted in identifying emerging and crucial research areas that require robust development to ultimately support a robotic mission to a planetary cave—principally the Moon and/or Mars. With the necessary financial investment and institutional support, the research and technological development required to achieve these necessary advancements over the next decade are attainable. Subsequently, we will be positioned to robotically examine lunar caves and search for evidence of life within martian caves; in turn, this will set the stage for human exploration and potential habitation of both the lunar and martian subsurface.

JGR Planets↗

USGS2001

Rock types of South Pole-Aitken basin and extent of basaltic volcanism

The enormous pre-Nectarian South Pole-Aitken (SPA) basin represents a geophysically and compositionally unique region on the Moon. We present and analyze the mineralogical diversity across this basin and discuss the implications for basin evolution. Rock types are derived from Clementine multispectral data based on diagnostic characteristics of ferrous absorptions in fresh materials. Individual areas are characterized as noritic (dominated by low-Ca pyroxene), gabbroic/basaltic (dominated by high-Ca pyroxene), feldspathic (<3-6% FeO), and olivine-gabbro (dominated by high-Ca pyroxene and olivine). The anorthositic crust has effectively been removed from the interior of the basin. The style of volcanism within the basin extends over several 100 Myr and includes mare basalt and pyroclastic deposits. Several areas of ancient (pre-Orientale) volcanism, or cryptomaria, have also been identified. The nonmare mafic lithology that occurs across the basin is shown to be noritic in composition and is pervasive laterally and vertically. We interpret this to represent impact melt/breccia deposits derived from the lower crust. A few localized areas are identified within the basin that contain more diverse lithologies (gabbro, olivine-gabbro), some of which may represent material from the deepest part of the lower crust and perhaps uppermost mantle involved in the SPA event.

Journal of Geophysical Research E: Planets↗

USGS2026

Ecological transformations of coastal wetlands of the conterminous United States in response to contemporaneous sea-level rise

Coastal wetlands are among the most important ecosystems on the planet but are increasingly imperiled by accelerating sea-level rise (SLR). In the past, biogeomorphic feedbacks have allowed coastal wetlands to adjust vertically and persist through periods of accelerated SLR. Recent rates of SLR are faster than any in recent geologic history, making the fate of coastal wetlands highly uncertain. Here, we synthesize surface elevation table marker-horizon data from 442 stations in coastal wetlands across the conterminous United States to evaluate if they are largely persisting in the face of accelerated SLR, or if they are undergoing ecological transformations of submergence and/or migration. Across the conterminous United States, 11% of coastal wetlands in this sample are on a trajectory of submergence whereas 73% of sites are lagging SLR, but may be able to migrate upslope, and 16% of sites are gaining elevation at rates which exceed SLR indicating persistence and an ability to migrate seaward. Vulnerability of these systems to ecological transformation varies across the three coasts of the conterminous United States which, span large biogeomorphic gradients. These results serve as one of the first national syntheses of coastal wetland elevation trends and may help focus conservation and restoration efforts in a rapidly changing future.

conterminous United States↗

USGS2006

Remote sensing studies of the Dionysius region of the Moon

The Dionysius region is located near the western edge of Mare Tranquillitatis and is centered on Dionysius crater, which exhibits a well-developed dark ray system. Proposed origins for these dark rays included impact melt deposits and dark primary ejecta. The region also contains extensive deposits of Cayley-type light plains. Clementine multispectral images and a variety of spacecraft photography were utilized to investigate the composition and origin of geologic units in the Dionysius region. The portions of the dark rays for which spectral and chemical data were obtained are composed of mare debris contaminated with minor amounts of highland material. Both five-point spectra and values of the optical maturity (OMAT) parameter indicate that the dark rays are dominated by mare basalts, not glassy impact melts. The high-albedo rays associated with Dionysius exhibit FeO and TiO2 values that are lower than those of the adjacent dark ray surfaces and OMAT values that indicate that bright ray surfaces are not fully mature. The high-albedo rays are bright largely because of the contrast in albedo between ray material containing highlands-rich ejecta and the adjacent mare-rich surfaces. The mafic debris ejected by Dionysius was derived from a dark, iron-rich unit exposed high on the inner wall of the crater. This layer probably represents a mare deposit that was present at the surface of the preimpact target site. With one possible exception, there is no evidence for buried mare basalts associated with Cayley plains in the region.

