Introduction: Athabasca Valles is the site of the most recent floods of water and lava on Mars [e.g., 1-3]. This makes the area interesting for understanding the current state of the interior of Mars, especially in regard to water and heat needed for life. However, the timing and magnitude of the possible aqueous flood is not well understood given that the surface is coated by the more recent lava. Here we lay out our plan to investigate this problem and present some initial results. Hypotheses: The idea that Athabasca Valles was carved by water and then draped by lava is popular [e.g., 1-6]. There are two variants of this model – the water could have immediately preceded the lava or the aqueous flood(s) could have been much earlier. A complete investigation needs to address two additional hypotheses: the channel was entirely carved by lava [7] and only aqueous flows were involved [e.g., 8-11]. Testing the Hypotheses: There are viable ways to test these four possible geologic histories. However, some new data, analyses and modeling are needed. Purely aqueous hypothesis. The purely aqueous hypothesis was rejected by Jaeger et al. [4] because of the compelling evidence that the floor of Athabasca Valles is coated with lava that has not been eroded by a subsequent flood. One of the key observation was that conical landforms were constructed on the flow’s surface while the flow was still moving. This is consistent with the cones being phreatovolcanic constructs but is incompatible with the cones being pingos or related periglacial features. Additionally, stable overhanging plates several meters long are compatible with the material properties of lava but are inconsistent with mud or ice. The delicate meter-scale volcanic features preserved on the floor of Athabasca Valles indicate that no significant flood of water post-dates this lava. Aqueous channel draped by lava. Features considered diagnostic of aqueous floods, such as streamlined islands and large bedforms, are abundant in Athabasca Valles [2,3,10,11]. While definitive evidence that these features pre-date the lava flow is lacking, there are observations that suggest a substantial time between the main valley-carving event and the lava emplacement. One of these is the presence of a number of uneroded craters on the floor of the valley that are covered by lava. These craters could have formed after the main valley was carved by water but before the emplacement of the lava. Six craters >1 km in diameter are found on the 18,000 km2 floor of Athabasca Valles, suggesting that hundreds of millions of years passed between the aqueous and lava floods. However, these craters are all found in the distal and marginal parts of Athabasca Valles (Fig. 1) where erosion was probably minimal. Until a more thorough investigation of the morphology of these craters and their setting in the channel is completed, this line of inquiry is supportive but inconclusive about the timing of the water and lava.

Introduction: The Athabasca Valles Lava was emplaced as a turbulent flood within the last 50 million years, making it the youngest known major volcanic event on Mars. The lava was fed by the Cerberus Fossae, coursed through Athabasca Valles, overfilled Cerberus Palus, and finally halted 1400 km from source. The peak discharge was of order 10 m/s and the eruption duration was only several weeks [1]. One of the most interesting remaining questions related to this remarkable eruption is “Could this catastrophic flood of lava erode the substrate?” A theoretical investigation of thermal erosion by this flow suggests that, because of the short eruption duration, only a few meters of material might have been removed by melting of the substrate [2]. Theoretical Considerations: However, mechanical erosion could plausibly be more effective [1,2]. As noted in [1], mechanical erosion by aqueous flows is often considered to be proportional to “stream power” or the basal shear stress created by the flow. The stream power for aqueous and lava floods filling Athabasca Valles would be nearly identical. Mechanical erosion by lava is also aided by the relatively small density contrast between rock and lava, making transport in suspension easier. However, a turbulent lava flow is not expected to have the intense eddies of a large aqueous flood, so current theory is insufficient to confidently conclude that there was significant mechanical erosion by lava in Athabasca Valles [1]. A simple thermal balance places rough but useful constraints on the amount of cold rock that could be entrained into a lava flow. Rock is of order 1000 K colder than the lava and the lava will freeze if cooled ~100 K, so the entrained rock should be no more than 10% of the erupted volume. Given the huge distance the lava continued past Athabasca Valles, an order of magnitude less entrainment is considered more realistic. Since the volume of the flow is 5000-7500 km [1], this corresponds to on order of ~100-1000 km of erosion. Athabasca Valles is roughly 300 km x 30 km in area (~10,000 km), so a maximum of <100 m, and more realistically ~10 m, average downcutting is allowed. Non-uniform erosion is likely, so some tens of meters of local downcutting is thermally plausible. Data: This scale of erosion would produce features resolvable in images from the Mars Reconnaissance Orbiter CTX and HiRISE cameras. The primary data we utilize are (a) the 1 m/post HiRISE DEM produced by the USGS from images PSP_002661_1895 and PSP_003294_1895 with corresponding 25 cm/pixel orthorectified images and (b) the 100 m/post CTX DEM (Fig. 1) produced by the USGS from images P01_001540_1889, P01_001606_1897, P03_002226_ 1895, P03_002371_1888, P19_008344_1894, and P20_008845_1894 with corresponding 6 m/pixel orthorectified images.

