Asteroid of the Half-Month: 21 Lutetia

Hooo boy… Lutetia’s a weird one. The body, like Uranus, is “tilted.” Either pole alternately faces the Sun for half its year. This isn’t too weird for a lander or settlement; if you’re near Lutetia’s equator, you would have solar power for the whole year by dialing solar panels into ‘summer’ and ‘winter’ angles. In spring and autumn, panels would be in between, and seeing day/night cycling every eight hours. If you’re on a peak or ridge, night shadowing would be even less.

It gets weirder- far weirder. When the asteroid classifications were first formalized in the early ’80s, Lutetia was put in the M-type based on its spectrum (color signature) and albedo (reflectivity or ‘shininess’). This is an oddball category, with only two other members at the time. It was suspected M asteroids were metallic, though an optical telescope alone won’t conclusively establish that. Nowadays ‘M’ is not officially short for metallic, and even at the time Lutetia was suspected of being borderline M- or C-type. The two classes have similar spectra, but M is brighter, C is dark.

Planetary radar was also getting big at the time. Some targets returned echoes that strongly indicate metal, but Lutetia gave no such ping. Passing asteroids did show a significant disturbance, indicating that Lutetia was a quite heavy object. Again, suspected but not conclusive. The radar return did indicate a surface layer of regolith (rock particles or ‘soil’), not bare slab. And there we sat for over twenty years, until the Rosetta comet mission passed Lutetia on the way to its primary target, Comet 67P/Churyumov-Gerasimenko.

Rosetta found, besides an angular shape, a better measurement of Lutetia’s mass. As a probe flies past a target, the Doppler shifts of its radio indicate the local pull of gravity. Shape divided by mass then gives density; Lutetia’s overall density, 3.4 grams/cubic cm, then worked out to be one of the highest ever measured for an asteroid. Again, this indicates high metal content, not simple rock or ice. Rock is ~2.7 g/cc, pure metal 7 g/cc. When you account for porosity- either caves, or just space between regolith grains- the material itself is something even denser. No big asteroid has ever been measured to be a slab without porosity.

What, then, is Lutetia? To complicate things further, a spectrum of water was reported (Rivkin et al. 2000) in ground telescopes; Rosetta saw none in its flyby. If ices were negating porosities, that would boost the numbers somewhat, but apparently not here.

By the way, the density numbers look good. The shape model of Lutetia is from both ground and Rosetta data, which agree pretty well. Though Rosetta went by one pole, and never shot the other (due to pole-on rotation), there isn’t a big, hidden volume on the unseen side that would skew the density lower. If anything, there seems to be a big crater on the ‘back,’ nicknamed Suspicio, so density figures may actually rise. Meanwhile the Doppler signal from the flyby is solid, with tens of thousands of data points. Humanity has figured out radio measurement, and as I mentioned the Rosetta data follows observations of Lutetia perturbing other asteroids.

Two meteorite analogues have been credibly proposed. Either Lutetia is like the parent body of CH chondrite meteorites (perhaps the parent body), or like enstatite chondrites. Also, some propose CO or CV chondrites, or possibly CK, but the evidence looks weaker.

CH chondrites are stony meteorites, but with a high metals content and density. Hence Moyano-Cambero et al. (2016) proposed them as Lutetia-like. In particular, they hold out CH3 chondrites as an analogue, and of them, the specimen PCA 91467. They claim spectral features match between Lutetia and PCA 91467, but leave open several possibilities. Lutetia appears to vary from place to place along its surface, while PCA 91467 may not be fully representative of CHs. For one, no CH meteorite has been obtained without contamination by Earth exposure.

After Rosetta, Vernazza et al. (2011) proposed enstatite chondrites as Lutetia analogues, as had others before the flyby (e. g., Chapman, Salisbury 1973). Again, asteroid and meteorite spectra seem to match, while enstatites (3.46 g/cc density) also have a high percentage of metal. And again, the authors leave open the possibility of aubrite meteorites instead, which also have a makeup as observations would suggest. And yet again, they point out the issue of Earth weathering of enstatites, changing the meteorites.

Of course, there are other possibilities:

-Lutetia has a crust of something else, over its metal-rich bulk
-Lutetia-analogue meteorites just haven’t been found, or found and catalogued

The Dawn probe found the surface of 4 Vesta to be “contaminated” by smaller, carbonaceous impactors… and then found 1 Ceres to be contaminated by them, too. Should 21 Lutetia be any different? As to missing meteorites, the M-type (or whatever) asteroids are clearly rare, so M-type fragments should be, too.

Let’s assume for argument that Lutetia is covered in CH/enstatite material. This has a big implication: The asteroid differentiated into a core and mantle (or at least partially). More metal deep inside than further out. If so, a high bulk density but a metal-poor surface would make perfect sense. If so, this all happened because Lutetia melted, at least partially. If so, this could only happen because it’s a surviving planetesimal from the birth of the solar system. Heavy.

Of course, this is still speculating; Rosetta observations weren’t definitive. Vincent et al. 2011 claim that landslides are evidence for differentiation. Rosetta images clearly show that regolith slid from crater rims, to crater floors. They claim is that this indicates the gravity of a core, versus the gravity of an undifferentiated body. However, their own error bars show their own conclusion isn’t very conclusive.

While we’re speculating, the water question is still open, with or without Rosetta. Possibilities:

-A carbonaceous chondrite, significantly large and/or damp, hit Lutetia
-A dry impactor (or not) pierced Lutetia’s surface, uncovering its water

Either case could have the water dissipate in the decade before Rosetta arrived, and if true could happen again some random time. Again, pure speculation.

Even more speculative: if enstatite chondrites truly represent Lutetia, then the asteroid is a tourist. Enstatites form in the inner Solar System, not the Main Belt, where it’s too cold. Vernazza et al. (2011) thus hypothesize that primordial Lutetia formed around Mercury, Venus, Earth, etc. but got slung out to the Belt. At the dawn of the Solar System, when there was more stuff in more bodies, it was like roller derby. Smaller bodies got thrown into the Sun, thrown out completely, or swallowed/destroyed in a collision. Lutetia, then, is lucky to survive to this day- assuming it’s a giant enstatite chondrite, that is. Still speculation.

If I haven’t already made it clear, the search for answers on Lutetia led to… more questions. Not as clear to many people: this is a good thing, and this is science at work. The fact that an answer leads to something else meant that the original question was a good question. The fact means Lutetia was a good subject for study; it, or a body like it, deserves to be studied further. NASA is sending a probe to 16 Psyche, an unambiguous metal asteroid, by 2026. This fills out a range, from stony bodies, through Lutetia, to Psyche. But only if Lutetia is part-metal, and in a way we’re assuming; let’s study further.