Comparative compositions of Pluto and friends, even long-distant friends.

Continuing coverage of the July 22, 2013 first day of the Pluto Science Conference being held this week in Laurel, MD.

Bill McKinnon (Washington University) next provided an engaging talk about implications for composition and structure for Pluto and Charon.

Where did Pluto Accrete (i.e. where was Pluto born)? Pluto is not alone in its location on that a/e plot for Trans-Neptunian Objects (see previous posting).  It’s part of an ensemble of bodies on the 2:3 resonance with Neptune, coined the group “Plutinos.” Was Pluto formed around 33 AU (Malhotra 1993, 1995) and then migrated outward? What does this Nice I Model (Levinson et al 2008) which migrates the giant planets predict for the KBO population? The Nice I Model implies that for Pluto, Pluto could have formed 20-29 AU (i.e. closer in) to allow it to achieve its high inclination. Then a subsequent model, Nice II (Levinson et al 2011), suggests Pluto may have formed in the 15-34 AU range. This is in okay-agreement with accretion models since Pluto, a 1000-km size body, would need 5-10 million years (i.e. within a nebular life) if it were formed in the 20-25 AU range. McKinnon’s best guess: Pluto formed between 15-30 AU.

How long did accretion take and what are the implications (i.e. how long for Pluto to grow up)? If we have an accretion time (10’s of million years), there is time enough to form Aluminum-26, which provides a form of heat through its decay. Heat then can melt ices and create a differentiated body (i.e., rocky core, icy mantle) and also drive water out. McKinnon’s best guess: Pluto formed rapidly and early.

What are Pluto & Charon made of? They are understood to be made of rock+metal, volatile ices, and organics, with rock+metal more than ice, and ice more than organics. The rock will be some combination of hydrated & anhydrous silicates, sulfides, oxides, carbonates, chondrules, CAIs (calcium-aluminum-rich inclusions), CHONPS (carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur). We don’t really know what sort of composition these KBO volatile ices: will they be more like Jupiter Family Comets or Oort Comets? And we know even less about organic components: will the Nitrogen to Carbon ratio tell us whether KBO N2 (nitrogen) comes from organics rather than NH3 (ammonia)? Solar models (which lock up CO (carbon-monoxide) into carbon) can influence understanding of what rocks in the outer solar system are made of but their models are not in agreement with the best understanding of Pluto/Charon make-up. McKinnon’s best guess: Rock/Ice nature of Pluto-Charon is 70/30.

What are the implications for Pluto & Charon internal structure? New Horizons will not directly detect the differentiation state of Pluto & Charon because it does not fly close enough.

Alain Doressoundiram (Paris, France) came next. Using MIOSTYS, multi-fiber front-end to a fast EMCCD camera, on a 193 cm telescope in France, they observe outer solar system bodies using stellar occultations. Other science objectives for variable stars, transiting exoplanets. They confirmed two TNO occultation events, one in 2009 and one in 2012 and continue monitoring.

Luke Burkhart (Johns Hopkins University) talked about his work on a “Non-linear satellite search around Haumea.” Haumea is another Trans-Neptunian Object (TNO) that has multiple satellite companion, like Pluto. Using HST (10 orbits) they observed the Haumea system and used a method of stacking & shifting to identify satellites. But this method fails to capture objects which are close in, moving fast, and on highly curved orbits. So they developed a new method using a non-linear shift-rate. Their approach, when applied to the Haumea system, had a null-result. However, this approach could be used on images of the Pluto system and other TNOs.  Specifically, in answer to a question from the audience, Luke would be eager to use his technique on any of those long-range KBO targets the New Horizons project is currently investigating.

Family portraits of the eight largest trans-Neptunian objects (TNOs).

Family portraits of the eight largest trans-Neptunian objects (TNOs) (From Pluto is shown with its 5 companions.

Andy Rivkin (JHU/APL) ended the afternoon’s lively discussion by addressingDistant Cousins: What the Asteroids Can Teach us About the Pluto System”. He started his talk with a comparison of sizes between Ceres (the largest asteroid in the asteroid belt between Mars and Jupiter) and the Pluto System. He used as his framework Emily Lakdawalla’s chart, which can be found on the Planetary Society blog

Here the relative sizes of objects in the Pluto system are represented by objects from the Saturn system. Saturn’s moon Rhea serves as Pluto, Dione as Charon, Prometheus as Nix, Pandora as Hydra, Helene as Kerberos, and Telesto as Styx. Superimposed is where Ceres (an asteroid in our asteroid belt between Jupiter & Mars) fits on this scale. Andy Rivkin did a comparison of his observations of Ceres to postulate what that might mean for the Pluto system.

Pluto system and Ceres shown to scale, represented by objects from the Saturn system.

Pluto system and Ceres shown to scale, represented by objects from the Saturn system.

Ceres has an icy interior, but much too warm to keep ice on surface. HST images reveals rather smooth surface. IR spectra (from reflected sunlight) are very rich and indicative of ice-type features. Could there some sort of layering? On Pluto, you could have the same thing, but it’s also cold enough for ice to remain on the surface. There is also a mystery that several large C asteroids have similar 3 micron spectra to Ceres like 10 Hygiea and 704 Interamnia.