Introduction: We have completed a new 1:15,000,000 global geologic map of Jupiter’s volcanic moon, Io, based on a set of 1 km/pixel combined Galileo-Voyager mosaics produced by the U.S. Geological Survey [1]. The map was produced over the last three years using ArcGISTM software, and has undergone peer-review. Here we report some of the key results from our global mapping efforts, and how these results relate to questions regarding the volcano-tectonic evolution of Io. Previous Work: Previously we reported our techniques for global mapping of Io [2] and on the development of an Io database [3] that will include most Io data sets to address the surface changes due to Io’s active volcanism. Previously we also reported the percentage of Io covered by each of 14 process-related geologic material units and structures [4], and last year we presented a stratigraphic correlation of these map units [5]. Here we report results from visual, graphic, and statistical analyses of the map units and structures and discuss insights into the formation of plains, lava flow fields, paterae, mountains, and diffuse deposits. Results I (Plains): Plains units cover 66.6% of the surface, and (with the exception of a few outliers) are geographically distributed on Io. Red-brown plains dominantly occur >±30 ̊ latitude, and are thought to result from enhanced radiation-induced alteration of other plains units. White plains (typically enriched in SO2) occur mostly in the equatorial antijovian region (±30 ̊, 90 ̊-230 ̊W), possibly indicative of a regionally colder part of the satellite to preserve the SO2. Why is this one region colder such that SO2 concentrates here? The answer may be related to variations in crustal distribution of magma sources or delivery mechanisms, or perhaps crustal thickness, relative to other parts of Io. Outliers of white, bright, and red-brown plains occurring in other regions likely result from long-term accumulation of white, yellow, and red diffuse deposits, respectively. Results II (Lava Flows): Lava flows cover 27.8% of the surface, the bulk of which (20.6%) are undivided flows whose original composition (dark silicate or bright sulfur) cannot be determined. Bright flow fields outnumber dark flow fields by a ratio of ~1.5 to 1; both of these are presumed to be the freshest and youngest lava flow types on Io. Only 16.8% of the bright flow fields are adjacent to dark flow fields. The association of adjacent bright and dark flows would be expected if sulfur flows are derived from secondary sulfur volcanism (i.e., melting of sulfur-rich country rock by heat from silicate magmas or lavas [6].) Thus, this result suggests that secondary sulfur volcanism may only have a minor role in Io’s current volcanic activity (although there may be a scale-dependence on these processes that requires further investigation). There is an unusual concentration of bright flows at ~45 ̊-75 ̊N, ~60-120 ̊W, perhaps indicative of past, extensive primary sulfur volcanism in this region. However, this stands in stark contrast to the current correlation of active hot spots with surface materials, in which only 1.7% of hot spots correlate with bright flows, suggesting that at present primary sulfur volcanism has a minor role in Io’s current activity. 20.3% of hot spots detected by telescopic and spacecraft observations correlate with dark flow fields and another 9.3% correlate with undivided flow fields. Thus, lava flows make up less than one-third of Io’s heat sources. Results III (Paterae): Paterae are circular to irregular volcano-tectonic depressions on Io, thought to be similar to terrestrial calderas [7]. Evidence suggests some of these contain periodically foundering lava lakes [8], whereas others are resurfaced by bright or dark lava flows. We have mapped a total of 425 paterae on Io, an increase from the 417 previously identified by [7]. Yet even though paterae cover only 2.5% of Io’s surface (and dark patera floor material covers only 0.5%), Io’s hot spots dominantly occur within paterae (63.9% of all hot spots, with 45.3% correlated with dark patera floor material). The fact that 93.5% of Ionian hot spots correlate with either dark (younger) or undivided (older) patera floors or lava flows suggests that silicate materials are the dominant component of Io’s recent volcanism. Results IV (Mountains): Mountains cover only 3.1% of Io’s surface, yet are some of the most dramatic features observed in spacecraft images. A majority of Io’s mountains, 37.9%, were mapped as undivided mountain materials, in contrast to mapping 27.1% as Lineated mountains, 22.1% as Layered plains, 3.6% as Tholi (volcanic mountains), and 2.9% as Mottled mountains. These results demonstrate that variable imaging coverage of Io’s mountains inhibits more accurate mapping of undivided mountains into the other units. As expected, lineated mountains, thought to be tectonically uplifted crustal blocks [9], are generally taller than the more degraded mottled

Recently acquired data from the High Resolution Imaging Science Experiment (HiRISE), Context (CTX) imager, and Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) onboard the Mars Reconnaissance Orbiter (MRO) spacecraft were used to investigate the emplacement of the youngest flood-lava flow on Mars. Careful mapping finds that the Athabasca Valles flood lava is the product of a single eruption, and it covers 250,000 km2 of western Elysium Planitia with an estimated 5000–7500 km3 of mafic or ultramafic lava. Calculations utilizing topographic data enhanced with MRO observations to refine the dimensions of the channel system show that this flood lava was emplaced turbulently over a period of only a few to several weeks. This is the first well-documented example of a turbulently emplaced flood lava anywhere in the Solar System. However, MRO data suggest that this same process may have operated in a number of martian channel systems. The magnitude and dynamics of these lava floods are similar to the aqueous floods that are generally believed to have eroded the channels, raising the intriguing possibility that mechanical erosion by lava could have played a role in their incision.

