🔭 astrophysics

SPHEREx Re-Observation of Interstellar Object 3I/ATLAS in December 2025: Detection of Increased Post-Perihelion Activity, Refractory Coma Dust, and New Coma Gas Species

In December 2025, NASA's SPHEREx spacecraft observed the interstellar object 3I/ATLAS post-perihelion undergoing a dramatic transformation into a fully active comet, characterized by a 40-fold increase in water vapor emission, the emergence of carbon-rich gas species (CN and organics), and a coma dominated by refractory organo-silicaceous dust as its original water ice sublimated.

Original authors: C. M. Lisse, Y. P. Bach, S. A. Bryan, P. M. Korngut, B. P. Crill, A. J. Cukierman, O. Doré, A. Cooray, B. Fabinsky, A. L. Faisst, H. Hui, G. J. Melnick, C. H. Nguyen, Z. Rustamkulov, V. Tolls, M. W. W

Published 2026-01-30

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: C. M. Lisse, Y. P. Bach, S. A. Bryan, P. M. Korngut, B. P. Crill, A. J. Cukierman, O. Doré, A. Cooray, B. Fabinsky, A. L. Faisst, H. Hui, G. J. Melnick, C. H. Nguyen, Z. Rustamkulov, V. Tolls, M. W. Werner

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a cosmic traveler named 3I/ATLAS. It's an interstellar object—a visitor from another star system—that passed through our solar system in late 2025. A NASA telescope called SPHEREx took a "before" picture in August and a "after" picture in December.

The story the paper tells is one of a frozen rock waking up, shedding its winter coat, and revealing a chaotic, colorful interior. Here is what happened, explained simply:

1. The "Before" Picture (August 2025)

When SPHEREx first looked at 3I/ATLAS in August, the object was still chilly. It was like a snowball that had just entered a warm room but hadn't started melting yet.

What it looked like: It was mostly just big, icy dust grains reflecting sunlight.
The activity: It was quiet. The only thing actively bubbling off was carbon dioxide (CO2), like a soda bottle that has been shaken but the cap is still tight.

2. The "After" Picture (December 2025)

By December, the object had spent a few months closer to the Sun. The heat finally penetrated deep inside. The paper describes this as the object "fully sublimating," which means its frozen ices are turning directly into gas.

The Explosion of Activity: The object is now 40 times more active than it was in August. It's no longer just a quiet snowball; it's a geyser.
New Ingredients: In August, we only saw carbon dioxide. In December, the telescope spotted a whole new menu of gases:
- Water vapor (H2O): The main ingredient, now flowing out in massive amounts.
- Organic "C-H" gases: These are carbon-based compounds (like methane or alcohol), which are the building blocks of life.
- Carbon Monoxide (CO): This gas increased by a staggering 80 times.

3. The "Pear" vs. The "Ball"

The paper uses a clever visual trick to explain how the gases are moving:

The Gas Clouds (CO2 and CO): These gases form a perfect, round ball around the object, like a fluffy cloud of smoke expanding evenly in all directions. This suggests they are coming straight from the core (the nucleus) of the object.
The Dust Tail: The dust and the new organic gases look different. They form a shape like a pear, with the narrow "stem" pointing toward the Sun. This suggests the dust is being pushed by sunlight and is carrying the heavier, stickier ingredients (like the organics and water) away from the core.

4. The Big Reveal: What Was Hidden?

The most exciting part of the paper is the theory of why the object changed so much.

The "Onion" Theory: The authors suggest that 3I/ATLAS is like a layered onion. In August, the heat only peeled off the outer skin, which was rich in carbon dioxide but poor in other things.
The Deep Thaw: By December, the "thermal wave" (heat) had traveled deep inside the object, past the outer crust. This deep layer was rich in water ice and trapped organic chemicals. As this deep layer melted, it released all the hidden ingredients at once.
The Dust Transformation: The dust grains themselves changed. In August, they were covered in fresh ice (like a snowman). By December, the ice melted away, leaving behind dark, dusty grains (like a dirty rock) that scatter blue light.

5. What Does This Mean?

The paper concludes that 3I/ATLAS has transformed from a dormant, icy rock into a fully active comet. It is now behaving very much like the long-period comets we see in our own solar system, spewing out a mix of water, carbon dioxide, carbon monoxide, and organic chemicals.

In short: The Sun's heat finally reached the "deep freeze" inside this interstellar visitor, causing it to wake up, melt its internal ice, and spray a rich, complex mixture of gases and dust into space, revealing a composition that is surprisingly similar to our own solar system's comets.

Technical Summary: SPHEREx Re-Observation of Interstellar Object 3I/ATLAS

Problem and Context
Following initial pre-perihelion observations in August 2025, the NASA SPHEREx mission conducted a re-observation of the interstellar object (ISO) 3I/ATLAS in December 2025. The primary objective was to characterize the object's post-perihelion state, specifically investigating changes in activity levels, coma composition, and dust properties as the object moved away from the Sun but remained within the solar system's ice line. Previous August data indicated an object dominated by large, icy dust grains with limited sublimation of volatile species beyond CO₂. The December observations aimed to determine if the thermal wave from the Sun's close approach (perihelion at 1.35 AU on October 30, 2025) had penetrated deeper layers of the object, triggering the sublimation of more volatile ices and altering the coma's morphology and spectral signature.

