Prediction: CO₂/H₂O inversion in 3I/ATLAS

Science
#1

Empirical Predictions for 3I/ATLAS (October–December 2025)

Posted for independent verification and discussion

These predictions concern the expected compositional and photometric behaviour of the interstellar object 3I/ATLAS as it approaches and passes perihelion (~29 October 2025, r ≈ 1.36 AU). They are model-agnostic and intended purely as empirical expectations for comparison with forthcoming JWST and VLT datasets.

  1. CO₂/H₂O Ratio Inversion
    • At heliocentric distances > ~2.8 AU, the coma will remain CO₂-rich with weak H₂O signatures.
    • Near perihelion (~1.4 AU), the CO₂/H₂O ratio should decline, marking activation of deeper H₂O ices.
    • A measurable inversion of the ratio (relative to early observations) is expected between ~1.8 – 1.5 AU.

  2. Early Nickel Emission
    • Spectral lines of neutral or ionised Ni are predicted to appear before any Fe features or strong H₂O lines, likely between 3.0 – 2.5 AU.
    • Nickel activity should plateau or decline as the H₂O phase begins.

  3. Rotational Compositional Modulation
    • CO₂ and Ni emissions are expected to vary strongly with rotational phase, reflecting surface heterogeneity.
    • H₂O emission should be more uniform once deeper layers activate.

  4. Polarimetric and Grain-Size Evolution
    • Pre-perihelion dust will show higher polarisation (finer, porous grains).
    • As perihelion approaches, larger and less-porous grains from deeper layers will reduce polarisation by ~10–20 %.

  5. Post-Perihelion Hysteresis
    • Outgassing ratios will not retrace inbound values: CO₂ emission will drop faster than H₂O, producing a distinct hysteresis loop in compositional trends.

  6. Possible Isotopic Offset
    • Isotopic ratios in CO₂ may differ slightly from those in later-released H₂O (Δ(¹³C/¹²C) or Δ(¹⁸O/¹⁶O) ≈ few × 10⁻³), indicating stratified material layers.

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Purpose:
To provide an observational reference for colleagues tracking 3I/ATLAS through perihelion. These outcomes are testable using existing or planned JWST MIRI/NIRSpec and VLT VISIR/CRIRES+ datasets.

Should any of these behaviours be confirmed or contradicted, please share updates here so that independent analyses can be refined accordingly.

— Christopher Portelli, Independent Researcher (UK)

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#2

Progress Note: Observational Status of 3I/ATLAS (10 October 2025, 10:19 BST)

As of this morning, current multi-instrument datasets and press releases provide several key empirical developments concerning interstellar object 3I/ATLAS:

  • CO₂-dominated coma: JWST and SPHEREx have both reported a high CO₂/H₂O mixing ratio (~8 ± 1) at heliocentric distances near 3 AU inbound, indicating that present activity is governed primarily by CO₂ sublimation rather than water-ice.
  • Early metallic emission: VLT spectroscopy continues to show strong atomic Ni I lines with no corresponding Fe I signatures, implying early release of metal-bearing volatiles while the object remains distant from the Sun.
  • Emergent water signature: Ultraviolet OH detections (Swift) confirm the onset of measurable water activity, though still an order of magnitude weaker than CO₂-driven outgassing.
  • Pre-perihelion brightening: ExoMars TGO and Mars Express have observed a rapidly expanding coma, consistent with increasing volatile activity ahead of perihelion.

Collectively, these observations describe a temporal sequence in which lower-binding-energy volatiles and metal species become active first, with water-ice outgassing expected to strengthen as solar heating deepens.
Monitoring through the upcoming late-October perihelion will be critical for determining whether subsequent compositional evolution—in particular any reduction of CO₂ dominance or fading Ni emission—occurs as projected.

(Compiled from recent JWST, VLT, SPHEREx, Swift, and ESA releases; summarised 10 October 2025, 10:19 BST.)

— Christopher Portelli
Independent Researcher | Isle of Wight, UK

#3

Progress Note: Observational Status of 3I/ATLAS (12 October 2025, 10:25 BST)

As of this morning, current multi-instrument datasets and mission releases provide the following empirical updates on interstellar object 3I/ATLAS:
• CO₂-dominated coma (inbound): JWST reports a CO₂/H₂O mixing ratio of ~8 ± 1 at r ≈ 3.32 au, confirming CO₂-led activity well before perihelion; H₂O, CO, OCS, water-ice and dust are also detected.
• Extended CO₂ coma with limited H₂O gas: SPHEREx resolves a CO₂ gas coma (Q₍CO₂₎ ≈ 9.4 × 10²⁶ s⁻¹) with only upper limits on H₂O, reinforcing CO₂ dominance at several au.
• Early metallic emission: VLT spectroscopy shows strong Ni I lines with no Fe I detection at r ≈ 2.85 au, consistent with efficient nickel release while iron remains inactive.
• Emergent water signature: UV OH detections confirm measurable H₂O production inbound, though still weaker than CO₂ at comparable distances.
• Geometry & timeline: Perihelion remains expected in late October (~30 Oct) at ~1.4 au.

