Pluto and Ceres: Young Dwarf Planets

New Horizons continues to surprise astronomers with evidence of active geology and youth at Pluto. Ditto for Ceres as seen from Dawn.

Pluto Update

Ever since the July 2015 flyby of Pluto by the New Horizons spacecraft (7/15/16), planetary scientists have struggled to understand the surprising, unexpected features on the dwarf planet. The final bits of data were received at earth in October 2016. Nature published a series of new papers in November, providing popular media reporters with opportunities to invent eye-catching headlines:

• Pluto could harbor a subterranean icy ocean (Fox News).
• The Ocean Beneath Pluto’s Wandering Heart (Astrobiology Magazine).
• Pluto may sport clouds of poisonous acid and flammable gases (New Scientist).
• Pluto may have tipped over when Charon tugged at its heart (New Scientist).
• Pluto’s Wandering Heart Hints at Subsurface Ocean (Mike Wall at Space.com).
• Pluto ‘has slushy ocean’ below surface (BBC News).
• Could there be life in Pluto’s ocean? (Phys.org). There’s one in every crowd: a hydrobioscopy imagineer.
• Pluto’s ‘heart’ may be cold as ice, but it’s in the right place, according to research (Phys.org).
• Cracked, frozen and tipped over: New clues from Pluto’s past (Science Daily).
• New analysis adds support for a subsurface ocean on Pluto (Science Daily).

The subtitle of that last headline indicates Pluto may not be alone: “Findings suggest other large objects in the Kuiper belt may also have liquid oceans beneath frozen shells.” This was not supposed to be. The bodies at the farthest reaches from the sun were supposed to be the oldest, coldest and deadest. What is this talk about liquid oceans, dynamic atmospheres and tip-overs? Let’s look into the scientific papers where objectivity should trump creative writing. First, the editorial summaries:

Alexandra Witze, “Icy heart could be key to Pluto’s strange geology” (Nature, 21 Oct 2016). Following a long tradition, astronomers are calling on the impact card to explain what they didn’t predict. “Sputnik Planitia [the heart-shaped feature] may be a crater punched by a giant meteorite impact, which later filled with ice,” Witze writes. It might have tilted Pluto over, keeping Sputnik Planitia permanently facing away from Charon.

Amy C. Barr, “Planetary science: Pluto’s telltale heart” (Nature, 30 Nov 2016). “Four papers published in this issue of Nature show that the heart formed as a result of the interplay of slow deposition of frozen noxious chemicals, bitterly cold winds, cracking icy crusts, cryogenic buried oceans and planetary cartwheels.” Subheading reads, “Studies of a large frost-filled basin on Pluto show that this feature altered the dwarf planet’s spin axis, driving tectonic activity on its surface, and hint at the presence of a subsurface ocean.” Barr says the phenomena at Pluto are not unique; Enceladus, Mars and our Moon “have undergone reorientation due to a loading of material on their crusts.” Pluto’s surface “is smooth and only 10 million years old,” she says; that would be 1/450th the assumed age of Pluto. What happened so recently?

Chris Arridge, “Why Pluto may have a large ocean beneath its icy surface” (The Conversation, 17 Nov 2016). A research fellow from Lancaster University, Arridge uses diagrams to show how an impact might have created a slushy ocean under Sputnik Planitia and re-oriented Pluto’s surface. But instead of explaining why this impact hit an improbably small body so recently in the history of the solar system, he distracts attention to speculations about hydrobioscopy. ” Extremophilic organisms are found to thrive wherever there is liquid water,” he tantalizes illogically. “So although the presence of life in these oceans is open for debate, the probability is high enough for us to try to look for it.” It’s doubtful, however, that any mission will return to Pluto in his lifetime, so he cannot be proven wrong. As for probability, he should watch Illustra Media’s new film Origin on that topic.

Here are links to the four papers published by Nature.

Grundy, Cruikshank et al., “The formation of Charon’s red poles from seasonally cold-trapped volatiles” (Nature, 14 Sept 2016). Tossing around speculations about processes at timescales ranging from centuries to billions of years, these scientists “model the surface thermal environment on Charon and the supply and temporary cold-trapping of material escaping from Pluto, as well as the photolytic processing of this material into more complex and less volatile molecules while cold-trapped.” Some things can happen quickly out there, because Pluto and Charon are exposed to various processes:

Our hypothesis requires energetic radiation to process the seasonally cold-trapped CH₄ [methane]. It is frozen on Charon’s surface only during the polar winter night, so it must be processed rapidly, on the timescale of a century, and only by radiation impinging on the night side. It need not be fully converted into macromolecular solids such as tholins on such a short timescale, only into molecules that are sufficiently non-volatile to remain on the surface after the pole re-emerges into sunlight and warms back up. Charon’s surface is subject to a variety of energetic radiation sources, including ultraviolet photons, solar wind charged particles, interstellar pickup ions and galactic cosmic rays

Bertrand and Forget, “Observed glacier and volatile distribution on Pluto from atmosphere–topography processes” (Nature, 19 Sept 2016). This paper reports “ongoing geological activity” with evidence of glaciers on Sputnik Planum (the heart-shaped feature) and movement of nitrogen frosts, “methane and carbon monoxide on Pluto over thousands of years.” But Pluto is supposed to be over four billion years old.

