ACCUMULATION of METEORITIC NICKEL on MARS. C.I. Fassett1 and M

46th Lunar and Planetary Science Conference (2015) 1875.pdf

ACCUMULATION OF METEORITIC NICKEL ON MARS. C.I. Fassett1 and M. D. Dyar1, 1Dept. of Astrono- my, Mount Holyoke College, South Hadley, MA 01075, [email protected].

Introduction and Background: Martian surface Table 1. Nickel abundances of selected materials materials could potentially include a significant exoge- Sample Ni abundance Ref. Mean C1 Chondrite nous component, as is the case on the Moon. If so, this 1.1% [12] should be reflected in the siderophile geochemistry of (typical of impactors) SNC meteorites 5-500 ppm the martian soil; many elements are enriched in mete- [13, 14] orites compared to planetary crusts, particularly nickel (martian crust) µ~80 ppm Adirondack basalt [1] and iron. Any iron signal that might result from 200 ppm [15] (e.g., primary igneous crust ) meteoritic influx to Mars would likely be obscured by 200-700 ppm Gusev basaltic soils (APXS) [2] Fe in endogenic basaltic materials common on the sur- µ~470 ppm face. In contrast, nickel is much more abundant in as- Meridiani soils (APXS) 600-1300 ppm teroidal material than the basaltic crust (Table 1). (*soils may have complicated [2] µ~840 ppm Thus, its abundance on the martian surface can be po- exhumation history) tentially used to trace meteoritic addition [1-3] and Gale Crater high-Ni layered tar- get Natkusiak 700-2900 ppm quantify the impact contribution to surface materials. N/A Several factors help bolster the likelihood that me- (ChemCam; calibration refine- µ~1800 ppm teoritic nickel should be concentrated at the martian ments in progress [16]) surface [see also 1-3]. First, rates inferred for surface With over 150,000 LIBS spectra now available erosion are very slow [4-6], so exogenous material has from the ChemCam instrument, there is potential to time to accumulate and mix with surface materials. provide useful new information about this topic. Thus, Second, surface materials commonly are very old. we are working on refining the calibration of LIBS Crater retention for billions of years is common on measurements of nickel, a topic that we explore in a Mars and sedimentary material at Gale crater were companion abstract [16]. Here, we focus on updating plausibly deposited in the Late Noachian/Early Hespe- theoretical expectations for the addition of meteoritic rian, >3.5 Ga [7]. Third, the impact flux was much nickel into martian soils and sedimentary rocks. higher in this early period, enhanced ~20-50× higher at Note that this work is preliminary and further re- the Late Noachian/Early Hesperian boundary than it is finements in these calculations are ongoing. today [8, 9]. Mass Flux: Most of the meteoritic mass that Previous in situ measurements of Ni on the martian reaches Mars can be thought of as doing so in one of surface have been somewhat contradictory. Mars Ex- two size regimes: micrometeorites, or (much) larger ploration Rover Alpha Particle X-Ray Spectrometer impactors (Fig. 1). In 1990, Flynn and McKay [3] es- results were used to suggest that ~1-3% chondritic timated the mass flux of micrometeoritic material as material had been added to martian regolith based on between 2,700 and 59,000 tons/yr by taking then- enhanced Ni concentration in soils compared to prima- current estimates of the micrometeoritic flux to Earth ry martian basalts [1-2]. However, measurements of (~15,000 to ~80,000 tons/yr) and scaling the results to other elements, such as Cr, are not enhanced as ex- Mars. pected if the all of the Ni results from of simple chon- Estimates for the micrometeoritic flux at Earth dritic addition to the soils. Viking major element data have subsequently been substantially refined and re- from X-ray fluorescence (XRF) measurements were duced to 7,400±1,000 tons/yr [17]. Scaling this to Mars interpreted as potentially implying much greater mete- would suggest a mass flux in the micrometeorite range oritic addition to the fines at the martian surface of ~1,250 tons/yr or ~9 g/km2/yr. At the current impact (~40%; [10]). flux, this would deliver the equivalent of ~3 mm/Ga of Despite these contradictions, quantifying the mete- meteoritic material to the surface. oritic contribution to martian surface materials has It is possible to assess the importance of larger im- substantial value. Constraining the addition of exoge- pactors by converting the Hartmann model for crater nous material has astrobiological implications because accumulation [9] back to the projectile mass using it can potentially provide an independent metric for Schmidt-Housen-Holsapple scaling [e.g., 18,19]; a how much exogenous organic matter is added to Mars similar approach was taken by [1] with different as- [11]. In addition, potential exists to use the concentra- sumptions about scaling/cratering efficiency. The re- tion of exogenous material observed in soils and sedi- sults are shown in Figure 1. These calculations illus- mentary sequences to constrain surface exposure histo- trate that the crossover point where the mass delivered ry, major hiatuses in deposition, and regolith evolution. to Mars from projectiles forming impact craters is

