STARGAZING: The oceans of Pluto

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, Penticton.

Just about a year ago the New Horizons spacecraft flew past Pluto.

It showed us a world like nothing we had expected. Out there in the cold, dark outer reaches of the Solar System, we expected Pluto to be a deep-frozen ball of rock and ice, unchanged over billions of years, apart from a long history of being cratered by collisions with other objects.

We had made temperature measurements. During the Plutonian winter, temperatures fall to about ñ240 C, and in summer, they rise to a not-very-balmy 220 C. Our main interest in Pluto was due to our idea that it would have changed little since the birth of the Solar System, and would be a preserved sample of the primordial material from which the Sun and other bodies formed. We were wrong. As the spacecraft approached Pluto, and the images got more and more detailed, it became clear that Pluto was very different from what we expected.

Instead of an icy, rocky ancient surface covered with craters we saw great areas with no craters, and other areas that were cratered, but relatively few of them. Plutoís surface is not ancient; it is geologically young, and still changing.

One area that really stood out was a large, pinkish plain, which has been provisionally named Sputnik Planum, the Plain of Sputnik, after the first manmade satellite to orbit the Earth. It was covered with huge polygon-shaped tiles, separated by grooves. At the edge of the plain, where it met the surrounding mountainous terrain, there were features that looked like the broken and refrozen masses of ice we see on the seashore or on Canadian lakes in winter, where the tide or wind piles up ice on the shore.

The tiled appearance looked very like the surfaces we see in the Canadian Arctic, where the ground overlying permafrost has repeatedly melted and refrozen. It also resembles the frozen tops of convection cells, like the ones we see in a pot of heating cooking oil. This leads to a surprising conclusion; the surface of Pluto is not permanently frozen. It is melting, rearranging and freezing again.

This raises two questions: what material is melting and re-freezing, and where is the heat coming from?

In the Canadian Arctic the terrain is moulded by the melting and refreezing of water. However, on Pluto the temperatures are so low that water is a permanently solid rock mineral. The best candidate is nitrogen. There is a lot of it on Pluto and at the temperatures on that world, it would be a solid. The melting point of nitrogen is 210 C. It is possible that on particularly warm days the nitrogen would melt, or at least become a soft, semi-molten slurry. That would explain the glacier-like features we see. However, it is likely that the Sputnik Plain is not a plain at all; it is the frozen cover of an ocean. In addition, the complete lack of craters on the Sputnik Plain suggests that the cover is not permanently frozen; it melts sometimes. However, even the Plutonian summers are not really warm enough, and to have a molten ocean below that icy cover requires some heat to be applied from below, at the seabed.

The most likely candidate is heat released in its core by the decay of radioactive elements collected when Pluto formed. Since these elements are usually dense, they tend to accumulate in the core, where they produce heat. Such elements contribute to our world having a molten core. Heat from the bottom of Plutoís ocean melts the nitrogen into a liquid or slurry, which convects to the surface, forming those huge polygons. Due to its great distance from us, the spacecraft signals are very weak and the information is being sent back very slowly, so it is likely that more surprises will be coming.

Jupiter is descending in the west, and Mars and Saturn lie in the southern sky. Mars is the bright one; Saturn is fainter and to Mars’s left. The Moon will reach First Quarter on the July 11.

Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astrophysical Observatory, Penticton.

 

Get local stories you won't find anywhere else right to your inbox.
Sign up here

Just Posted

These nails were collected off the Campbell Mountain bike trails in Penticton this weekend. Someone placed them all over the trail. (Facebook)
Hundreds of nails placed on popular Penticton bike trail

A mountain biker took to Facebook to warn others about the nails

Chrystina Barnard, owner of Lucky’s Pet Supply, has made it her mission to visit as many patios in Penticton as a way to promote restaurants. Here she is enjoying an eggs benny with her best fur friend at Loki’s Garage in Penticton. (Facebook)
Penticton foodie commits to 19-day patio crawl to promote local restaurants

The small business owner wanted to help out eateries hurt by the new restrictions

Flight with COVID
Another Kelowna flight with COVID-19 exposure

Westjet flight on April 5 from Kelowna to Edmonton

Penticton fire truck
Residents evacuated after apartment fire in Penticton

The fire started in an apartment on Government Street Saturday night

Lori Jantz snapped this picture of a fight between a bald eagle and an osprey above Osoyoos Lake on Friday. (Lori Jantz photo)
Battle in the sky erupts above South Okanagan lake

Bald eagle and osprey fight mid-air in Osoyoos

B.C. Health Minister Adrian Dix and Premier John Horgan describe vaccine rollout at the legislature, March 29, 2021. (B.C. government)
1,262 more COVID-19 infections in B.C. Friday, 9,574 active cases

Province’s mass vaccination reaches one million people

A second case of COVID-19 has been confirmed at Vernon’s BX Elementary School. (Kerry Hutter photo)
Second COVID case confirmed at Okanagan elementary school

Exposure at Vernon’s BX Elementary happened April 6 and 7

People walk past the Olympic rings in Whistler, B.C., Friday, May 15, 2020. Whistler which is a travel destination for tourists around the world is seeing the effects of travel bans due to COVID-19. THE CANADIAN PRESS/Jonathan Hayward
Adults living, working in Whistler, B.C., eligible for COVID-19 vaccine on Monday

The move comes as the province deals with a rush of COVID-19 and variant cases in the community

RCMP crest. (Black Press Media files)
UPDATE: RCMP investigating after child, 6, dies at motel in Duncan, B.C.

The BC Coroners Service is conducting its own investigation into the circumstances around the child’s death

Highway 97 being converted to four lanes in April 1990. This photo taken in Lake Country. (Greater Vernon Museum and Archives Photo #14025)
HISTORY: How the old Highway 97 in Lake Country got new name

Pelmewash Parkway recognizes the First Nations history in Lake Country

RCMP display some of the fish seized from three suspects who pleaded guilty to violating the Fisheries Act in 2019, in this undated handout photo. THE CANADIAN PRESS/HO - RCMP
3 banned from fishing, holding licences after overfishing violations near Vancouver Island

Mounties seized the group’s 30-foot fishing vessel and all equipment on board at the time

B.C. Premier John Horgan responds to questions during a postelection news conference in Vancouver, on Sunday, October 25, 2020. British Columbia’s opposition Liberals and Greens acknowledge the COVID-19 pandemic has presented huge challenges for Horgan’s government, but they say Monday’s throne speech must outline a coherent plan for the province’s economic, health, social and environmental future. THE CANADIAN PRESS/Darryl Dyck
Horgan’s NDP to bring in throne speech in B.C., Opposition wants coherent plan

Farnworth said the budget will include details of government investment in communities and infrastructure

Most Read