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In-situ study of comet 1P/Halley during its 1986 apparition revealed a surprising abundance of organic coma species. It remained unclear, whether or not these species originated from polymeric matter. Now, high-resolution mass-spectrometric data collected at comet 67P/Churyumov-Gerasimenko by ESA’s Rosetta mission unveil the chemical structure of complex cometary organics. Here, we identify an ensemble of individual molecules with masses up to 140 Da while demonstrating inconsistency of the data with relevant amounts of polymeric matter. The ensemble has an average composition of C 1 H 1.56 O 0.134 N 0.046 S 0.017 , identical to meteoritic soluble organic matter, and includes a plethora of chain-based, cyclic, and aromatic hydrocarbons at an approximate ratio of 6:3:1. Its compositional and structural properties, except for the H/C ratio, resemble those of other Solar System reservoirs of organics—from organic material in the Saturnian ring rain to meteoritic soluble and insoluble organic matter –, which is compatible with a shared prestellar history. A new analysis of Rosetta mass spectra reveals an ensemble of complex organic molecules with striking similarities to other organic reservoirs in the Solar System, including Saturn’s ring rain material, pointing at a likely joint prestellar history.

Molecular nitrogen (N2) is thought to have been the most abundant form of nitrogen in the protosolar nebula. It is the main N-bearing molecule in the atmospheres of Pluto and Triton and probably the main nitrogen reservoir from which the giant planets formed. Yet in comets, often considered the most primitive bodies in the solar system, N2 has not been detected. Here we report the direct in situ measurement of N2 in the Jupiter family comet 67P/Churyumov-Gerasimenko, made by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis mass spectrometer aboard the Rosetta spacecraft. A N2/CO ratio of (5.70 ± 0.66) × 10–3 (2σ standard deviation of the sampled mean) corresponds to depletion by a factor of ∼25.4 ± 8.9 as compared to the protosolar value. This depletion suggests that cometary grains formed at low-temperature conditions below ∼30 kelvin.

The Solar Wind Anisotropies all-sky hydrogen Lyman-alpha camera on the Solar and Heliosphere Observatory observed the hydrogen coma of interstellar comet 3I/ATLAS, also called C/2025 N1 (ATLAS), beginning on 2025 November 6, 9 days after perihelion. Water production rates were calculated from each image of 3I/ATLAS using the methodology of J. T. T. Mäkinen and M. R. Combi, and fluorescence rates and g-factors were calculated using the daily solar Lyman-alpha fluxes from the LASP database (https://lasp.colorado.edu/lisird/data) corrected for solar rotation and for the comet’s heliocentric velocity. The method has been used for over 90 comet apparitions. A water production rate of 3.17 × 1029 s−1 was found on November 6 when the comet was at a heliocentric distance of 1.40 au and at a sufficient solar elongation angle. It decreased over time after that, down to 1–2 × 1028 s−1 around 40 days postperihelion (December 9).

The origin of cometary matter and the potential contribution of comets to inner-planet atmospheres are long-standing problems. During a series of dedicated low-altitude orbits, the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) on the Rosetta spacecraft analyzed the isotopes of xenon in the coma of comet 67P/Churyumov-Gerasimenko. The xenon isotopic composition shows deficits in heavy xenon isotopes and matches that of a primordial atmospheric component. The present-day Earth atmosphere contains 22 ± 5% cometary xenon, in addition to chondritic (or solar) xenon.

