A hazy atmosphere on Sedna?

Abstract

Introduction

Sedna has a very elliptical orbit (Brown et al. 2004), taking it from ~76 AU at perihelion to ~1000 AU at aphelion. This makes it the most remote solar system object currently known. It spends the vast majority of its time much further from the sun than its present 89 AU distance, and even at perihelion it is well outside of the Kuiper Belt.

As a physical object, Sedna exhibits a number of curious features atypical of other outer solar system bodies. [Summaries of Sedna's properties can be found on its discoverers' web pages at www.gps.caltech.edu/~mbrown/sedna/ and hepwww.physics.yale.edu/quest/sedna/sedna.html.] It has a surprisingly red colour (Brown et al. 2004) It exhibits a weak opposition surge (brightening near the phase of full illumination), comparable to that of Pluto. The amplitude of brightness variation attributed to rotation (Gaudi et al. 2005) is small (~1%).

The peculiar light scattering properties of Sedna are similar to what one would expect from the illumination of a nearly uniform billiard ball. Such an appearance is suggestive of a hazy atmosphere (e.g., Titan). Indeed, Pluto is known to have a thin atmosphere which is suspected to have a layer of haze (Elliot & Young 1992). Here, I explore the possibility that Sedna might possess a similar atmosphere.

Physical conditions on Sedna

Over the large range of heliocentric distances spanned by Sedna's orbit, it is expected that its temperature will vary from 11 to 38 K (Trujillo et al. 2005). with a present temperature of ~33 K. Most gases familiar in remote planetary atmospheres like ammonia and carbon dioxide are frozen as ices at the very cold temperature of Sedna, even at the extremely low pressure appropriate to the surface of a planetoid devoid of a significant atmosphere (Young 1992). Spectral observations of Sedna place upper limits on the surface coverage by ices of water and methane (Trujillo et al. 2005).

Of other familiar atmospheric gases, only molecular nitrogen and oxygen have triple point temperatures (63 and 54 K, respectively - Young 1992) approaching the low temperatures expected on Sedna. Oxygen is presumably bound entirely into ices and minerals and would not be present as O₂. Nitrogen should still be frozen on Sedna. One can imagine that it might enter the gas phase as a result of some local heating mechanism such as occurs on Triton. This would be interesting, but would hardly suffice to support a global atmospheric haze.

Common elements that remain gaseous below 40 K include hydrogen, helium, and neon. Hydrogen and helium are too light to retain, but it might be possible for Sedna to retain neon. The typical thermal velocity of a neon atom of mass m_Ne = 3.35 x 10^-26 kg is V_th = (3kT/m_Ne)^1/2. For T = 33 K, this works out to V_th = 0.2 km/s. The escape velocity must exceed this value by a significant factor to prevent rapid gas loss; a common (if somewhat arbitrary) condition for long term retention is V_esc > 5 V_{th} ~ 1 km/s for Sedna. [ Strictly speaking, we are concerned with the high velocity tail of the Maxwellian distribution. We will see that seasonal considerations may somewhat relax the usual requirement for gas retention in the case of Sedna.] The escape velocity depends on the size R and density p: V_esc = R (8 (3.14) G p/3)^1/2. While bracketing limits on Sedna's size exist, its density is essentially unknown. By analogy with other outer solar system bodies, we might guess p ~ 2 g/cc (i.e., the density of Pluto), in which case V_esc -> 1 km/s as R -> 900 km. We caution that such analogies have been poor predictors of Sedna's other properties. There does exist a plausible range of density over which Sedna may retain a neon atmosphere (Fig. 1).

Escape velocity as a function of density. Heavy curves bracket the limits on Sedna's size: it most likely lies in between. The right axis shows the equivalent temperature for the condition V_esc = 5 V_th. The horizontal lines show the expected range of temperatures on Sedna from aphelion (lower line) to perihelion (upper line) and the triple point temperature of neon (dashed line). Neon would be frozen most of the time, thawing only as Sedna approached perihelion. The leakage rate then might be small, depending on its size and density.

The Seasons of Sedna

The triple point of neon is 24.5 K and 0.4 bar (Young 1992), with the phase transition temperature having a very weak pressure dependence. As Sedna proceeds along its orbit, it will cross through the temperature boundary between solid and gaseous phases. This will occur when Sedna is ~180 AU from the sun. Consequently, Sedna may experience a `brief' (~1800 year) `summer' over ~14% of its orbit.

These estimates are very uncertain for many reasons, not least of which is the assumption of pure neon. Presumably there is some admixture of other materials even if neon is the primary atmospheric gas. Whether and how these affect the phase diagram is hard to guess, though large perturbations seem unlikely given the unique conditions under consideration. Complicated behaviour is possible near the triple point.

