We give up: Pluto is out to surprise us. This time it’s the ice. We thought it’d be easy once we confirmed ice caps of methane and nitrogen, but the story is much more complicated. The entire world has methane ice, but it’s concentrated at the equator and relatively thin at the pole.
Yesterday, NASA released the first data back from the Linear Etalon Imaging Spectral Array (LEISA). You can think of the spectra as a methane distribution map for the dwarf planet. Both the red and blue bands cover where shortwave infrared light is strongly absorbed by methane ice, while the green band is unaffected.
In our first detailed spectra of Pluto, we can see an abundance of unevenly distributed methane ice. The ice is present across the world, but in a complex distribution we’re just barely starting to describe, and certainly don’t yet understand. Both the pole and equator have ice, but what that ice is made of, what it looks like, and how it behaves are totally different. This is Pluto’s version of playing with how textures and contamination impact an an ice’s behaviour and appearance. Although this spectra is looking at methane and nitrogen ice, you can think of it being somewhat analogous to how flawless deep blue ice and fluffy clean white snow look different despite both being pure, and how pristine snowy wilderness is immediately distinguishable from grungy street-clearing snowbanks.
New Horizons scientist Will Grundy describes the north polar region, confirmed as an ice cap just days ago, as an uneven mix of methane and nitrogen:
We just learned that in the north polar cap, methane ice is diluted in a thick, transparent slab of nitrogen ice resulting in strong absorption of infrared light.
From the spectra, the cap has a deep methane absorption. Meanwhile, equatorial patches that are so dark in optical light have shallow infrared absorption. This led Grundy to speculate:
The spectrum appears as if the ice is less diluted in nitrogen, or that it has a different texture in that area.
The more shallow dip in the spectra suggests that something is scattered more infrared light.
Methane, a carbon atom bonded to four hydrogen atoms, is considered one of the simplest organic compounds—although the term ‘organic’ is often confused to mean that methane can only be produced biologically. As we’ve learned exploring our own solar system, and by pointing our telescopes into distant clouds of interstellar dust, methane forms spontaneously all over the galaxy. Geochemically, it can be formed via a process known as serpentenization, involving water, carbon dioxide, and the mineral olivine—this is thought to be the dominant methane-forming process on Mars. It can also be formed by inorganic reactions with strong oxidizing agents, things like chlorine and fluorine, and biologically, through a type of anaerobic (oxygen-free) metabolism known as methanogenesis.
The discovery of methane on Pluto actually places the dwarf planet in line with every single other planet in our solar system—we’ve found methane everywhere, from trace quantities in Mercury’s thin atmosphere, to small amounts in the Martian crust, to literal oceans of the stuff on Saturn’s moon Titan (methane turns condenses from gas to liquid at a blistering −161.49 °C or −258.68 °F.) So abundant is liquid methane on Titan that scientists have run computer simulations to model the formation of a living cell based on methane rather than water.
But again, methane-based life is only speculation. Using Earth biology as an example, extraterrestrial methane would only be taken as strong potential evidence of life if it was found in disequilibrium in the atmosphere—that is, in combination with something like oxygen that would naturally consume methane. Liquid water and a heat source would make that case even stronger.
We don’t yet know what the frozen methane spread across Pluto’s surface means—this is just the very barest beginning of a hint of the science story going on here. It’s going to get more interesting as more spectra are returned back to Earth, and downright fascinating once we get a glimpse at what happened when LEISA spun around to peek at Charon. With spectra of both worlds, we’ll finally be able to start playing the matching game of seeing if we can find the same materials on both worlds, hopefully drawing connections between the two.
LEISA is a spectrometer on Ralph that operates in infrared wavelengths (1.25-2.50 micrometers) to map surface compositions. A different instrument, Alice, keeps an eye on the composition of atmospheres. Both instruments have their individual strengths and weakness, but between them we’ll hopefully slowly build a more complete map of how different elements are scattered around these alien worlds. For example, later spectra from LEISA will be able to distinguish between methane, ethane, and propane, but it will always be blind to any argon on the surface.
Only a portion of the first spectrum was returned for this initial downlink. This is a false colour image, where three infrared wavelength bands are mapped into the optical spectra for us to be able to see them. Red is remapped to capture the longest wavelengths (2.30 to 2.33 micrometers), followed by green (1.97 to 2.05 micrometers) and blue at the short end of infrared (1.62 to 1.70 micrometers). Pluto glows brightly in these short infrared bands, with a distinctly green northern cap with a limb reaching down to the equator, an a much more red southern hemisphere. To the right are individual spectral lines, recordings of the exact distribution of infrared light at the regions outlined with a thick dashed line. The northern pole (green) has a more extreme spectra with both higher peaks and troughs than the equator (red). The notable dips mark methane absorptions.
The data was acquired on July 12, 2015.
Top image: Methane absorption spectra on Pluto. Credit: NASA/JHUAPL/SwRI