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Why Does Mars’s Moon Look Like That? A Dive into Moon Formation

April 29, 2024

The other day, I came across a funny meme on Instagram.

 

 

 

I laughed, I gave it a like, and I moved on with my life- kind of. As I continued with my day, one thought kept creeping up in the back of my mind- “Why does Mars’s moon look like that?” It doesn’t even look like a moon, from the standards of as average Earth-dweller, who is so used to the beautiful, spherical, and almost perfect moon that we have been spoiled by. Why is Earth’s moon so perfect, while Mars’s moon looks like it just hopped off the nearest asteroid belt into Mars’s orbit?

 

Mars has two moons, Phobos and Deimos. There is no absolute consensus among the scientific community on how these two moons came to be, but there are two main theories. The first theory, known as the Capture Theory, is that Mars’s moons are captures asteroids, meaning debris created during the early formation of the solar system were pulled into Mars’s orbit by the planet’s gravitational pull. The fact that the two moons are so small and irregularly shaped supports the Capture Theory, along with the fact that they are visibly much darker in color than the Red Planet. The opposing theory is known as the Impact Theory, proposes that Mars underwent some impact from some collision during its early years, uplifting debris that aggregated into two moons. Recent photographic evidence has suggested that Deimos is composed of a rocky material, very similar to what makes up Mars, which supports the Impact Theory and opposes the Capture Theory, which would expect the moons to be made of a more carbon-rich material. There is also debate to whether or not these two moons are even made of the same material, or moreover, were even formed in the same event.

There are a great deal of unknowns about how Mars’s moons were formed, but however they came to be, they sure look a lot different from our moon. While formation process may play a role in this, the main reason for this difference, is mass. Our moon has an approximate mass of 7.348 × 1022 kilograms, while Mars’s larger moon, Phobos, has a mass of only 1.060×1016 kilograms. While these both seem like huge numbers, the masses of these two moons are on vastly different scales, with Phobos being about 0.000014% the mass of Earth’s moon. In summary, Mars’s moons are tiny, and this accounts for the fact that they look so nonuniform.

 

When the Earth’s moon was forming, it became rounded due to the same principle that causes every other sizable celestial body to become spherical- self gravity. Self-gravity is the gravitational force that an object exerts on itself. Once a body accumulates enough mass, this force will pull in towards its center, forming a spherical shape. This final stage is known as hydrostatic equilibrium. Every object that has mass has some amount of self-gravity, even people! But of course, we do not have nearly enough mass for our self-gravity to pull us into a sphere. So that’s the main reason Mars’s moons are so misshapen; they’re too small to exert a strong enough self-gravitational pull on themselves. But, there is one more piece to the puzzle. Mass isn’t the only things that cause celestial bodies to become rounded. The other secret ingredient is material.

 

For bodies to reach a spherical state, their self-gravity must be strong enough to warp and squish the physical material that they are composed. For example, a hypothetical body made of only liquid water would not need much mass in order to achieve hydrostatic equilibrium, because water molecules can be moved around with very little force. There are other materials, such as iron, that resist gravitational pulls even more, and require even more mass to become spherical. The smallest known celestial body that has reached hydrostatic equilibrium is Saturn’s moon, Mimas, which has a diameter of only 396 km. This moon has a very low density, and is composed of mainly ice, and a small amount of rock. This composition gives Mimas a low resistance to its own self-gravity, hence allowing it to become spherical, even with its small mass.

While the answer to my original question turned out to be pretty simple, topics involving space always have a way of becoming more complex. I hope you were able to learn something from this article, or maybe find a topic that you would like to learn more about. There are still so many unknowns in this topic, including exactly what Mars’s moons are even made of, but there is also new and exciting research coming out every day.

 

 

 

Sources

Coffey, D. (2023, November 13). Why are things in Space round?. LiveScience. https://www.livescience.com/space/why-are-things-in-space-round#:~:text=Gravity’s%20rounding%20effect%20is%20a,into%20a%20sphere%2Dlike%20shape.

Giant impact may explain origin of martian moons phobos and deimos. Smithsonian Insider. (n.d.). https://insider.si.edu/2010/12/giant-impact-may-explain-origin-of-martian-moons/

Jonti Horner                          Professor (Astrophysics). (2024, March 5). I’ve always wondered: Why are the stars, planets and Moons Round, when comets and asteroids aren’t? The Conversation. https://theconversation.com/ive-always-wondered-why-are-the-stars-planets-and-moons-round-when-comets-and-asteroids-arent-160541

Metcalfe, T. (2023, May 18). A new look at Mars’ Moon Deimos highlights its mysterious origin. Astronomy Magazine. https://www.astronomy.com/space-exploration/a-new-look-at-mars-moon-deimos-highlights-its-mysterious-origin/

 

 

 

 

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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.