Seasonal behaviour has some interesting implications. Unless its density is surprisingly high, Sedna is unlikely to retain a neon atmosphere indefinitely. However, most of the time neon will be frozen and deposited on the surface like other ices. This greatly enhances the period over which neon may be retained.

As Sedna nears the sun, neon would sublimate, forming a tenuous atmosphere. If there is enough neon that the atmospheric pressure grows high enough to exceed the triple point pressure, there could be a period with open pools of liquid neon on the surface. While it is compelling to imagine such a situation, a more important consequence of the phase transition is that as neon boils into the atmosphere, it may bring with it particles of other materials like hydrocarbons, dust, or common ices. These might make the atmosphere hazy, providing a natural explanation for the weak opposition surge and small rotational brightness variation. The colour would depend on the properties of the particles in the haze.

A long term prediction of this scenario is that as Sedna retreats from the sun, the atmosphere will freeze out. As it does so, its appearance should change appreciably as it loses its haze, presumably resulting in a closer resemblance to other outer solar system bodies. This process would also lead to widespread resurfacing as the haze particles rain out and the neon re-condenses. Analogous processes occur on Pluto and Triton with methane and nitrogen being the relevant gases.

In the shorter term, the most obvious way to test this scenario is with stellar occultation. Neon itself is transparent, but a haze layer should be detectable if present. Whether Sedna will pass in front of a sufficiently bright star for this to be an interesting experiment any time soon must await a more accurate determination of its orbit.

Is a neon atmosphere viable?

In terms of cosmic abundance, neon is a very common element. It ranks ahead of nitrogen, being fifth behind hydrogen, helium, oxygen, and carbon in number (Grevesse & Sauval 1998). Its exact solar abundance is currently controversial, with estimates ranging from A_Ne/A_O = 0.15 (Grevesse & Sauval 1998; Schmelz et al. 2005) to 0.41 (Drake & Testa 2005) or even 0.52 (Antia & Basu 2005). Irrespective of the correct value, there is clearly a lot of it out there.

So the first question is why neon is not more common in planetary atmospheres. This presumably has to do with its chemistry and volatility. Hydrogen is reactive, so is common in compounds with other common elements like oxygen, carbon, and nitrogen even though pure hydrogen is too light to be retained by sub-Jovian planets. Helium is similarly too light to retain in pure form, and is presumably rare in terrestrial settings because it is chemically inert: unlike hydrogen, there are no bonds to tie it down. The same goes for neon --- in the inner solar system. At the low temperature of Sedna, it is quite conceivable that neon could be retained.

While neon might well be retained under current conditions, it is another matter whether it would have survived the formation of Sedna. Much depends on where Sedna formed. The further out, the colder, and presumably the fewer barriers to retaining neon.

One possibility is that Sedna is a scattered disk object, having formed closer in than its present location. In this case, one would naively expect it to be more like other Kuiper Belt objects. However, its large perihelion (76 AU) never brings it within the Kuiper Belt proper, nor within the gravitational influence of Neptune, the great scatterer of the outer solar system. It is therefor quite possible that it is native to the realm beyond the Kuiper Belt, as emphasised by (Brown et al. 2004).

Irrespective of the particular origin of Sedna, neon is so inert and volatile it is hard to see how it can be retained in any collisional aggregation process. Of course, no such process will be perfectly efficient in ejecting volatiles. There is, after all, some helium and neon left on Earth. Intriguingly, the isotopic signature in rocks derived from deep and shallow mantle sources suggest a rather large reservoir of primordial noble gases could be preserved deep in the interior (Trieloff & Kunz 2005).

Of more relevance to Sedna is that some noble gases are preserved in primitive meteorites (Lewis, Srinivasan, & Anders 1975; Ott, Mack, & Chang 1981; Nittler 2003). Recent experiments (Marrocchi et al. 2005) indicate that in conditions appropriate to the early solar nebula, noble gases could be adsorbed onto carbonaceous material much more readily than previously appreciated, especially at lower temperatures. This, together with the high cosmic abundance of neon and the large amount of carbonaceous material revealed within comet 9P/Tempel 1 by the recent Deep Impact mission (A'Hearn et al. 2005) all combine to suggest that it is worthwhile to consider the possibility of a neon atmosphere on Sedna even if it may seem farfetched.

Conclusions

Whether a neon atmosphere actually exists on Sedna depends sensitively on how and where the planetoid formed. Any presumptive formation mechanism is unlikely to retain neon efficiently. Yet the cosmic abundance of neon is so high that it is worth considering the possibility that enough might be available to provide a thin atmosphere.

If neon can be retained on Sedna, it might be generically more important in the outer solar system than we might otherwise expect. There are many curious bodies in the solar system which exhibit unanticipated properties. The thick atmosphere of Titan and the thin ones of Pluto and Triton remind us that surprising things are possible. The curious nature of Sedna, an unusual object in a previously unexplored region of space, emphasises the continuing importance of exploring new regimes.

References