Here’s an interesting bit of news from the New Horizons team. Remember that the spacecraft, having made its pass by the Pluto/Charon system in 2015, will be moving ever deeper into the Kuiper Belt. It’s been the hope of mission planners that a close study of one or more objects there might be possible. Now astronomer Scott Sheppard (Carnegie Institution of Washington) has announced that he has detected the first asteroid in Neptune’s trailing Trojan zone (the planet’s L5 point), an area New Horizons will fly through before arriving at the Pluto/Charon binary.
2008 LC18 is not itself in range for a New Horizons flyby, but mission principal investigator Alan Stern notes its significance in a recent report on the mission’s Web site: “ …its discovery shows that additional and potentially closer Neptune Trojans that New Horizons might be able to study could be discovered in the next three years.” And that gives us an interesting mission extension for New Horizons, to take advantage of the instrumentation we’ve put deep into the outer system. Closest approach to Pluto occurs in 1719 days.
Colors of a KBO
Thus far we’ve been able to image about 1000 objects in the Kuiper Belt, and it’s interesting to note that they come in a range of colors. Work at NASA GSFC (Greenbelt, MD) now offers up a computer model that tells us how the icy bodies acquire their red, blue or white tints. The model is based on incoming radiation and its effects on the different layers of a KBO.
Image: This cutaway model shows a red “shelf” layer of a Kuiper Belt object peeking through the thin, darkened crust above so that the object appears red in telescopes. Credit: NASA/Conceptual Image Lab/Tyler Chase.
Active Processes Deep in the Outer System
I find Kuiper Belt objects utterly fascinating, and not least because of the possible astrobiological interest. Eris, for example, has a bright, icy surface. According to the GSFC model, deeper layers of relatively pure water ice could erupt upwards to form new outer layers on KBOs, accounting for Eris’ brightness. The potential for active processes deep within a KBO is intriguing, and I remember discussing astrobiological possibilities with Joel Poncy at last year’s Aosta conference. Poncy and team (Thales Alenia Space) were investigating a fast orbiter mission to Haumea, which is, like Eris, highly reflective and evidently covered with water ice.
Poncy’s point was that if you’re going to study KBOs in terms of astrobiology, Haumea isn’t the place, because its unusual shape (a flattened ellipsoid) probably derives from a major collision, which would have disrupted the interior processes we want to study. But we now have numerous objects with diameters over 500 kilometers beyond the orbit of Neptune, and we’ll doubtless find hundreds more in coming years. Some smaller KBOs, like 2002 TX300, are possibly the result of the same collision that produced Haumea. 2002 TX300 is highly reflective, its ice covering evidently fresh and thus somehow resurfaced periodically. The case for life inside a KBO is slim, but these intriguing objects may teach us something about life’s early chemistry.
Explaining How Colors Emerge
But back to the GSFC work. The colors of Kuiper Belt Objects seem related to the different sizes and orbits we’ve observed since 1992, when the first KBO, 1992 QB1, was discovered. John Cooper, a physicist at Goddard, takes note of KBO diversity:
“There’s a group called the Cold Classicals that move in relatively circular orbits, and are nearly aligned in the same plane as the orbits of the other planets. These are all consistently reddish. Other objects, which might range from red to blue to white, tend to move in more elliptical or inclined orbits, which suggest they came from a different location within the solar system early in its history. So, it’s possible that the uniformly red Cold Classicals represent a more pristine sample, showing the original composition of the Kuiper Belt with minimal disturbances.”
Cooper’s work tells us that radiation should affect different objects in different ways, depending on their location. The so-called Cold Classicals would have formed in an area where plasma ions from the Sun aren’t intense enough to darken the KBO’s outermost surface. Instead, the plasma ions ‘sandblast’ the topmost layer to expose the layer immediately below, with further erosion being produced by dust grains from collisions in the belt. Given enough time, simple chemical reactions producing organic molecules, a kind of radiation ‘cooking’ process mediated by radiation from interstellar space, can then produce the red tint we see on many KBOs.
Ice and Complex Molecules
White Kuiper Belt objects also fit within Cooper’s layer model, which assumes water ices in a deep mantle layer that can erupt onto the surface to leave bright icy patches. “So these may not be dead icy objects,” says Cooper, “they may be volcanically active over billions of years.” Usefully, New Horizons’ pass through this region may yield better surface observations not just of Pluto and Charon but objects beyond that will help us confirm what materials are present. And its readings of the energy distribution and particle count may confirm the factors needed to make Cooper’s model work.
And about astrobiology in the Kuiper Belt?
“When you take the right mix of materials and radiate them, you can produce the most complex species of molecules,” says Cooper. “In some cases you may be able to produce the components of life — not just organic materials, but biological molecules such as amino acids. We’re not saying that life is produced in the Kuiper Belt, but the basic chemistry may start there, as could also happen in similar Kuiper Belt environments elsewhere in the universe and that is a natural path which could lead toward the chemical evolution of life.”
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My interest in KBO’s is because I agree with Freeman Dyson that they will be the ultimate abode of O’neill style space colonization. The estimate numbers of and total mass of the Kuiper belt is at least 100 times greater than the asteroid belt.
Does Cooper specify the mechanism he thinks will keep KBO’s volcanically active over billions of years?
The EKB is fascinating and I’m intrigued by the astrobiological angle, but I do wonder if Michael Papagiannis’ speculations on Main Belt ETI asteroid habitats in the 1970s might not be more applicable to the EKB.
Are We all Alone, or could They be in the Asteroid Belt ?
The obvious problem with KBOs is the absence of energy of any kind, for life or colonization. They are too small to be heated by readioactive decay, I suppose, and there isn’t much sunshine, either. Colonies could perhaps deal with that using artificial nuclear energy, but life certainly couldn’t.
John Freeman writes:
John, I’ve got a message in to GSFC about this and will post any response as soon as I get it.
Huge thin-film solar-collectors are perfectly feasible if you can make a space-colony, thus the Solar-powered habitable zone extends quite aways out from the Sun well into the Oort Cloud. The EKB is toasty by comparison.
Re John Freeman’s Question: Anywhere one finds ices and some energy source to make the ices either melt to liquid in high pressure (e.g., subsurface) environments or sublimate directly to gas in low pressure environments (e.g., KBO surfaces and near-surface), there is the potential for cryovolcanism. Comets approaching the Sun outgas due to thermal heating from the infrared part of sunlight. A large KBO like Eris might retain enough heat in its rocky core from primordial formation and subsequent decay of radioisotopes to maintain a subsurface liquid layer. Impacts could generate local heating to the liquid or gas phase. My personal favorite for research is the irradiation of KBO surfaces by high-energy cosmic rays, as we referred to in the media article. The kinetic energy of the cosmic rays is converted to chemical energy in terms of production of reactive oxidizing species in the ice. If these species later come into contact with ammonia, methane, or other hydrocarbons in the ice, the result can be production of gases such as nitrogen and carbon dioxide that could be the drivers for cryovolcanic emissions. If there is an existing body of liquid water, as probably exists below the south polar cap of Saturn’s moon Enceladus, then, as I proposed in a research paper last November. so much the better since this provides a warm thermal environment to increase chemical reaction rates and a liquid for the resultant gases to push on to produce the jets of water vapor and ice that we see shooting out of this active moon. So, a similar process might occur on some KBOs such as Eris and account for bright icy surfaces of such objects.
Thank you for the clarification, Dr. Cooper. Much appreciated!
My thanks to Dr Cooper.