Event and conditions that produced the iron meteorite Block Island on Mars

Abstract

[1] The Mars Exploration Rover Opportunity has discovered four large iron-nickel meteorites that range in size from 50 to 240 kg dispersed over 10 km of Meridiani Planum, Mars. Because these meteorites are covered with hollows that resemble regmaglypts, their surfaces record their ablation through the atmosphere, and they must have landed at speeds below hypervelocity (<2 km s −1 ) to survive. Slowing massive iron meteorites requires a minimum atmospheric density, which was quantified using a numerical model that integrates the equations of motion for incoming meteoroids through an atmosphere of a given surface density and scale height and records their outcomes as direct (generally hypervelocity impacts that form craters), longer over the horizon and fallback flight paths, and skip outs. The present atmosphere of Mars is sufficient to slow iron meteoroids as large as Block Island (the most massive meteorite) via drag and significant ablation on long flight paths, although for standard distributions of entering meteoroid masses, velocities, and entry angles, such events are rare (0.007% of incoming iron meteoroids). Such events require entry angles of 10°-13°, entry velocities of 6-18 km s −1 , and entry masses of 225-710 kg. The absence of large stony meteorites is probably at least partially because they are much weaker and thus broken up into smaller fragments on impact. Although differential drag deceleration on long flight paths could disperse fragments of an entering meteoroid by tens of kilometers, dynamic pressures are too low to break up an iron meteorite, leaving the possibility that they are paired an open question.

Meteorites at Meridiani Planum provide evidence for significant amounts of surface and near‐surface water on early Mars

“…Regarding hypothesis 1, it is now accepted that observations of rounded shapes and smooth surfaces confirm that the meteorites are not spall fragments that fractured off the impactor during impact, because such fragments would have surfaces dominated by fracture planes, broken edges, and planar or angular features (Chappelow and Sharpton 2006; Chappelow and Golombek 2010). In addition, the meteorites do not show any signs of impact damage or deformation.…”

“…Finally, considering hypothesis 3, Chappelow and Sharpton (2006) and Chappelow and Golombek (2010) analyze the possibility that iron meteorites could be landed recently without forming a primary crater and being destroyed or severely deformed. Only a small impact pit or structure near or under the meteorites would be expected, and meteorites could have ricocheted away from the point of landing to their current location.…”

“…Only a small impact pit or structure near or under the meteorites would be expected, and meteorites could have ricocheted away from the point of landing to their current location. Chappelow and Sharpton (2006) identified the conditions for impact survival in the case of HSR, and Chappelow and Golombek (2010) did the same for BI. They conclude that such landings could have occurred even during a low‐density Martian atmosphere similar to the one at present, although only quite rarely and in very specific and limiting conditions concerning initial mass, speed, and entry angle.…”

“…The six meteorites have portions of their surfaces that are smooth, and they show ubiquitous hollows and pits that appear similar to regmaglypts, probably formed by ablation during the descent through the atmosphere (Schröder et al. 2008; Chappelow and Golombek 2010) and/or by subsequent aqueous interaction once on the surface (Fleischer et al. 2010a; Johnson et al.…”

“…2011). This information suggests that at least BI and SI, probably MI, OR, and Ir, and possibly HSR and some others yet to be discovered, may be pieces of a single (or 2 or 3) parent body that fragmented in the atmosphere before landing, although this possibility faces some problems (Chappelow and Golombek 2010; and also see the section below). As the hypothesis that the iron meteorites are paired is an important subject, yet not affecting the results of this work, we leave this alternative as an open question.…”