Journal of Geophysical Research E: Planets↗

USGS2011

Transient and steady state creep response of ice I and magnesium sulfate hydrate eutectic aggregates

Using uniaxial compression creep experiments, we characterized the transient and steady state deformation behaviors of eutectic aggregates of system ice I and MgSO4 11H2O (MS11; meridianiite), which has significance because of its likely presence on moons of the outer solar system. Synthetic samples of eutectic liquid bulk composition, which produce eutectic colonies containing 0.35-0.50 volume fraction MS11, were tested as functions of colony size and lamellar spacing, temperature (230-250 K), and confining pressure (0.1 and 50 MPa) to strains ???0.2. Up to a differential stress of 6 MPa, the ice I-MS11 aggregates display an order of magnitude higher effective viscosity and higher stress sensitivity than do aggregates of pure polycrystalline ice at the same conditions. The creep data and associated microstructural observations demonstrate, however, that the aggregates are additionally more brittle than pure ice, approaching rate-independent plasticity that includes rupture of the hydrate phase at 6-8 MPa, depending on the scale of the microstructure. Microstructures of deformed samples reveal forms of semibrittle flow in which the hydrate phase fractures while the ice phase deforms plastically. Semibrittle flow in the icy shell of a planetary body would truncate the lithospheric strength envelope and thereby decrease the depth to the brittle-ductile transition by 55% and reduce the failure limit for compressional surface features from 10 to ???6 MPa. A constitutive equation that includes eutectic colony boundary sliding and intracolony flow is used to describe the steady state rheology of the eutectic aggregates. Copyright ?? 2011 by the American Geophysical Union.

Journal of Geophysical Research E: Planets↗

USGS2015

Examining spectral variations in localized lunar dark mantle deposits

The localized lunar dark mantle deposits (DMDs) in Alphonsus, J. Herschel, and Oppenheimer craters were analyzed using visible-near-infrared spectroscopy data from the Moon Mineralogy Mapper. Spectra of these localized DMDs were analyzed for compositional and mineralogical variations within the deposits and were compared with nearby mare basalt units. Spectra of the three localized DMDs exhibited mafic absorption features indicating iron-rich compositions, although the DMDs were spectrally distinct from nearby mare basalts. All of the DMDs contained spectral signatures of glassy materials, suggesting the presence of volcanic glass in varying concentrations across the individual deposits. In addition, the albedo and spectral signatures were variable within the Alphonsus and Oppenheimer crater DMDs, suggesting variable deposit thickness and/or variations in the amount of mixing with the local substrate. Two previously unidentified localized DMDs were discovered to the northeast of Oppenheimer crater. The identification of high concentrations of volcanic glass in multiple localized DMDs in different locations suggests that the distribution of volcanic glass across the lunar surface is much more widespread than has been previously documented. The presence of volcanic glass implies an explosive, vulcanian eruption style for localized DMDs, as this allows volcanic glass to rapidly quench, inhibiting crystallization, compared to the larger hawaiian-style eruptions typical of regional DMD emplacement where black beads indicate a higher degree of crystallization. Improved understanding of the local and global distributions of volcanic glass in lunar DMDs will further constrain lunar degassing and compositional evolution throughout lunar volcanic history.

Journal of Geophysical Research E: Planets↗

USGS2025

The pre-maria geologic history of the Imbrium basin preserved by remnant highlands massifs

The Imbrium basin is one of the largest and youngest impact basins on the Moon. It has experienced multiple phases of volcanism that filled the basin with basaltic lavas, obscuring most evidence of geologic activity prior to the emplacement of mare basalts. Elevated basin ring massifs, however, can retain some of that history due to their higher topographic elevation compared to the maria. In this work, we use thermal infrared and radar data sets in conjunction with compositional data sets to establish the presence of external material that has been deposited on top of several remnant basin massifs of Imbrium. These massifs originally formed as part of the Imbrium basin ring structure, but their material properties indicate that they have since experienced modification from outside sources. In southwest Imbrium, we present evidence that Mons Vinogradov was mantled by rock-poor, glassy pyroclastic material prior to the deposition of Eratosthenian-era basalts immediately surrounding the mons. In northern Imbrium, we find that Montes Recti and Montes Teneriffe were not affected by pyroclastic volcanism but rather were mantled by rock- and glass-poor ejecta materials likely related to the Iridum basin impact. At Mons Piton in eastern Imbrium, we see weaker glass signatures than those found at Mons Vinogradov, which we suggest could be due to a thin layer of reworked or partially buried glassy pyroclastic material. These results indicate that basin ring massifs provide a mechanism for studying the geologic history of lunar impact basins.