46th Lunar and Planetary Science Conference (2015) 1875.pdf

~1.6 cm of meteoritic material incorporated into the regolith over 3.5 Ga. If this were uniformly mixed into a 10-m thick regolith layer, it would represent a ~0.2% contaminant; leading to 10% enhancement in the concentration of Ni in the soil (220 ppm), much less than estimated from APXS (Table 1). Reaching the factor of ~2.35× enhancement that is observed would require the accumu- lating meteoritic material be concentrated in the upper 68 cm of soil. Because gardening of the regolith is expected to be particularly effective at and near the surface, this is plausible. Alternative explanations are that (1) nickel abundances might be enhanced in the soil during aeolian deflation of the soil [5], (2) the basaltic soil on the Gusev plains may not be entirely lo- cally derived, or (3) the observed Ni concentrations

may have been enhanced or concentrated by fluids. Figure 1. Mass flux of meteoritic material to Mars with estimates scaled to Mars from [17] and from the impact flux of The ChemCam laser-induced breakdown spec- [9]. trometer (LIBS) instrument also has the capability of measuring Ni in martian soils and rocks, though it is larger than the mass from micrometeorites occurs at a not yet calibrated for such measurements. Our prelimi- mass of ~2×1014 g, corresponding to impactors that nary calibration based on doped samples and multiple form craters of ~5.75 km in diameter. Because impacts matrices [16] suggests that Ni can be measured with an at these scales are comparatively rare, the contribution accuracy of ca. ±200 ppm if the major element chemis- of meteoritic material from these events will be highly try of the materials being lased is a good match to the stochastic – enhanced in the crust in the vicinity of matrix used in the Ni standards. Once the LIBS Ni large craters, and relatively unimportant elsewhere. For calibration is completed, further constraints on the dis- this reason, we neglect the contribution of larger im- tribution of Ni among materials probed by MSL should pactors in the calculations that follow and leave an provide additional useful insights into the distribution assessment of their role for future work; this contribu- and abundance of Ni among materials on the surface. tion nonetheless is likely to be important for ancient Acknowledgments: This work was supported by crustal material on Mars (Noachian and pre-Noachian NASA grants NNX09AL21G, NNX12AK84G, and crust). NNX14AG56G from the MFR Program. Mixing of Ni with surface materials: Once the References: [1] Yen A. S. et al. (2006) JGR, 111, meteoritic influx of material to the surface is estimated, E12S11. [2] Yen A. S. et al. (2005) Nature, 436, 49- it is important to consider how this material is mixed 54. [3] Flynn G.J. and McKay, D. S. (1990) JGR, 95, with the regolith. The behavior, growth, and gardening 14497-14509. [4] Golombek M. P. and Bridges N. T. of Mars regolith is much less studied than that of the (2000) JGR, 105, 1841-1853. [5] Golombek M. P. et Moon [20-21]. There are also complications in how al. (2006) JGR, 111, E12S10. [6] Golombek M. P. et meteoritic material is delivered; virtually all microme- al. (2014) JGR, 119, in press. [7] Thomson B. J. et al. teorites delivered to Mars interact with, and are poten- (2011) Icarus, 214, 413-432. [8] Neukum G. et al. tially ablated by, the martian atmosphere [3]. (2001) Space Sci. Rev., 96, 55-86. [9] Hartmann W. K. With these caveats in mind, we can make a new (2005) Icarus, 175, 294-320. [10] Boslough M. B. estimate for how much nickel we might expect to find. (1988) LPS XIX, 120-121. [11] Becker, L. et al. (1999) We consider as an example the Hesperian volcanic EPSL, 167, 71-79. [12] Anders E. and Grevesse N. plains of Gusev crater. First, we assume that all of the (1989) Geochim. Cosmo. Acta, 53, 197-214. [13] Lod- mass delivered as micrometeorites ultimately reaches ders K. (1998) MAPS, 33, A183-A190. [14] Meyer C. the surface and mixes into the regolith. The regolith (2013) Mars Meteorite Comp. [15] Ming D. W. et al. thickness in Gusev is uncertain, but ~10 m is generally (2006) JGR, 111, E02S12. [16] Lepore, K. H. et al. thought to be consistent with the site’s geology [5]. (2015) this meeting. [17] Cremonese G. et al. (2012) Second, we use the Hartmann chronology function to ApJL, 749, L40. [18] Schmidt R. M. and Housen K. R. determine the flux of incoming material [22; fn. 1]. (1987) Int. J. Impact Engng., 5, 543-560. [19] Holsap- This implicitly requires that the micrometeorite flux is ple K. A. (1993) Ann. Rev. Earth Pl. Sci., 21, 333-373. proportional to the flux of crater-forming impactors. [20] Hartmann W. K. et al. (2001) Icarus, 149, 37-53. Combined, these assumptions lead to an estimate of [21] Warner N. et al. (2014) LPSC 45, 2217. [22] Kite E. S. et al. (2013) Icarus, 225, 850-855.