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We present  temperatures in Mars' ionosphere derived from data measured by the SupraThermal And Thermal Ion Composition instrument onboard the Mars Atmosphere and Volatile EvolutioN spacecraft. We focus on data obtained during nine special orbit maneuvers known as Deep Dips, during which MAVEN lowered its periapsis altitude from the nominal 150 to 120\u00a0km for 1\u00a0week in order to sample the ionospheric main peak and approach the homopause. We use two independent techniques to calculate ion temperatures from the measured energy and angular widths of the supersonic ram ion beam. After correcting for background and instrument response, we are able to measure ion temperatures as low as 100\u00a0K with associated uncertainties as low as 10%. It is theoretically expected that ion temperatures will converge to the neutral temperature at altitudes below the exobase region (\u223c180\u2013200\u00a0km) due to strong collisional coupling; however, no evidence of the expected thermalization is observed. We have eliminated several possible explanations for the observed temperature difference between ions and neutrals, including Coulomb collisions with electrons, Joule heating, and heating caused by interactions with the spacecraft. The source of the energy maintaining the high ion temperatures remains unclear, suggesting that a fundamental piece of physics is missing from existing models of the Martian ionosphere.<\/jats:p>","DOI":"10.1029\/2021ja029531","type":"journal-article","created":{"date-parts":[[2021,11,30]],"date-time":"2021-11-30T21:30:25Z","timestamp":1638307825000},"update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":37,"title":["In Situ Measurements of Thermal Ion Temperature in the Martian Ionosphere"],"prefix":"10.1029","volume":"126","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-6250-7665","authenticated-orcid":false,"given":"K. G.","family":"Hanley","sequence":"first","affiliation":[{"name":"Space Sciences Laboratory University of California Berkeley  Berkeley CA USA"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"J. 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