Scientists are continuing to report startling diversity on the surface of Pluto as close-up images trickle in from NASA's New Horizons probe.
In the latest views, released during a Friday news briefing in Washington, the discussion has shifted from Pluto's surprisingly tall ice mountains to an adjacent plain that appears to be carved into polygon-shaped segments.
The segments are about 20 to 30 kilometres across and are separated by shallow troughs, some of which contain knobby mounds of unknown origin. The features may have been formed by the sublimation of frozen gases from Pluto's surface, but scientists are quick to add that any ideas they put forward at this early stage amount to mere speculation.
"I'm still having to remind myself to take deep breaths," said Jeffrey Moore of the mission's geology, geophysics and imaging team. "The landscape is just astoundingly amazing."
New Horizons flew through the Pluto system on Tuesday at a blinding 14 kilometres per second. It is now crossing through the Kuiper belt, a zone of small icy bodies beyond the outer planets. The probe, now nearly five billion kilometres from Earth, will eventually leave the solar system altogether.
But while Pluto is receding in New Horizons' rear-view mirror, for scientists, the bulk of the mission still lies ahead. That's because most of the information gathered during this week's close encounter has yet to be transmitted back to NASA ground stations – a process that will take more than a year to complete.
The pictures seen so far are part of what is called the probe's "browse dataset," which is meant to give scientists a highly compressed sampling of what is to come.
The polygonal features represent the best look yet at Pluto's most distinctive feature, Tombaugh Regio, a bright heart-shaped area named after Pluto's discoverer, astronomer Clyde Tombaugh. It sits near Pluto's equator, precisely opposite the hemisphere that faces Pluto's large moon Charon. Newly released spectral data from New Horizons show that the region is rich in frozen carbon monoxide. Frozen methane has also been detected there.
Tombaugh Regio may be a place where gases are escaping from below Pluto's surface. Science team member William Grundy said it reminded him of the froth that forms on hot chocolate.
One other New Horizons image released on Friday shows dark spots that appear to be aligned, a possible indication of wind blowing across the surface.
Members of New Horizons atmospheric team said that as the spacecraft flew by it scooped up ions that are created when nitrogen gas escaping from Pluto's thin atmosphere reacts with charged particles from the sun. Scientists estimate Pluto's atmosphere is losing a staggering 450 tonnes of material every hour. At such a rate, they say, the atmosphere must be replenished, possibly by gas escaping from below the surface.
Scientists are continuing to marvel at the range of diversity in Pluto's features. There are strong hints that it has been geologically active in the recent past, including the relative absence of craters in some regions and its tall ice mountains, first seen on Wednesday.
Francis Nimmo, a science team member who initially predicted Pluto's surface would look far less interesting based on its size and projected heat loss over billions of years, said he was not at all disappointed by the curveball New Horizons has flung at his theories.
"I'm delighted," Dr. Nimmo said. "It's much more fun to be wrong than to be right because if you're wrong that means you've learned something."
This week's trove of data is particularly poignant for those who started off studying Pluto when it was little more than a dot in ground-based telescopes.
"I always knew that some day I would have to give up my beloved Pluto to the geologists," said Marc Buie, a science team member and an astronomer whose own observations of Pluto date back to 1982.
But Dr. Buie added that now that New Horizons is pulling away from Pluto, it will fall to astronomers like him to continue to monitor the distant world and look for signs of changes that can be related to the features New Horizons has revealed.