A dark, red spot on 2003EL61

EL61, a large, fast-spinning KBO

The Kuiper belt object known as 2003EL61 (we'll call it EL61 here) is very interesting because of its large size and very fast rotation. Given its large size, about 2000 km in diameter, and if not rotating, EL61 should be nearly spherical like the Earth or the Moon. This is because its self-gravity would compress it equally in all directions and so force it into a giant ball. But EL61 spins so fast (once every 3.9 hours, i.e., more than 6 times as fast as the Earth) that its shape has become elongated due to the centripetal force. We know this from EL61's lightcurve which shows a characteristic, approximately sinusoidal shape (see Figure 1).

EL61 has a dark, red spot

Two more subtle features in the lightcurve, namely that the two peaks (and the two troughs) are unequal, and that the blue data and the red data do not perfectly align in the last quarter of the phase plot, tell us even more about EL61. If the object were uniformly colored the lightcurve would have to be perfectly symmetric, with perfectly equal maxima and minima.

What the asymmetry tells us is that this is not the case: EL61 has a region which is darker than elsewhere on its surface. This dark spot is visible all the way from when EL61 shows its flank to when it shows the trailing tip (see Figure 2, and remember the rugby ball shape).

Furthermore, the spot is redder than the rest of EL61. This we know because the blue lightcurve data points around the region of the spot are below the red lightcurve data points. This means that EL61's dark spot reflects more red light than blue light from the spot, so it is red.

One thing we do not know is the size of the spot. It could be very small, smaller than in the leftmost animation in Fig. 2, but extremely dark and red, or just slightly darker and redder than the rest of the surface, but much larger, for instance taking up almost half of the surface of EL61. These two extreme possibilities would produce similar effects on the lightcurve, and our theory is based on the lightcurve.

Why do we care?

KBOs are so distant that it is very difficult to understand even the simplest things about them. After Pluto, EL61 is only the second KBO we know that shows signs of marks on its surface. Knowing that EL61 has a dark, red spot will raise new questions: What is the origin of the spot? How is the spot material different from the rest of the surface? Is the spot a scar of a collision? If so, is the spot a peek into a more interior layer of EL61?

EL61 is suspected to have suffered a large collision, more than 1 billion years ago. That collision may be what gave the object its extremely fast current rotation. The collision is also supposed to have ejected large chunks, and indeed other KBOs have been identified with close orbits and similar surface properties to those of EL61. Could these be the ejecta from the collision? Their large number (~10) and size (up to ~1000 m in diameter) imply a collision of an almost unimaginable magnitude.

Do you want to know more?

The paper describing this discovery in more detail can be found here. The authors are myself, David Jewitt, and Nuno Peixinho.

Figure 1 — Lightcurve of KBO 2003EL61 in three broadbands, blue B and red R. The regular, quasi-sinusoidal shape of the lightcurve, together with the rapid rotation (period P=3.9 hr) are strong indication that this object is elongated like a rugby ball. Two other important pieces of information are the different heights (and depths) of the 2 peaks (and the 2 troughs), and the misalignment of the red and blue data points between the phases 2.7 and 3.9 hr, approximately.

Figure 2 — Three simple models for the EL61 spot. All these reproduce the lightcurve data well (see Fig. 3). Although color is not shown, the darker the spot the redder it is.

Figure 3 — Lightcurves produced by the models in Fig. 2 plotted on top of the data. All three models are equally good. The thick gray line is for a model with no spot.