Observations of water ice on the surface of comet 67P/Churyumov–Gerasimenko show the ice appearing and disappearing in a cyclic pattern that follows local illumination conditions, providing a source of localized activity and leading to cycling modification of the ice abundance on the surface. A cometary hydrologic cycle Maria Cristina De Sanctis et al . report observations from the VIRTIS imaging spectrometer onboard the Rosetta mission that show a diurnal water ice on the surface of comet 67P/Churyumov–-Gerasimenko. Surface water ice appears and disappears in a cyclic pattern that follows local illumination conditions, providing a source of localized activity. The authors suggest that the cyclic sublimation–condensation of ice triggered by varying illumination conditions may be a general process acting on cometary nuclei. Observations of cometary nuclei have revealed a very limited amount of surface water ice 1 , 2 , 3 , 4 , 5 , 6 , 7 , which is insufficient to explain the observed water outgassing. This was clearly demonstrated on comet 9P/Tempel 1, where the dust jets (driven by volatiles) were only partially correlated with the exposed ice regions 8 . The observations 6 , 7 of 67P/Churyumov–Gerasimenko have revealed that activity has a diurnal variation in intensity arising from changing insolation conditions. It was previously concluded that water vapour was generated in ice-rich subsurface layers with a transport mechanism linked to solar illumination 1 , 2 , 3 , 5 , but that has not hitherto been observed. Periodic condensations of water vapour very close to, or on, the surface were suggested 3 , 9 to explain short-lived outbursts seen near sunrise on comet 9P/Tempel 1. Here we report observations of water ice on the surface of comet 67P/Churyumov–Gerasimenko, appearing and disappearing in a cyclic pattern that follows local illumination conditions, providing a source of localized activity. This water cycle appears to be an important process in the evolution of the comet, leading to cyclical modification of the relative abundance of water ice on its surface.

The Solar Wind ANisotropies (SWAN) all-sky hydrogen Lyα camera on the Solar and Heliosphere Observatory (SOHO) observed the hydrogen coma of Halley-type comet (HTC) 12P/Pons–Brooks from SOHO’s halo orbit around L1 throughout its 2023–2024 apparition from the beginning of 2023 November to just before the mid-November outburst. There was a gap in coverage during December and January when the comet was in the galactic plane, after which coverage then continued until 29 July. SWAN also observed another HTC comet, 13P/Olbers, from 2024 April 26 until 9 August. Water production rates were calculated from each image of both comets using the standard methodology with fluorescence rates calculated using the daily solar Lyα fluxes from the LASP database corrected for solar rotation. The water production rate of 12P reached a maximum of 1.7 × 1030 s−1 7 days before the comet perihelion on 2024 April 21. Eight outbursts were detected with released water masses from 4.66 × 108 to 8.06 × 109 kg. The comet’s water production rate varied with heliocentric distance (r) with an exponent of −2.5 before perihelion and −2.9 after, covering heliocentric distances from 2.747 au before perihelion to 1.861 au after, when the activity is driven by water sublimation. The maximum production rate of 13P/Olbers was 1.1 × 1029 s−1, reached 27 days before perihelion on 2024 June 30. Its variation with heliocentric distance was more irregular and very scattered compared to that of 12P but generally increased with decreasing heliocentric distance. The variation of activity for 12P was generally very similar to that of the more famous HTC, 1P/Halley, in that both vary with heliocentric distance more similarly to most long-period Oort cloud comets than to most Kuiper belt Jupiter-family comets, many of which vary with very large negative slopes.

Two of the primary goals of the MAVEN mission are to determine how the rate of escape of Martian atmospheric gas to space at the current epoch depends upon solar influences and planetary parameters and to estimate the total mass of atmosphere lost to space over the history of the planet. Along with MAVEN’s suite of nine science instruments, a collection of complementary models of the neutral and plasma environments of Mars’ upper atmosphere and near-space environment are an indispensable part of the MAVEN toolkit, for three primary reasons. First, escaping neutrals will not be directly measured by MAVEN and so neutral escape rates must be derived, via models, from in situ measurements of plasma temperatures and neutral and plasma densities and by remote measurements of the extended exosphere. Second, although escaping ions will be directly measured, all MAVEN measurements are limited in spatial coverage, so global models are needed for intelligent interpolation over spherical surfaces to calculate global escape rates. Third, MAVEN measurements will lead to multidimensional parameterizations of global escape rates for a range of solar and planetary parameters, but further global models informed by MAVEN data will be required to extend these parameterizations to the more extreme conditions that likely prevailed in the early solar system, which is essential for determining total integrated atmospheric loss. We describe these modeling tools and the strategies for using them in concert with MAVEN measurements to greater constrain the history of atmospheric loss on Mars.

