Deciphering Denshal Dog Data

standby for nakhla photo

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 ejecta translates to masses of 150–270 kg—too high to be a limiting factor when considering a Nakhla strewn field 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 martian meteorite 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 fall weights given in parentheses: Nakhla (10 kg), Zagami (18 kg), Shergotty (5 kg), and Chassigny (4 kg). In addition, the recovered mass of the Tissint fall is reported to be 7 kg.

The minimum pre-atmospheric radius of the Nakhla meteoroid was initially calculated to be 22 cm, and it was considered to have experienced a low mass ablation consistent with a very high entry angle close to vertical. Newer analyses utilizing gamma-ray spectrometry (Povinec et al., 2020) provided estimates of the pre-atmospheric meteoroid radii of both the Nakhla and Chassigny meteorite samples in the Vatican Observatory Collection, and additionally data for the shergottite Tissint obtained by Povinec et al. (2016) and Schulz et al. (2019). These estimates reflect the calculated depth of each sample on a spherical body that would correspond to its measured 26Al activity. It was determined that the Nakhla meteoroid had a radius of 30 (±5) cm, the Chassigny meteoroid had a radius of 17 (±4) cm, and the Tissint meteoroid had a radius of 20 (±3) cm. The mass of each of these meteoroids that corresponds to the respective measured bulk density is 210–580 kg for Nakhla, 30–130 kg for Chassigny, and 60 (±150) kg for Tissint. These estimated masses indicate that significant material was lost in the ablation process and/or much of the material that made it to the ground was not recovered.

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 representative 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). Although no pattern is obvious, I would not expect the Nakhla fall to be much larger 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 well.

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 shergottites. Following a determination of CRE ages from known profiles, cosmic ray track densities of specific samples were used to calculate the sample's shielding depth and ablation characteristics on the pre-atmospheric meteoroid. 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 corresponds to a mass of 26 kg of which only 5 kg was recovered, inferring an ablation rate of 80%. Ablation rates of 50–80% have also been reported for other shergottite falls.

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.