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The vertical distribution of water vapor is key to the study of Mars' hydrological cycle. To date, it has been explored mainly through global climate models because of a lack of direct measurements. However, these models assume the absence of supersaturation in the atmosphere of Mars. Here, we report observations made using the SPICAM (Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars) instrument onboard Mars Express that provide evidence of the frequent presence of water vapor in excess of saturation, by an amount far surpassing that encountered in Earth's atmosphere. This result contradicts the widespread assumption that atmospheric water on Mars cannot exist in a supersaturated state, directly affecting our long-term representation of water transport, accumulation, escape, and chemistry on a global scale.

The Rosetta spacecraft has investigated comet 67P/Churyumov-Gerasimenko from large heliocentric distances to its perihelion passage and beyond. We trace the seasonal and diurnal evolution of the colors of the 67P nucleus, finding changes driven by sublimation and recondensation of water ice. The whole nucleus became relatively bluer near perihelion, as increasing activity removed the surface dust, implying that water ice is widespread underneath the surface. We identified large (1500 square meters) ice-rich patches appearing and then vanishing in about 10 days, indicating small-scale heterogeneities on the nucleus. Thin frosts sublimating in a few minutes are observed close to receding shadows, and rapid variations in color are seen on extended areas close to the terminator. These cyclic processes are widespread and lead to continuously, slightly varying surface properties.

We use vertical profiles of Martian atmospheric density, pressure, and temperature from the Mars Express SPICAM UV spectrometer to study thermal tides in the poorly studied middle atmosphere region at 70–120 km. Here we show that nonmigrating tides cause zonal pressure variations of tens of percent and zonal temperature variations on the order of 10 K in these observations. Wave‐2 and wave‐3 components are dominant, consistent with previous work at lower and higher altitudes and with theoretical predictions. Normalized pressure amplitudes tend to increase with altitude for the cases and altitudes studied here. Phases of the pressure variations vary little with altitude, indicating long vertical wavelengths for the underlying tidal modes. We derive theoretical relationships between zonal variations in temperature and in pressure and find that they are generally satisfied. Failure of these relationships can be used to infer the presence of multiple tidal modes contributing to a single observed wave component. The wave‐2 component is dominated by the diurnal Kelvin wave 1 (DK1) above about 80 km but contains multiple tidal modes below this altitude. In one unusual instance, 40°S–30°S, Ls = 150°–180°, and local time of 22–24 h, the usually strong wave‐2 component is extremely weak. The wave‐3 component is always dominated by a single tidal mode, which for tropical and extratropical latitudes we identify as the diurnal Kelvin wave 2 (DK2). Key Points Thermal tides seen by SPICAM in middle atmosphere of Mars DK1 dominates wave‐2 component of zonal variations DK2 dominates wave‐3 component of zonal variations in tropics

Using an absorption cell, we measured the Doppler shifts of the interstellar hydrogen resonance glow to show the direction of the neutral hydrogen flow as it enters the inner heliosphere. The neutral hydrogen flow is found to be deflected relative to the helium flow by about 4°. The most likely explanation of this deflection is a distortion of the heliosphere under the action of an ambient interstellar magnetic field. In this case, the helium flow vector and the hydrogen flow vector constrain the direction of the magnetic field and act as an interstellar magnetic compass.

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.

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.

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 Rosetta spacecraft spent ~2 years orbiting comet 67P/Churyumov-Gerasimenko, most of it at distances that allowed surface characterization and monitoring at submeter scales. From December 2014 to June 2016, numerous localized changes were observed, which we attribute to cometary-specific weathering, erosion, and transient events driven by exposure to sunlight and other processes. While the localized changes suggest compositional or physical heterogeneity, their scale has not resulted in substantial alterations to the comet’s landscape. This suggests that most of the major landforms were created early in the comet’s current orbital configuration. They may even date from earlier if the comet had a larger volatile inventory, particularly of CO or CO₂ ices, or contained amorphous ice, which could have triggered activity at greater distances from the Sun.

SOIR is a high‐resolution spectrometer flying on board the ESA Venus Express mission. It performs solar occultations of the Venus high atmosphere, and so defines unique vertical profiles of many of the Venus key species. In this paper, we focus on the Venus main constituent, carbon dioxide. We explain how the temperature, the total density, and the total pressure are derived from the observed CO2 density vertical profiles. A striking permanent temperature minimum at 125 km is observed. The data set is processed in order to obtain a Venus Atmosphere from SOIR measurements at the Terminator (VAST) compilation for different latitude regions and extending from 70 up to 170 km in altitude. The results are compared to many literature results obtained from ground‐based observations, previous missions, and the Venus Express mission. The homopause altitude is also determined. Key Points Venus terminator mesosphere and thermosphere Carbon dioxide measurement Venus terminator model

The Mars Atmosphere and Volatile Evolution (MAVEN) mission instrument suite includes an ultraviolet echelle spectrograph with high‐spectral resolution designed to resolve D and H Lyman‐α emissions. The high‐spectral resolution mode was previously characterized in the lab and in the cruise phase to Mars and had been calibrated using observations and models of interplanetary hydrogen Lyman‐α emissions. This work presents improved characterizations of the high‐spectral resolution mode using in‐orbit observations that allow for more robust detections of the faint D Lyman‐α emission line. Additionally, the instrument was re‐calibrated using simultaneous and comparable observations made with the Hubble Space Telescope high‐spectral resolution instrument. Comparisons to Lyman‐α observations made with the low‐resolution UV channel on the spectrometer, that had been calibrated with stars, showed consistency in the brightness values for measurements obtained at similar observational conditions. The combined upgrades to the faint‐emission fitting and new calibration techniques of the MAVEN echelle channel have resulted in an improved data‐reduction pipeline with favorable implications for the science utility of D and H Lyman‐α emissions. Plain Language Summary The MAVEN high resolution echelle (ECH) instrument has been used to observe ultraviolet emissions from Mars. Measurements made with MAVEN/ECH were made available to the public by using a data reduction pipeline that had been developed with in‐lab and theoretical analysis to convert observations into a form that was useable by the broader scientific community. Since MAVEN's orbit insertion in 2014, the ECH detector has been better characterized with lessons learned from in‐orbit performance. These improvements produced a new data reduction pipeline that has enhanced the detection of faint D Lyman‐α emissions that are critical to interpreting water loss from the upper atmosphere of Mars. Key Points MAVEN/ECH uses HST/STIS, the only other UV echelle instrument deployed to space, for calibration and independent verification The MAVEN/ECH data reduction algorithm has been upgraded to improve detection of faint D Lyman‐α emissions The scientific implications of this work include doubling the previous data set of D measurements available for reliable interpretation