JGR Planets↗

USGS2014

Volcanic glass signatures in spectroscopic survey of newly proposed lunar pyroclastic deposits

Moon Mineralogy Mapper spectroscopic observations are used to assess the mineralogy of five sites that have recently been proposed to include lunar dark mantle deposits (DMDs). Volcanic glasses have, for the first time, clearly been identified at the location of three of the proposed pyroclastic deposits. This is the first time that volcanic glasses have been identified at such a small scale on the lunar surface from remote sensing observations. Deposits at Birt E, Schluter, and Walther A appear to be glassy DMDs. Deposits at Birt E and Schluter show (1) morphological evidence suggesting a likely vent and (2) mineralogical evidence indicative of the presence of volcanic glasses. The Walther A deposits, although they show no morphological evidence of vents, have the spectroscopic characteristics diagnostic of volcanic glasses. The deposits of the Freundlich-Sharonov basin are separated in two areas: (1) the Buys-Ballot deposits lack mineralogical and morphological evidence and thus are found to be associated with mare volcanism not with DMDs and (2) the Anderson crater deposits, which do not exhibit glassy DMD signatures, but they appear to be associated with possible vent structures and so may be classifiable as DMDs. Finally, dark deposits near the crater Kopff are found to be associated with likely mare volcanism and not associated with DMDs. The spectral identification of volcanic glass seen in many of the potential DMDs is a strong indicator of their pyroclastic origin.

Journal of Geophysical Research E: Planets↗

USGS2011

Optical maturity variation in lunar spectra as measured by Moon Mineralogy Mapper data

High spectral and spatial resolution data from the Moon Mineralogy Mapper (M3) instrument on Chandrayaan-1 are used to investigate in detail changes in the optical properties of lunar materials accompanying space weathering. Three spectral parameters were developed and used to quantify spectral effects commonly thought to be associated with increasing optical maturity: an increase in spectral slope ("reddening"), a decrease in albedo ("darkening"), and loss of spectral contrast (decrease in absorption band depth). Small regions of study were defined that sample the ejecta deposits of small fresh craters that contain relatively crystalline (immature) material that grade into local background (mature) soils. Selected craters are small enough that they can be assumed to be of constant composition and thus are useful for evaluating trends in optical maturity. Color composites were also used to identify the most immature material in a region and show that maturity trends can also be identified using regional soil trends. The high resolution M3 data are well suited to quantifying the spectral changes that accompany space weathering and are able to capture subtle spectral variations in maturity trends. However, the spectral changes that occur as a function of maturity were observed to be dependent on local composition. Given the complexity of space weathering processes, this was not unexpected but poses challenges for absolute measures of optical maturity across diverse lunar terrains. Copyright 2011 by the American Geophysical Union.

Journal of Geophysical Research E: Planets↗

USGS2000

Compositional analyses of small lunar pyroclastic deposits using Clementine multispectral data

Clementine ultraviolet-visible (UVVIS) data are used to examine the compositions of 18 pyroclastic deposits (15 small, three large) at 13 sites on the Moon. Compositional variations among pyroclastic deposits largely result from differing amounts of new basaltic (or juvenile) material and reworked local material entrained in their ejecta upon eruption. Characterization of pyroclastic deposit compositions allows us to understand the mechanisms of lunar explosive volcanism. Evidence for compositional differences between small pyroclastic deposits at a single site is observed at Atlas crater. At all sites, compositional variation among the small pyroclastic deposits is consistent with earlier classification based on Earth-based spectra: three compositional groups can be observed, and the trend of increasing mafic absorption band strength from Group 1 to Group 2 to Group 3 is noted. As redefined here, Group 1 deposits include those of Alphonsus West, Alphonsus Southeast, Alphonsus Northeast 2, Atlas South, Crüger, Franklin, Grimaldi, Lavoisier, Oppenheimer, Orientale, and Riccioli. Group 1 deposits resemble lunar highlands, with weak mafic bands and relatively high UV/VIS ratios. Group 2 deposits include those of Alphonsus Northeast 1, Atlas North, Eastern Frigoris East and West, and Aristarchus Plateau; Group 2 deposits are similar to mature lunar maria, with moderate mafic band depths and intermediate UV/VIS ratios. The single Group 3 deposit, J. Herschel, has a relatively strong mafic band and a low UV/VIS ratio, and olivine is a likely juvenile component. Two of the deposits in these groups, Orientale and Aristarchus, are large pyroclastic deposits. The third large pyroclastic deposit, Apollo 17/Taurus Littrow, has a very weak mafic band and a high UV/VIS ratio and it does not belong to any of the compositional groups for small pyroclastic deposits. The observed compositional variations indicate that highland and mare materials are also present in many large and small pyroclastic deposits, and they suggest that volcanic glasses or spheres may not be dominant juvenile components in all large pyroclastic deposits.

