JUICE-JANUS observations of Earth in preparation for the JANUS investigation of Jupiter&rsquo;s atmosphere

Hueso, Ricardo; Antuñano, Arrate; Lara, Luisa María; Stephan, Katrin; Zinzi, Angelo; Coustenis, Athena; Yair, Yoav; Sato, Mitsuteru; Haruyama, Junichi; Simon, Amy; Tubiana, Cecilia; Penasa, Luca; Agostini, Livio; Luchetti, Alice; Aboudan, Alessio; Aye, Michael; Kersten, Elke; Matz, Klaus-Dieter; Politti, Romolo; Trauthan, Frank; Evill, Ry; Belgacem, Ines; Yukihiro, Takahashi; Castro-Marín, Jose María; Della Corte, Vincenzo; Hviid, Stubbe; Roatsch, Thomas; Schmitz, Nicole; Patel, Manish; Portyankina, Ganna; Palumbo, Pasquale

doi:10.5194/egusphere-2026-710

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https://doi.org/10.5194/egusphere-2026-710

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17 Feb 2026

| 17 Feb 2026

Status: this preprint is open for discussion and under review for Annales Geophysicae (ANGEO).

JUICE-JANUS observations of Earth in preparation for the JANUS investigation of Jupiter’s atmosphere

Ricardo Hueso, Arrate Antuñano, Luisa María Lara, Katrin Stephan, Angelo Zinzi, Athena Coustenis, Yoav Yair, Mitsuteru Sato, Junichi Haruyama, Amy Simon, Cecilia Tubiana, Luca Penasa, Livio Agostini, Alice Luchetti, Alessio Aboudan, Michael Aye, Elke Kersten, Klaus-Dieter Matz, Romolo Politti, Frank Trauthan, Ry Evill, Ines Belgacem, Takahashi Yukihiro, Jose María Castro-Marín, Vincenzo Della Corte, Stubbe Hviid, Thomas Roatsch, Nicole Schmitz, Manish Patel, Ganna Portyankina, and Pasquale Palumbo

Abstract. JANUS (Jovis, Amorum ac Natorum Undique Scrutator), the high-resolution camera on the JUICE mission (JUpiter ICy moons Explorer), obtained observations of the Earth during and short-after a Lunar and Earth Gravitational Assist maneuver (LEGA) that was run on August 19–20, 2024. We report on the observations of the Earth that were acquired on August 20, 2024 at a closest approach to the surface of 8,408 km, and on September 9, 2024 at a distance of 564,300 km. The close approach observations covered a narrow strip of the Earth, starting in the night-side of the planet and moving over the terminator and day-side. The later observation provided a low spatial resolution portrait of the Earth and the Moon. Here we examine JANUS observations of the Earth as an example of the different science topics that will be addressed in future observations of Jupiter's atmosphere.

The Earth night-side images show atmospheric airglow, clouds illuminated by a full Moon, fires in rural areas, lights over the ocean from maritime traffic, city lights, no firm detections of lightning, and two meteor candidates compatible with meteoroids of 1–30 g entering Earth's atmosphere. The day-side images show crepuscular rays under extreme incidence angles, atmospheric gravity waves on elevated cirrus, sun glint on multi-filter images of the tropical Western Pacific, convective storms, internal waves in the ocean and multiple cloud systems at a variety of spatial resolutions. Some images acquired with the panchromatic filter showed islands such as Luzon and the Big Island of Hawaii. We compare spectral trends of the ocean and various cloud systems extracted from JANUS multi-spectral images with spectra from the EnMAP and PRISMA satellites. JANUS images acquired on September 9, 2024 allow to form a multi-spectral view of the Earth at low spatial resolution. The data confirm the expected instrument performance in terms of optical quality and multi-wavelength radiometry precision, and the ensemble of observations contains a wide variety of features that are good analogs to multiple systems in Jupiter's atmosphere.

