arXiv subjects

Alexander G. Hayes

Publications and source records attributed to Alexander G. Hayes.

14 recordsLinked to original sources

arXiv2025

The Atmosphere of Titan in Late Northern Summer from JWST and Keck Observations

Saturn's moon Titan undergoes a long annual cycle of 29.45 Earth years. Titan's northern winter and spring were investigated in detail by the Cassini-Huygens spacecraft (2004-2017), but the northern summer season remains sparsely studied. Here we present new observations from the James Webb Space Telescope (JWST) and Keck II telescope made in 2022 and 2023 during Titan's late northern summer. Using JWST's mid-infrared instrument, we spectroscopically detected the methyl radical, the primary product of methane break-up and key to the formation of ethane and heavier molecules. Using the near-infrared spectrograph onboard JWST, we detected several non-local thermodynamic equilibrium CO and CO2 emission bands, which allowed us to measure these species over a wide altitude range. Lastly, using the near-infrared camera onboard JWST and Keck II, we imaged northern hemisphere tropospheric clouds evolving in altitude, which provided new insights and constraints on seasonal convection patterns. These observations pave the way for new observations and modelling of Titan's climate and meteorology as it progresses through the northern fall equinox, when its atmosphere is expected to show notable seasonal changes.

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arXiv2025

Titan's Fluvial and Lacustrine Landscapes

In this chapter we begin with a review of Titan's fluvial and lacustrine landscapes as observed with Cassini remote sensing data, and what the many discoveries have revealed about Titan's surface materials and climate. Yet Cassini remote sensing data are coarse, topographic data are largely lacking, and the absence of in situ field measurements means we have little understanding of what the surface is composed of. At present, our knowledge of Titan's hydrology is comparable to that of Mars in the 1970's during the Viking era. Fortunately, the coming decades promise many new and exciting discoveries that can be achieved through Earth-based experiments, numerical modeling, and a continued commitment to the exploration of Titan by future missions, including both Dragonfly and orbiting assets. We therefore close the chapter with a discussion about what can be done with the current Cassini data and how new data, from both Dragonfly and a potential future orbiter, would allow us to leverage Titan to help solve some of the largest problems both here on Earth and on hydrologic planets and exoplanets more generally.

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arXiv2023

Spatiotemporal patterns of Io's bright transient eruptions, 1978-2022

This study analyzes Io's thermally detected volcanic outbursts and mini-outbursts, generally called bright transient eruptions. We examine their evolving characteristics over the history of outburst observations between the Voyager flybys in 1978 and 2022. We catalog, compare, and interpret the data of these bright transient eruptions from several spacecraft flybys and numerous ground-based observation campaigns. To test the spatiotemporal behavior of these events, we compare them to a population of randomly spaced, stochastic events with an equal likelihood of occurrence anywhere on Io's surface. We find that the aggregate of all outbursts is consistent with a random distribution across Io, whereas mini-outbursts strongly prefer the trailing hemisphere (180 to 360 W). On shorter timescales, however, outbursts show a significant change in spatiotemporal behavior before and after the year 2012. Outbursts from 1995 to 2007 favor the northern leading hemisphere, while outbursts from 2013 to 2021 favor the southern trailing hemisphere. These temporally separated clusters of outbursts are remarkably similar to Io's two primary mountainous regions, indicating that outbursts may be related to mountain-forming activity. These trends show how bright transient eruptions are distinct from Io's other forms of volcanism. These could be essential constraints to assess models of Io's interior heat transport between tidal generation and volcanic distribution.

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arXiv2023

Discovery of seven volcanic outbursts on Io from an IRTF observation campaign 2016 to 2022

This study analyzes near-infrared measurements of Io, Jupiter's moon, observed over 170 nights from 2016 to early 2022 using the NASA Infrared Telescope Facility (IRTF). During this period, seven new volcanic outbursts, the most energetic volcanic events on Io, were discovered and characterized, increasing the total number of observed outburst events from 18 to 25. We also present simplified criteria for the thermal detection of an outburst, requiring it to be both confined to a specific location of Io and above a threshold intensity in the Lp-band (3.8 micron). Our measurements use 2 to 5 micron photometry in eclipse, Jupiter occultation, and reflected sunlight. In addition to extending the observational dataset of Io's dynamic activity, these data provide insights into the temporal and spatial distribution of outbursts on Io. Notably, all seven outbursts were detected in Io's trailing hemisphere. These include Pillan Patera and a newly discovered repeating outburst location at Acala Fluctus. We add these events to the rare category of recurring outbursts, before which Tvashtar was the only known example. We observed that another outburst at UP 254W decreased in Lp-band intensity by a factor of two in 4.5 hours. In August 2021, Io exhibited high volcanic activity when two powerful outbursts rapidly appeared, propagating East. Our findings underscore IRTF's ongoing contributions to the study of Io.

