I didn’t really get a chance to discuss the A&A papers from last week. They were kind of important, both by the purview of this blog, and in general:
Liu, Z. Hestroffer, D. Desmars, J. et al. Asteroid (4337) Arecibo: Two ice-rich bodies forming a binary… letter L23 /202450586
Schröder, S. Schörghofer, N. Mazarico, E. et al. Spectral properties of bright deposits in permanently s… article A178 /202450247
Geem, J. Ishiguro, M. Naito, H. et al. Study of hydrated asteroids via their polarimetric properties at lo… A195 /202450384
Water, water, EVERYWHERE. After hydrogen and helium, the two ‘bulk’ atoms of the universe, water is one of the most common molecules in… oh… all of existence. After H and He, next in abundances come C, N, and O, from nucleogenesis (the triple-alpha process). Helium is nonreactive, so we can dismiss it in this context. The ‘second-rank’ elements C, N, and O then commonly form hydrides (CH4, NH3, and H2O), less often mixed molecules (i. e., HCN, H2CO, and so on). But of all these, H2O is the one that stabilizes itself at reasonable conditions (“condensation temperature”). Water, then, is a major constituent of stable (here, solid) objects in space; the others exist where colder temperatures will allow it (like on deep-space dust grains), or as solutions with water or rock (basically, contaminants).
The upshot: water isn’t a question of ‘where is it,’ but more like ‘where isn’t it’; where has water been lost since the formation of solids, and conversely, where is water still present?
The answer: there is water at asteroids (1) Ceres, (4337) Arecibo, and… numerous other asteroids.
Schröder et al. used direct imagery by the Dawn spacecraft at Ceres. Looking ‘down’ at polar craters, some are cold because they’re in permanent shadow versus the Sun (PSRs, Permanently Shadowed Regions). The group selected craters where, despite shadowing, the camera still works. Sunlight bounces off a far rim of that crater, providing scattered light into the bowl and illuminating what’s inside. And what’s inside is water (as ice). Of the four craters selected for the rim-bounce technique, some are unambiguously water-bearing, and one is borderline. And this is all aside from subsurface ice at lower latitudes. Even without PSR craters as shelters, ice can persist at Ceres if it’s buried deep enough; the rock overburden acts as a shield, and stabilizes the temperature enough (across the diurnal cycle) that primordial water deposits aren’t lost. But I digress from Schröder et al.’s paper.
Liu et al. studied the outer-Belt asteroid (4337) Arecibo, and found it’s a binary- actually two bodies. With a parent-satellite pair like this, one can deduce more properties of the system. Timing the orbit of one about the other, we can apply Kepler’s Laws. Kepler’s orbit laws can deduce masses in the system, from the orbit times. And the diameters of one or both can be estimated- even measured directly- via multiple lines of evidence. The result? Mass, with size, gives density. The density of asteroid (4337) Arecibo (or should that be “asteroid” Arecibo?) is ~1.3 grams per cubic centimeter. This low value is virtually impossible for a pure-rock body. Rocks are, variously, ~3-7 gram/cc. Sure, some asteroids hold organics, and some have porosities (density ~0.0 g/cc). But of asteroids we’ve seen, none are overwhelmingly organics, and porosity levels over ~0.5 are implausible- that’s half empty space. Ergo, (4337) Arecibo contains, variously, a little or a lot of ice. This is plausible- Arecibo is in the Themis family of asteroids, and other Themis-family objects show signs of ice. Multiple signs of ice.
Geem et al. approach this from the other direction. Meteorites land on Earth with, in some cases, water content. Asteroids (definitely NOT on Earth) show signs of water content, in our telescopes, and to our space probes. How do we connect the two lines of argument? Can we link water-bearing meteorites to (possible or likely) parent bodies in space, through either telescopic or probe data, without physically taking samples? The research group says this is possible. “Rock” surfaces show various polarizations, indicating various properties. The researchers show a link between the polarizations of certain, key meteorite classes, and the polarizations of certain, well-studied asteroids. It appears (let’s not count the chickens before) that the two are related. If this work pans out (can an independent group reproduce their results? is there an independent line of evidence that pans out?), then we have a method in hand to find good asteroid targets. Targets for detailed study, mining, colonization, etc.
Water is EVERYWHERE… including (some of) the asteroids. In case you didn’t get that, water IS EVERYWHERE.