Launched in January 2006, the New Horizons spacecraft zoomed past Pluto and its moons in July 2015, before conducting the first reconnaissance of a Kuiper Belt object (KBO), Arrokoth, on New Year's 2019. The data New Horizons gathered and beamed home during these historic encounters continues to yield new insights into the previously unexplored regions.
New Horizons team members shared some of these discoveries with media on March 14 at the 54th Lunar and Planetary Science Conference in The Woodlands, Texas. Among them: new clues to the origins of Arrokoth, two new findings about ancient and evolving geology of Pluto, and a preview of unique observations of Uranus and Neptune that could improve our knowledge of those worlds and influence how we interpret data on similar planets in other solar systems.
The Mounds of Arrokoth
Both the farthest and the most primitive object ever explored by a spacecraft, KBO Arrokoth is considered a "contact binary," meaning it's made of two objects - or lobes - that once orbited each other until they gently merged long ago.
New Horizons scientists focused on large mounds on the larger lobe to learn more about how Arrokoth came together, and found that the mounds may be the building blocks of the lobe itself. They observed that the lobe consists of 12 distinct mounds clustered around a larger center mound - examining each mound's shape, size and orientation, reflectance and colors. They used stereo imaging to identify the troughs, scarps, pit chains and stretches of bright material indicating where the mounds actually merged.
"We discovered that the mounds are similar in many respects, including their sizes, reflectivities and colors," said Alan Stern, New Horizons principal investigator from Southwest Research Institute in Boulder, Colorado. "We believe the mounds were likely individual components that existed before the assembly of Arrokoth, indicating that like-sized bodies were formed as precursors to Arrokoth itself. This is surprising, and a new piece in the puzzle of how planetesimals - building blocks of the planets, like Arrokoth and other Kuiper Belt objects come together."
Looking into Pluto's Past . . .
At some early point in its history - just like Earth - Pluto flipped on its side. Scientists describe this as "true polar wander," when the surface of a planet or moon tilts with respect to its spin axis, causing the latitudes and longitudes of surface features to change.
New Horizons researchers have tied the massive surface realignment that occurred on Pluto to the formation of Sputnik Planitia, the large, nitrogen ice-filled impact basin that forms half of the distinctive heart-shaped region New Horizons observed on Pluto's surface. The question of how much true polar wander Pluto underwent has been an ongoing subject of debate in the almost eight years since New Horizons flew past Pluto. New Horizons researchers have used computer models to try to determine Pluto's orientation before Sputnik formed, the amount of nitrogen ice that is filling the basin, the effect of Pluto's axial tilt on how fast the basin filled with nitrogen ice, and the push of Pluto's underground ocean against the bottom of the basin.
In addition to modeling efforts, the New Horizons team has been revealing the path of Pluto's true polar wander by clues in its early geology. "We're seeing signs of ancient landscapes that formed in places and in ways we can't really explain in Pluto's current orientation," said Oliver White, a New Horizons co-investigator from SETI Institute in Mountain View, California. "We suggest the possibility is that they formed when Pluto was oriented differently in its early history, and were then moved to their current location by true polar wander."
. . . and into Pluto's Present
A fascinating discovery from the Pluto flyby was an extensive field of jagged landforms made almost entirely of methane ice, at the edge of the hemisphere visible to New Horizons during closest approach. Now scientists have evidence that this so called "bladed terrain" likely stretches across much wider swaths of Pluto - and say it shows how Pluto's landscape responds to a changing climate.
Analyzing images and data gathered on Pluto's "far side" during approach, scientists detected surface roughness, methane properties and high altitudes similar to what they saw in the other area of bladed terrain - leading them to believe the terrain extends across a large part of Pluto's equator.
"The presence of these bladed terrains on Pluto provides important new insights into the geology and dynamics of Pluto's surface," said Ishan Mishra, a postdoctoral researcher and science team contributor from NASA's Jet Propulsion Laboratory in Pasadena, California. "That they appear under certain conditions in distinct locations also tells us how the surface responds to environmental changes. The discovery of these features just adds to our understanding of the processes that shape Pluto and other icy planets in our solar system, and highlights the complexity, dynamic nature, and diversity of planetary surfaces like Pluto's."
Eyes on Uranus and Neptune
From New Horizons' unique perch in the Kuiper Belt, it can observe planets and other bodies at angles and distances like no other spacecraft or telescope on Earth. As New Horizons begins its second mission extension, scientists are building on long-distance observations the spacecraft made of Uranus and Neptune to provide new insight into the atmospheres of each of these two planets.
New Horizons imaged both planets in 2019, and the team plans additional observations this year and next. "We'll build on Voyager's observations of Uranus and Neptune, seeing them at unique geometries, in longer wavelengths, for example, and through new seasons, that Voyager could not" said Will Grundy, and New Horizons co-investigator from Lowell Observatory in Flagstaff, Arizona.
New Horizons and Hubble Space Telescope observations of each world are complementary, Grundy said, particularly on studies of the planets' atmospheres and the transfer of heat from their rocky cores through their gaseous exteriors. "The science return is better than either spacecraft can provide on its own," Grundy said. "It also sets the stage for observations of similar ice giant planets around other stars."
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