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Penitentes

April 23, 2024

If you wander over to the high-altitude regions of the Andes, you might spot fields of white figures, kneeling as if in prayer. Known as penitentes (based on the Spanish for penitent, one in penance), these are snow formations in the form of tall spikes.

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First described in literature by Charles Darwin in his book *The Voyage of the Beagle*, penitentes are structures that form at altitudes higher than 4,000 meters in snow-covered regions of the Dry Andes. He encountered them on his way from Santiago to Mendoza, and reported the local belief that these eerie formations were caused by the winds.

This belief persisted while scientists came up with many alternative explanations such as electromagnetic orientation of snow, thermal eddies, and the descent of snow from landslides, none of which had proper backing until Lliboutry published his seminal “The Origina of Penitents” in the Journal of Glaciology. Lliboutry studied a region of penitentes and hypothesized that differential ablation was the cause of the structures.

Ablation refers to the combined processes removing ice from a glacier surface. The main loss of ice is a result of sunlight hitting it. Although it feels like a fair assumption that sunlight hits the ground evenly, because of the way light reflects, certain regions of the snow cover receive more light and conversely energy than others.

The crests remain cool enough to maintain their structure, but the valleys and spaces between individual penitentes have concentrated sunlight, causing them to wear down and melt over time. What starts off as random differences in the speed of melting snow cover accelerates into a spiky formation as more and more concentrated regions begin to melt over time.

Penitentes are an eerie sight to look at, and could potentially impact our travels in space too!

A 2018 paper by Hobley et al. theorized the presence on penitente-like structures on the surface of Europa, one of Jupiter’s moons. Europa is a promising target for astrobiology research due to its presence of a subglacial ocean, and the news of penitentes on its surface creates challenges for aerospace engineers designing landing gear. While the penitentes on Earth reach a meter or so tops, the modeled structures on Europa would be 15 meters tall!

Discussion and design of Europa landers has now shifted to consider penitentes, but their presence on Europa is still hotly contested. In a 2023 study, Warren goes in depth why the Hobley study might be incorrect. He contests that Hobley misrepresents Lliboutry’s statements, and that the evidence Hobley sites can be reattributed to subsurface irregularities.

The court is still out on whether or not penitentes are a hazard for flying to Europa, but they could still pose benefit to astrobiology research. A study in 2017 identified communities of microbes living at the tips of penitentes, sometimes giving them a reddish tint. This visual feature is something prominent in painted depictions of the structures. Studying the distribution of microbes here can be a valuable source of data for understanding the presence of bacteria in extreme environments.

 

Works Cited

Hobley, Daniel E. J., et al. “Formation of Metre-Scale Bladed Roughness on Europa’s Surface by Ablation of Ice.” Nature Geoscience, vol. 11, no. 12, 8 Oct. 2018, pp. 901–904, https://doi.org/10.1038/s41561-018-0235-0. Accessed 20 Apr. 2024.

Kaushik Patowary. “Penitentes: Peculiar Spikey Snow Formation in the Andes.” Amusing Planet, 25 May 2012, www.amusingplanet.com/2012/05/penitentes-peculiar-spikey-snow.html.

Kristine De Abreu. “Natural Wonders: Penitentes» Explorersweb.” Explorersweb, 11 Dec. 2022, explorersweb.com/natural-wonders-penitentes/.

Lliboitry, Louis. “The Origin of Penitents.” Journal of Glaciology, vol. 2, no. 15, 1954, pp. 331–338, www.cambridge.org/core/journals/journal-of-glaciology/article/origin-of-penitents/FACAAF8EF9A38B0F14AE85DC6805B8C7/core-reader, https://doi.org/10.3189/s0022143000025181. Accessed 20 Apr. 2024.

Solon, Adam J., et al. “Microbial Communities of High-Elevation Fumaroles, Penitentes, and Dry Tephra “Soils” of the Puna de Atacama Volcanic Zone.” Microbial Ecology, vol. 76, no. 2, 5 Jan. 2018, pp. 340–351, https://doi.org/10.1007/s00248-017-1129-1. Accessed 20 Apr. 2024.

Warren, Stephen G. “Snow Spikes (Penitentes) in the Dry Andes, but Not on Europa: A Defense of Lliboutry’s Classic Paper.” Annals of Glaciology, 23 Mar. 2023, pp. 1–5, https://doi.org/10.1017/aog.2023.12. Accessed 20 Apr. 2024.

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  Timothy Durham was convicted of rape when a test showed his DNA genotype matched the DNA recovered from the crime scene. Eye witnesses testified that Durham was in a different state at the time of the incident, but he wasn’t exonerated from his 3000 year sentence until follow up DNA analysis showed that the preliminary forensic analysis used in the trial was misinterpreted (Thomas et al.). DNA testing serves an invaluable part in our criminal justice system, but it comes with complications and bias that can lead to wrongful convictions and misinformed juries.  

