Mercury's Exosphere study

Figure 1: The plot shows the meteoritic flux, as mass per unit time unit area and unit velocity, as a function of the impact velocity, for the two size ranges considered for the possible contribution to the exosphere.

In the last few years we are studying the Mercury's exosphere making ground-based observations, from the TNG telescope and THEMIS on Canary Islands, and CFHT on Hawaii, and modeling the meteoroid impacts on its surface.
We have developed a dynamical model to estimate the flux of meteoroid larger than 1 cm for the first time at the heliocentric distance of Mercury (Marchi, et al., 2005). Figure 1 shows the meteoritic flux for the two populations considered in the estimate to the contribution to the exosphere, where the larger particles come from our model and the smaller one from the Cintala (1992) work. Utilizing these estimates of the meteoritic flux we have developed a model of impact of meteoroid on the Mercury surface obtaining the vapour and neutral sodium atoms production rates, that is of the order of 1.5x10²⁴ atoms per second (Cremonese, et al., 2005) as it is reported in figure 2.
The production rate calculated suggest that the meteoroid impacts represent the main contributor to the exosphere, then almost the 99% of the sodium is produced by meteoroid smaller than 1 cm.
Up to now the meteoroid smaller than 1 cm have been estimates from old models derived and extrapolated from calculations made for the Earth-Moon system.

Figure 2: Log-log plot of the cumulative production rate of the neutral Na atoms for the complete size range (10^-8 - 0.10 m for Mercury and 10^-8 - 0.15 m for the Moon), as a function of the minimum meteoroid radius. Each production rate value is due to all the meteoroids having a size larger than the corresponding radius in the x-axis.

This is the reason why we have started to make a complex model following the dynamical evolution of particles smaller than 1 cm coming from the Asteroid Main Belt till Mercury, taking into account the non gravitational Poynting-Robertson effect being the dominant force for this size range.
The model is providing a velocity distribution of the particles, that is smaller than utilized up to now, and we are going to estimate the flux.
This work will be very useful not only for the exospheric studies of Mercury, but also for the space mission working at smaller heliocentric distances as it will provide the fluence of meteoroids that may impact the satellites.

People: G. Cremonese, P. Borin

Collaboration: C. Barbieri, F. Marzari, S. Marchi (Padova Univ.), A.Doressoundiram (Observatoire de Meudon, Paris), E.Chassefiere, F.Leblanc (Université P & M Curie, Paris), M.Mendillo (Boston Univ.), A.Sprague, D.Hunten (LPL Arizona)

Recent Publications: Bruno et al. (2007), P&SS 55,1494; Killen et al. (2007), SSRv 132,433; Sprague et al. (2007), SSRv 132,399