I never really understood mars return in witch queen
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Savathun gives conflicting testimony about the subject, but given that the Witness deployed the Sol Divisive Vex to the Spire of the Watcher I like to lean towards this explanation given by her: Savathun returned Mars to our system as a means of luring Guardians in after studying the Tablet of Ruin she had in her possession and managing to reverse the abduction that the Witness had done the year prior. The ship we board is quite literally called the Lure and Savathun does not actually appear to be doing much of anything on Mars (that has been touched on yet at least) so this would be my thought. An add-on that might lend credence to this factor is that the Relic on Mars was instrumental to us acquiring the ability of Deepsight, which we used to inadvertently restore Savathun's memories. It was all on great manipulation on her part, whether she knew that it was of greater importance to the Witness than the other planets and intentional sought to sabotage it's efforts is unclear, but possible.