Journal of Geophysical Research E: Planets↗

USGS2012

Galileo observations of Post-Imbrium lunar craters during the first Earth-Moon flyby

Copernican‐age craters are among the most conspicuous features seen on the far side and western limb of the Moon in the Galileo multispectral images acquired in December 1990. Among the new morphologic observations of far‐side craters are bright rays, continuous ejecta deposits, and dark rings associated with probable impact‐melt veneers. These observations suggest that the mapped age assignments of several large far‐side craters (Ohm, Robertson, and possibly Lowell and Lenz) need revision. New crater size‐frequency measurements on Lunar Orbiter images suggest the following age reassignments: Hausen (170 km diameter), Pythagoras (120 km), and Bullialdus (61 km) from Eratosthenian to Upper Imbrian, and Carpenter (60 km) and Harpalus (39 km) from Copernican to Eratosthenian. Colors and albedos of craters (away from impact‐melt veneers) are correlated with their geologic emplacement ages as determined from counts of superposed craters; these age‐color relations are used to estimate the emplacement age (time since impact event) for other Copernican‐age craters. These age‐color relations indicate a probable Copernican age for 27 far‐side or western limb craters larger than 10 km diameter that were not previously mapped as Copernican. The apparent deficiency of Copernican craters on the far side compared with the near side in published geologic maps is not present in our data. Age‐color trends differ between mare and highland regions and between the interiors and continuous ejecta of the craters. Similar trends are established for color and albedo versus soil‐maturity indices for the returned lunar samples, with distinct trends for mare and highland soils. However, the mare versus highland offsets are reversed in the two comparisons. These relations can be explained by variations in regolith thicknesses and rates of mixing with relatively fresh, crystalline ejecta. Therefore, the soil‐maturity trends represent longer geologic time periods in regions with thinner regoliths, such as the maria.

Journal of Geophysical Research E: Planets↗

USGS1997

The Phanerozoic impact cratering rate: Evidence from the farside of the Moon

The relatively recent (<1 b.y.) flux of asteroids and comets forming large craters on the Earth and Moon may be accurately recorded by craters with bright rays on the Moon's farside. Many previously unknown farside rayed craters are clearly distinguished in the low-phase-angle images returned by the Clementine spacecraft. Some large rayed craters on the lunar nearside are probably significantly older than 1 Ga; rays remain visible over the maria due to compositional contrasts long after soils have reached optical maturity. Most of the farside crust has a more homogeneous composition and only immature rays are visible. The size-frequency distribution of farside rayed craters is similar to that measured for Eratosthenian craters (up to 3.2 b.y.) at diameters larger than 15 km. The areal density of farside rayed craters matches that of a corrected tabulation of nearside Copernican craters. Hence the presence of bright rays due to immature soils around large craters provides a consistent time-stratigraphic basis for defining the base of the Copernican System. The density of large craters less than ∼3.2 b.y. old is ∼3.2 times higher than that of large farside rayed craters alone. This observation can be interpreted in two ways: (1) the average cratering rate has been constant over the past 3.2 b.y. and the base of the Copernican is ∼1 Ga, or (2) the cratering rate has increased in recent geologic time and the base of the Copernican is less than 1 Ga. We favor the latter interpretation because the rays of Copernicus (800–850 m.y. old) appear to be very close to optical maturity, suggesting that the average Copernican cratering rate was ∼35% higher than the average Eratosthenian rate. Other lines of evidence for an increase in the Phanerozoic (545 Ga) cratering rate are (1) the densities of small craters superimposed on Copernicus and Apollo landing sites, (2) the rates estimated from well-dated terrestrial craters (≤120 m.y.) and from present-day astronomical observations, and (3) the Proterozoic rate suggested by the crater record of Australia. The hypothesis most consistent with several key observations is that the cratering rate has increased by ∼2x during the past ∼300 m.y‥

