The bright plain on the image-left side of Pluto\u2019s heart is Sputnik Planitia, a basin roughly 1,000 kilometres across filled mainly with nitrogen ice. Across all of the terrain New Horizons could inspect, no impact crater has been confirmed within it.<\/p>\n
Any crater large and old enough to have been visible has been removed. The surface is not preserving its history in the way Pluto\u2019s older, heavily cratered landscapes do.<\/p>\n
Crater statistics set a broad upper limit on the surface age, while computer models describe a much faster cycle of renewal. They are not the same clock.<\/p>\n
No crater is still an upper limit, not a precise age<\/p>\n
Planetary scientists estimate a surface\u2019s age by counting craters over a known area and comparing the result with models of the impact rate. A surface exposed for longer should, all else being equal, accumulate more scars.<\/p>\n
Pluto provides its own contrast. New Horizons mapped more than a thousand craters across the dwarf planet, including terrains thought to be about four billion years old. Sputnik Planitia stands out because its nitrogen plains contain none that can be identified in the available images.<\/p>\n
A 2016 analysis by Kelsi Singer and colleagues found no craters down to about two kilometres in the wider low-resolution coverage and 625 metres in the best high-resolution strip<\/a>. Using estimates of the small objects crossing Pluto\u2019s orbit, they placed the plain\u2019s crater-retention age below roughly 10 million years.<\/p>\n That is an order-of-magnitude ceiling and depends on estimates of a sparse Kuiper Belt impact population. \u201cCrater-free\u201d should therefore be read as \u201cno crater detected at New Horizons resolution\u201d, not as a claim about features too small for the spacecraft to resolve.<\/p>\n The plain is a glacier made from volatile ice<\/p>\n Sputnik Planitia is the western lobe of Tombaugh Regio, the bright formation commonly called Pluto\u2019s heart. The two lobes look joined in global images, but they are geologically different terrains.<\/p>\n The basin contains molecular nitrogen mixed with smaller amounts of methane and carbon monoxide ice. At Pluto\u2019s surface temperature, nitrogen ice is far softer than the water ice that forms the surrounding crust and mountains. Over long periods it can deform and flow like a very cold glacier.<\/p>\n Blocks of water ice broken from the surrounding uplands can be carried by the nitrogen glacier and gathered at the margins of its cells. Space Daily\u2019s earlier report on Pluto\u2019s floating hills<\/a> described these blocks as the local equivalent of icebergs, except the supporting sea is solid nitrogen rather than liquid water.<\/p>\n A detailed geological map of Sputnik Planitia<\/a> concluded that its mapped units are being affected by convection, glacial flow and sublimation. Condensation can also lay down a bright nitrogen mantle. The missing craters therefore do not have to be erased by one process alone.<\/p>\n The polygons record slow overturning<\/p>\n Much of Sputnik Planitia is divided into cells roughly 10 to 40 kilometres wide. Their centres are comparatively smooth and raised, while narrow troughs form many of the boundaries. This is the surface pattern expected when a layer of soft solid material circulates.<\/p>\n In a 2016 Nature model led by William McKinnon<\/a>, nitrogen ice more than about one kilometre thick could convect under plausible conditions on Pluto. A layer several kilometres deep reproduced the broad cells seen by New Horizons.<\/p>\n Warm, buoyant nitrogen ice rises towards a cell centre, spreads sideways and eventually descends along the margins after cooling. Average surface speeds of a few centimetres a year were enough to move or renew material across a cell in about 500,000 years.<\/p>\n A crater does not need to be explosively destroyed. A bowl excavated into material that continually flows can be distorted, carried towards a downwelling boundary, buried by condensation or lost as the ice sublimates and is deposited elsewhere.<\/p>\n The human comparison belongs to the model<\/p>\n