In 2017, 2018, and 2019, comets 46P/Wirtanen, 45P/Honda-Mrkos-Pajdusakova, and 41P/Tuttle-Giacobini-Kresak all had perihelion passages. Their hydrogen comae were observed by the Solar Wind ANisotropies (SWAN) all-sky hydrogen Ly camera on the SOlar and Heliospheric Observer (SOHO) satellite: comet 46P for the fourth time and comets 45P and 41P for the third time each since 1997. Comet 46P/Wirtanen is one of a small class of so-called hyperactive comets whose gas production rates belie their small size. This comet was the original target comet of the Rosetta mission. The SWAN all-sky hydrogen Ly camera on the SOHO satellite observed the hydrogen coma of comet 46P/Wirtanen during the apparitions of 1997, 2002, 2008, and 2018. Over the 22 yr, the activity decreased and its variation with heliocentric distance has changed markedly in a way very similar to that of another hyperactive comet, 103P/Hartley 2. Comet 45P/Honda-Mrkos-Pajdusakova was observed by SWAN during its perihelion apparitions of 2001, 2011, and 2017. Over this time period, the activity level has remained remarkably similar, with no long-term fading or abrupt decreases. Comet 41P/Tuttle-Giacobini-Kresak was observed by SWAN in its perihelion apparitions of 2001, 2006, and 2017 and has decreased in activity markedly over the same time period. In 1973 it was known for large outbursts, which continued during the 2001 (two outbursts) and 2006 (one outburst) apparitions. However, over the 2001 to 2017 time period covered by the SOHO/SWAN observations the water production rates have greatly decreased by factors of 10-30 over corresponding times during its orbit.

We investigate the plasma environment of comet 67P/Churyumov‐Gerasimenko, the target of the European Space Agency's Rosetta mission. Rosetta will rendezvous with the comet in 2014 at almost 3.5 AU and follow it all the way to and past perihelion at 1.3 AU. During its journey towards the inner solar system the comet's environment will significantly change. The interaction of the solar wind with a well developed neutral coma leads to the formation of an upstream bow shock and, closer to the comet, the inner shock separating the solar wind, with cometary pick‐up ions mass‐loaded, from the inner cometary ions which are dragged outward through abundant collisions and charge exchange with the expanding neutral gas. As a consequence the interplanetary magnetic field is prevented from penetrating the innermost region of the comet, the so‐called magnetic cavity. We use our magnetohydrodynamics model BATSRUS (Block‐Adaptive‐Tree‐Solarwind‐Roe‐Upwind‐Scheme) to simulate the solar wind – comet interaction. The model includes photoionization, ion‐electron recombination, and charge exchange. Under certain conditions our model predicts an unstable plasma flow at the inner shock. We show that the plasma shear flow around the magnetic cavity can lead to Kelvin‐Helmholtz instabilities. We investigate the onset of this phenomenon with change of heliocentric distance and furthermore show that a previously stable magnetic cavity boundary can become unstable when the neutral gas is predominately released from the dayside of the comet. Key Points MHD model of a comet's plasma‐solar wind interaction Peculiar plasma flows at the magnetic cavity boundary associated to jets Kelvin‐Helmholtz instabilities predicted to occur under certain conditions

Under favorable conditions, high-resolution long-slit infrared spectroscopic observations of comets can provide information on how ices are associated within the nucleus from spatially resolved properties of volatiles in the coma. In this study, we demonstrate the utility of this technique by investigating two Jupiter-family comets that were observed close to Earth—73P-Schwassmann–Wachmann (fragments B and C) and 103P/Hartley 2—providing high spatial resolution from ground-based observations. Spectra of these comets were acquired with NIRSPEC at the W. M. Keck Observatory. Molecular emissions were sufficiently strong to obtain spatial distributions of column densities and rotational temperatures for H2O, C2H6, HCN, and CH3OH (for 103P only). Comparison of 73P-B and 73P-C coma properties and nucleus associations test the heterogeneity of processes in comets that were originally part of a single nucleus. The spatial distributions of molecular column densities and rotational temperatures are notably similar in 73P-B and 73P-C despite different activity levels, providing additional and independent evidence of a remarkably homogeneous 73P parent nucleus. Comparison of coma properties and nucleus associations in 73P and 103P test the heterogeneities of processes between comets with very different chemical compositions. Both comets show strong evidence of abundant coma icy grains that contain less-abundant volatiles in addition to H2O. Because icy grains are constituents of the nucleus, the presence of volatiles within these grains provides evidence for how ices are associated within the nuclei of these comets. The spatial distributions of molecular column densities and rotational temperatures suggest different nucleus ice associations for 73P and 103P.