Mapping Comet 67P/Churyumov-Gerasimenko

If you’ve ever wondered about the surface of a comet — and why not? — wonder no more. The Rosetta mission has figuratively ripped the fuzzy veil away from Comet 67P/Churyumov-Gerasimenko (Comet 67P for short) mapped its surface, and done preliminary analysis of what that surface is like. That’s just for starters!

It turns out this particular comet (and probably most comets) has a planetary scientist’s wonderland of features. To get you started exploring the comet, check out this amazing image of the comet released over the weekend by the Rosetta mission’s OSIRIS team. OSIRIS is an imaging instrument sensitive to optical as well as infrared light (essentially heat radiation).

When I first looked at this image a couple of hour or so ago, it reminded me of the top of a craggy mountain peak. However, this is no rocky mountain. It’s a chunk of ice the size of a big city, following an orbit that takes it from just outside the orbit of Jupiter (at its most distant) to outside Earth’s orbit over the course of 6.5 years. (Earth orbits at 1 A.U. from the Sun (150,000 million kilometers or 93 million miles.  The comet gets as close as 1.24 A.U. (185,940,000 kilometers or 115 million miles from the Sun).

(This is to reassure anyone who was worried that the comet will somehow intersect Earth’s orbit. It won’t. During perihelion passage (closest approach to the Sun) the comet and Earth will be 3.3 A.U. (490,000,000 million kilometers or 304 million miles). That occurs in August, 2015.)

Like its other cometary siblings, Comet 67P was formed in the earliest epochs of solar system history and contains ancient ices and dust grains that date back to the messy birth of our planets, moons, rings, comets, and asteroids.

To study a comet is to delve into that primordial time. The ices and dust will be studied minutely to help planetary scientists understand the conditions that existed in those early epochs. The isotopes (the atoms of elements with different numbers of electrons) of the chemicals that make up the comet, for example, can exist under certain conditions. That makes the ices and dust very important clues in the hunt for an understanding of how our solar system (and others) came to be.

The appearance of the surface also has a story to tell of what the comet has experienced in its billions of years wandering the solar system. Craterlike features could indicate collisions or possibly outgassing explosions. Canyons and/or grooves may tell a tale of shock waves faulting (cracking) a surface.

What Does 67P Tell Us About Itself?

We can see that it has cliffs, craters, boulders, grooves, and odd-looking depressions. All of these terrains tell a story about the comet’s history and activity. Now that Rosetta mission scientists have gotten good imagery, they can map the surface and THEN begin figuring out just how all this terrain was formed.

When planetary scientists and geologists look at the surface of a world, they automatically divide it into terrain units. That is, they say, “those are mountains, these are dry lakebeds, over here are grooves (or deep canyons), and these look like craters.”  Think of it as a preliminary scouting report, with descriptions of the shapes (morphologies) of features included.

For Comet 67P, the Rosetta scientists have created a preliminary surface map of part of the comet to describe the morphologies they’ve found. The different colors delineate different surface “regimes”, such as the relatively smooth areas (in brown) and the cratered regions (in light, turquoise blue).

There’s a lot more mapping to be done, and later this week, the team expects to announce where the Philae lander portion of the mission will settle down on the comet. That landing will occur in November. Team members are looking for the safest, yet most interesting spot for the lander, which is expected to take samples of the surface and return data about the ices and dust that make up the comet.

There are already preliminary indications about the chemical makeup of the surface. The U.S.-based ALICE ultraviolet imaging spectrometer determined that the outer coating on the comet is very dark at ultraviolet wavelengths. It found no large water-ice patches in the otherwise dark, dust-and-ice surface, and detected hydrogen and oxygen in the comet’s outer atmosphere (called the coma). So far, the instrument found that the surface material is darker than charcoal and doesn’t reflect much light.

In addition to the mapping, the spacecraft’s COSIMA instrument captured some cometary dust grains for further study, and the VIRTIS instrument team has recorded more than 3 million spectra of the surface, and the data have allowed them to construct thermal (temperature) maps. The average surface temperature of the comet is around 205 K (-90F/-68C). This means the comet is not exclusively covered in ice, which nicely goes along with the ALICE findings. Also, as the comet rotates, it gets more sunlight warming in some areas than others. Knowing where these “hotspots” are will help in the decision about where to land Philae in a few months.

Everything I’ve talked about so far is just the tip of the iceberg (so to speak) of the discoveries being made (and waiting to be made) at this comet. As a former comet researcher myself, I’m fascinated by the things we’re finding out at this cometary nucleus. I can’t wait to see what else the Rosetta mission scientists and its instrument teams find next!