Pluto Has A Northern Polar Cap
And Other Early Findings By New Horizons
The data New Horizons will collect over the next few days, Alan Stern told reporters Monday morning, is "a gift for the ages to all mankind." He described the early returns as "mouthwatering," and gave a peak at what New Horizons can already tell us about the Pluto-Charon system, having only just passed the one million mile mark to Pluto late Sunday night.
The first comes from the RALPH instrument's infrared mapping spectrometer, LEISA. As with the visible light that humans can detect with their eyes, infrared light is a form of electromagnetic energy. Its wavelength is just below that of visible light, in fact, and some animals like snakes are able to detect infrared light, which can be very useful in situations where there is little visible light to go by (eg. nighttime). LEISA scans the sunlight that is reflected by Pluto, Charon, Nix, and Hydra in the infrared region of the spectrum, observing which wavelengths of the infrared light are absorbed at Pluto's surface before being reflected back to the spacecraft. Scientists have a library of the absorption "signature" of every compound -- that is, which wavelengths of light, infrared and otherwise, it absorbs. So studying the spectra returned by LEISA allows them to conclude what compounds are present on Pluto's surface.
In mid-April, when New Horizons had only just been able to resolve Pluto's disk, a visible splotch of white heavily suggested to the team that a polar cap was present at Pluto's northern pole. The spacecraft is now close enough that LEISA can tell the difference, compositionally, between the bright spot suspected to be a polar cap and the rest of Pluto, and it has detected the presence of methane and nitrogen ices, all but confirming that it is indeed a polar cap.
Pluto, imaged by New Horizons' LORRI instrument on 11 July. Credit: NASA/JHUAPL
Keep in mind how cold that region must be -- methane freezes into a solid a -295 degrees Fahrenheit, and nitrogen at -346 degrees Fahrenheit. And that's at the pole that is right now experiencing "summer." LEISA is too distant at the moment to determine the exact composition of the dark bands in the equatorial regions (what mission scientists have been calling "the whale,"see above image in the lower right), but that will be accomplished at close approach.
As New Horizons gets closer, LEISA will be able to do a lot more than just tell large regions apart. At Pluto, LEISA will map the composition of the surface down to a resolution of 10 kilometers per pixel, meaning it can determine differing compositions for areas as small as a Manhattan neighborhood. It will also map the composition of Charon's surface, and will be able to tell us the composition of the prominent dark spot showing up on recently obtained photos of the moon. The surfaces of Nix and Hydra will be similarly mapped, though to a lower resolution since they are more distant.
Another observation that the team has been on the lookout for is where the solar wind ends and Pluto's escaping atmosphere begins. As detailed yesterday, the dwarf planet is losing its atmosphere, and one goal of New Horizons is to find out exactly how fast it is escaping the weak clutches of Pluto's gravity. To do this, New Horizons is on the lookout for the boundary at which escaping nitrogen meets the incoming solar wind. This morning, Alan Stern reported that they had detected that boundary; the PEPSSI instrument detected nitrogen that had escaped from the atmosphere and become ionized by interaction with the solar wind five days ago, which is much earlier than the team expected. This mean's that either Pluto's atmosphere escapes more quickly than expected, the process by which escaping nitrogen becomes ionized is more efficient than expected, or some other mechanism they were previously unaware of is at work. More data and more interpretation will be required to suss that out.
Finally, it turns out that Pluto is larger than expected. Observations from Earth had allowed us to conclude that the body was somewhere between 1150 kilometers and 1200 kilometers in radius, and it was expected to be on the low end of that. The exact figure has never been known, mostly because of Pluto's atmosphere. Without knowing the exact properties of the atmosphere, it was impossible to tell how much of its apparent radius was caused by the thin envelope of gas surrounding it. Now with fresh on-site data from New Horizons, and combining a few image interpretation techniques, the New Horizons team was able to conclude that Pluto's radius is on the upper end of the expected range, at 1185 kilometers in radius. This has a few important implications.
Since the mass of Pluto was already known, its volume being larger than expected means its also less dense than expected. This could point to a higher proportion of the body being ice rather than rock. It also means that different layers of the atmosphere could begin at different altitudes, which the occultation experiments during close approach should solve conclusively.
As New Horizons draws closer to the Pluto system, the road gets more dangerous. Models tell the team that the greatest density of debris that could impact the spacecraft is in the regions of the smaller moons -- Nix, Hydra, Styx, and Kerberos. When these small moons are impacted by other objects, the debris that is kicked up easily escapes their very weak gravities, but isn't fast enough to escape the Pluto system, so it ends up in orbit around Pluto. Though Charon sweeps up most of this debris, outside its orbit the danger still lurks. Even a small pebble-sized dust grain, at New Horizons' extreme velocity (14 kilometers per second relative to Pluto), could be enough to cripple the spacecraft. But Stern isn't worried. Though he calls the Kuiper belt a "shooting gallery," space is very large, and, well, spacious. The probability of even a tiny debris impact is small. Stern placed the odds of mission failure due to an event like that at 1 in 10,000.
"The energy level," Stern said of the Mission Operations Center, "is just electric." With New Horizons already making several new substantial contributions to our understanding of the Pluto system, still 20 hours from closest approach, it's easy to see why.