Background:

Forensic genetics aims to identify the origin of a biological sample. It can be traced back to Karl Landsteiner who discovered human ABO blood groups. With blood groups, suspects could be eliminated based on their blood type. For example, if a suspect had type A blood and there was type B blood left at the scene by the perpetrator, the investigators could rule out the suspect. Unfortunately, the reverse didn’t work. If the suspect and perpetrator both had the same blood type, it can’t be concluded that the suspect was guilty. Blood identifiers evolved into 8 distinct categories which improved the ability to eliminate suspects, but could only conclude that (if the system matched) it might be the perpetrator or this person might be related to the perpetrator (Jobling and Gill). The field evolved in the 1980s with the discovery of Minisatellites, lines of repetitive DNA that don’t encode proteins. Minisatellite patterns in DNA create what we know today as the DNA fingerprint, and it has a much higher accuracy rate than previous methods (Augustyn et al.). Around the same time, a breakthrough for victims of sexual assault was discovered. In the case of assault, bodily fluids of both the victim and the assaulter can get mixed. A process called differential lysis was created which could selectively increase the concentration of sperm in a fluid sample thereby allowing for accurate DNA testing (Jobling and Gill). Forensics evolved to use single-locus probe testing after several years because of its ease of interpretation. It looked at specific minisatellites that had high variability between subjects. This was the first DNA-based method to be used in a criminal investigation. Two murder homicides from several years apart were connected using DNA evidence. The evidence was then used to exonerate an innocent man, and the guilty party was found when he tried to evade mandatory DNA testing for all men in the small town. His guilt was then reinforced when the DNA profiles matched, and he gave a confession (Jobling and Gill). Finding DNA at a crime scene doesn’t guarantee that you’ll find a murderer. There are issues with non-pure samples and limited samples that make DNA work difficult, and accreditation of facilities and new techniques and take time (Jobling and Gill).  

Subjectivity of DNA Interpretation:

In a 2009 publication, the National Academy of Science discusses the possibility for subjective influence in lab based forensics. The interpretation bias happens in complex cases where, for example, there could be a mixture of DNA. A study published in 2011 looked specifically at this issue. A gang rape happened in Georgia, and the mixture in DNA from the case was used to prosecute one suspect after another suspect took a plea bargain to testify against him. With DNA mixtures, there are three possible conclusions: suspect excluded, suspect cannot be excluded, and inconclusive. If the suspect is excluded, it means that there’s little to no possibility that his DNA is part of the sample. If the suspect cannot be excluded, it means that there’s a possibility that his DNA is part of the sample. In the trial, the DNA examiners who knew the context of the case concluded that the suspect being prosecuted could not be excluded. The 2011 study gave the DNA evidence from this case and the DNA profiles of the suspects to 17 forensic examiners. These examiners had no context of the case, and they all worked independently. Of the 17, only 1 came to the same conclusion (suspect cannot be excluded) as the examiners who knew the context of the case. Twelve of the 17 concluded that the suspect who was prosecuted and convicted had no relation to the DNA found in the sample. Not only does this show that subjectivity occurs due to the context of the case, but the 17 examiners came up with different findings based on their past experiences meaning that not all DNA analysis is objective. Some organizations like the Scientific Working Group on DNA Analysis Methods say that quantitative statistical evidence should be presented with every categorical conclusion, but the US doesn’t require this (Dror and Hampikian).

False Positives:

There are two categories that cause wrongful identification in a DNA report: coincidental matches and false positives. Coincidental matches are DNA matches between two unrelated people. The probability of these matches is about 1 to 500,000 for white individuals and about 1 to 350,000 for black individuals. This small probability of coincidental matches makes people feel confident in DNA evidence. False positives, on the other hand, are matches between DNA and test DNA that result from issues in the forensics lab. There have been multiple cases of forensics labs accidentally switching victim and suspected rapist DNA which lead to convictions that were later overturned when the lab realized the error. There are also cases of misreading or misunderstanding a sample. In a case from 1993, a man was convicted of rape because someone interpreted a mixed sample of victim and perpetrator DNA as an individual sample. The conviction was again overturned. To prevent false convictions, retesting DNA is allowed, but it’s not an efficient solution. Many people are pushing for statistical information on the probability of false positives, so that the jurors can make accurate judgements based on DNA evidence (Thomas et al.).  

Conclusion:

DNA analysis is a useful tool for the criminal justice system, but subjectivity can be an issue in forensic testing. To combat wrongful convictions, there are organizations like The Innocence Project who use DNA testing to free the innocent, but reformed forensic testing standards should be put in place to prevent subjectivity and to ensure juries are correctly informed during trials. https://www.innocenceproject.org/    

Citation:

Augustyn , Adam, et al. “DNA Finerprinting.” Encyclopedia Britannica , 26 Apr. 2018, www.britannica.com/science/DNA-fingerprinting#ref153186. Dror, Itiel E., and Greg Hampikian. “Subjectivity and Bias in Forensic DNA Mixture Interpretation.” Science & Justice, vol. 51, no. 4, 2011, pp. 204–208., doi:10.1016/j.scijus.2011.08.004. Jobling, Mark A., and Peter Gill. “Encoded Evidence: DNA in Forensic Analysis.” Nature Reviews: Genetics, vol. 5, no. 10, Oct. 2004, pp. 739–751., doi:10.1038/nrg1455. Thompson, William C, et al. “How the Probability of a False Positive Affects the Value of DNA Evidence.” Journal of Forensic Sciences, vol. 48, no. 1, Jan. 2003, pp. 1–8., doi:10.1520/jfs2001171.