How to cite. Hueso, R., Antuñano, A., Lara, L. M., Stephan, K., Zinzi, A., Coustenis, A., Yair, Y., Sato, M., Haruyama, J., Simon, A., Tubiana, C., Penasa, L., Agostini, L., Luchetti, A., Aboudan, A., Aye, M., Kersten, E., Matz, K.-D., Politti, R., Trauthan, F., Evill, R., Belgacem, I., Yukihiro, T., Castro-Marín, J. M., Della Corte, V., Hviid, S., Roatsch, T., Schmitz, N., Patel, M., Portyankina, G., and Palumbo, P.: JUICE-JANUS observations of Earth in preparation for the JANUS investigation of Jupiter’s atmosphere, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2026-710, 2026.

Received: 06 Feb 2026 – Discussion started: 17 Feb 2026

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'Comment on egusphere-2026-710', Anonymous Referee #1, 20 Apr 2026 reply
General Comments
The manuscript "JUICE-JANUS observations of Earth in preparation for the JANUS investigation of Jupiter’s atmosphere," by Hueso et al., is a comprehensive summary of Earth imaging observations during the August 2024 flyby of the JUICE spacecraft. The encounter provided a demonstration of the JANUS instrument capabilities in advance of future science activities in the Jupiter system. The novelty of the manuscript lies in the data---which are the first science observations with JANUS---rather than any new ideas or scientific conclusions. The Earth observations reveal stray light (Fig. 1a) and photometric (Fig. 20c) calibration issues to be addressed before JUICE arrives at Jupiter. The manuscript is somewhat long, but all of the material is equally interesting, so there is no clear pathway to shorten it without losing key insights from the dataset. Also, the current manuscript should provide a reference point for the planning and analysis of future flyby encounter data, enabling more concise future publications that can cite the work here. I do not recommend cutting any of the figures or tables, although the detailed "Image ID" numbers are of unclear usefulness given the inability of readers outside the JANUS team to actually access the images.
The paper is mostly publishable in its current form, but I have identified a few minor technical comments that could further improve the usefulness of the manuscript. This is my first time attempting to review a manuscript using the novel approach of commenting in a web form, rather than corresponding with an editor, so please excuse any irregularities due to the unfamiliar system.
Specific comments
The concentric ring artifact is a prominent feature in Fig. 1a. It should be briefly explained in the caption (in addition to the discussion already present in the text at line 103). The text should give more detail on planned methods to remove such artifacts from JANUS observations in the Jupiter system, either operationally (observation geometry restrictions) or in post-processing (e.g., flatfielding). The explanation of the artifact ("internal reflections in the baffles") should be clarified, because of the preconception that baffles are typically light-absorbing, rather than reflecting. Any light leaks in the baffles could be expected to produce a gradient or generally unstructured pattern. Could there be internal reflections within optical elements contributing to the artifact?

Somewhere in Sec. 1, the proprietary status of the data should be mentioned. Currently, this is covered only in the "Data availability" section on Line 639. Although it is understandable to keep the entire full/raw dataset proprietary, would it be possible for the authors to publish the selected map data used in the paper, as a supplemental collection? The images (especially the cloud maps later in the paper) are beautiful, and providing them in a high-level map format could increase the reach and impact of the JANUS Earth flyby dataset.

The lightning analysis is intriguing, but some details could be clarified. Could the authors rule out other reasons for lightning to go undetected besides exceptionally weak signals as claimed on line 171, such as gaps in the ground-based network? Figure 6 shows some areas with lightning and some without lighting, but it does not show the locations or coverage areas of ground stations. On line 215, it should be clarified that the intrinsic optical energy of the flash is unknown, only the observed energy can be described. It is possible for a bright flash to be completely undetectable from space if there is sufficient cloud obscuration. I am curious whether the Jupiter campaign will include scanned exposures capable of characterizing temporal distribution of optical flashes within a storm.

