3D Monte-Carlo Simulation of Ganymede&#8217;s Water-Related Exosphere
Authors: Vorburger Audrey , Fatemi Shahab , Galli André , Carberry Mogan Shane , Roth Lorenz , Wurz Peter 
Published: September 2022

&lt;p&gt;&lt;span lang=&quot;en-US&quot;&gt;Ganymede&amp;#8217;s water exosphere has been observed &lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;spectroscopically &lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;on several occasions since the first detection of atomic hydrogen in Ganymede&amp;#8217;s exosphere in 1997 [1]. &lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;From these observations a consistent picture of Ganymede&amp;#8217;s exosphere has emerged: Ganymede&amp;#8217;s day-side exosphere is dominated by sublimated water with inferred column densities of up to several 1e15 cm&lt;/span&gt;&lt;sup&gt;&lt;span lang=&quot;en-US&quot;&gt;-2&lt;/span&gt;&lt;/sup&gt;&lt;span lang=&quot;en-US&quot;&gt;, while elsewhere the exosphere is dominated by sputtered molecular oxygen (inferred column densities of several 1e14 cm&lt;/span&gt;&lt;sup&gt;&lt;span lang=&quot;en-US&quot;&gt;-2&lt;/span&gt;&lt;/sup&gt;&lt;span lang=&quot;en-US&quot;&gt;) at low altitudes and by atomic oxygen (inferred column densities of several 1e13 cm&lt;/span&gt;&lt;sup&gt;&lt;span lang=&quot;en-US&quot;&gt;-2&lt;/span&gt;&lt;/sup&gt;&lt;span lang=&quot;en-US&quot;&gt;) at higher altitudes &lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;[2-&lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;9&lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;]&lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;. In addition, atomic hydrogen has been observed with inferred column densities of a few 1e12 cm&lt;/span&gt;&lt;sup&gt;&lt;span lang=&quot;en-US&quot;&gt;-2 &lt;/span&gt;&lt;/sup&gt;&lt;span lang=&quot;en-US&quot;&gt;[1, 4, 10]&lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;. Whereas many modeling approaches have been able to reproduce the inferred H&lt;/span&gt;&lt;sub&gt;&lt;span lang=&quot;en-US&quot;&gt;2&lt;/span&gt;&lt;/sub&gt;&lt;span lang=&quot;en-US&quot;&gt;O and O&lt;/span&gt;&lt;sub&gt;&lt;span lang=&quot;en-US&quot;&gt;2&lt;/span&gt;&lt;/sub&gt;&lt;span lang=&quot;en-US&quot;&gt; densities, they have struggled to re-create the high inferred atomic oxygen and hydrogen column densities [&lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;11-13&lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;].&lt;/span&gt;&lt;/p&gt;
&lt;p&gt;&lt;span lang=&quot;en-US&quot;&gt;In this work, we present new &lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;simulations of Ganymede&amp;#8217;s water-related exosphere. Our modeling &lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;results&lt;/span&gt; &lt;span lang=&quot;en-US&quot;&gt;reproduce&lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt; the observed H&lt;/span&gt;&lt;sub&gt;&lt;span lang=&quot;en-US&quot;&gt;2&lt;/span&gt;&lt;/sub&gt;&lt;span lang=&quot;en-US&quot;&gt;O emissions well by sublimating water &lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;at&lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt; Ganymede&amp;#8217;s day-s&lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;i&lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;de surface temp&lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;e&lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;rature. The observed &lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;OI &lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;emission lines, &lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;which were interpreted as &lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;an &lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;O&lt;/span&gt;&lt;sub&gt;&lt;span lang=&quot;en-US&quot;&gt;2&lt;/span&gt;&lt;/sub&gt;&lt;span lang=&quot;en-US&quot;&gt; atmosphere, &lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;on the other hand, agree with a sputter source, &lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;with Jupiter&amp;#8217;s magnetospheric plasma acting as the &lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;sputter agent&lt;/span&gt;&lt;span lang=&quot;en-US&quot;&gt;.&lt;/span&gt;&lt;/p&gt;
&lt;p&gt;Ganymede has a complex magnetic field, that shields part of the surface (mainly the equatorial regions) from impinging plasma ions and electrons, leaves the polar caps exposed to unhindered plasma precipitation, and accelerates the precipitating plasma in the separatrix, resulting in strong auroral emissions [2-8]. We show that the thermal electrons reaching the polar caps are insufficient to produce the amount of atomic oxygen and hydrogen inferred from the observations, and that an interaction between the water atmosphere and auroral electrons is necessary. In addition, we will discuss how the Particle Environment Package [14] onboard ESA&amp;#8217;s JUpiter and ICy moons Explorer [15] will help us learn more about Ganymede&amp;#8217;s atmosphere and plasma environment.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
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