Investigating Pluto's Atmosphere
Randy Gladstone, New Horizons' Co-Investigator with a focus on atmospheric study, is a man on a "mission of delayed gratification," as he puts it. The observations that New Horizons will make relevant to his field are some of the most precise of the closest approach sequence, and the wealth of data they produce may not be available to him until the end of 2015.
New Horizons will not be the first attempt at investigating Pluto's atmosphere. From Earth, using tools like the Hubble Space Telescope and other powerful optical and spectrographic instruments, we've been able to draw some basic conclusions about the very thin envelope of gasses that surrounds the tiny dwarf planet. We've learned that it's primarily nitrogen, with tiny amounts of methane and carbon monoxide.
"It's sort of like the atmosphere is 50 miles above our heads," says Gladstone, drawing a comparison to Earth. It's extremely cold, the pressure is very low, but "stuff still happens up there."
For starters, the particles of solar wind and UV radiation generated by the Sun break up the nitrogen and other trace molecules to create simple chemical reactions. Gladstone expects that these reactions create hydrocarbons like ethylene, acetylene, and hydrogen cyanide high in Pluto's atmosphere, which they hope to detect when New Horizons flies by. These heavier molecules, it's expected, sink from the high atmosphere to lower altitudes, "warming" on the way from around 40 Kelvin (-390 degrees Fahrenheit) high up to 100 Kelvin (-280 degrees Fahrenheit) closer to the surface. As they sink and warm up, they condense out, so investigators will be looking for any signs of resulting smog or frost in their observations.
The bottom line, says Gladstone, is "there's a lot of transport." Pluto's atmosphere is not a static ball of gas, but an active, evolving system, and Gladstone's team hopes to use the data from New Horizons' fly-by to characterize what drives the transport of these compounds, and how it shapes the composition of Pluto's atmosphere, from top to bottom.
The observations that will enable this will mostly occur after New Horizons has made its closest pass of Pluto. The key moment is when Pluto passes between New Horizons and the Sun -- what's referred to as an "occultation." This is when, from New Horizons' perspective, sunlight will pass directly through Pluto's atmosphere. New Horizons can then use the Alice instrument to observe it. Alice is a spectrometer; it collects light, in this case sunlight that has passed through Pluto's atmosphere, splits it into its constituent parts (think of light passing through a prism and showing a rainbow), and then analyzes the parts of the spectrum where the light has been absorbed by Pluto's atmosphere. Since we know what chemicals absorb which regions of light's spectrum, it will allow the team to deduce the composition of the atmosphere.
This technique is a powerful tool, but will only work in observing the upper part of Pluto's atmosphere, where enough sunlight will pass through and reach New Horizons to be captured by Alice's telescope. To characterize the lower atmosphere, New Horizons' REX instrument will be performing a very similar experiment, monitoring a powerful microwave uplink signal sent from the Deep Space Network here on Earth, and measuring the ways in which it changes after passing through Pluto's lower atmosphere. Between Alice's solar occultation and REX's radio occultation observations, Gladstone's team hopes to map the entire composition of Pluto's atmosphere, characterizing how it changes according to altitude, and how it interacts with the solar wind. When Charon passes in front of the Sun from New Horizons' perspective, similar observations will be conducted to answer the standing question of whether it has an atmosphere.
There's another quality of Pluto's atmosphere they hope to explain -- why it's still there. Pluto is a very small body, with a very weak gravitational pull. Bodies of its size are not expected to retain atmospheres, and in fact we know now that Pluto's atmosphere is escaping. It's suspected that it escapes at the rate of about 70 kilograms per second, but that's an approximation. New Horizons hopes to refine that to an exact number using the SWAP instrument. As Pluto's atmosphere escapes, it creates a sort of outward pressure that is eventually balanced by the solar wind, the stream of particles extending away from the Sun in all directions. The distance from Pluto at which these two forces balance is the key variable in an equation that will tell scientists the exact rate of escape of the atmosphere. SWAP will be constantly measuring the solar wind to determine where this boundary is.
If the 70 kilogram per second figure is correct, at that pace, Pluto's entire atmosphere would escape in about 6,000 years. Pluto has been around for billions of years now and the atmosphere is still there, so Gladstone's team wants to know how the constant loss is being replaced, by processes taking place either on or below the dwarf planet's surface.
New Horizons will be collecting mountains of data during its brief fly-by, and it will take months following the encounter to compress it and transmit it all back to Earth. Unfortunately for Gladstone, the answers to his team's questions may be a while in coming. He expects to get only a "hint" of the occultation data in September, and hopes to get all of his data by the end of 2015. From that point, it will take months and years of interpretation and study by the larger scientific community to fully grasp what New Horizons was able to find out about Pluto's atmosphere. Alan Stern, New Horizons' principle investigator, said today that the story of Pluto is in large part the transport of volatile materials. Characterizing its atmosphere will be a pivotal chapter in that story.