A New “Giant Comet” Model Of Pluto Formation

• Scientists built a cosmochemical model that expands our understanding of Pluto’s origin and evolution. 
• They perform an in-depth investigation of whether molecular nitrogen N2 on Pluto could be a primordial species. 

Pluto was first discovered in 1930 by an American astronomer, Clyde Tombaugh. At the time of the discovery, it was considered a planet, but was later in 2006, reclassified as a dwarf planet.

Since then, tons of studies have been done on Pluto to uncover its mysteries. Recently, scientists at Southwest Research Institute developed a new model about how the dwarf planet might have formed at the solar system’s edge.

They have built a cosmochemical model, what they call a giant comet model, that expands our understanding of Pluto’s formation and evolution. They found a fascinating consistency between the volume of nitrogen present inside Sputnik Planum [a high-albedo ice-covered basin on Pluto, mostly lies in the Northern hemisphere and extends across the equator] and the volume that would be presumed if the planet was formed by billions of comets and other Kuiper Belt objects.

Ratio of N2 and CO on Pluto

Researchers have used data provided by Rosetta (ESA) and New Horizons (NASA) mission to perform an in-depth investigation of whether molecular nitrogen N2 on Pluto could be a primordial species. They have emphasized the mass balance that relies on atmospheric, photochemical and surficial reservoirs of N2.

The team made an assumption for N2 on the Pluto. They considered 2 theoretical models – solar and cometary.

The cometary model exhibits and interesting consistency with past-like-present inventory represented by N2 ice in Sputnik Planum. This justifies the need of excessive outgassing of N2 from the interior, and less escape of N2 from the atmosphere of Pluto.

Sputnik Planum Credit: NASA/Southwest Research Institute 

However, for solar model consistency can be achieved for the opposite of the previous requirement; i.e. minimum outgassing and excessive escape. According to the team, at the most basic level, Pluto should have begun with enough N2, and this fact supports the primordial N2 hypothesis.

Read: Uranus’ Atmosphere Smells Like Rotten Egg Due To Hydrogen Sulfide

Now this brings the issue of missing carbon monoxide (CO), which should be confronted by any model that relates the Pluto composition to those of more primitive celestial bodies, like comets.

The one solution to the missing CO is that N2 in Pluto is not primordial. Researches showed that if Pluto had a subsurface ocean, destruction of CO to formate or carbonate species would be highly favored.

This approach could be applied to the cometary model as the ratio of carbon monoxide and water is large in the latter. Thus, cometary model seems appropriate with more options to reconcile the ratio of CO and N2.

Composition of Pluto’s atmosphere and surface Credit: NASA/Southwest Research Institute 

Moreover, it’s not necessary that the aqueous and burial destruction hypothesis for missing carbon monoxide are mutually exclusive. The observed Pluto’s composition can’t be fully primitive, even if the N2 is indeed primordial. This justifies the dynamic geology seen by NASA’s New Horizons Spacecraft.

Reference: ScienceDirect | doi:10.1016/j.icarus.2018.05.007 | SwRI 

Overall, scientists envision the evolution of dwarf planet as

1. It began with cometary inventories of CO and N2.
2. Subsurface aqueous chemistry destroyed CO.
3. Then, N2 was outgassed effectively.
4. There was no loss of N2 at the surface and it gathered in Sputnik Planum.
5. CO came from comets and gets mixed with N2.

Unanswered Question

This study has led to several subsequent questions that should be addressed to build an efficient and reliable model.

1. Is there any other source(s) of nitrogen that is consistent with the mass balance constraint on Pluto?
2. What’s the amount of N2 present in the Pluto, and how it’s distributed among subsurface reservoirs?
3. What’ the Sputnik Planum’s compositional structure in 3 dimensions, and what processes control it?
4. What does the presence of Carbon dioxide on Triton (Neptune’s Moon) and its lack of detection on Pluto mean with context of solving missing Carbon monoxide problem on both bodies?

Read: Will We Ever Make It to Stars Outside Our Solar System?

While some of these questions could be answered by further investigating the data obtained from New Horizons, others will require new data.