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arXiv2021

Hypotheses for Triton's Plumes: New Analyses and Future Remote Sensing Tests

At least two active plumes were observed on Neptune's moon Triton during the Voyager 2 flyby in 1989. Models for Triton's plumes have previously been grouped into five hypotheses, two of which are primarily atmospheric phenomena and are generally considered unlikely, and three of which include eruptive processes and are plausible. These hypotheses are compared, including new arguments, such as comparisons based on current understanding of Mars, Enceladus, and Pluto. An eruption model based on a solar-powered, solid-state greenhouse effect was previously considered the leading hypothesis for Triton's plumes, in part due to the proximity of the plumes to the subsolar latitude during the Voyager 2 flyby and the distribution of Triton's fans that are putatively deposits from former plumes. The other two eruption hypotheses are powered by internal heat, not solar insolation. Based on new analyses of the ostensible relation between the latitude of the subsolar point on Triton and the geographic locations of the plumes and fans, we argue that neither the locations of the plumes nor fans are strong evidence in favor of the solar-powered hypothesis. We conclude that all three eruption hypotheses should be considered further. Five tests are presented that could be implemented with remote sensing observations from future spacecraft to confidently distinguish among the eruption hypotheses for Triton's plumes. The five tests are based on the: (1) composition and thickness of Triton's southern hemisphere terrains, (2) composition of fan deposits, (3) distribution of active plumes, (4) distribution of fans, and (5) surface temperature at the locations of plumes and/or fans. The tests are independent, but complementary, and implementable with a single flyby mission such as the Trident mission concept. We note that, in the case of the solar-driven hypothesis, the 2030s and 2040s may be the last ...

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arXiv2021

Science goals and new mission concepts for future exploration of Titan's atmosphere geology and habitability: Titan POlar Scout/orbitEr and In situ lake lander and DrONe explorer (POSEIDON)

In response to ESA Voyage 2050 announcement of opportunity, we propose an ambitious L-class mission to explore one of the most exciting bodies in the Solar System, Saturn largest moon Titan. Titan, a "world with two oceans", is an organic-rich body with interior-surface-atmosphere interactions that are comparable in complexity to the Earth. Titan is also one of the few places in the Solar System with habitability potential. Titan remarkable nature was only partly revealed by the Cassini-Huygens mission and still holds mysteries requiring a complete exploration using a variety of vehicles and instruments. The proposed mission concept POSEIDON (Titan POlar Scout/orbitEr and In situ lake lander DrONe explorer) would perform joint orbital and in situ investigations of Titan. It is designed to build on and exceed the scope and scientific/technological accomplishments of Cassini-Huygens, exploring Titan in ways that were not previously possible, in particular through full close-up and in situ coverage over long periods of time. In the proposed mission architecture, POSEIDON consists of two major elements: a spacecraft with a large set of instruments that would orbit Titan, preferably in a low-eccentricity polar orbit, and a suite of in situ investigation components, i.e. a lake lander, a "heavy" drone (possibly amphibious) and/or a fleet of mini-drones, dedicated to the exploration of the polar regions. The ideal arrival time at Titan would be slightly before the next northern Spring equinox (2039), as equinoxes are the most active periods to monitor still largely unknown atmospheric and surface seasonal changes. The exploration of Titan northern latitudes with an orbiter and in situ element(s) would be highly complementary with the upcoming NASA New Frontiers Dragonfly mission that will provide in situ exploration of Titan equatorial regions in the mid-2030s.

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arXiv2021

Explaining Bright Radar Reflections Below The Martian South Polar Layered Deposits Without Liquid Water

Recent discoveries of anomalously bright radar reflections below the Mars South Polar Layered Deposit (SPLD) have sparked new speculation that liquid water may be present below the ice cap. The reflections, discovered in data acquired by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on board the Mars Express orbiter, were interpreted as reflections from damp materials or even subsurface ponds and lakes similar to those found beneath Earth's ice sheets. Recent studies, however, have questioned the feasibility of melting and maintaining liquid water below the SPLD. Herein, we compare radar simulations to MARSIS observations in order to present an alternate hypothesis: that the bright reflections are the result of interference between multiple layer boundaries, with no liquid water present. This new interpretation is more consistent with known conditions on modern Mars.