Summary:
The inbound sequence remains Ni → strong CO₂ → weak H₂O (OH) — precisely the ordering outlined in the initial empirical predictions (6 October 2025, 4:25 PM). While no confirmed CO₂/H₂O inversion has yet been reported, the current data trajectory continues to support that forecast. Monitoring through late October perihelion remains critical to determine whether a transition from CO₂-dominated to H₂O-driven activity and/or declining Ni emission occurs as predicted.

(Compiled from recent JWST, VLT, SPHEREx and NASA sources; summarised 12 October 2025, 10:25 BST.)
— Christopher Portelli
Independent Researcher | Isle of Wight, UK

#4

Observational Update Schedule (Pre-Perihelion Status)

This thread currently summarises three empirical checkpoints in the inbound evolution of interstellar object 3I/ATLAS (6 Oct – 10 Oct – 12 Oct 2025), covering early nickel activation and CO₂-dominated outgassing prior to perihelion.

As no major compositional deviation is expected before perihelion, further interim updates will be paused.

Unless a significant transition is detected — such as a confirmed CO₂/H₂O inversion or rapid decline in Ni emission — the next full update will appear near 28 October 2025, coinciding with perihelion approach.

Continuous monitoring will continue, and any major deviations from the projected sequence will be reported immediately.

— Christopher Portelli

Independent Researcher | Isle of Wight, UK

#5

Appreciate the continued interest — a perihelion update will follow once new compositional data become available,

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#6

Observational Note – 3I/ATLAS Forward-Facing Jet (Pre-Perihelion Phase)

Recent imaging and spectroscopic reports (JWST / VLT) describing a prominent sunward-facing jet are consistent with earlier pre-perihelion expectations of CO₂-dominated outgassing. The single, highly collimated structure implies a thermally active region aligned near the sub-solar latitude, likely corresponding to the low-binding-energy volatiles identified in prior datasets (~3 AU inbound).

Such directional activity suggests significant compositional heterogeneity across the nucleus and supports continued monitoring for subsequent transition phases. Should water-driven emission begin to dominate as the object approaches ~1.8 – 1.5 AU, a measurable inversion in the CO₂/H₂O ratio would mark the activation of deeper ices and define the next key observational threshold.

Ongoing spectroscopy through the perihelion window (late October) will determine whether this transition occurs concurrently with a decline in nickel emission or a modification in jet morphology.

— Christopher Portelli

Independent Researcher | Isle of Wight, UK

#7

Status Update — Approaching Perihelion (29 Oct 2025 ~11:47 GMT)

3I/ATLAS is currently at perihelion but in solar conjunction, positioned almost directly behind the Sun from Earth’s perspective.

This geometry means no ground-based or near-Earth telescopic observations are presently possible; the object will re-emerge into the pre-dawn sky around 10 – 15 November 2025, when elongation increases beyond ~30°.

The next compositional assessment (CO₂/H₂O ratio and any post-perihelion inversion) will therefore be made after reliable spectra become available.

Monitoring continues through mission and space-based instruments where possible.

— Christopher Portelli

Independent Researcher | Isle of Wight, UK

#8

Empirical Context — Comparative Volatile Behaviour in Interstellar Visitors (Pre-Reemergence Note)

Posted for independent verification and discussion

As 3I/ATLAS remains in solar conjunction (perihelion ~30 Oct 2025 @ 06:06 GMT, q ≈ 1.36 AU), direct Earth-based observation is currently suspended.

This interim note provides a comparative context for volatile behaviour across the three known interstellar visitors — 1I/ʻOumuamua, 2I/Borisov, and 3I/ATLAS — outlining empirical patterns relevant to the upcoming post-perihelion phase.

1I/ʻOumuamua (2017)

• Volatile-suppressed object; no detectable coma or gas bands (CO, CO₂, H₂O absent).

• Exhibited small non-gravitational acceleration with no clear volatile driver.

• Represents a volatile-poor or refractory-dominated end-member.

2I/Borisov (2019)

• Displayed classical cometary activity with strong CO, CO₂, and H₂O emission.

• CO/H₂O ratio elevated relative to typical Solar-system comets.

• Serves as a volatile-rich interstellar analogue to Solar-born comets.

3I/ATLAS (2025)

• Inbound spectra show CO₂-dominated coma at r ≈ 3 AU; weak H₂O/OH signatures.

• VLT and JWST data reveal early Ni I emission; Fe I absent.

• Represents an intermediate composition between the extremes of 1I and 2I.

Anticipated Post-Perihelion Sequence

• Monitoring for a CO₂ → H₂O inversion as deeper ices sublimate.

• Potential isotopic or polarimetric changes indicating stratified layering.

• Comparative CO₂/H₂O evolution will test whether volatile sequencing forms a continuum or suggests distinct formation environments.

Outlook

Re-emergence from solar conjunction is expected around 10–15 November 2025, at which point new ground-based and space-based spectroscopy (JWST / VLT / Swift / ESA assets) should resolve the next phase in compositional evolution.

3I/ATLAS, with its strong inbound CO₂ signature, offers an opportunity to assess whether volatile sequencing across interstellar objects forms a continuous spectrum or points to multiple formation regimes.

— Christopher Portelli

Independent Researcher | Isle of Wight, UK