On page 10, difficulties determining whether a feature is a TLE or a cosmic ray artifact are mentioned. Is it possible to rule out cosmic rays based on the point spread function of the instrument? Depending on the size of the PSF compared to the pixel scale, it may be possible to say that a point source (lightning) would be spread out more than the observed signal. Little et al. (1999) used the FWHM of lightning features to estimate source depth below a screening cloud layer, and van Dokkum (2001, DOI: 10.1086/323894) used the sharpness of cosmic ray features to remove them from images where the PSF is sufficiently well sampled by the detector. Similarly, in the Fig. 14 caption, "hot pixel" seems to refer to an actual image feature, but this term usually refers to pixels with a higher background level or quantum efficiency compared to neighboring pixels in the context of astronomical detectors.

Some additional references would give a better summary of how the anticipated Jupiter observations relate to prior work. On line 415, other work focused on the 727-nm band (JANUS F9) and its sensitivity for screening deep clouds: Banfield et al. (1998, DOI: 10.1006/icar.1998.5985), Vasavada et al. (1998, DOI: 10.1006/icar.1998.5984), and West et al. (2004, in the Bagenal et al. Jupiter book). Wong et al. (2023, DOI: 10.3390/rs15030702) found that in the HST/WFC3 filter system, a red filter (at 631 nm) provided a more effective continuum reference for JANUS-F9 than 750 nm (JANUS F5) due to ammonia absorption within the 750-nm filter bandpass. On line 441, Fletcher et al. (2018, DOI: 10.3847/1538-3881/aace02) also analyzed mesoscale waves on Jupiter in visible-wavelength (HST) data, and correlated them with thermal imaging at 5 microns. Similar types of multiwavelength studies could be conducted at Jupiter by JUICE if it will be able to conduct coordinated thermal infrared and visible wavelength imaging (e.g., with MAJIS). On line 565, Dyudina et al. (2001, DOI: 10.1006/icar.2000.6582) should be listed as an earlier application of PCA to Jupiter prior to Simon et al. (2015).

Technical comments
Table 1 - State whether bandwidth means FWHM (full width at half maximum) or some other meaning.
Line 140 - Observations are not serendipitous if planned/anticipated many years in advance.
Line 149 - An early mention could be added here regarding the search for bolides in Jupiter's atmosphere.
Fig. 6 - Image 15 is mentioned in the caption but image 14 has the red outline on the map.
Line 176 - Would "light extinction" be a better word choice because it includes scattering, or did the authors really mean absorption only?
Line 192 - "on the WWS direction" should be "in the WSW direction" if you mean azimuth 248 degrees east of north. There are other small English issues throughout the manuscript (e.g., should be "examination" rather than "examen" on line 235), but in every case the intended meaning is clear, so I have not listed them all here.
Line 206 - Add to say "highlighted with a red circle"
Line 296 - Is there also an implication on the size distribution of meteoroids in the outer solar system?
Fig. 8 - Please provide the gamma correction formula, or perhaps just reference Irwin et al. (2024, DOI: 10.1093/mnras/stad3761, see appendix A).
Fig. 8 - Add to the caption to say "highlighted by the gold box in (a)"
Line 324 - It may be more accurate to say "diffuse dawn light" which could be interpreted to include scattering as well as refraction.
Line 342 - Cumulus or cumulonimbus?
Fig. 23 - Although it would break the nice 2x3 arrangement, the authors should show PC1 separately, and define luminance, which could potentially be a weighted average of brightness in different filters. If this is the case, the weights should be stated.

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Citation: https://doi.org/10.5194/egusphere-2026-710-RC1

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Short summary

JANUS, the high-resolution camera on ESA’s JUICE mission, observed Earth during the August 2024 Lunar–Earth Gravity Assist. Images from a close flyby captured night- and day-side atmospheric phenomena, and a distant view resulted in a low-resolution Earth portrait. These observations validate JANUS performance and demonstrate the instrument’s capabilities. We focus our analysis on characterizing atmospheric systems that provide relevant examples for future studies of Jupiter’s atmosphere.


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