IMPACT OF A PALEOMAGNETIC FIELD ON SPUTTERING LOSS OF MARTIAN ATMOSPHERIC ARGON AND NEON

We examine the implications of including a paleomagnetic field on sput tering loss of argon and neon from the Martian atmosphere. In a previo us investigation [Hutchins and Jakosky, 1996], we found that collision al sputtering from the exobase by oxygen pickup ions dramatically modi fied the evolution of atmospheric argon and neon (removing greater tha n 85% of outgassed Ar-36 over time and the present atmospheric allotme nt of Ne-20 in < 100 Myr) and could easily reproduce the anomalous iso topic fractionation of Ar-36/Ar-38. However, the existence of an intri nsic magnetic field could limit sputtering loss by deflecting the sola r wind around the upper atmosphere, reducing the number of oxygen pick up ions produced. Evaluation of argon and neon atmospheric evolution i ncluding a magnetic field results in lower sputtering loss rates if th e magnetic field shut off between 2.5 and 3.6 Gyr. Nonetheless, the ex tent of sputtering loss requires atmospheric input from sources, in ad dition to outgassing by intrusive and extrusive volcanism, capable of providing 4 - 100 times more argon and 40 - 1800 times more neon (than provided by volcanic outgassing), dependent on the time that sputteri ng begins. The additional volatile source(s) must also preferentially outgas neon relative to argon by a factor of between 10 and 26.