This blog post digs into a recent scientific study that shares the first near-infrared polarimetric observations of the interstellar object 3I/ATLAS. By pushing polarization-effects-in-telescope-optics-and-instrumentation/”>polarimetric measurements beyond the optical and into the near-infrared, researchers found some pretty odd dust-scattering properties. These set 3I/ATLAS apart from your average Solar System comet and offer rare clues about the stuff that makes up planetesimals from other stars.
Extending Polarimetry into the Near-Infrared
Polarimetry gives us a window into the physical nature of dust in comets and small bodies. It checks how sunlight gets polarized after bouncing off dust grains, which tells us a lot about their size, structure, and what they’re made of.
In this study, the team grabbed the first near-infrared polarimetric measurements of 3I/ATLAS. They covered wavelengths all the way from the optical RC band at 0.64 µm up to the Ks band at 2.25 µm. These new data build on previous optical observations and, for the first time, let us see how polarization shifts across a wide wavelength range for an interstellar visitor.
Anomalous Polarization Phase Curve
The heart of the study is the object’s polarization phase curve (PPC). This curve shows how the degree of linear polarization changes with solar phase angle.
For most comets in our Solar System, PPCs stick to familiar patterns that reflect their dust and activity.
A Polarization Signature Unlike Solar System Comets
3I/ATLAS stands out with an unusually large polarization amplitude. This is a big departure from what we see in typical cometary nuclei.
Even more surprising, this odd PPC didn’t budge as the object swung through perihelion.
Perihelion happened inside the water snow line, where you’d expect solar heating to spark strong outgassing in comets. But there was no measurable change in the PPC during this key stretch. That suggests quick outbursts or sudden dust production aren’t behind the polarization we see here.
Intrinsic Dust Properties as the Key Explanation
Since the polarization didn’t change with solar distance, the authors argue that intrinsic optical properties of refractory dust particles must drive the signal.
So, the dust in the coma or just under the surface of 3I/ATLAS is probably quite different from what we find in most Solar System comets. It might even hold onto a “memory” of its birth environment in another planetary system.
Wavelength-Dependent Scattering and the Polarization Color Curve
The team also looked at the polarization color curve (PCC), which tracks how polarization shifts with wavelength. This method gives us another angle on dust structure and size.
Evidence for Submicron Aggregate Dust
The PCC for 3I/ATLAS climbs steadily from 0.6 to 1.2 µm and then peaks somewhere between 1.5 and 2.0 µm. That’s classic wavelength-dependent scattering by dust aggregates, not tight, compact grains.
The data fit best if we assume the aggregates are made of submicron-sized monomers. That’s a size range that matches up with:
Implications for Interstellar Objects and Planet Formation
The PPC and PCC results together hint that 3I/ATLAS is a primitive cometary planetesimal formed in another planetary system. Its dust structure looks a lot like known cometary aggregates, but its refractory composition stands out from most Solar System objects.
This study nudges planetary science toward something intriguing: interstellar objects can retain unique, intrinsic material properties even after a close pass through the inner Solar System. As we spot more of these visitors, polarimetry—especially in the near-infrared—will likely help us figure out how planet formation differs across the galaxy.
Here is the source article for this story: Dust Properties of the Interstellar Object 3I/ATLAS Revealed by Optical and Near-Infrared Polarimetry