Jupiter's volcanic moon Io is burning much hotter than expected, according to a new study that challenges long-held assumptions about the moon's heat flow. The research, led by Federico Tosi of Italy's National Institute for Astrophysics, highlights a critical issue with how scientists have been interpreting data from NASA's Juno spacecraft's JIRAM instrument. The study reveals that some widely used measurements can distort the true scale of Io's heating, potentially leading to a significant underestimation of the moon's energy output.
The focus of the reassessment is on data from JIRAM, which observes the moon at specific wavelengths to detect volcanic activity. The study finds that relying solely on a narrow slice of the infrared spectrum, known as the M band, can lead to inaccurate conclusions. This region of light near 4.8 micrometers is highly effective at identifying the hottest lava, but it struggles to capture the cooler surfaces that cover a much larger area.
Tosi compares this approach to judging the size of a bonfire by only looking at the flames while ignoring the embers. The brightest parts of a volcano capture our attention, but most of the heat escapes from the broader, cooler crusts that are not visible in the M band. When the team re-examined how the M band responds to different temperatures, they discovered that the same M-band brightness can represent vastly different total power levels. Without temperature measurements, the M band alone cannot provide a reliable estimate of the total heat.
Io's volcanoes are not uniform; they consist of lava lakes surrounded by very hot rims and cooler, solidified crusts. The crust, still warm enough to glow in the infrared, radiates most of its energy at longer wavelengths. For instance, at Chors Patera, the M-band image suggests a power output of about one gigawatt, primarily from the thin ring. However, when the team estimated the total heat from the interior crust, the number skyrocketed to about 420 gigawatts. This implies a ratio of total heat to M-band heat that is far higher than the small correction factors assumed in many studies.
Similar patterns were observed at other sites like Catha Patera and Pfu1063. The M-band powers in these locations range between 0.12 and 0.71 gigawatts, while their true thermal outputs range between 100 and 414 gigawatts. This discrepancy highlights that the crust, not the ring, drives the total heat. These findings are not isolated cases but represent a broader truth, emphasizing the significant contribution of cooler terrain to Io's heat.
Another issue lies in the instrument's electronics. The M-band camera can saturate when viewing very bright objects, and past papers have attempted to avoid saturated pixels. However, the new analysis reveals that the camera leaves its linear range earlier than expected, causing some images to already distort the true signal downward by the time a pixel appears 'safe' on paper. The spectrometer, which collects light across a broader range, does not suffer from this problem and can be used as a reference.
The study also challenges earlier claims that Io emits more heat at low latitudes than near the poles. By sorting M-band radiance by latitude and applying various statistical tests, the team found that small shifts in the thresholds could wash out or reverse the signal. This instability arises from the fact that about half of the M-band output comes from only 17 of the 266 known volcanoes, making the location of these volcanoes far more critical than any global trend.
The implications of this research extend to the debate about Io's interior magma ocean. The study demonstrates that existing M-band trends cannot support firm conclusions about the interior structure. While independent radio science data from Juno also points away from a global ocean, the authors caution that infrared evidence alone is insufficient to settle the issue.
The study emphasizes the need for caution in interpreting data that relies solely on the M band. It encourages the use of broader spectral data to make more accurate interpretations. This research offers a fresh perspective on understanding volcanic activity on Io and other worlds, revealing the limitations of single-band measurements in capturing a planet's full energy budget.
Future missions like Europa Clipper and Juice will observe Io from a distance, and the method developed in this study will aid scientists in making better use of limited data. These findings also guide the design of future instruments capable of handling both bright and faint signals without distortion. Ultimately, improving heat flow estimates will contribute to our understanding of how worlds with active interiors evolve and how tidal heating shapes planets in other solar systems.
