356 research outputs found
Lightning and charge processes in brown dwarf and exoplanet atmospheres
The study of the composition of brown dwarf atmospheres helped to understand
their formation and evolution. Similarly, the study of exoplanet atmospheres is
expected to constrain their formation and evolutionary states. We use results
from 3D simulations, kinetic cloud formation and kinetic ion-neutral chemistry
to investigate ionisation processes which will affect their atmosphere
chemistry: The dayside of super-hot Jupiters is dominated by atomic hydrogen,
and not HO. Such planetary atmospheres exhibit a substantial degree of
thermal ionisation and clouds only form on the nightside where lightning leaves
chemical tracers (e.g. HCN) for possibly long enough to be detectable. External
radiation may cause exoplanets to be enshrouded in a shell of highly ionised,
H-forming gas and a weather-driven aurora may emerge. Brown dwarfs enable
us to study the role of electron beams for the emergence of an extrasolar,
weather-system driven aurora-like chemistry, and the effect of strong magnetic
fields on cold atmospheric gases. Electron beams trigger the formation of
H in the upper atmosphere of a brown dwarf (e.g. LSR-J1835) which may
react with it to form hydronium, HO, as a longer lived chemical tracer.
Brown dwarfs and super-hot gas giants may be excellent candidates to search for
HO as an H product.Comment: 16 pages, accepted for publication in the Philosophical Transactions
A of the Royal Society, (some typos corrected
Small hydrocarbon molecules in cloud-forming Brown Dwarf and giant gas planet atmospheres
We study the abundances of complex carbon-bearing molecules in the
oxygen-rich dust- forming atmospheres of Brown Dwarfs and giant gas planets.
The inner atmospheric re- gions that form the inner boundary for thermochemical
gas-phase models are investigated. Results from Drift-phoenix atmosphere
simulations, which include the feedback of phase- non-equilibrium dust cloud
formation on the atmospheric structure and the gas-phase abun- dances, are
utilised. The resulting element depletion leads to a shift in the
carbon-to-oxygen ratio such that several hydrocarbon molecules and
cyanopolycyanopolyynene molecules can be present. An increase in surface
gravity and/or a decrease in metallicity support the increase in the partial
pressures of these species. CO, CO2, CH4, and HCN contain the largest fraction
of carbon. In the upper atmosphere of low-metallicity objects, more carbon is
contained in C4H than in CO, and also CH3 and C2H2 play an increasingly
important role as carbon-sink. We determine chemical relaxation time-scales to
evaluate if hydrocarbon molecules can be affected by transport-induced
quenching. Our results suggest that a considerable amount of C2H6 and C2H2
could be expected in the upper atmospheres not only of giant gas planets, but
also of Brown Dwarfs. However, the exact quenching height strongly depends on
the data source used. These results will have an impact on future
thermo-kinetic studies, as they change the inner boundary condition for those
simulations.Comment: 17 pages, 13 figures, 1 table, accepted to MNRA
Exo-lightning radio emission: the case study of HAT-P-11b
Lightning induced radio emission has been observed on solar system planets.
Lecavelier des Etangs et al. [2013] carried out radio transit observations of
the exoplanet HAT-P-11b, and suggested a tentative detection of a radio signal.
Here, we explore the possibility of the radio emission having been produced by
lightning activity on the exoplanet, following and expanding the work of
Hodos\'an et al. [2016a]. After a summary of our previous work [Hodos\'an et
al. 2016a], we extend it with a parameter study. The lightning activity of the
hypothetical storm is largely dependent on the radio spectral roll-off, ,
and the flash duration, . The best-case scenario would
require a flash density of the same order of magnitude as can be found during
volcanic eruptions on Earth. On average, times larger flash
densities than the Earth-storms with the largest lightning activity is needed
to produce the observed signal from HAT-P-11b. Combined with the results of
Hodos\'an et al. [2016a] regarding the chemical effects of planet-wide
thunderstorms, we conclude that future radio and infrared observations may lead
to lightning detection on planets outside the solar system.Comment: Accepted to the Conference Proceedings of the 8th International
Workshop on Planetary, Solar and Heliospheric Radio Emissions (PRE 8), held
in Seggauberg near Leibnitz/Graz, Austria, October 25-27, 2016. 12 pages, 2
figure
Jupiter as a Giant Cosmic Ray Detector
We explore the feasibility of using the atmosphere of Jupiter to detect
Ultra-High-Energy Cosmic Rays (UHECR's). The large surface area of Jupiter
allows us to probe cosmic rays of higher energies than previously accessible.
