Evidence of Ion Heating at Low Altitudes in the Dayside Ionosphere at Mars
Monday, 14 December 2015: 10:50
3002 (Moscone West)
One of the outstanding questions at Mars is how the surface water was lost. MAVEN, the newly arrived spacecraft at Mars, is dedicated to investigate how water can be lost via the upper atmosphere. The upper atmosphere is cold but the Mars gravity well is small meaning that light atoms can be lost through Jeans escape while heavier atoms such as O only need to gain a small amount of energy to escape. The critical region is at the exobase, the region (in altitude) at which the highly collisional atmosphere becomes collisionless. Below the exobase particles will collide frequently with the neutral atmosphere (which is ~10^5 orders of magnitude more dense than the charged ionosphere) and thermalize to the neutral temperature (~300 K at Mars). Thus, heating well below the exobase will be inefficient at enhancing escape as collisions will dominate and the heated particles will thermalize before escaping. The most efficient region to deposit energy is at and just above the exobase: low enough in altitude so that there are ample ions to heat but not so low that collisions dominate and prevent heated particles from escaping. For neutrals, recombination can for some species provide the escape energy while for ions plasma waves can efficiently deposit energy into them above the exobase. Using data from the MAVEN mission we can for the first time demonstrate that there are significant plasma waves close to the exobase that are heating the ions to high energies. Event studies will be presented demonstrating these plasma process close to the exobase and the implications to Mars’ escape rates will be discussed.