High-velocity cometary dust enters the atmosphere of Mars

When comet C/2013 A1 (Siding Spring) passed nearbyMars in 2014, it offered an unprecedented opportunity to observe the interaction between the dust tail of the comet and the atmosphere of Mars. Here I provide an overview of a recent series of four articles reporting observations from three satellites fortuitously orbiting Mars at the appropriate time (MAVEN, MEX, and MRO). These observations reveal high-velocity ablation and ionization of metals from the comet, the diffusion and transport processes that operated in Mars’ atmosphere, and the abundance of these metals in the comet. Soon after the discovery of comet C/2013 A1 (Siding Spring) in early 2013 by McNaught et al. [2013], orbital solutions indicated that it would pass close to Mars in late 2014. As more observations were made and the orbital solution of the comet was refined [Farnocchia et al., 2014], it became clear that the comet’s coma would interact significantly with Mars’ atmosphere. The comet would pass Mars at a high relative velocity of 56 km/s. At this velocity, metallic atoms in the cometary dust are expected to be subject to efficient ablation and ionization upon entering Mars’ atmosphere, which would enhance the chances of determining the abundance of these individual elements in the comet itself [Withers, 2014]. The hyperbolic orbit of the comet and the formation of a coma at large heliocentric distance (7 AU) imply that this comet is likely making its first pass through the inner solar system. As such, this event offers a rare opportunity to look at the primary chemical composition of an object from the far reaches of the solar system. By early 2014, precise observations of the level of activity of the comet [Li et al., 2014; Stevenson et al., 2015] enabled the detailed modeling of the possible flux of dust particles into Mars’ atmosphere [Tricarico et al., 2014; Kelley et al., 2014; Ye and Hui, 2014]. Key parameters in the model are the ejection velocity of the dust grains from the comet nucleus and the size-frequency distribution (SFD) of the dust grains, which are expected to follow a power law. As shown in Tricarico et al. [2014], small changes in the estimates of these parameters corresponded to changes in dust fluence at Mars by a few orders of magnitude. The nominal model results seemed to indicate that the dust cloud would nearly miss Mars but also included the possibility that a small fraction of grains, of the order of a few percent, would be ejected at higher velocities and thus reach Mars. The predicted time tpeak of peak dust flux [Tricarico et al., 2014; Kelley et al., 2014] was approximately 20 UT on 19 October 2014 or approximately 90min after the close approach of the comet. Although the close passage of the comet by Mars represented a special scientific opportunity, it also posed a threat to Mars orbiters. Estimating tpeak as accurately as possible was of paramount importance in order to adjust the orbiters’ position so they could use Mars as a shield from the peak flux, and modeling the duration of the dust flux had implications on when it was safe to resume the spacecraft operations and observations, including determining the chemical composition of the dust and observing the interaction of the dust with Mars’ atmosphere and the evolution of the atmosphere in the hours and days after the initial dust exposure. The earliest observations after the passage of the comet came from the Mars Reconnaissance Orbiter (MRO). With its nearly circular and short period orbit, MRO was able to resume the observations at tpeak + 1 h in the Mars region exposed to dust, just south of the equator; see Figure 1. Restano et al. [2015] report shallow radar observations in the nightside of Mars, where the presence of a sporadic ionized layer is inferred by the observed dispersion in the radar pulse signal, as this effect depends on the ionized particle density along the signal path. A similar observation confirming this effect was repeated at tpeak + 8 h, this time near the north pole. A similar effect was not observed before or afterward. TRICARICO COMET DUST REACHES MARS 1 PUBLICATIONS Geophysical Research Letters