Cosmic ray extensive air showers in Jupiter's atmosphere could in principle be
detected by the Large Area Telescope (LAT) on the Fermi observatory. In order
to be observed, these air showers would need to be oriented toward the Earth,
and would need to occur sufficiently high in the atmosphere that the gamma rays
can penetrate. We demonstrate that, under these assumptions, Jupiter provides
an effective cosmic ray "detector" area of km. We predict
that Fermi-LAT should be able to detect events of energy eV with
fluence erg cm at a rate of about one per month. The observed
number of air showers may provide an indirect measure of the flux of cosmic
rays eV. Extensive air showers also produce a synchrotron
signature that may be measurable by ALMA. Simultaneous observations of Jupiter
with ALMA and Fermi-LAT could be used to provide broad constraints on the
energies of the initiating cosmic rays.Comment: 8 pages, 5 figures. Accepted for publication in the Astrophysical
Journal Letter
Mediating exposure in public interactions
Mobile computing and public interactions together open
up a new range of challenges in interaction design. To
date a very gregarious model of interaction has been
assumed. However, the public setting will invoke feelings
of shyness and a desire to control the personal exposure
associated with interactions. In this paper we discuss
these issues and our initial tests of a system which affords
a control beyond "engage or don't engage"
Nitrogen Oxide Concentrations in Natural Waters on Early Earth
A key challenge in origins-of-life studies is estimating the abundances of
species relevant to the chemical pathways proposed to have contributed to the
emergence of life on early Earth. Dissolved nitrogen oxide anions
(NO), in particular nitrate (NO) and nitrite
(NO), have been invoked in diverse origins-of-life chemistry, from
the oligomerization of RNA to the emergence of protometabolism. Recent work has
calculated the supply of NO from the prebiotic atmosphere to the
ocean, and reported steady-state [NO] to be high across all plausible
parameter space. These findings rest on the assumption that NO is
stable in natural waters unless processed at a hydrothermal vent. Here, we show
that NO is unstable in the reducing environment of early Earth. Sinks
due to UV photolysis and reactions with reduced iron (Fe) suppress
[NO] by several orders of magnitude relative to past predictions. For
pH and C, we find that it is most probable that
NO]M in the prebiotic ocean. On the other hand, prebiotic
ponds with favorable drainage characteristics may have sustained
[NO]M. As on modern Earth, most NO on prebiotic
Earth should have been present as NO, due to its much greater
stability. These findings inform the kind of prebiotic chemistries that would
have been possible on early Earth. We discuss the implications for proposed
prebiotic chemistries, and highlight the need for further studies of
NO kinetics to reduce the considerable uncertainties in predicting
[NO] on early Earth.Comment: In review for publication at Geochemistry, Geophysics, and Geosystems
(G-cubed). Comments, questions, and criticism solicited; please contact
corresponding author at [email protected]. SI at:
https://web-cert.mit.edu/sukrit/Public/nox_si.pdf. GitHub at:
https://github.com/sukritranjan/no
Can comets deliver prebiotic molecules to rocky exoplanets?
In this work we consider the potential of cometary impacts to deliver complex
organic molecules and the prebiotic building blocks required for life to rocky
exoplanets. Numerical experiments have demonstrated that for these molecules to
survive, impacts at very low velocities are required. This work shows that for
comets scattered from beyond the snow-line into the habitable zone, the minimum
impact velocity is always lower for planets orbiting Solar-type stars than
M-dwarfs. Using both an analytical model and numerical N-body simulations, we
show that the lowest velocity impacts occur onto planets in tightly-packed
planetary systems around high-mass (i.e. Solar-mass) stars, enabling the intact
delivery of complex organic molecules. Impacts onto planets around low-mass
stars are found to be very sensitive to the planetary architecture, with the
survival of complex prebiotic molecules potentially impossible in
loosely-packed systems. Rocky planets around M-dwarfs also suffer significantly
more high velocity impacts, potentially posing unique challenges for life on
these planets. In the scenario that cometary delivery is important for the
origins of life, this study predicts the presence of biosignatures will be
correlated with i) decreasing planetary mass (i.e. escape velocity), ii)
increasing stellar-mass, and iii) decreasing planetary separation (i.e.
exoplanets in tightly-packed systems).Comment: Accepted by Proceedings A of the Royal Society. 17 pages, 5 figure
Prebiosignature Molecules Can Be Detected in Temperate Exoplanet Atmospheres with JWST
The search for biosignatures on exoplanets connects the fields of biology and
biochemistry to astronomical observation, with the hope that we might detect
evidence of active biological processes on worlds outside the solar system.
Here we focus on a complementary aspect of exoplanet characterisation
connecting astronomy to prebiotic chemistry: the search for molecules
associated with the origin of life, prebiosignatures. Prebiosignature surveys
in planetary atmospheres offer the potential to both constrain the ubiquity of
life in the galaxy and provide important tests of current prebiotic syntheses
outside of the laboratory setting. Here, we quantify the minimum abundance of
identified prebiosignature molecules that would be required for detection by
transmission spectroscopy using JWST. We consider prebiosignatures on five
classes of terrestrial planet: an ocean planet, a volcanic planet, a
post-impact planet, a super-Earth, and an early Earth analogue. Using a novel
modelling and detection test pipeline, with simulated JWST noise, we find the
detection thresholds of hydrogen cyanide (HCN), hydrogen sulfide (H2S),
cyanoacetylene (HC3N), ammonia (NH3), methane (CH4), acetylene (C2H2), sulfur
dioxide (SO2), nitric oxide (NO), formaldehyde (CH2O), and carbon monoxide (CO)
in a variety of low mean molecular weight (<5) atmospheres. We test the
dependence of these detection thresholds on M dwarf target star and the number
of observed transits, finding that a modest number of transits (1-10) are
required to detect prebiosignatures in numerous candidate planets, including
TRAPPIST-1e with a high mean molecular weight atmosphere. We find that the
NIRSpec G395M/H instrument is best suited for detecting most prebiosignatures.Comment: 28 pages, 12 figures, accepted for publication in A
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