In Situ Measurements of Thermal Ion Temperature in the Martian Ionosphere

In situ measurements of ionospheric and thermospheric temperatures are experimentally challenging because orbiting spacecraft typically travel supersonically with respect to the cold gas and plasma. We present O-2(+) 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 km for 1 week 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 K 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 (similar to 180-200 km) 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. Plain Language Summary A small fraction of a planet's atmosphere, the ionosphere, exists as a plasma made of positive ions produced when sunlight removes an electron from a neutral particle. We measured the temperatures of ions in Mars' lower ionosphere for the first time since 1976 using an instrument called SupraThermal And Thermal Ion Composition on the Mars Atmosphere and Volatile EvolutioN spacecraft. In the part of the ionosphere with the most ions, ions were expected to have the same temperature as the neutral atmosphere because ions and neutrals collide with each other many times per second. However, we found that during the day, the ions were more than 100 K hotter than the neutrals. We eliminated many possible sources of energy for the ions, including sunlight, collisions with electrons, chemical reactions, and electric fields in Mars' atmosphere. The source of the energy that keeps the ions hot remains unclear, suggesting that a fundamental piece of physics is missing from existing models of the Martian ionosphere.