Atmospheres, Origins, and Moons Get Spotlight on Conference's Second Day

The second day of the Pluto Science Conference, July 23, brought more detailed and exciting discussion of Pluto, now focusing on the atmospheres of both Pluto and Charon and then proceeding to the subject of Pluto’s satellites—all five of them. In the evening, Alan Stern gave a public talk, “New Horizons to Planet Pluto: Exploring the Frontier of Our Solar System,” which can be viewed here: http://www.youtube.com/watch?v=iPyRAmviIuE&feature=c4-overview-vl&list=PL2335EF9F07B1DE5E .

Between staying late for Alan’s talk and having problems of my laptop constantly freezing on me, this post is actually a day late.

My friends and family find it hard to believe I found a five-day conference entirely about Pluto. Sure enough, participants continue talking about Pluto in between the sessions! If only this could be a regular thing, preferably closer to home!

Being here is an amazing, exciting opportunity. I am actually meeting people whose works I have read and whom I have watched online giving talks about Pluto and other areas of planetary science. Everyone here is incredibly friendly—while many don’t know me, not a single person has been negative, cold, or insulting. The conference is thankfully free of ego issues, a place where many people share the same interest, and a tremendous learning opportunity for all.

At times there is disagreement, but it is always friendly and respectful.

The discussion on atmospheres reviewed current knowledge, noting Pluto has a nitrogen-dominated atmosphere and also contains methane and carbon monoxide. Until 1985, no one knew this small planet has an atmosphere. This was revealed first when Pluto occulted a star in 1985. Three years later, mutual eclipses by Pluto and Charon of one another, enabling astronomers to obtained improved spectral measurements and observe ongoing changes, such as increasing pressure.

Talks addressed the interaction between surface methane and atmospheric methane on Pluto and the question of whether haze is present in its atmosphere. The size of that atmosphere has increased significantly between 1988 and 2012. And there is a difference between the planet’s upper and lower atmospheres.

Researchers presented their work and that of others in the form of spectral lightcurves (graphs of light intensity, plotted for celestial objects or regions as a function of time), data learned from occultations by Pluto of stars, and through observations using the world’s largest telescopes, including Hubble.

While we know little about Pluto just two short years before the New Horizons flyby, the information we have shows that Pluto’s atmosphere is fully formed and global.

Speakers addressed the issue of whether Pluto’s atmosphere will at some point collapse as it recedes from the Sun. Collapsing in this case means the nitrogen gets so cold that it falls back onto the surface.

The planet has volatile organic compounds (organic chemicals that have a high vapor pressure at ordinary, room temperature conditions) in its atmosphere, and these can condense and evaporate when clouds are present.

New Horizons will make map the distribution of volatile organic compounds in Pluto’s atmosphere through the REX (Radio Science Experiment) thermal imaging instrument. It will also the source of any hazes on Pluto and whether they come from condensates or from sources higher in the atmosphere.

Various researchers presented the findings learned from different occultations of stars by Pluto. A study this year used global circulation models to predict that the atmosphere would have much more cold methane than warm methane in localized regions.

The atmospheres of Pluto and Charon have been studied in the infrared and near-infrared portions of the electro-magnetic spectrum. Stern pointed out that comet impacts could cause a temporary, transient atmosphere on Charon. The large moon has no permanent atmosphere since it lost its volatile organic compounds long ago.

With Pluto’s moon count now up to five, there was no shortage of areas to discuss regarding Pluto’s origins and satellites. Pluto likely did not form where it currently is located. A model of the evolution of the solar system, known as the Nice model, proposes that the four giant planets, Jupiter, Saturn, Uranus, and Neptune, migrated to their current positions, having initially formed between 15 and 20 astronomical units or AU (one AU equals the distance from the Sun to the Earth, about 93 million miles). Jupiter likely formed closer to the Sun and moved outward, leading Uranus and Neptune to go unstable and scatter into the proto-planetary disk around the early Sun.

Where did Pluto come from? Dr. Hal Levison stated that Pluto was already in that disk, at least according to this model, when the giant planets migrated. Yet the model has flaws. The only way it works without resulting in total destruction of Earth and the terrestrial planets is if a third ice giant is added, a planet that subsequently was scattered out of the solar system altogether by Jupiter.

Pluto’s Kuiper Belt neighborhood should have more mass than it does. If all Kuiper Belt Objects are put together, they result in a mass less than 0.1 Earth masses. Far more mass is needed to have formed what is there now. Where did it go?

There are “cold classical Kuiper Belt Objects (KBOs) that have orbits of low eccentricity and inclination as well as plutinos, objects in various resonances with Neptune, and “hot” or highly eccentric KBOs with high inclinations, such as those in the Scattered Disk.

Astronomers presented different theories of planet formation to explain how these KBOs accreted. But do processes work the same at 20 AU from the Sun as they do at Earth’s distance (one AU)?

Notice how many talks end up with more questions than answers. That is part of the fascination of Pluto. We know a lot about this very cold, distant world, yet that knowledge is the tip of the iceberg, showing us how much we don’t yet know.

The New Horizons mission initially motivated the search for additional moons of Pluto. From 1978-2005, only Charon was known. Nix and Hydra were discovered through a deep exposure using Hubble. Pluto’s fourth moon, Kerberos, was found during an intense search for rings around Pluto in 2011, and Hydra was found a year later, its fifth, Styx, was discovered through use of a broader filter that allowed astronomers to probe even more deeply.

All five moons orbit roughly in the same plane as Pluto and Charon, and all are very close to being in mean motion resonances with Charon. Nix and Hydra are estimated to be about 50 kilometers in diameter while Kerberos and Styx are smaller, about 10-15 kilometers in diameter.

The position of these moons was discussed in the context of New Horizons and possible alternate trajectories in the event there is debris present that could destroy the small spacecraft on impact. New Horizons will measure the surface temperatures of Nix and Hydra and obtain high resolution images of all four small moons.

Also addressed were the origins of Pluto’s satellites, likely through an impact by another planetary body with “proto-Pluto,” similar to the impact believed to have formed Earth’s moon. It is unlikely that the small moons were captured rather than formed this way. Charon is thought to be composed of material from the impacting object.

That impact may have heated Pluto to temperatures between 50 and 100 Kelvin, causing Pluto to lose ice.

The orbits, physical properties, and chaotic rotations of Pluto’s moons were subjects of extensive discussion. Just about every speaker tied his or her talk to New Horizons, outlining how the mission will answer the many unresolved questions.

Current plans call for New Horizons to fly through an area interior to Charon known as the Charon Instability Strip (CIS), an area of about 2500 kilometers where no satellites are expected. Hubble will allow further constraining of that region, to less than 25 kilometers in one small area. The LORRI instrument will look for hazards upon approach seven days before the flyby. LORRI will continue to search for satellites, down to much more narrow areas.

From the flyby, there will be a “final answer” to the question of whether or not there are more moons of Pluto out there, Alan Stern said.

His talk that evening on the New Horizons mission was well attended and broadcast live online. One proud Pluto fan from India noted he got up at 5 AM (his time) to hear the talk live. During the question and answer section, a boy asked about planet definition and whether objects have to clear their orbits to be planets. Dr. Stern responded by confirming this is an ongoing debate, adding his view that the solar system now has a third region of planets where prior to 20 years ago, it consisted of four terrestrials, four gas giants, and one misfit, Pluto.