Origin of water on Earth

To address such crucial problems in planetary science as the origin of water on Earth and the formation process of our Solar System, as well as other planetary systems, it is important to conduct in-depth research in the most primitive materials – leftovers from planetary system formation, such as comets. Estimation of the deuterium-to-hydrogen (D/H) isotopic ratio in water sublimating from their nuclei, provides a chemical tracer of water’s source region and, especially when compared to the Earth’s oceans value of 1.5576×10⁻⁴ (Vienna Standard Mean Ocean Water – VSMOW) potentially indicates an insight into our planet’s past. However, through objective research difficulties, this problem still remains open. Because of the faint signatures of deuterated species, D/H ratios were only obtained in 12 comets to this date, among which ground-based observations were possible mostly to the very bright ones (visible to the naked eye). And for only two of them, D/H ratio is known from direct measurements of the photodissociation products of deuterated water. Despite the use of the most powerful available instruments, in many instances these measurements are not free of inconsistencies or significant errors.

Figure 1 summarizes the progress made so far in investigating the deuterium content of cometary water. These efforts were started by the European Giotto spacecraft, aimed at 1P/Halley during its most recent return. The space mission was equipped with two instruments capable of estimating D/H ratio in two independent regimes via Ion Mass Spectrometer (IMS) and Neutral Mass Spectrometer (NMS). As stated by Balsiger et al. 1995 and Eberhardt et al. 1995, both measurements provided consistent D/H ratio of ∼3×10⁻⁴, however Brown et al. 2012 reevaluated the D/H value obtained with NMS to 2.1×10⁻⁴, relatively closer to the value for water on Earth. Later in 20^th century there were two rare opportunities to observe extremely bright comets, C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp), making first cometary HDO measurements possible from Earth-based facilities. From 464.925 GHz line observed by Caltech Submillimeter Observatory and the James Clerk Maxwell Telescope (JCMT), another two D/H values were obtained, remaining in line with original values from Giotto spacecraft.

In case of two other comets: C/2002 T7 (LINEAR) and C/2001 Q4 (NEAT), an attempt was made to obtain D/H ratio through the observation of the photodissociation products of deuterated water, as it results mainly in the production of OH and H. At 310 nm Hutsemékers et al. 2008 measured OH ultraviolet bands with the VLT UVES in the Oort-cloud comet C/2002 T7 (LINEAR), but no individual OD line was detected, however a marginal 3σ detection of OD was obtained by co-adding contributions of the brightest lines, resulting in OD/OH value (2.5±0.7)×10⁻⁴. In 2004 with Hubble Space Telescope the D/H ratio was measured directly by atomic deuterium and atomic protium Lyman-α emission from comet C/2001 Q4 (NEAT), using Space Telescope Imaging Spectrograph. Weaver et al. 2008 obtained a clearly greater value (4.6±1.4)×10⁻⁴, which to this day remains one of the largest derived ratios in comets. Another relatively high D/H, but consistent with previous values was obtained for a periodic comet 8P/Tuttle, which is likely to be an Oort Cloud interloper. In 2009 another instrument was used with D/H=(4.09±1.45)×10⁻⁴ from HDO transitions at 3.7 μm spectral region observed by Cryogenic Infrared Echelle Spectrograph (CRIRES) of the VLT.

Determination of D/H in the Jupiter-family comet 103P/Hartley 2 provided a close match to water from terrestrial oceans: 1.61×10⁻⁴ (Hartogh et al. 2011), acquired due to the HDO transition at 509.292 GHz using the Heterodyne Instrument for the Far-Infrared (HIFI) onboard the ESA Herschel space observatory. Similar measurements were obtained in case of the Oort Cloud comet C/2009 P1 (Garradd) by Bockelée-Morvan et al. 2012, resulting in D/H=2.06×10⁻⁴. Use of the same instrument confirms a significant difference in isotopic composition of water in the Oort-cloud comet and the Kuiper Belt object (although 103P is currently short-period comet, it is thought to originate from the population of comets formed in Kuiper Belt). Furthermore, it is vital to mention that Herschel detections require the comparison between emissions from deuterated water and a rare isotopic variant of water, H₂¹⁸O, to avoid problems generated by the severe optical thickness of the main water isotopologue (therefore the H₂¹⁸O/H₂O ratio had to be assumed).

After the retirement of Herschel in 2013, available instruments limited the detection window for cometary deuterium only to the most active comets. Paganini et al. (2017) and Biver et al. (2016) provided two independent ground-based measurements of D/H in comet C/2014 Q2 (Lovejoy), separated not only by the time interval of observation, but also by a scientific approach. Paganini’s measurements rely on sensing HDO simultaneously with water at IR wavelengths at the 10-meter W. M. Keck Observatory (Keck II). In contrast, Biver used radio/sub-mm observations of HDO line at 241.561 GHz from IRAM 30 m radio telescope and two water isotopologues: the H₂¹⁶O line at 556.936 GHz, and H₂¹⁸O line at 547.676 GHz through Odin 1.1 m submillimeter satellite, obtained one week later, as IRAM cannot detect H₂O. Both results agrees only at the 2σ level, which is a matter of great consequences, though the post-perihelion D/H from Keck is nearly 2 times greater than the Earth’s ocean value (1.94±0.56 VSMOW), while the radio measurements indicated that it should be in line with terrestrial water (0.89±0.25 VSMOW). To complete this picture, in the same work Biver et al. (2016) reported D/H measure in yet another Oort-cloud comet C/2012 F6 (Lemmon), exceeding over 4 VSMOW, or (6.5±1.6)×10⁻⁴ – the highest value ever recorded, even though the detection of HDO was at marginal 4σ level. It was revealed as a great contrast to C/2014 Q2 (Lovejoy), although both objects should have similar origin and were studied using the same instruments and similar methods. As discussed by Paganini et al. (2017), the D/H ratio might have been strongly influenced by a systematic bias as different experimental setups were used and the use of two telescopes with different beam sizes by Biver et al. (2016), together with comparing observations from different days, could provide a possible explanation for the varying D/H estimates in this comet, as physical isotopic variation seems unlikely due to Rosetta mission results (Müller et al. 2022).

The close 2018 apparition of another Jupiter-family comet 46P/Wirtanen provided an opportunity to use SOFIA for detection of 547 and 509 GHz transitions of H₂¹⁸O and HDO, although requiring assumptions similar to those of Herschel. Obtained D/H=(1.61±0.65)×10^-4 shows similarity to VSMOW and led Lis et al. (2019) to the conclusion about possible correlation between cometary nucleus’ active area fraction and isotopic structure of their water vapours. Other ground-based attempts to quantify deuterated water in comets ended with non-detections, e.g. 153P/Ikeya-Zhang, C/2007 N3 (Lulin), D/2012 S1 (ISON) and 45P/Honda–Mrkos–Pajduskova, where limits on D/H were determined.

To this date, the most precise measurements of cometary isotopic abundances were obtained in comet 67P/Churyumov-Gerasimenko, using Double Focusing Mass Spectrometer (DFMS) onboard Rosetta spacecraft (Müller et al. 2022). Apart from water-based D/H=(5.01±0.41)×10^-4, it provided a wider possibilities to measure the deuteration of a handful of other molecules, including linear alkanes (methane, ethane, propane, butane) and methanol, but also prior to that H₂S and NH₃. Until then most search of D/H ratio from non-water deuterated species were concluded with estimation of upper limits. The only exceptional case of the detection of DCN was for C/1995 O1 (Hale-Bopp) from IRAM observations.