Australian Sky & Telescope — July 2017

http://www.skyandtelescope.com.au 27

COMET AND SPIN DIAGRAM: ESA / ROSETTA / MPS FOR OSIRIS TEAM; LAYER DIAGRAM: M. MASSARONI ET AL. /

NATURE

REALITY CHECK

Mission scientists envisioned Philae

landing effortlessly on a smooth

tract of the comet’s nucleus. But

the landing went awry, and Rosetta

eventually found Philae wedged

sideways in a craggy outcrop.

hemisphere (we sometimes call it the ‘hemiduck’) and the

other side is the southern one. The 52° obliquity (tilt) of 67P’s

rotation axis relative to its orbit keeps the northern hemisphere

in sunlight during the long, distant part of the orbit, and the

southern hemisphere is illuminated during the fast, close part

of the orbit. The upshot is that northern summer lasts 5.5 years

and is relatively cold, whereas the southern summer is much

shorter, less than a year — but it’s much more intense because it

occurs when the comet is closest to the Sun.

Because of this seasonal asymmetry, we see distinct

differences between the two hemispheres. The mass loss

in the south during its short, hot summer is significantly

stronger, potentially eroding many metres in some regions

during each pass around the Sun. This material mostly

escapes, but what falls back preferentially settles on the

cold northern hemisphere. Many regions in the north end

up thickly mantled with dust, leaving the southern regions

mostly dust-free and exposing the more solid surface.

We also see chemical differences in the gas coming from

the two hemispheres — the ratio of carbon dioxide to water

escaping from the south is higher. This perhaps has less to

do with the lobes having different compositions than with

the fact that they experience different heating, erosion, and

chemical evolution histories.

Rosetta also recorded daily effects due to the comet’s

rotation — a kind of cometary ‘water cycle’ — in which two

processes might be at work. In one, ice sublimates from the

dayside, and that gas subsequently freezes as a frost on the

colder, nightside surface. This frost then sublimates again as

the Sun rotates into view, and the process repeats. Another

possibility is that, after local sunset, residual heat inside the

nucleus liberates some buried water, which then freezes out

again on the surface experiencing night.

Either way, these constant flows are punctuated by

sudden events, jets of gas and dust, which are likely due to

the Sun hitting freshly exposed ice on part of the surface

that collapsed from a cliff or from the walls of a pit. Other

speculation is that jets also form when the thermal pulse

reaches subsurface pockets of ice that then quickly sublimate,

building up pressure and explosively escaping.

In physics, we often refer to a simplified version of a

problem as a ‘spherical cow’ because, as the old joke goes, to a

first approximation a cow is roughly spherical. The nucleus of

comet 67P definitely is not behaving like a spherical duck, and

once we got close and saw all the details, we no longer could

use many of our simplifying assumptions about comets. That’s

not a bad thing — it means the analysis and complex modelling

of Rosetta’s results likely will continue for decades to come.

Clues to 67P’s origin

One of the foremost questions in planetary science concerns the

origin of Earth’s water. Did it mostly come from comets, and if

so were they like 67P? Key to figuring out the answer is the ratio

of deuterium (‘heavy hydrogen’) to normal hydrogen in water

molecules. This D:H ratio varies based on where it’s measured

in the Solar System. Meteorites and asteroids have D:H values

similar to Earth’s. Although we think of these objects as very

53°

47°

41°

36°

30°

24°

19°

13°

7°

1°

SWHIRLING “DUCK” Comet 67P/Churyumov-Gerasimenko spins

every 12 hours around an axis through the narrow neck that connects its

two lobes. Left: The upper (northern) portion of each lobe spends more

time in sunlight, on average, but at times when the comet is farthest from

the Sun. The lower (southern) portions experience a shorter but more

intense exposure to sunlight when the comet is near perihelion.

STELLTALE SHAPES Careful analysis shows that each of the comet’s

lobes is draped with a unique set of layered deposits — indicating that

the two lobes formed apart and then became joined. Colours show how

well each layer’s plane matches the local gravity vector, with angular

deviations ranging from 1° (green) to 53° (red).