Dunes on Pluto?

The New Horizons team has released a few pictures of the surface of Pluto that have fairly regularly-spaced linear features somewhat similar to desert dunes on Earth.

Just a few years ago, nobody would have ever considered calling landforms like these dunes without a great deal more work, but the discovery of vast dune fields on Saturn’s icy moon, Titan, has brought windswept surfaces to the forefront of outer Solar System planetary science. Now when we see similar features on these cold, ice-covered worlds, our first thought is to wonder if they are windblown dunes.

But can the nearly airless world of Pluto really make “wind”? Yes, it can! The atmosphere is just thick enough – or at least it can be at certain times of the Plutonian year – to support strong winds able to blow grains along the ground. My colleague Ryan Ewing of Texas A&M University has proposed that sand grains on Pluto would be about 1 mm in size; a little larger than those typically found on Earth (or Venus, Mars, or Titan, for that matter), and that winds as strong as 140 m/s (that’s 313 miles per hour or 504 kilometers per hour) are needed to be able to move these grains.

It sounds strange to think of Pluto as a windswept world, but then again windblown features have been found on comet 67P/Churyumov-Gerasimenko, so why not Pluto too? The images we’ve seen from Hew Horizons show that sediment has flowed on Pluto’s surface, probably as glaciers, and this process could easily create enough fine-grained material to be blown by any wind that’s strong enough.

Figure 1

The large dark linear features in the Baré Montes (Figure 1) were the first to be considered dunes. It’s possible that’s what they are, but it’s tough to say for sure. If they are dunes, then they have formed on rough topography so that we see a mix of the dark, linear dune-like features among other mottled surface features. That makes it hard to pick out any characteristics that would be diagnostic of dunes.  If they are dunes, then they are likely to be lee dunes, forming from winds blowing from the upper right to the lower left. In this case, the wind would carry sediment over the rough terrain and let dark sand pile up on the lee side, forming dunes that would extend downwind. Many dunes like this have been found on bothMars and Earth.

What are they, if not dunes? Good question. They could be wind-eroded features calledyardangs. Or they could be joints: rocks broken into a regular pattern by tectonic stresses. Regular patterns abound in geology; perhaps in time, better imagery will help determine what they are. Equally interesting to me are the much smaller, regularly-spaced bright features on the surrounding plains. Could those be dunes? If so then they’re made of very different materials than the larger, darker features of the Baré Montes.

Figure 2

A new spectacular image near Pluto’s terminator shows the Tartarus Dorsa, mountains with a distinctive linear texture the New Horizon’s team calls “snakeskin” (Figure 2). Could these be dunes? Maybe, but then again maybe not. On the one hand, there are some aspects of these structures that are similar to windblown dunes: They are regularly-spaced, linear features that are superposed on (i.e., sitting on top of) everything else, just like dunes tend to be. On the left side of this field of tan-colored features are some small bright grayish, arcuate shapes that are reminiscent of barchan dunes. On the other hand, there are some aspects of the “snakeskin” that are unlike windblown dunes: they seem unusually sharp-tipped, and they’re located on high ground (dunes tend to form in valleys).

So again, the question is what are they, if not dunes?  Different conditions on other planets make for different landforms. It could be something we’ve never seen on Earth. Their sharp tops make me wonder if they’re some sort of erosional feature, like sastrugi. As every scientist has said, “we need more and better data to know for sure.”

NASA’s K2 Finds Dead Star Vaporizing a Mini “Planet”

Scientists using NASA’s repurposed Kepler space telescope, known as the K2 mission, have uncovered strong evidence of a tiny, rocky object being torn apart as it spirals around a white dwarf star. This discovery validates a long-held theory that white dwarfs are capable of cannibalizing possible remnant planets that have survived within its solar system.

“We are for the first time witnessing a miniature “planet” ripped apart by intense gravity, being vaporized by starlight and raining rocky material onto its star,” said Andrew Vanderburg, graduate student from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and lead author of the paper published in Nature.

In this artist’s conception, a tiny rocky object vaporizes as it orbits a white dwarf star. Astronomers have detected the first planetary object transiting a white dwarf using data from the K2 mission. Slowly the object will disintegrate, leaving a dusting of metals on the surface of the star.

Credits: CfA/Mark A. Garlick

As stars like our sun age, they puff up into red giants and then gradually lose about half their mass, shrinking down to 1/100th of their original size to roughly the size of Earth. This dead, dense star remnant is called a white dwarf.

The devastated planetesimal, or cosmic object formed from dust, rock, and other materials, is estimated to be the size of a large asteroid, and is the first planetary object to be confirmed transiting a white dwarf. It orbits its white dwarf, WD 1145+017, once every 4.5 hours. This orbital period places it extremely close to the white dwarf and its searing heat and shearing gravitational force.

During its first observing campaign from May 30, 2014 to Aug. 21, 2014, K2 trained its gaze on a patch of sky in the constellation Virgo, measuring the minuscule change in brightness of the distant white dwarf. When an object transits or passes in front of a star from the vantage point of the space telescope, a dip in starlight is recorded. The periodic dimming of starlight indicates the presence of an object in orbit about the star.

The diagram depicts a model of light curve shapes. The red line indicates the symmetric shape of a hypothetical Earth-size planet transit while the blue line is the asymmetric shape of the tiny disintegrating planet and its comet-like trailing dusty tail. The black dots are measurements recorded by the K2 mission of WD 1145+017.

Credits: CfA/A. Vanderburg

A research team led by Vanderburg found an unusual, but vaguely familiar pattern in the data. While there was a prominent dip in brightness occurring every 4.5 hours, blocking up to 40 percent of the white dwarf's light, the transit signal of the tiny planet did not exhibit the typical symmetric U-shaped pattern. It showed an asymmetric elongated slope pattern that would indicate the presence of a comet-like tail. Together these features indicated a ring of dusty debris circling the white dwarf, and what could be the signature of a small planet being vaporized.

“The eureka moment of discovery came on the last night of observation with a sudden realization of what was going around the white dwarf. The shape and changing depth of the transit were undeniable signatures,” said Vanderburg.

In addition to the strangely shaped transits, Vanderburg and his team found signs of heavier elements polluting the atmosphere of WD 1145+017, as predicted by theory.

Due to intense gravity, white dwarfs are expected to have chemically pure surfaces, covered only with light elements of helium and hydrogen. For years, researchers have found evidence that some white dwarf atmospheres are polluted with traces of heavier elements such as calcium, silicon, magnesium and iron. Scientists have long suspected that the source of this pollution was an asteroid or a small planet being torn apart by the white dwarf's intense gravity.

Analysis of the star's atmospheric composition was conducted using observations made by the University of Arizona's MMT Observatory. 

“For the last decade we’ve suspected that white dwarf stars were feeding on the remains of rocky objects, and this result may be the smoking gun we’re looking for,” said Fergal Mullally, staff scientist of K2 at SETI and NASA’s Ames Research Center in Moffett Field, California. “However, there's still a lot more work to be done figuring out the history of this system.”

“This discovery highlights the power and serendipitous nature of K2. The science community has full access to K2 observations and is using these data to make a wide range of unique discoveries across the full range of astrophysics phenomena,” said Steve Howell, K2 project scientist at Ames.

Ames manages the Kepler and K2 missions for NASA’s Science Mission Directorate. NASA's Jet Propulsion Laboratory in Pasadena, California, managed Kepler mission development. Ball Aerospace & Technologies Corporation operates the flight system with support from the Laboratory for Atmospheric and Space Physics at the University of Colorado in Boulder.