Laboratory experiments to constrain the identity of Venus&#8217;s unknown UV absorber
Authors: Egan Joanna , James Alexander , Plane John , Murray Benjamin , Feng Wuhu 
Published: July 2022

&lt;p&gt;Background&lt;/p&gt;
&lt;p&gt;In situ probe measurements and remote sensing have revealed that Venus has a highly organised cloud system. Comparisons between models of the expected spectra and observations reveal unexplained absorption in the near-UV to blue region of the spectrum. While many candidates for this &amp;#8220;unknown absorber&amp;#8221; have been proposed over the years, none have been conclusively demonstrated to match the physical and optical behaviour observed (P&amp;#233;rez-Hoyos et al., 2018, &lt;em&gt;JGR Planets, 123&lt;/em&gt;).&lt;/p&gt;
&lt;p&gt;One such candidate is ferric chloride (Krasnopolsky, 2017, &lt;em&gt;Icarus, 286&lt;/em&gt;; Zasova et al., 1981, &lt;em&gt;ASR&lt;/em&gt;, &lt;em&gt;1&lt;/em&gt;). Attempts to reliably determine its suitability have been hampered by the scarcity of representative spectra available. Absorbance spectra generally used in the literature are measured in ethyl acetate (Aoshima et al., 2013, &lt;em&gt;Polymer Chemistry, 4&lt;/em&gt;), and therefore may not be representative of the absorption produced by ferric chloride in the Venusian clouds.&lt;/p&gt;
&lt;p&gt;In addition to the absorption spectrum produced, the behaviour of absorber candidates must also be considered, including their rates and locations of production and loss, transport mechanisms, and lifetimes in the atmosphere. While much of this behaviour must be examined in atmospheric models, laboratory studies to establish reaction pathways and measure rates are needed to provide as much quantitative data as possible for model development.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;Method and results&lt;/p&gt;
&lt;p&gt;Literature spectra for ferric chloride employ UV-visible spectrometry using ethyl acetate as a solvent. We present absorption spectra of ferric chloride in sulphuric acid. This change of solvent produces an environment more closely aligned to that on Venus, where ferric chloride, if present, may exist as an impurity in the micron-sized sulphuric acid cloud droplets (Petrova, 2018, &lt;em&gt;Icarus, 306&lt;/em&gt;).&lt;/p&gt;
&lt;p&gt;In addition, mass spectrometry was used to investigate the kinetics and products of reactions of ferric chloride that could occur in the Venusian atmosphere. Behaviour predicted by these experiments can then be included in atmospheric models to test the lifetime and transport of ferric chloride and its reaction products in the atmosphere.&lt;/p&gt;
&lt;p&gt;&amp;#160;&lt;/p&gt;
&lt;p&gt;Conclusions&lt;/p&gt;
&lt;p&gt;The unknown absorption was first observed close to 100 years ago, yet the mystery of its cause remains unsolved. More representative spectra of ferric chloride and a greater understanding of its behaviour in the atmosphere of Venus are critical to advancing the identification of the unknown absorber. As the absorber is located towards the top of the clouds and absorbs in the near-UV to blue region, it is responsible for large amounts of absorption of incident sunlight, and therefore has a significant impact on the Venusian energy budget. Accurate atmospheric modelling of the planet therefore requires an understanding of the absorber which can only be achieved once it has been conclusively identified.&lt;/p&gt;