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arXiv2021

Titan: Earth-like on the Outside, Ocean World on the Inside

Thanks to the Cassini-Huygens mission, Titan, the pale orange dot of Pioneer and Voyager encounters has been revealed to be a dynamic, hydrologically-shaped, organic-rich ocean world offering unparalleled opportunities to explore prebiotic chemistry. And while Cassini-Huygens revolutionized our understanding of each of the three layers of Titan--the atmosphere, the surface, and the interior--we are only beginning to hypothesize how these realms interact. In this paper, we summarize the current state of Titan knowledge and discuss how future exploration of Titan would address some of the next decade's most compelling planetary science questions. We also demonstrate why exploring Titan, both with and beyond the Dragonfly New Frontiers mission, is a necessary and complementary component of an Ocean Worlds Program that seeks to understand whether habitable environments exist elsewhere in our solar system.

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arXiv2020

Modeling transmission windows in Titan's lower troposphere: Implications for infrared spectrometers aboard future aerial and surface missions

From orbit, the visibility of Titan's surface is limited to a handful of narrow spectral windows in the near-infrared (near-IR), primarily from the absorption of methane gas. This has limited the ability to identify specific compounds on the surface -- to date Titan's bulk surface composition remains unknown. Further, understanding of the surface composition would provide insight into geologic processes, photochemical production and evolution, and the biological potential of Titan's surface. One approach to obtain wider spectral coverage with which to study Titan's surface is by decreasing the integrated column of absorbers (primarily methane) and scatterers between the observer and the surface. This is only possible if future missions operate at lower altitudes in Titan's atmosphere. Herein, we use a radiative transfer model to measure in detail the absorption through Titan's atmosphere from different mission altitudes, and consider the impacts this would have for interpreting reflectance measurements of Titan's surface. Over our modeled spectral range of 0.4 - 10 micron, we find that increases in the width of the transmission windows as large as 317% can be obtained for missions performing remote observations at the surface. However, any appreciable widening of the windows requires onboard illumination. Further, we make note of possible surface compounds that are not currently observable from orbit, but could be identified using the wider windows at low altitudes. These range from simple nitriles such as cyanoacetylene, to building blocks of amino acids such as urea. Finally, we discuss the implications that the identifications of these compounds would have for Titan science.

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arXiv2020

The Case for Non-Cryogenic Comet Nucleus Sample Return

Comets hold answers to mysteries of the Solar System by recording presolar history, the initial states of planet formation and prebiotic organics and volatiles to the early Earth. Analysis of returned samples from a comet nucleus will provide unparalleled knowledge about the Solar System starting materials and how they came together to form planets and give rise to life: 1. How did comets form? 2. Is comet material primordial, or has it undergone a complex alteration history? 3. Does aqueous alteration occur in comets? 4. What is the composition of cometary organics? 5. Did comets supply a substantial fraction of Earth's volatiles? 6. Did cometary organics contribute to the homochirality in life on Earth? 7. How do complex organic molecules form and evolve in interstellar, nebular, and planetary environments? 8. What can comets tell us about the mixing of materials in the protosolar nebula?

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arXiv2015

Titan Science with the James Webb Space Telescope (JWST)

The James Webb Space Telescope (JWST), scheduled for launch in 2018, is the successor to the Hubble Space Telescope (HST) but with a significantly larger aperture (6.5 m) and advanced instrumentation focusing on infrared science (0.6-28.0 $μ$m ). In this paper we examine the potential for scientific investigation of Titan using JWST, primarily with three of the four instruments: NIRSpec, NIRCam and MIRI, noting that science with NIRISS will be complementary. Five core scientific themes are identified: (i) surface (ii) tropospheric clouds (iii) tropospheric gases (iv) stratospheric composition and (v) stratospheric hazes. We discuss each theme in depth, including the scientific purpose, capabilities and limitations of the instrument suite, and suggested observing schemes. We pay particular attention to saturation, which is a problem for all three instruments, but may be alleviated for NIRCam through use of selecting small sub-arrays of the detectors - sufficient to encompass Titan, but with significantly faster read-out times. We find that JWST has very significant potential for advancing Titan science, with a spectral resolution exceeding the Cassini instrument suite at near-infrared wavelengths, and a spatial resolution exceeding HST at the same wavelengths. In particular, JWST will be valuable for time-domain monitoring of Titan, given a five to ten year expected lifetime for the observatory, for example monitoring the seasonal appearance of clouds. JWST observations in the post-Cassini period will complement those of other large facilities such as HST, ALMA, SOFIA and next-generation ground-based telescopes (TMT, GMT, EELT).