Methodology
Between December 8 and December 15, 2025, SPHEREx observed 3I/ATLAS at heliocentric distances ( $r_h$ ) of 2.0–2.2 AU and geocentric distances ( $\Delta$ ) of 1.809–1.845 AU. The mission utilized its spectrophotometric imaging capabilities across 102 discrete spectral channels covering the 0.75–5.0 $\mu$ m range with a spectral resolution ( $R$ ) of 40–130.

The observational strategy involved 104 special pointings with an integration time of 115 seconds per frame, centering the object on the Large Field of View (LVF) patterned detectors. Data processing followed the established protocol from the August 2025 campaign (Lisse et al., 2025a,b):

Photometry: A 2-pixel radius aperture was placed at the object's location to collect source counts.
Background Subtraction: Background counts were derived from a surrounding annulus and subtracted from the source aperture.
Analysis: The aggregate data were analyzed for spectral continuum features, gas emission lines, and spatial morphology. Geometry-corrected flux measurements were plotted against time to assess temporal variability.

Key Results

Increased Activity and Sublimation: The December observations revealed a significantly more active object compared to August. The H₂O gas emission feature, barely detected in August, increased by a factor of ~40, reaching a production rate ( $Q_{gas, H2O}$ ) of $1.4 \pm 0.28 \times 10^{28}$ molecules/sec. This indicates that water ice is now fully sublimating.
New Gas Species:
- CN: A new emission line was detected at 0.925 $\mu$ m, coincident with CN gas.
- Organics (C-H): Strong emission bands between 3.2–3.6 $\mu$ m indicate the presence of abundant gaseous organics (identified as CH₃OH, H₂CO, CH₄, and/or C₂H₆), with a production rate ( $Q_{gas, organics}$ ) of $2.0 \pm 0.5 \times 10^{27}$ molecules/sec.
- CO: Carbon monoxide flux increased by ~80x since August, with a production rate ( $Q_{gas, CO}$ ) of $7.6 \pm 1.5 \times 10^{27}$ molecules/sec. This shifts the CO/CO₂ abundance ratio to ~2.5, a value typical for CO-dominated comets in the solar system.
- CO₂: The CO₂ flux increased only slightly (~2x) compared to August, suggesting this species was already near-fully active pre-perihelion.
Spectral Continuum and Dust Properties: The spectral continuum shifted from an ice-dominated reflectance spectrum (similar to Kuiper Belt Objects) to one dominated by scattered sunlight (0.75–3.0 $\mu$ m) and thermal emission (4–5 $\mu$ m) from organo-silicaceous dust. The broad H₂O-ice absorption feature largely disappeared, replaced by signatures of dust grains that have lost their ice mantles. The scattered light continuum is significantly bluer than zodiacal dust, suggesting the presence of amorphous carbon and olivine-rich dust.
Morphology:
- Gas Comae: The CO₂ and CO gas comae are circularly symmetric, suggesting a source centered on the nucleus.
- Dust and Volatile Comae: The dust, H₂O, CN, and organic (C-H) comae exhibit a pear-shaped morphology with the "stem" pointing sunward, distinct from the symmetric gas comae. This suggests these species are sourced from the dust grains rather than directly from the nucleus.
- Tail Structure: No obvious jets or radiation-pressure-dominated anti-solar dust tails were observed, implying the presence of large (mm to dm scale) dust grains.
Temporal Stability: Continuum flux measurements at 1.0–1.5 $\mu$ m showed no variability exceeding 20% over the observation period (Dec 8–15).

Significance and Interpretation
The paper posits that 3I/ATLAS has transitioned into a fully active cometary body, sublimating all its ice constituents, including water, which was previously too cold to sublimate in August. The authors attribute this change to the thermal wave from the Sun's close perihelion passage penetrating the object's surface layers (estimated alteration depth ~10m), exposing deep, unprocessed material.

The observed composition changes—specifically the high ratios of CO-to-H₂O (0.55) and CO₂-to-H₂O (0.23), and a total [CO+CO₂]-to-H₂O ratio of 0.78—suggest the gas is being produced from CO-rich ice phases as well as H₂O-impurity phases. The authors note that the high organic abundance relative to water ( $Q_{Organics}/Q_{H2O} = 0.14$ ) and the emergence of CN and C-H features suggest these carbon-rich species were either contained within H₂O phases or trapped beneath them.

The paper concludes that the material making up 3I/ATLAS has fundamentally changed from an ice-dominated reflector to a low-albedo, dust-dominated emitter. While the authors acknowledge a tension regarding the size of the "icy dust grains" required to explain the August data versus the depth of thermal penetration needed to explain the December data, they maintain that the current data supports a model where bulk matrix comet material is now evaporating, releasing the full inventory of the object's constituents. These findings align the composition of 3I/ATLAS with that of long-period solar system comets.

Future Work
The authors state that the reported spectrophotometric measurements are preliminary. A more comprehensive analysis is planned prior to the object's next passage through SPHEREx's survey pattern in April 2026.

1. The "Before" Picture (August 2025)

2. The "After" Picture (December 2025)

3. The "Pear" vs. The "Ball"

4. The Big Reveal: What Was Hidden?

5. What Does This Mean?

More like this