Thursday, July 25, 2013

Rings, Interiors, Surface Composition--and One Man's Personal Agenda

Who would have imagined there is SO much to discuss about one small planet?

This five-day conference is truly a gathering of great minds, the most knowledgeable people in the world regarding Pluto. With every aspect of the Pluto system under intense analysis and discussion, those of us present are witnesses to the frontier of all we know about these worlds, at the same time knowing that in two years, that knowledge and understanding will dramatically change.

Today, discussion moved from satellites to rings, addressing the possibility of rings around Pluto and their potential sources. Most dust ejected by the small moons is thrown out of the Pluto system altogether while some lands on Pluto and Charon.

Naturally, computer simulations were conducted to determine potential hazards to New Horizons, as the dust particles present in planetary rings could destroy the spacecraft in the event of an impact.

Alternative flyby routes, known as Safe Haven Bail Out Trajectories, or SHBOTS, have been designed by the New Horizons team in the event such hazards are discovered. One such alternative, the Deep Inner SHBOT, actually presents opportunities not afforded on the original trajectory. This path goes through the best locations for higher density particles, meaning it potentially yields the best opportunity for discovery of moons and/or rings. But are potential hazards worth it?

There was much discussion on the process that drives the formation of rings, including the influx of material from the Kuiper Belt. Additionally, as Pluto passes the ecliptic plane twice during each of its orbits, it passes through a thick patch of dust.

The remote search for rings is done in two ways—by direct imaging and by measurements made when Pluto occults (passes in front of) a star. So far, no rings have been found. On its original path, New Horizons faces a reduced risk but is not entirely out of the danger zone.

From dust and rings, discussion moved to the composition of the surfaces of Pluto and its four moons. There is evidence for methane frost on Pluto; various researchers described spectroscopic studies in wavelengths ranging from the infrared to the ultraviolet in the quest to discover the makeup of these objects.

We know methane exists in a pure state and in a diluted state on Pluto; some of it is dissolved in nitrogen. It is distributed uniformly across the surface, as are nitrogen and carbon monoxide. Neptune’s moon Triton contains carbon dioxide, and there is clear evidence it also harbors water ice. Evidence has not been found for carbon dioxide on Pluto though the James Webb Space Telescope, once in operation, should be able to do a more thorough search for it.

What do we know so far? For one thing, ultraviolet light does not make it to Pluto’s surface due to the shielding effects of methane in its atmosphere. Methane is present in Pluto’s atmosphere and on its surface.

The surface of Charon is different from that of Pluto. Pluto’s surface is colored, meaning it has more material than pure ice, which is white. Charon does not appear to have carbon monoxide, carbon dioxide, or methane; it does have water ice and a hybrid form of ammonia. Pluto’s coloring is likely due to organic residue resulting from solar irradiation of its surface. Other hydrocarbons must be present, but we don’t have access to all parts of the electromagnetic spectrum to distinguish them.

Observations with IRAC/Spitzer confirm that Pluto’s surface is heterogeneous. Participants learned about the many methods used to identify what is on that surface through measurements of Pluto’s thermal lightcurve and observations at long wavelengths.

In general, the physical properties of Trans-Neptunian Objects (TNOs) are not well known. Spectroscopy shows that Quaoar, which is probably large enough to be spherical, has a surface dominated by water ice. A current goal of Earth-based observations with ALMA is to obtain accurate information on the size and densities of medium-sized TNOs. Measuring in long wavelengths is important in determining these objects’ characteristics and surfaces.

Makemake, a small Kuiper Belt planet, is known to contain large slabs of methane. Quaoar and Eris also have methane on their surfaces. A total of 56 hydrocarbons have been identified on the surfaces of TNOs.

Pluto has also been measured in the mid-ultraviolet portion of the electromagnetic spectrum. Laboratory studies have been used to simulate conditions on Charon. In some ways, Charon’s surface resembles that of Saturn’s moon Tethys. No variation in Charon’s color or lightcurve has been seen since 1992. Charon is believed to have a reasonably cratered surface, and very subtle albedo and color features. The side facing Pluto looks different than the side facing away from Pluto. Cryovolcanism is possible; the presence of nitrogen is uncertain.

Speakers presented studies on the spectra and chemistry of Pluto’s ices, the formation of high mass hydrocarbons on Kuiper Belt Objects, and radiation chemistry on Pluto. A long-term goal is to obtain optical data for nitrogen, methane, and methane ices on Pluto.

A huge question explored is the possibility that Pluto harbors a subsurface ocean. The presence of an ocean depends on the presence of minor compounds and on whether the processes of convection and dehydration have occurred. Studies of the interiors of both Pluto and Charon play a crucial role in determining whether either body could have such an ocean.

To that end, researchers discussed constraints on the interior structures of both Pluto and Charon, mass determination for both objects by New Horizons’ REX instrument, the process involved through which Pluto’s interior structure evolved, and distinguishing formation scenarios for both objects based on the question of whether the impactor that created them was partially differentiated.

If Pluto and/or Charon have subsurface oceans, how did they evolve and cool?

One way that could help determine whether either body has a subsurface ocean is a look at the objects’ shape, which New Horizons will make possible. If Pluto and/or Charon show fossil bulges on either side, chances are no ocean is present. But if they don’t show these fossil bulges, there is a good chance an ocean exists. The formation of subsurface oceans depends on heat production and heat removal.

One of the most interesting subjects raised was presented as background information by Dr. Marc Buie, whose talk was titled “The Surface of Charon.” Buie revealed an interesting factoid. While Charon was discovered in July 1978, the late Brian Marsden, head of the IAU Minor Planet Center, did not officially recognize it as a satellite of Pluto until February 1985! Buie displayed copies of several IAU circulars issued between July 1978 and February 1985 that referred to Charon as an “unconfirmed” satellite of Pluto. During this time period, astronomers, despite IAU reluctance to recognize Pluto's moon, all recognized its existence and even referred to it by the name Charon—long before that name became “official.”

The display drew repeatedly drew laughter and recognition by those in attendance.

Did Marsden have a personal agenda? This is the same man who told the late Clyde Tombaugh that he “will torpedo your planet” and see that Pluto gets a minor planet number, even if Tombaugh did not live to see that. For decades, Marsden wanted Pluto under his auspices at the Minor Planet Center. And in 2006, it was Marsden who pushed hard for demoting Pluto.

It took seven years for an established astronomer to be certain that Pluto has a large moon, well beyond the time this was accepted as common knowledge, not just by astronomers, but by the general public?

More likely, Marsden didn't want to know that the object he was so determined to downgrade was actually half of a binary system.

Personal agendas should not come into scientific discussions. This is just one more reason that the minor planet numbers assigned to Pluto and the other dwarf planets should be removed—in spite of the fact that Marsden will not see it.

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