Journal of Geophysical Research E: Planets↗

USGS2011

Volcanism on Io: New insights from global geologic mapping

We produced the first complete, 1:15 M-scale global geologic map of Jupiter’s moon Io, based on a set of monochrome and color Galileo – Voyager image mosaics produced at a spatial resolution of 1 km/pixel. The surface of Io was mapped into 19 units based on albedo, color and surface morphology, and is subdivided as follows: plains (65.8% of surface), lava flow fields (28.5%), mountains (3.2%), and patera floors (2.5%). Diffuse deposits (DD) that mantle the other units cover ∼18% of Io’s surface, and are distributed as follows: red (8.6% of surface), white (6.9%), yellow (2.1%), black (0.6%), and green (∼0.01%). Analyses of the geographical and areal distribution of these units yield a number of results, summarized below. (1) The distribution of plains units of different colors is generally geographically constrained: Red–brown plains occur >±30° latitude, and are thought to result from enhanced alteration of other units induced by radiation coming in from the poles. White plains (possibly dominated by SO 2 + contaminants) occur mostly in the equatorial antijovian region (±30°, 90–230°W), possibly indicative of a regional cold trap. Outliers of white, yellow, and red–brown plains in other regions may result from long-term accumulation of white, yellow, and red diffuse deposits, respectively. (2) Bright (possibly sulfur-rich) flow fields make up 30% more lava flow fields than dark (presumably silicate) flows (56.5% vs. 43.5%), and only 18% of bright flow fields occur within 10 km of dark flow fields. These results suggest that secondary sulfurous volcanism (where a bright-dark association is expected) could be responsible for only a fraction of Io’s recent bright flows, and that primary sulfur-rich effusions could be an important component of Io’s recent volcanism. An unusual concentration of bright flows at ∼45–75°N, ∼60–120°W could be indicative of more extensive primary sulfurous volcanism in the recent past. However, it remains unclear whether most bright flows are bright because they are sulfur flows, or because they are cold silicate flows covered in sulfur-rich particles from plume fallout. (3) We mapped 425 paterae (volcano-tectonic depressions), up from 417 previously identified by Radebaugh et al. (Radebaugh, J., Keszthelyi, L.P., McEwen, A.S., Turtle, E.P., Jaeger, W., Milazzo, M. [2001]. J. Geophys. Res. 106, 33005–33020). Although these features cover only 2.5% of Io’s surface, they correspond to 64% of all detected hot spots; 45% of all hot spots are associated with the freshest dark patera floors, reflecting the importance of active silicate volcanism to Io’s heat flow. (4) Mountains cover only ∼3% of the surface, although the transition from mountains to plains is gradational with the available imagery. 49% of all mountains are lineated and presumably layered, showing evidence of linear structures supportive of a tectonic origin. In contrast, only 6% of visible mountains are mottled (showing hummocks indicative of mass wasting) and 4% are tholi (domes or shields), consistent with a volcanic origin. (5) Initial analyses of the geographic distributions of map units show no significant longitudinal variation in the quantity of Io’s mountains or paterae, in contrast to earlier studies. This is because we use the area of mountain and patera materials as opposed to the number of structures, and our result suggests that the previously proposed anti-correlation of mountains and paterae (Schenk, P., Hargitai, H., Wilson, R., McEwen, A., Thomas, P. [2001]. J. Geophys. Res. 106, 33201–33222; Kirchoff, M.R., McKinnon, W.B., Schenk, P.M. [2011]. Earth Planet. Sci. Lett. 301, 22–30) is more complex than previously thought. There is also a slight decrease in surface area of lava flows toward the poles of Io, perhaps indicative of variations in volcanic activity. (6) The freshest bright and dark flows make up about 29% of all of Io’s flow fields, suggesting active emplacement is occurring in less than a third of Io’s visible lava fields. (7) About 47% of Io’s diffuse deposits (by area) are red, presumably deriving their color from condensed sulfur gas, and ∼38% are white, presumably dominated by condensed SO 2 . The much greater areal extent of gas-derived diffuse deposits (red + white, 85%) compared to presumably pyroclast-bearing diffuse deposits (dark (silicate tephra) + yellow (sulfur-rich tephra), 15%) indicates that there is effective separation between the transport of tephra and gas in many Ionian explosive eruptions. Future improvements in the geologic mapping of Io can be obtained via (a) investigating the relationships between different color/material units that are geographically and temporally associated, (b) better analysis of the temporal variations in the map units, and (c) additional high-resolution images (spatial resolutions ∼200 m/pixel or better). These improvements would be greatly facilitated by new data, which could be obtained by future missions.

Icarus↗