From Halopedia, the Halo wiki
The purpose of the solute was to cause every animal killed by the Halo Array to instantly decay into their component molecules. This was meant to prevent pollution of the atmosphere due to decomposition which had happened on Faun Hakkor following the test firing of Installation 07. It was spread over many of the planets before the Halos were activated. Catalog observed solute being released on Erde-Tyrene during the Forerunner-Flood war as the Lifeworkers oversaw the indexing of the planet as part of the Conservation Measure. While acknowledging the intended purpose of the substance, Catalog also contemplated the possibility that the solute could have been used as a convenient means to obscure the firing of the Halos and the subsequent mass extinctions from future investigators.
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