Darkening of Mercury's surface by cometary carbon

Abstract

Mercury's surface is darker than that of the Moon 1,2 . Ironbearing minerals and submicroscopic metallic iron produced by space weathering are the primary known darkening materials on airless bodies. Yet Mercury's iron abundance at the surface is lower than the Moon's 3,4 ; another material is therefore likely to be responsible for Mercury's dark surface 1,2,5-8 . Enhanced darkening by submicroscopic metallic iron particles under intense space weathering at Mercury's surface 9-12 is insu cient to reconcile the planet's low reflectance with its low iron abundance 12 . Here we show that the delivery of cometary carbon by micrometeorites provides a mechanism to darken Mercury's surface without violating observational constraints on iron content. We calculate the micrometeorite flux at Mercury and numerically simulate the fraction of carbonaceous material retained by the planet following micrometeorite impacts. We estimate that 50 times as many carbon-rich micrometeorites per unit surface area are delivered to Mercury, compared with the Moon, resulting in approximately 3-6 wt% carbon at Mercury's surface (in graphite, amorphous, or nanodiamond form). Spectroscopic analysis of products of hypervelocity impact experiments demonstrates that the incorporation of carbon e ectively darkens and weakens spectral features, consistent with remote observations of Mercury 1,2,5-8,12 . Carbon delivery by micrometeorites provides an explanation for Mercury's globally low reflectance and may contribute to the darkening of planetary surfaces elsewhere.Efforts to characterize the composition of Mercury's surface over the past several decades revealed a planet described by dark and featureless visible to near-infrared reflectance spectra; multispectral data acquired by the MESSENGER spacecraft recently reconfirmed the absence of a 1-µm absorption feature at Mercury 1,2,5-8 . The

Global Distribution and Spectral Properties of Low‐Reflectance Material on Mercury

“…In cases where it is not clearly associated with specific craters, LRM occurs in patchy spots within broad regions of heavily cratered, ancient terrain where the ejecta from numerous small craters overlap and extensive regolith gardening has taken place. Although exogenic carbon likely contributes in some fashion to the overall darkening of Mercury as a whole (Bruck Syal et al, 2015), it is irreconcilable with the geological context of LRM. Although exogenic carbon likely contributes in some fashion to the overall darkening of Mercury as a whole (Bruck Syal et al, 2015), it is irreconcilable with the geological context of LRM.…”

“…Visible to near-infrared reflectance spectra of LRM exhibit a flatter slope than the average reflectance for Mercury, which is strongly red sloped (increasing in reflectance with longer wavelength). Given the very dark nature of LRM, it has been proposed that LRM could be the result of extensive space weathering (Riner & Lucey, 2012), accumulation of cometary carbon (Bruck Syal et al, 2015), or endogenic carbon from a primordial flotation crust Murchie et al, 2015;Peplowski et al, 2016;Vander Kaaden & McCubbin, 2015). Given the very dark nature of LRM, it has been proposed that LRM could be the result of extensive space weathering (Riner & Lucey, 2012), accumulation of cometary carbon (Bruck Syal et al, 2015), or endogenic carbon from a primordial flotation crust Murchie et al, 2015;Peplowski et al, 2016;Vander Kaaden & McCubbin, 2015).…”

Cometary impact effects at the Moon: Implications for lunar swirl formation

“…The MESSENGER mission to Mercury, however, resolved some of these features to be clusters of ''hollows'' (Blewett et al, 2011a(Blewett et al, , 2013 and, as a result, challenged the comet-impact hypothesis for lunar swirls. At the same time, it has been argued that the space-weathering rate is much higher on Mercury due to the planet's intense, complex solar wind environment (McClintock et al, 2008) and enhanced bombardment by darkening micrometeorites (Cintala, 1992;Bruck Syal et al, 2015). The balance between the mechanism and rate of swirl formation (through cometary impacts) and destruction (through space weathering) at Mercury will be considered in a future study.…”

Impacts of Cosmic Dust on Planetary Atmospheres and Surfaces

“…MESSENGER measurements limit the northern hemisphere C abundance to the range 0-4.1 wt% at the three-sigma significance level (Peplowski et al 2015). Bruck Syal et al (2015) suggested the C contributed by micrometeorites could explain Mercury's globally low reflectance and might also contribute to the darkening of other planetary surfaces. Modelling by Bruno et al (2007) indicates that the neutral Na "atmosphere" detected on Mercury and the Moon could be produced by the impact of meteoroids and that the mass of meteoroids that have impacted the whole surfaces of the Moon and Mercury in the last 3.8 billion years are 8.9 × 10 18 g and 2.7 × 10 19 g, respectively.…”

“…Hence, inferences of the meteoritic contribution are based on modelling, which suggests that 50 times as many carbon-rich micrometeorites per unit surface area are delivered to Mercury, compared with the Moon, resulting in approximately 3-6 wt% carbon at Mercury's surface (Bruck Syal et al 2015). MESSENGER measurements limit the northern hemisphere C abundance to the range 0-4.1 wt% at the three-sigma significance level (Peplowski et al 2015).…”