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Vertical Structure of the Callisto Ionosphere from Galileo Radio Occultation Data and Its Implication on the Moon's Interior.
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  • Ashok Verma,
  • Krishan Khurana,
  • Raymond Walker,
  • Jean-Luc Margot
Ashok Verma
University of California Los Angeles

Corresponding Author:ashokverma@ucla.edu

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Krishan Khurana
University of California at Los Angeles
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Raymond Walker
University of California Los Angeles
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Jean-Luc Margot
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Using the magnetic field observed by Galileo during two flybys of Callisto, Khurana et al. (1998) demonstrated that Callisto generates a strong induction response to the time-varying primary field, indicative of the presence of a subsurface ocean. In contrast, Hartkorn and Saur (2017) modeled the atmosphere and ionosphere of Callisto and suggested that the ionosphere could be responsible for a significant part of the observed magnetic fields. Thus, they concluded that the water ocean might be located much deeper than previously thought or might not exist at all. While Khurana et al. (1998) did not account for the induction within a conductive ionosphere, Hartkorn and Saur (2017) overestimated the conductivity of the ionosphere by using Cowling conductivity which is not applicable for the situation at Callisto. In this paper, we re-analyzed the S-band open-loop one-way Doppler data of the Galileo spacecraft with the aim to derive the electron density (ED) and neutral density (ND) profiles of Callisto and address its implication in terms of moon’s conductivities and interiors. Using modern orbit determination software, MONTE, and the most up-to-date information on the Jovian system, we reconstructed the Galileo orbit with a full dynamical approach. The estimated rms values of the Doppler residuals for baseline measurement vary from 0.01-0.08 Hz, well within the expected noises of the radio signals. We used these residuals to derive the ED profiles using the technique discussed in Verma et al., (2019). We found an appreciable ionosphere for C22 and C23 Ingress occultations with peak densities of 15600±900 cm-3 and 17700±600 cm-3, respectively. For other cases, the detections do not exceed the 3-σ level. While the general features of the EDs are consistent with Kliore et al. (2002), our estimated 1-σ formal uncertainties are 2-3 times better presumably because of the constrained Galileo’s orbit. Assuming O2 as the major component of the Callisto’s atmosphere, the estimated ND (weighted mean) at the surface is 2.0±0.33 x10-10 cm-3 which corresponds to a column density of 3.9±0.35 x10-16 cm-2 (see Figure). Finally, we will use these density profiles to constrain the ionospheric conductivities and address their implications in terms of the presence of a subsurface ocean.