Implications for Pluto: Large main-belt asteroids could serve as comparisons for KBOs. Geophysical comparisons may be easier than compositional ones.

So the big take-away from the introductory talks on the “Kuiper Belt Context” is that we can learn more by sharing the knowledge: Learning from Other Bodies  (Other TNOs, Comets, Asteroids) will help us learn more about Pluto & Charon, and vice versa.

Finding that distant KBO needle in a deep space haystack.

Next up at the Pluto Science Conference were a series of talks dedicated to recent work in the searches for another Kuiper Belt Object (KBO) for the New Horizons spacecraft to fly by after its Pluto fly-by. Fuel on board the New Horizons spacecraft after the July 2015 Pluto fly-by could enable a fly-by of a distant KBO in the late 2010s through 2020s, pending identification of targets reachable within New Horizon’s remaining fuel budget.

John Spencer (SwRI) has been leading the ground based campaign to search for New Horizons’ next target. With an on board ~0.13 km/s delta-v (measure of propellant), traveling at 14 km/s, this translated to a ~0.5 deg half-angle cone through the Kuiper Belt for accessible targets, a type of “survey beam.” Previous KBO searches had been for R>26.0 over 1-2 degrees. But right now Pluto is in Sagittarius which is in the direction of the Galactic Center and there are a lot of other stars in the field that make searching for a slowly-moving object, this KBO, difficult.

KBO Survey Star Field

Above are what the star fields the team is inspecting look like. They observe the same star field night after night and look for shifts in a object between frames, indicating it’s a KBO and not one of the “fixed stars.”

Outer Solar System“Known objects in the Kuiper belt, derived from data from the Minor Planet Center. The pronounced gap at the bottom is due to difficulties in detection against the background of the plane of the Milky Way.” This is exactly where John Spencer and his team are focusing their efforts because a subsection of that part of the sky is what is reachable by the New Horizons spacecraft after 2015. Image taken from

The ground based search program area is entering a sweet spot, where they can cover a smaller area of sky from the Earth that falls within the expected New Horizons travel zone.

KBO Search Zone

The team has found 31 objects from 2011 data including a TNO. However, as of 2012 season, they have not found an object that could have a fly-by encounter by the New Horizons spacecraft.  But there are three objects (2011 JW31, 2011 JY31, 2011 HZ102) that New Horizons could get to within 0.15-0.2 AU of in 2018-2019. The team is in the middle of the 2013 observing season and based on the current number densities they are predicting to see 1.78 objects down to  R=26.0mag and 4.15 objects in 26.5 mag.

Alex Parker (one more month at Harvard before moving to Berkeley) provided a more in depth view of unique observations New Horizons can still make of these long-distance KBO fly-by, that is, a fly-by in the 0.1-0.2 AU range of the spacecraft. At 0.2 AU range, New Horizons’ LORRI will have 140 km/pixel range compared to our “sharpest eyes” by Hubble at 1200 km /pixel from low-Earth orbit.

His excitement over the unique discovery space New Horizons provides that you cannot get from anywhere else: Proximity. High angular resolution. High phase angles.

He’s been studying trans-Neptunian binaries as binaries provide a direct mechanism to measure their masses. “Wide” Kuiper Belt binaries have been discovered already (e.g.  Gemini observations of wide binaries 2006 BR284 separated by 0.82 arcseconds; 2000 CF105 separated by 0.95 arcseconds).

To visualize a ride on looking over the New Horizons shoulders as it journeys into the Kuiper belt, check out this one of Alex Parker’s visualizations at

Make note to hold onto your seat when the craft enters the Cold Kuiper Belt region in 2018!

Susan Benecchi (Carnegie Institute) rounded out the talks with HST Follow-Up Observations of Long-Range Candidates for New Horizons post Pluto. They observed 2011 JW31, 2011 JY31, and 2011 HZ102 with HST. Those objects had been detected with the KBO ground based search program described by John Spencer and Alex Parker (previous presentations). Her team has not confirmed detection of 2011 JW31. Her team has confirmed the colors of the two other objects being “red” which is consistent with the Cold Classical Population (i.e. primordial). Implications for New Horizons: HST can provide this extra characterization step for new candidates.

Gustavo Beneditti-Rossi (Brazil) described a summary of “Astrometric Analysis of 15 years of Pluto Observations.” Using the Pico dos Dias Observatory (1.6m and 0.6m telescopes), they monitored Pluto-Charon (which is not separated in their data) for 154 nights over 1995 to 2012. They do refraction correction (due to viewing angle from earth) and photo-center correction (due to the fact they cannot separate Pluto from Charon). And show that their tracking of Pluto’s location is in agreement with occultation data.

To end this post, I could not resist showing you Alex Parker’s vision of what New Horizons brings to this field of study. He created this montage of images illustrating the proximity (within artistic license) and equally important the high phase (objects as crescents) and high angular resolution (we can see surface features), all that New Horizons will provide in 2015 that no other observation platform can.

NH Silhouette

2015 will be the “Year of Pluto” and so much more!