Geologists Smash Pluto to Make it Look Old
Planetary scientists pull out their tried-and-true
all-purpose explanation: impacts.
Assuming Pluto is old when it looks young is like telling a preteen girl, “Why Grandma, you look so young for your age.”
Pluto is supposed to be 4.5 billion years old, circling the sun in a highly inclined orbit far from other gravitational influences. Mission scientists for New Horizons expected to see a crater-saturated ice ball when the spacecraft flew by in 2015. Instead, they got an active body with nitrogen glaciers, tectonic activity and a thin atmosphere. Now they even think it has a liquid ocean under its surface. Quick! Bring in the Theory Fixers! In come the asteroids. They added some heat and scarred Pluto’s face to make it look older.
The heart-shaped basin named Sputnik Planitia is a particular area of concern, because much of the activity occurs there. It has very few craters. If it was gouged out by an impact, how did it get filled in with nitrogen ice? How long did that take?
Clues to Pluto’s History Lie in Its Faults (EOS Science News from the AGU). A news item from the American Geophysical Union summarizes research by the Lunar and Planetary Institute. They decided that the “cold Pluto” theory doesn’t explain the young features, so a “hot Pluto” model is needed— with a little help from (Incoming rock alert!… WHAM!!) impactors.
This liquid water ocean has huge implications for how Pluto formed and retained enough heat to melt all that ice. In the years since the New Horizons flyby, two general formation hypotheses have emerged. The first starts with a “cold” Pluto, which involves Pluto forming over millions of years by the slow accretion of cold objects. This version of Pluto eventually would have coalesced enough material that radiative heating from the inside would melt the subsurface ocean. The other hypothesis involves a “warm” or “hot” Pluto, in which Pluto formed over a shorter time period in violent collisions that heated its interior, formed the ocean, and eventually cooled the planet into the majority ice ball we know today.
If Pluto had formed by slow accretion (the old standard theory), its only heat source would have been radiogenic heat. All the short-period radionuclides should have burned out long ago. Another source of heat out in the solar system’s icebox zone comes from the kinetic energy of impacts.
Tectonism and Enhanced Cryovolcanic Potential Around a Loaded Sputnik Planitia Basin, Pluto (McGovern, White and Schenk, JGR Planets 126:12, December 2021, open access). Here is the paper. Let’s see if they can keep Pluto billions of years old. Feel the surprise at Pluto’s revelation during the flyby:
Following its flyby of the Pluto-Charon system in 2015, NASA’s New Horizons spacecraft returned high quality images that revealed complex worlds with an unexpectedly diverse range of terrains and a correspondingly diverse range of resurfacing mechanisms, including broad tectonic systems, suggestions of cryovolcanic activity, and even ongoing surface renewal in the form of the convecting and glacially flowing nitrogen ice deposits of Sputnik Planitia on Pluto (Moore et al., 2016; Stern et al., 2015).
Such processes should not be going on so far away from the sun. Have they been steady for billions of years?
Global climate modeling of Pluto (Bertrand et al., 2018) indicates that the massive scale of the Sputnik basin would cause it to be filled by nitrogen ice migrating into it from across Pluto after its excavation over a timescale of some tens of millions of years (less than 1% of Pluto’s total age). If the basin is filled so rapidly, it follows that the formation of the Sputnik basin and its subsequent infilling (i.e., loading) are essentially coincidental to within some tens of millions of years.
Notice the assumption: “1% of Pluto’s total age.” That embeds the 4.5-billion-year age assumption into the discussion. They take it as undisputed fact, but if the impact occurred near the beginning of Pluto’s old-age story, and if Sputnik Planitia filled in within tens of millions of years (several 450ths of the assumed age), why is activity still going on today?
According to the “hot start” model in Figure 1 of Bierson et al. (2020) (which they determined to be a more plausible model of Pluto’s thermal evolution than the “cold start” model), the thickness of the elastic layer expanded from 40 to 75 km between 4.45 and 4.33 Ga, which causes us to conclude that the Sputnik basin formed at some point within this period. However, this finding assumes that the entire nitrogen ice inventory of the Sputnik basin today was available on the surface for infilling immediately following the Sputnik impact, and not (for instance) released gradually from Pluto’s subsurface over a protracted timescale, which would have correspondingly lengthened the timescale of infilling of the basin.
They focus on other matters: basin was pan-shaped and shallow, about 3 km deep, to account for the amount of infilling. They never address, though, why Sputnik Planitia looks so young and relatively crater-free today. If the impact and infilling was rapid, why the activity now? It seems they can only hope that there is enough radiogenic heat left.
No Help from Tidal Flexing
Perhaps the Pluto-Charon system could get heat from tidal flexing to sustain its activity for billions of years. Another recent paper looked into that.
The tidal–thermal evolution of the Pluto–Charon system (Baheri et al., Icarus 376, April 2022). Icarus does its readers a favor by putting the main findings in bullet points at the top.
- Pluto is predicted to harbor a subsurface ocean with a thickness of 40–150 km.
- Charon’s subsurface ocean refroze because of a lower heat budget relative to Pluto.
- The orbital evolution of the Pluto–Charon binary takes less than 1 Myr.
- Tides, despite intense for a short time, have not affected the thermal history much.
The bad news for old agers is summarized here: “Generally, we find that in all of our simulations the orbital evolution does not exceed ~106 years and, relative to the contribution from radioactive decay, tidal heating has little effect on the final state of the Pluto–Charon system.”
The Nuclear Option
Tidal heating can add a lot of energy to a system, but it is quickly dissipated. The only other heat source capable of sustaining a liquid ocean is radiogenic heating. These authors rely on a 2016 paper that promised enough long-lived radioisotopes (uranium-235 [nuclear bomb stuff], U-238, thorium-232, potassium-40) to keep Pluto active for billions of years. That paper (Hammond et al., Geophysical Research Letters, AGU 2016), however, assumed enough parent material at the time of formation: “We assume an initial abundance of parent isotopes appropriate for CI chondrites.” But how much un-decayed uranium would have been drifting around in the outer fringes of the solar nebula to make it into Pluto? See also seven other assumptions made in the paper.
All the old-agers are trusting that last hope, because tidal heating can’t do it, and neither can impacts.
It would liberate scientists’ thinking to do away with the assumption of 4.5 billion Darwin years, so that they could look at each solar system object independently to consider its age. That assumed age of the solar system is a ball and chain around their legs. But they will not liberate themselves, because they fear the Law of the Misdeeds and Perversions, which cannot be altered. The solar system MUST be that old to give Darwin time to invent and evolve life on Earth. Nobody wants to anger King Charles, because his Cancel Culture is quick and ruthless. The enforcers of Darwin Sharia, found throughout Big Science, make sure of that.