The recent flyby of Pluto by NASA’s New Horizons spacecraft revealed the distant planet to be a surprisingly active world, replete with geologically recent flows of nitrogen ice that may be reshaping its surface even today and a dynamic atmosphere which exhibits a much more complex structure than previously thought. Now, a new study undertaken by members of the mission’s science team provides indirect evidence that the presence of nitrogen, which dominates the planet’s atmospheric and surface chemistry, may be the result of cryovolcanism on the distant alien world.
The presence of an atmosphere around Pluto was known to astronomers and planetary scientists for decades, from observations of various stellar occultations by the distant planet during the 1980s. Albeit a rarefied one compared to terrestrial standards, the Plutonian atmosphere plays a crucial role in the dynamics and overall evolution of the planet. Ever since its discovery, Pluto’s atmosphere has been the subject of countless studies, albeit theoretical ones, in the absence of more detailed data, with each one of them giving different values for Pluto’s atmospheric properties, like its pressure, rate of escape, and overall dynamics.
During its closest approach to the Pluto system on July 14, New Horizons made the first-ever, detailed, in-situ observations of the distant planet and its assortage of moons, while returning a treasure trove of data to eagerly awaiting scientists back on Earth. Analysis of the preliminary data beamed back by the spacecraft to date showed, among other things, that due to Pluto’s weaker gravity the rate with which its atmosphere slowly escapes into space is in the order of 500 tonnes of atmospheric material per hour, which is substantially greater than what has been measured on planets much closer to the Sun, like Mars. Based on these data, the New Horizons’ science team has calculated that in the course of the planet’s 4.5-billion-year history, the equivalent of at least 1,000 to 9,000 feet of nitrogen ice must have evaporated from the surface and subsequently escaped into space. Yet, since nitrogen is the dominant constituent on Pluto’s atmosphere, amounting to 90 percent of its total chemical composition, it must be constantly replenished for it to have remained present in such large quantities over time. But what could that process be?
In a new study, recently published on The Astrophysical Journal Letters, Dr. Kelsi Singer, a postdoctoral researcher and member of the New Horizons science team, and Dr. Alan Stern, principal investigator for the mission, both at the Southwest Research Institute in Boulder, Colo., set out to address this question: “More nitrogen has to come from somewhere to resupply both the nitrogen ice that is moving around Pluto’s surface in seasonal cycles, and the nitrogen that is escaping off the top of the atmosphere as the result of heating by ultraviolet light from the Sun,” says Singer. The researchers examined various possible mechanisms for the replenishment of nitrogen on Pluto, like cometary impacts and the subsequent excavation of underground sources of nitrogen by cratering.
Based on the currently known and hypothesized population of Kuiper Belt Objects, as revealed by observational studies as well as the estimated impact velocity of KBOs on Pluto’s surface, Singer and Stern calculated that the latter must have been hit by more than 600 million impacts from cometary objects as large as 60 km in size since the formation of the Solar System 4.5 billion years ago. Even though short-period comets, which are thought to originate from the Kuiper Belt, are abundant in volatile compounds like methane, nitrogen, and carbon monoxide (all of which are also found on Pluto as well), they were found to be insufficient as a source of nitrogen for replenishing Pluto’s surface, with the researchers calculating that they would provide three to four orders of magnitude less material than necessary to keep nitrogen abundant on the distant planet.
A second mechanism that the researchers considered as a possible source was the excavation of subsurface nitrogen on Pluto as a result of cometary impacts. In this case, in order for an impactor to be able to effectively bring any nitrogen deposits to the surface, it would have to be larger than 3 km in size, so that it could dig through the layers of other less volatile compounds that may also be present on Pluto’s surface, which can cover up any underground sources of nitrogen. Furthermore, Pluto’s weak gravity as an effect results in cometary objects having very low impact speeds, which in turn, according to the researchers, would cause less than 1 percent of the impacting objects to have velocities higher than the planet’s escape velocity for any released amounts of nitrogen, cometary or otherwise, to be able to sublimate away or escape into space.
Taking all these factors into consideration, Singer and Stern suggest that even though a certain amount of nitrogen could be delivered into Pluto’s atmosphere through cometary impacts, it wouldn’t be sufficient to resupply the atmosphere on a constant basis. For this reason, the researchers conclude that most of the nitrogen on Pluto must come through endogenic geologic processes like cryovolcanism, not unlike what has been observed on Neptune’s largest moon Triton, which could provide with abundant quantities of the volatile material. “Based on the first-order analysis conducted here, it does not seem that either cometary import or cratering-related excavation/sublimation effects can resupply Pluto’s atmospheric nitrogen escape losses,” write the researchers in their study.
“Given the also demonstrated difficulty of delivering enough nitrogen with comets, we suggest that either atmospheric escape rates have been overestimated or cryovolcanism or another tectonic or geodynamic means of nitrogen resupply may be necessary to resupply Pluto’s atmosphere against escape and the buildup of an involatile lag deposit resulting from the escape process. We found that all of these effects, which are the major ones from cratering, do not seem to supply enough nitrogen to supply the escaping atmosphere over time,” says Singer. “While it’s possible that the escape rate was not as high in the past as it is now, we think geologic activity is helping out by bringing nitrogen up from Pluto’s interior.”
Even though the results of this study are based on previous theoretical predictions, both researchers are confident that the constant stream of data expected from the New Horizons mission during the next 16 months will help to shed more light to the geologic and atmospheric processes of Pluto, which are revealed to be even more fascinating as time goes by. “Our pre-flyby prediction, made when we submitted the paper, is that it’s most likely that Pluto is actively resupplying nitrogen from its interior to its surface, possibly meaning the presence of ongoing geysers or cryovolcanism,” comments Stern. “As data from New Horizons comes in, we will be very interested to see if this proves true.”
Far from a boring chunk of rock and ice in the outer reaches of the Solar System, Pluto is proving to be one of the most fascinating members of our planetary family.
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