Ion sputtering and surface erosion at Europa

摘要

New results from the energetic particle detector (EPD) experiment on the Galileo mission are used to update the ion sputtering rate at Europa. It is found that the resultant gas emission rates are 5.7 x 10(27) O-2 s(-1) and 9.5 x 10(27) H2O s(-1) in total if the maximum self-sputtering effect of the exospheric pickup ions is taken into consideration. This means thatthe surface number density of Oz molecules could be as much as 1.8 x 10(9) cm(-3) and the average column density is 3.4 x 10(15) O-2 molecules cm(-2). The corresponding ion production rate is on the order of 1.3 x 10(27) ions s(-1) probably leading to a plasma density of about 2000 ions cm(-3) in the ionospheric wake region. The magnetospheric particle bombardment effect is also effective in redistributed mass across Europa's surface. The maximum mass redistribution rate can be estimated to be approximate to 80 m per 100 Myrs with the associated net erosion rate approximate to 20 m/100 Myrs. At the same time, mass resurfacing rate by liquid water or outgassing could reach similar magnitude. [References: 30]