Meteor showers from comet 209P/Linear at Earth and comet C/2013 A1 (Siding Spring) at Mars

Thursday, 18 December 2014
Petrus M M Jenniskens, SETI Institute, Mountain View, CA, United States
The exceptionally close encounter of long-period comet comet C/2013 A1 (Siding Spring) to Mars on October 19, 2014, may result in the first meteor observations in the atmosphere of Mars. Large (few-mm to cm-sized) meteoroids are not easily observed by other techniques. The manner in which these meteoroids fragment will shed light on the physical properties of large freshly-ejected dust grains of a long-period comet, which carry most of the comet's mass loss. Their physical properties may be very different than those of the much more aged meteoroids typically encountered in annual meteor showers and the sporadic meteor background. The 2014 May 24 close encounter of comet 209P/LINEAR to Earth, for example, showed that the meteoroids ejected by this short-period comet about 100 years ago were still unusually fragile, some breaking vigorously into a cloud of dust towards the end of their entry trajectories. Moreover, the meteoroid stream encountered by Earth was dominated by small meteoroids, with a steep differential mass distribution index of s = 2.42+/-0.15 (0.003-4 g) and a differential size distribution index of a = 5.3 +/- 0.4 (0.2-2 cm). The fragile nature of the grains is likely the reason for the lack of large meteoroids in the stream. The encounter with comet Siding Spring may shed light onto the question whether this is because they do not survive the ejection process (in which case the shower on Mars would also lack bright meteors) or because they do not survive the thermal cycling in interplanetary space for more than a century or so (in which case larger meteoroids may still be present in the stream).