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arXiv2015

The fate of ethane in Titan's hydrocarbon lakes and seas

Ethane is expected to be the dominant photochemical product on Titan's surface and, in the absence of a process that sequesters it from exposed surface reservoirs, a major constituent of its lakes and seas. Absorption of Cassini's 2.2 cm radar by Ligeia Mare however suggests that this north polar sea is dominated by methane. In order to explain this apparent ethane deficiency, we explore the possibility that Ligeia Mare is the visible part of an alkanofer that interacted with an underlying clathrate layer and investigate the influence of this interaction on an assumed initial ethane-methane mixture in the liquid phase. We find that progressive liquid entrapment in clathrate allows the surface liquid reservoir to become methane-dominated for any initial ethane mole fraction below 0.75. If interactions between alkanofers and clathrates are common on Titan, this should lead to the emergence of many methane-dominated seas or lakes.

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arXiv2015

Meridional variation in tropospheric methane on Titan observed with AO spectroscopy at Keck and VLT

The spatial distribution of the tropospheric methane on Titan was measured using near-infrared spectroscopy. Ground-based observations at 1.5$μ{\rm m}$ (H-band) were performed during the same night using instruments with adaptive optics at both the W. M. Keck Observatory and at the Paranal Observatory on 17 July 2014 UT. The integral field observations with SINFONI on the VLT covered the entire H-band at moderate resolving power, $R=λ/Δλ\approx1,500$, while the Keck observations were performed with NIRSPAO near 1.55254$μ{\rm m}$ at higher resolution, $R\approx25,000$. The moderate resolution observations are used for flux calibration and for the determination of model parameters that can be degenerate in the interpretation of high resolution spectra. Line-by-line calculations of CH$_4$ and CH$_3$D correlated $k$ distributions from the HITRAN 2012 database were used, which incorporate revised line assignments near 1.5$μ{\rm m}$. We fit the surface albedo and aerosol distributions in the VLT SINFONI observations that cover the entire H-band window and used these quantities to constrain the models of the high-resolution Keck NIRSPAO spectra when retrieving the methane abundances. Cassini VIMS images of the polar regions, acquired on 20 July 2014 UT, are used to validate the assumption that the opacity of tropospheric aerosol is relatively uniform below 10 km. We retrieved methane abundances at latitudes between 42$^{\circ}$ S and 80$^{\circ}$ N. The tropospheric methane in the Southern mid-latitudes was enhanced by a factor of $\sim$10--40% over the nominal profile that was measured using the GCMS on Huygens. The Northern hemisphere had $\sim$90% of the nominal methane abundance up to polar latitudes (80$^{\circ}$N). These measurements suggest that a source of saturated polar air is equilibrating with dryer conditions at lower latitudes.

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arXiv2014

Simulations of Titan's paleoclimate

We investigate the effects of varying Saturn's orbit on the atmospheric circulation and surface methane distribution of Titan. Using a new general circulation model of Titan's atmosphere, we simulate its climate under four characteristic configurations of orbital parameters that correspond to snapshots over the past 42 kyr, capturing the amplitude range of long-period cyclic variations in eccentricity and longitude of perihelion. The model, which covers pressures from the surface to 0.5 mbar, reproduces the present-day temperature profile and tropospheric superrotation. In all four simulations, the atmosphere efficiently transports methane poleward, drying out the low- and mid-latitudes, indicating that these regions have been desert-like for at least tens of thousands of years. Though circulation patterns are not significantly different, the amount of surface methane that builds up over either pole strongly depends on the insolation distribution; in the present-day, methane builds up preferentially in the north, in agreement with observations, where summer is milder but longer. The same is true, to a lesser extent, for the configuration 14 kyr ago, while the south pole gains more methane in the case for 28 kyr ago, and the system is almost symmetric 42 kyr ago. This confirms the hypothesis that orbital forcing influences the distribution of surface liquids, and that the current observed asymmetry could have been partially or fully reversed in the past. The evolution of the orbital forcing implies that the surface reservoir is transported on timescales of $\sim$30 kyr, in which case the asymmetry reverses with a period of $\sim$125 kyr. Otherwise, the orbital forcing does not produce a net asymmetry over longer timescales, and is not a likely mechanism for generating the observed dichotomy.

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