Impact Ionization as a Potential Heating Mechanism for Ca in Mercury's Exosphere

Calcium vapor ejected into Mercury's exosphere has been found to be of extreme temperature, exceeding 20,000 K (Killen et al., 2005; Killen, 2016; Burger et al. 2012). Killen (2016) suggested that Ca is first ejected from the surface as a molecule (e.g., CaO, CaS) by impact vaporization, and subsequently gains energy by photodissociation. The problem with this scenario is that photodissociation fails to impart quite enough energy to result in a 20,000 K vapor. We suggest another possible mechanism for heating the Ca: impact ionization with subsequent recombination. The peak of the impact velocity distribution at the Moon is 10 km/s and that for Mercury is nearly 20 km/s, thus more than half of impacts onto the Moon and most impacts onto Mercury have impact velocities >10 km/s (Cintala, 1992).

Kurosawa et al. (2010) used a 1030 J laser with 2.5 ns shots onto diopside to simulate >10 km/s impacts. The results of these experiments showed that, initially, 25 - 50 ns after the laser shot, emission lines of Mg⁺, O⁺, and Ca⁺ were observed. The number of electrons then drastically decreased, suggesting that electron recombination was a key factor in the evolution of the vapor cloud. Kurosawa et al. suggest that ion-recombination plays an important role in heating the neutral vapor, which they observed at ~20,000 K, with the heating caused by release of the specific heat of ionization.

To the extent that these experiments simulate an impact, they may be directly relevant to the potential heating of Ca vapor in Mercury's exosphere.

References

• Burger, M. H., R. M. Killen, W. E. McClintock, R. J. Vervack, Jr., A. W. Merkel, A. L. Sprague, and M. Sarantos, Modeling MESSENGER Observations of calcium in Mercury's exosphere, Journal of Geophysical Research, 117, 0L11B, doi:10.1029/2012JE004158, 2012.
• Cintala, Mark J., Impact-Induced thermal effects in the lunar and Mercurian regoliths. Journ. Geophys. Res. 97 E1, 947-973, 1992.
• Killen, R. M. Pathways for energization of Ca and Mg in Mercury's exosphere. Icarus, 268, 32–36, 2016. doi: 1016/j.icarus.2015.12.035
• Killen, R. M., T. Bida, and T. H. Morgan, The calcium exosphere of Mercury, Icarus, 173#2, 300-311, 2005.
• Kurosawa, S. Sugita, T. Kadono et al., Roles of Shock-Induced Ionization due to >10 km/s Impacts on Evolution of Silicate Vapor Clouds. Abstract 1785. 41st Lunar and Planetary Science Conference, 2010.