Separating Myth from Fact: Is the Nakhla Dog Real?
A Personal Analysis By David Weir
Although my skeptical nature leads me to scrutinize the impact dog event, I remain open-minded to new evidence supporting either side. Despite the absence of eyewitnesses and newspaper articles, I am of the mind that the probability of such a dog impact in Denshal can be further assessed through the scientific method, using the data and theoretical applications currently available.
While looking through the literature for any helpful data, I found a non-peer-reviewed paper published by Eugster et al. in LPSC 33 (2002), in which they describe research on ‘The Pre-Atmospheric Size Of Martian Meteorites’. The upper limit of the radii of martian Fractured and/or molten rocky debris thrown out of a crater during a meteorite impact event, or, alternatively, material, including ash, lapilli, and bombs, erupted from a volcano. Click on Term to Read More translates to masses of 150–270 kg–too high to be a limiting factor when considering a Nakhla Area on the surface containing meteorites and fragments from a single fall. Also applied to the area covered by tektites, which are produced by large meteorite impacts. Strewnfields are often oval-shaped with the largest specimens found at one end. Given that the largest specimens go the greatest distance, a meteoroid's that might extend all the way to Denshal. However, in a diagram that compares the minimum pre-atmospheric weights of several Martian meteorites–including Nakhla, Zagami, Shergotty, QUE 94201, Chassigny, Los Angeles, and SaU 005–it is Nakhla that has the lowest, i.e., the smallest size. Therefore, one might reasonably expect Nakhla to also be at the low end of the range of weights of all Over 30 of the meteorites found on Earth almost certainly came from Mars (see http://www.imca.cc/mars/martian-meteorites.htm and http://www2.jpl.nasa.gov/snc/). All but one belongs to the group known as SNC meteorites, which includes the shergottites, nakhlites, and chassignites. SNC meteorites contain minerals that crystallized within the past 1.35 to 0.15 Ga, making them Click on Term to Read More falls, especially if a pattern is evident. The falls include the following four meteorites, listed in order from the smallest to the largest minimum calculated pre-atmospheric size, with the actual Meteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering (Weathering = 0). Beginning in 2014 (date needs confirmation), the NomComm adopted the use of the terms "probable fall" and "confirmed fall" to provide better insight into the meteorite's history. If Click on Term to Read More weights given in parentheses: Nakhla (10 kg), Zagami (18 kg), Shergotty (5 kg), and Chassigny (4 kg).
For those Martian meteorites that are finds, the two with the largest minimum pre-atmospheric masses, again with the actual fall weights given in parentheses, are SaU 005 (1.3 kg, but 10.6 kg with paired masses included) and then Los Angeles (0.7 kg), either of which may or may not be representive of their cumulative fall weights. In addition, having a minimum pre-atmospheric size similar to that of Chassigny, the Antarctic QUE 94201 (0.012 kg) likely does not represent its total fall weight. Although not included in this study, two other martians with large recovered weights can be mentioned for comparison–EET 79001 (7.9 kg) and the DaG 476 grouping (6.3 kg).
While I don’t observe a pattern, I would not expect the Nakhla fall to be much bigger than these. To my speculation, a greatly extended strewn field for Nakhla, with the usual pattern of larger masses falling further down range (into Denshal and the dog), would significantly increase the fall weight of Nakhla–a weight that presently seems to fit among the others quite comfortably, especially considering it was ascribed the lowest minimum pre-atmospheric weight.
While this is admittedly only a rudimentary stab at resolving the issue, I think there are other data out there, which taken together, could establish a preponderance of evidence and tip the scale one way or the other. For instance, in The Shergotty Consortium, published in Geochimica vol. 50, 1986, there are peer-reviewed papers concerning the pre-atmospheric and final fall sizes of certain Igneous stony meteorite with a Martian origin consisting mainly of plagioclase (or a shocked glass of plagioclase composition) and pyroxene. They are the most abundant type of SNC meteorites and the type member is the Shergotty meteorite, which fell in India in 1865. Shergottites are igneous rocks of volcanic or Click on Term to Read More. Following a determination of CRE ages from known profiles, cosmic ray track Mass of an object divided by its volume. Density is a characteristic property of a substance (rock vs. ice, e.g.). Some substances (like gases) are easily compressible and have different densities depending on how much pressure is exerted upon them. The Sun is composed of compressible gases and is much Click on Term to Read More of specific samples were used to calculate the sample’s shielding depth and Gradual removal of the successive surface layers of a material through various processes. • The gradual removal and loss of meteoritic material by heating and vaporization as the meteoroid experiences frictional melting during its passage through the atmosphere. The resulting plasma ablates the meteor and, in cases where a meteor Click on Term to Read More characteristics on the pre-atmospheric Small rocky or metallic object in orbit around the Sun (or another star).. This information was then used to calculate the size of the pre-atmospheric mass. From this calculated meteoroid size, the production rate of cosmogenic nuclides at different depths was used to better constrain the CRE age. For Shergotty, a pre-atmospheric size of ~12 cm was calculated. This is equal to a mass of 26 kg, of which only 5 kg was recovered, inferring an ablation rate of 80%. Ablation rates of 50–80% were determined for other shergottites.
This type of study could be done for Nakhla. Each piece of Nakhla studied would have cosmic ray track densities that were consistent with a specific shielding geometry, which should be consistent with the pre-atmospheric size as calculated from production rates of cosmogenic and radiogenic nuclides. An examination of a representative sampling of Nakhla fragments should be able to constrain its size and ablation characteristics, and perhaps determine if any anomalies in its fall weight are present. If not, it would be evidence tipping the scale in favor of a limited strewn field, thus ruling out an impact on a dog 33 km downrange in Denshal.