The Jovian moon Io hosts the most powerful persistently active volcano in the Solar System, Loki Patera1,2. The interior of this volcanic, caldera-like feature is composed of a warm, dark floor covering 21,500 square kilometres3 surrounding a much cooler central ‘island’4. The temperature gradient seen across areas of the patera indicates a systematic resurfacing process4,5,6,7,8,9, which has been seen to occur typically every one to three years since the 1980s5,10. Analysis of past data has indicated that the resurfacing progressed around the patera in an anti-clockwise direction at a rate of one to two kilometres per day, and that it is caused either by episodic eruptions that emplace voluminous lava flows or by a cyclically overturning lava lake contained within the patera5,8,9,11. However, spacecraft and telescope observations have been unable to map the emission from the entire patera floor at sufficient spatial resolution to establish the physical processes at play. Here we report temperature and lava cooling age maps of the entire patera floor at a spatial sampling of about two kilometres, derived from ground-based interferometric imaging of thermal emission from Loki Patera obtained on 8 March 2015 ut as the limb of Europa occulted Io. Our results indicate that Loki Patera is resurfaced by a multi-phase process in which two waves propagate and converge around the central island. The different velocities and start times of the waves indicate a non-uniformity in the lava gas content and/or crust bulk density across the patera.
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A comprehensive investigation of the Galilean moon, Io, by tracing mass and energy flows
Experimental Astronomy Open Access 05 July 2021
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The Large Binocular Telescope (LBT) is an international collaboration among institutions in the United States, Italy and Germany. The LBT Corporation partners are: the University of Arizona on behalf of the Arizona university system; Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; The Ohio State University; and The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota and University of Virginia. The LBT Interferometer is funded by NASA as part of its Exoplanet Exploration program. The LMIRcam instrument is funded by the US NSF through grant NSF AST-0705296. A.G.D., K.d.K. and I.d.P. are partially supported by the NSF grant AST-1313485 to UC Berkeley and by the NSF Graduate Research Fellowship to K.d.K. under grant DGE-1106400. A.G.D. thanks the NASA Outer Planets Research Program for support under grant OPR NNN13D466T.
The authors declare no competing financial interests.
Reviewer Information Nature thanks F. Marchis and J. Spencer for their contribution to the peer review of this work.
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Extended data figures and tables
Extended Data Figure 1 Io disk subtraction.
a, b, Image of Io from the LBTI during the occultation, both before (a) and after (b) the subtraction of the reflected light from Io’s disk. In each image, Loki Patera is at upper left and Pillan Patera at lower right. The residual intensity around the limb in the subtracted image is the result of imperfect limb darkening correction.
Extended Data Figure 2 Uniform-intensity models.
Main panels, ingress and egress light curves from models with a uniform intensity distribution within Loki Patera, with (bottom row) and without (top row) a central island, plotted with the data and 1σ uncertainties. The patera outline is derived from Voyager imaging data19, and the model light curves in each row correspond to the image in the left panel of that row. It can be clearly seen from a comparison between the rows that the patera shape and the central island produce the broad-scale features of the ingress light curve, even without postulating a non-uniform intensity distribution. However, the addition of a non-uniform intensity distribution is required to match the egress light curve, indicating that the main temperature gradient is in the direction of motion of Europa’s limb during egress (roughly northwest to southeast).
Extended Data Figure 3 Intensity maps corresponding to progressively closer fits.
Shown are intensity reconstructions with independently-fitted pixels sized 8 km × 8 km based on fits to the light curve. The series demonstrates the artefacts from systematic noise in the light curve that become increasingly prominent as the fits tighten. Panels a–f correspond to Fits A–F discussed in the text. The fit metrics are summarized in Extended Data Table 2; Fit C is the preferred model at this resolution. The arrows at the left show the approximate direction of movement of Europa’s limb during ingress and egress, demonstrating that the orientation of the limb corresponds to the striping artefacts in Fits E and F.
Extended Data Figure 4 Model light curves and residuals.
Shown are model light curves for ingress and egress (with residual plots under) corresponding to the intensity maps for Fits A–F shown in Extended Data Fig. 3, plotted with the observed light curve and 1σ uncertainties (error bars).
Extended Data Figure 5 Preferred models for a range of pixel sizes and two pixel shapes.
The image titles indicate the size of each pixel in kilometres; top row, square pixels; bottom row, diamond pixels. The edges of the diamond pixels are parallel to the limb of Europa as it passes that portion of Loki Patera. Note the artefacts in the small-pixel models with diamond pixel shapes; in these models, as few as two timesteps influence the value of each pixel, and the model is thus very sensitive to noise in the light curve. These images demonstrate that the retrieved intensity distribution is consistent across different model parameters and resolutions. See Methods for details.
Extended Data Figure 6 Retrieval of simulated temperature maps.
The simulated maps (top row) include discrete hot features of Gaussian brightness distribution on top of a uniformly bright background patera. The bright spot constitutes about 15% of the patera’s total intensity in panels a–e, and approximately 7.5% of the intensity in panels f–h, and ranges in size from 10 km to 40 km (full-width at half-maximum of the feature). The retrieved maps (bottom row) are generated using the same methods applied to the LBT data, based on light curves corresponding to each intensity map with realistic noise added. The figure demonstrates that the presence and location of localized hot features of these brightness levels can be accurately retrieved by the analysis methods, but not their exact shape or size.
Extended Data Figure 7 Retrieval of simulated temperature maps with global gradient.
The simulated maps (a, b) demonstrate two possible models with similar azimuthally smoothed intensity profiles and an overall increase in brightness towards the southeast of the patera. The retrieved maps (c, d) were produced using the same procedure by which the LBT map was produced, based on the simulated light curves (e, f), which include realistic noise. The retrieved maps demonstrate that these scenarios are clearly distinguished by the retrieval process, despite the similarities between the model light curves.
Extended Data Figure 8 Model fits without the central island.
Shown are intensity reconstructions based on light curve fits using models with 8 km × 8 km square pixels, treating the intensity of the pixels in the island as free parameters in the fit. Without making any assumptions about the island, the fits recover its presence. Panels a–d demonstrate increasingly tight fits to the light curve; the preferred model on the basis of the corresponding light curve residuals is c here. Panels a–d correspond to Fits A–D discussed in the text.
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de Kleer, K., Skrutskie, M., Leisenring, J. et al. Multi-phase volcanic resurfacing at Loki Patera on Io. Nature 545, 199–202 (2017). https://doi.org/10.1038/nature22339
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A comprehensive investigation of the Galilean moon, Io, by tracing mass and energy flows
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