EPSC 2015 – Small Bodies

EPSC 2015 – Small Bodies

Probably the most exciting aspect of EPSC 2015 for me was the coming of age of geological science for the two new kids on the block, the comet 67P and the dwarf planet Ceres. Data interpretations were necessarily pretty tentative at the Lunar and Planetary Science Conference in Texas in March, but now much more solid work has been done and we’re starting to get a handle on how these two bodies work.

First, a disclaimer: the discussion below is based on the parts of these sessions I attended myself, and so is particularly focussed on the geological findings. There were many other presentations on every aspect of these bodies, many of which fall way outside my speciality – you can see the abstracts for Rosetta and Ceres at these links, though of course they were written several months back, so they don’t contain the most recent work presented at the conference.

67P: independent onions or a sun-throttled whole?

Comet 67P (credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

Why does 67P have its duck-shape? And how do cometary dust jets occur? Two ways of addressing these related questions are by determining the composition of active and inactive parts of the comet and by analysing its structural fabric. The problem is that looking at it from these two different directions has led to conclusions that could potentially be at odds with each other.

On the one hand, there are structural observations. Matteo Massironi showed that thick sets of layers envelop the two lobes of the comet, and concluded that it is formed of two bodies with independent onion-like layering that have come together in a non-destructive manner. This would fit with Olivier Groussin’s work, which indicates that the comet’s low tensile strength is consistent with its formation by relatively gentle accretion.

Jets coming from the neck region of 67P (credit: ESA)

On the other hand, Maria Cristina de Sanctis used spectral data to show that ice forms in shadowed regions of the (very actively jetting) neck, and then sublimes quickly as the shadows move. Unless this is a very closed cycle of sublimation and recondensation (which in this high energy environment seems unlikely), this would indicate that the presence of the topography at the neck favours loss of material there, as it provides shadows where volatiles can be drawn up to the surface, which then move, causing the volatiles and associated dust to be lost to space. This accords with Alí-Lagoa et al.’s1 recent thermal modelling, which indicates that rapid thermal cycling due to shadow movement on the neck causes cracking of the surface and allows easier rise of volatiles. This could mean that 67P is shaped as it is because shadows have favoured outjetting activity in the neck region through a process of positive feedback, and not because it was originally two bodies.

So how do we reconcile these two stories? One possibility would be that it is indeed two bodies and that gravitational attraction keeps them together despite focussed material-loss at the region where they meet. Another is that the neck is formed in a single body by the sublimation/recondensation cycle alone, and that the layering in the two lobes results from an erosional process, rather than being primordial. Any such process would need to account not only for the up to 500 m sequences Massironi has recorded, but for Jean-Baptiste Vincent’s work showing that volatiles are preferentially lost from fractured cliffs and Sebastien Besse’s finding that jetting occurs from the walls of pits where there is side-on heating. It’s likely to be a while before we reach a consensus of exactly what we’re seeing in the geology of 67P, and we can look forward to some spirited discussion in the meantime.

Ceres: salts and ammonia

Bright deposits at the centre of impact craters on Ceres (above) and Mercury (below)

I was absolutely wowed when the Dawn mission returned the first images from Ceres: here was a whole new little world with bright spots glinting at the centre of its impact craters, begging for explanation. Working, as I do, on Mercury, I couldn’t help but make comparisons with the pits and bright deposits at the centre of Mercury’s impact craters, which I’ve written about at length23. I had to remind myself, though, that they’re very different bodies – what works for a Moon-sized body up close to the Sun doesn’t necessarily apply to a relatively icy dwarf planet out beyond Mars.

Data from Ceres was too new to be anything more than tantalizing at March’s LPSC meeting, so the EPSC session was a revelation. Firstly, though it was initially thought that the bright spots could be water ice, it turns out their albedo isn’t consistent with that, and that their spectrum is most consistent with perchlorate salts mixed with minor amounts of other materials. Ralf Jaumann suggested that they are salt domes, familiar from Earth, and that they occur within impact craters because the impacts have exposed salt diapirs in the subsurface. While this is potentially a good explanation, I would also point to the way in which impact crater structures localise magma ascent and subsurface storage on Mercury and the Moon4 – could they have a similar effect on subsurface salts on Ceres?

Ceres in the Solar System (credit: The International Astronomical Union/Martin Kornmesser)

The other really interesting discovery about Ceres is that, according to Maria Cristina de Sanctis, its overall surface spectrum accords best with that of ammonia-bearing clays. This is strange because ammonia is very unstable at Ceres’ distance from the Sun – it’s usually seen much further out in the Solar System, for instance on Neptune’s moon Triton. To explain the ammonia, she suggests that Ceres accreted ammonia-bearing pebbles that had migrated inwards from the outer Solar System, or even that the dwarf planet itself did so.

I could go on about the unusual features of Ceres: its strangely-shaped impact craters, its possible cryovolcanism (discussed at EPSC by Thomas Platz) or its surprisingly large topographic range, which indicates that its crust is stronger than we first expected. This little dwarf world has a lot of secrets yet to tell us, and should prove a fruitful arena for geological research for many years to come.

 

  1. Alí-Lagoa, V, Delbo’, M. and Libourel, G.  The Astrophysical Journal Letters, 810.2 (2015): L22 []
  2. Thomas, R. J. et al. (2014), J. Geophys. Res. Planets, 119, 2239–2254. []
  3. Thomas, R. J. et al. (2015), Planet. Space Sci., 108, 108–116. []
  4. Thomas, R. J. et al. (2015), Earth Planet. Sci. Lett., 431, 164–172. []