Keane, Matsuyama et al., “Reorientation and faulting of Pluto due to volatile loading within Sputnik Planitia” (Nature, 16 Nov 2016). The first sentence reveals the scientists’ surprise: “Pluto is an astoundingly diverse, geologically dynamic world.” At one point, they estimate “it would take approximately 5 million years to grow a 5 km N₂ ice cap given Pluto’s present average atmospheric pressure and temperature.” That would be only about one thousandth the assumed age of Pluto.

Nimmo, Hamilton et al., “Reorientation of Sputnik Planitia implies a subsurface ocean on Pluto” (Nature, 16 Nov 2016). This team argues that Sputnik Planitia resulted from an impact. How to keep Pluto active? A little imagination can help. “A rigid, conductive shell could be reconciled with putative cryovolcanic surface features by appealing to ocean pressurization caused by progressive thickening of the ice shell.” Given that assumption, oceans might be common. None of this was predicted before the flyby.

Hamilton, Stern et al., “The rapid formation of Sputnik Planitia early in Pluto’s history” (Nature, 30 Nov 2016). This team thinks Sputnik is an ice cap, not an impact basin. Ice caps don’t require billions of years. “Over many seasonal cycles of sublimation and deposition, the runaway albedo effect (discussed above and in Methods) will cause a single ice cap to form in at most a few hundred thousand years,” they say. Although they think Charon became tidally locked to Pluto early in its history, in just a million years, how do they account for the activity reported by other teams?

Ceres Update

Latest news reports about Ceres, the second asteroid being explored by the Dawn spacecraft, indicate surprisingly youthful features. (Like Pluto, Ceres is currently classified as a dwarf planet.)

Where is the ice on Ceres? (Alicia Chang, Phys.org). Ice should not survive on the airless surface of Ceres, but scientists believe that permanently-shadowed craters could store ice deposits, like at Mercury. If water molecules jump around on the surface, they could land in these cold traps and stay there for billions of years, the story goes. “With every hop there is a chance the molecule is lost to space, but a fraction of them ends up in the cold traps, where they accumulate.” OK, then, but ices are still exposed to cosmic rays and other energetic sources over those billions of years.

Ceres: Water ice in eternal polar night (Science Daily). An icy interior was predicted based on density measurements, but not ice on the surface. It would sublimate in short periods of time. That’s why astronomers had to invent the cold-trap hypothesis.

Solar System’s biggest asteroid is an ancient ocean world (Alexandra Witze, Nature News). Subsurface oceans are trendy these days (10/14/16). “Asteroids might look dry and barren, but the Solar System’s biggest asteroid — Ceres — is chock full of water, NASA’s Dawn spacecraft has found.” Again, this was not expected. The observations do not require billions of years.

Today, the water is either frozen as ice, filling pore spaces deep inside Ceres, or locked inside hydrated minerals at the surface. But billions of years ago, early in Ceres’s history, heat left over from the Solar System’s formation probably kept the asteroid warm inside. This allowed the water to churn and flow, helping to separate Ceres into layers of rock and ice….

The discovery adds to a growing awareness of Ceres as an active, wet world that pushes the boundary of what it means to be a planet. Today it sports a 4-kilometre-high ice volcano and bright spots of salt mixed with ice and rock.

Water, Water Everywhere on Dwarf Planet Ceres (Calla Cofield, Space.com). Water is ubiquitous on Ceres, close to the surface, and may exist in a subsurface ocean. “The fact that so much water is still present on Ceres ‘confirms predictions that water ice can lie for billions of years within a meter of the surface,'” she writes, but wait: was that really a prediction, or an after-the-fact rationalization? We quoted a scientist on 6/29/16 who said it was “absolutely incredible” to think an ocean could exist under Pluto after billions of years. Just a couple of months ago, scientists were astonished to think that Ceres might have active geology, even erupting geysers (9/10/16). The brightness of surface features was hard to explain last March (3/28/16). In August, they complained about missing craters and inexplicable mountains (8/05/16). It’s disingenuous to turn around now and say that the science “confirms predictions.”

Planetary scientists are like Senators. They find which way the data is going, then run in front of it and call themselves the leader. We remember how flabbergasted they were at the first images. Don’t let the moyboys rewrite history.