We provide an overview of features indicative of the interaction between water and lava and/or magma on Mars as seen by the High Resolution Imaging Science Experiment (HiRISE) camera during the Primary Science Phase of the Mars Reconnaissance Orbiter (MRO) mission. The ability to confidently resolve meter-scale features from orbit has been extremely useful in the study of the most pristine examples. In particular, HiRISE has allowed the documentation of previously undescribed features associated with phreatovolcanic cones (formed by the interaction of lava and groundwater) on rapidly emplaced flood lavas. These include "moats" and "wakes" that indicate that the lava crust was thin and mobile, respectively [Jaeger, W.L., Keszthelyi, L.P., McEwen, A.S., Dundas, C.M., Russel, P.S., 2007. Science 317, 1709–1711]. HiRISE has also discovered entablature-style jointing in lavas that is indicative of water-cooling [Milazzo, M.P., Keszthelyi, L.P., Jaeger, W.L., Rosiek, M., Mattson, S., Verba, C., Beyer, R.A., Geissler, P.E., McEwen, A.S., and the HiRISE Team, 2009. Geology 37, 171–174]. Other observations strongly support the idea of extensive volcanic mudflows (lahars). Evidence for other forms of hydrovolcanism, including glaciovolcanic interactions, is more equivocal. This is largely because most older and high-latitude terrains have been extensively modified, masking any earlier 1–10 m scale features. Much like terrestrial fieldwork, the prerequisite for making full use of HiRISE's capabilities is finding good outcrops.

The High Resolution Imaging Science Experiment (HiRISE) on the Mars Reconnaissance Orbiter (MRO) recently discovered multi-tiered columnar jointing on Mars [1, 2]. Since the initial discovery image, more columns, some with entablature, have been observed at 13 sites. Nearly all of these sites occur in the uplifted walls of impact craters in regions with histories of flood volcanism. Blind extension of the modeling of terrestrial columnar lavas by [3] to the martian columnar lavas and entablature discussed in [2] (and seen in observation PSP 005917 2020) suggest that they cooled over approximately five years, during which time they were subject to multiple episodes of inundation by liquid water.

We report on the discovery of columnar jointing in Marte Valles, Mars. These columnar lavas were discovered in the wall of a pristine, 16-km-diameter impact crater and exhibit the features of terrestrial columnar basalts. There are discontinuous outcrops along the entire crater wall, suggesting that the columnar rocks covered a surface area of at least 200 km(2), assuming that the rocks obliterated by the impact event were similarly jointed. We also see columns in the walls of other fresh craters in the nearby volcanic plains of Elysium Planitia-Amazonis Planitia, which include Marte Vallis, and in a well-preserved crater in northeast Hellas.

A new global geologic map of Jupiter's volcanic moon, Io is being prepared, with the focus being on completion of a draft map by July 2008. Here initial results of the mapping are reported: a preliminary distribution of material units in terms of areas and a visual representation. Additionally, the mapping hopes to address some of the problems in Io geology. Thus far it has been discovered that Io's surface is dominated by plains material, thought to consist of Io's silicate crust covered by pyroclastic deposits and lava flows of silicate and sulfur-bearing composition. Many plains areas contain flow fields that cannot be mapped separately due to a lack of resolution or modification by alteration processes. Discrete lava flows and flow fields are the next most abundant unit, with bright (sulfur?) flows in greater abundance than dark (silicate?) flows. The source of most of Io's heat flow, the paterae, are the least abundant unit in terms of areal extent.Upon completion of the draft map for peer review, it will be used to investigate several specific questions about the geological evolution of Io that previously could not be well addressed, including: comparison of the areas versus the heights of Ionian mountains to assess their stability and evolution; correlation and comparison of Galileo Near-Infrared Mapping Spectrometer and Photopolarimeter-Radiometer hot spot locations with the mapped location of dark versus bright lava flows and patera floors to assess any variations in the types of sources for Io's active volcanism; and the creation of a global inventory of the areal coverage of dark and bright laval flows to assess the relative importance of sulfur versus silicate volcanism in resurfacing Io, and to assess whether there are regional concentrations of either style of volcanism that may have implications on interior processes.

We are approaching the end of the third year of mapping the Athabasca Valles region of Mars. The linework has been adjusted in response to new CTX images and we are on schedule to submit the 4 MTM quads (05202, 05207, 10202, 10207) and ac-companying paper by the end of this fiscal year.