Kevin Zahnle's research while affiliated with NASA and other places
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Publications (292)
The origin of life on Earth would benefit from a prebiotic atmosphere that produced nitriles, like HCN, which enable ribonucleotide synthesis. However, geochemical evidence suggests that Hadean air was relatively oxidizing with negligible photochemical production of prebiotic molecules. These paradoxes are resolved by iron-rich asteroid impacts tha...
Previous studies of cometary impacts in the outer solar system used the spatial distribution of ecliptic comets (ECs) from dynamical models that assumed ECs began on low-inclination orbits (≲5°) in the Kuiper Belt. In reality, the source population of ECs—the trans-Neptunian scattered disk—has orbital inclinations reaching up to ∼30°. In Nesvorný e...
Previous studies of cometary impacts in the outer Solar System used the spatial distribution of ecliptic comets (ECs) from dynamical models that assumed ECs began on low-inclination orbits (<5 deg) in the Kuiper belt. In reality, the source population of ECs - the trans-Neptunian scattered disk - has orbital inclinations reaching up to ~30 deg. In...
The origin of life on Earth would benefit from a prebiotic atmosphere that produced nitriles, like HCN, which enable ribonucleotide synthesis. However, geochemical evidence suggests that Hadean air was relatively oxidizing with negligible photochemical production of prebiotic molecules. These paradoxes are resolved by iron-rich asteroid impacts tha...
The Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging (DAVINCI) mission described herein has been selected for flight to Venus as part of the NASA Discovery Program. DAVINCI will be the first mission to Venus to incorporate science-driven flybys and an instrumented descent sphere into a unified architecture. The anticipated...
The impact heating hypothesis has been explored as a means of warming early Mars and inducing rainfall through the potential injection of water, energy, and reducing greenhouse gases to the atmosphere. We simulate H2-rich post impact scenarios with the 3D NASA Ames legacy early Mars Global Climate Model (eMGCM) for 100-km and 250-km diameter impact...
The Great Oxidation Event (GOE), arguably the most important event to occur on Earth since the origin of life, marks the time when an oxygen-rich atmosphere first appeared. However, it is not known whether the change was abrupt and permanent or fitful and drawn out over tens or hundreds of millions of years. Here, we developed a one-dimensional tim...
The Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging (DAVINCI) mission described herein has been selected for flight to Venus as part of the NASA Discovery Program. DAVINCI will be the first mission to Venus to incorporate science-driven flybys and an instrumented descent sphere into a unified architecture. The anticipated...
Trapped, paleoatmospheric xenon (Xe) in the Martian regolith breccia NWA 11220 is mass-dependently fractionated relative to solar Xe by 16.2 ± 2.7‰/amu. These data indicate that fractionation of atmospheric Xe persisted for hundreds of millions of years after planetary formation. Such a protracted duration of atmospheric Xe mass fractionation, whic...
Mars today has no active volcanism and its atmosphere is oxidizing, dominated by the photochemistry of CO2 and H2O. Using a one-dimensional photochemical model, we consider whether plausible volcanic gas fluxes could have switched the redox-state of the past martian atmosphere to reducing conditions. In our model, the total quantity and proportions...
We present a new scaling law to predict the loss of atmosphere from planetary collisions for any speed, angle, impactor mass, target mass, and body compositions, in the regime of giant impacts onto broadly terrestrial planets with relatively thin atmospheres. To this end, we examine the erosion caused by a wide range of impacts, using 3D smoothed p...
The origin of life on Earth seems to demand a highly reduced early atmosphere, rich in CH 4 , H 2 , and NH 3 , but geological evidence suggests that Earth's mantle has always been relatively oxidized and its emissions dominated by CO 2 , H 2 O, and N 2 . The paradox can be resolved by exploiting the reducing power inherent in the “late veneer,” i.e...
The atmosphere of the Archean eon—one-third of Earth’s history—is important for understanding the evolution of our planet and Earth-like exoplanets. New geological proxies combined with models constrain atmospheric composition. They imply surface O 2 levels <10 ⁻⁶ times present, N 2 levels that were similar to today or possibly a few times lower, a...
We report new elastic and inelastic cross-sections for O(3P) + CO2 scattering at collision energies from 0.03 to 5 eV, of major importance to O escape from Mars, Venus, and CO2-rich atmospheres. The cross-sections were calculated from first principles using three newly constructed ab initio potential energy surfaces correlating to the lowest energy...
The origin of life on Earth seems to demand a highly reduced early atmosphere, rich in CH4, H2, and NH3, but geological evidence suggests that Earth's mantle has always been relatively oxidized and its emissions dominated by CO2, H2O, and N2. The paradox can be resolved by exploiting the reducing power inherent in the "late veneer," i.e., material...
Impacts on early Mars can produce H2 and CH4 in the thermal plume. In a thick CO2 atmosphere, collision‐induced absorptions between CO2‐H2 and CO2‐CH4 can boost the greenhouse effect. We construct a simple model of the impact history of Mars and show that for a variety of impactor types and CO2 surface pressures >0.5 bars, postimpact surface temper...
The current goals of the astrobiology community are focused on developing a framework for the detection of biosignatures, or evidence thereof, on objects inside and outside of our solar system. A fundamental aspect of understanding the limits of habitable environments (surface liquid water) and detectable signatures thereof is the study of where th...
The current goals of the astrobiology community are focused on developing a framework for the detection of biosignatures, or evidence thereof, on objects inside and outside of our solar system. A fundamental aspect of understanding the limits of habitable environments (surface liquid water) and detectable signatures thereof is the study of where th...
We report new elastic and inelastic cross sections for O($^3P$)+CO$_2$ scattering at collision energies from 0.03 to 5 eV, of major importance to O escape from Mars, Venus, and CO$_2$-rich atmospheres. The cross sections were calculated from first principles using three newly constructed ab-initio potential energy surfaces correlating to the lowest...
Tidal heating is key to the evolution and habitability of many worlds across our solar system and beyond. However, there remain fundamental gaps in our understanding of tidal heating and coupled orbital evolution, which motivated a Keck Institute for Space Studies (KISS) workshop on this topic. The Cassini mission has led to many recent results abo...
We present an atmospheric transmission spectrum for the ultra-hot Jupiter WASP-121b, measured using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. Across the 0.47–1 wavelength range, the data imply an atmospheric opacity comparable to—and in some spectroscopic channels exceeding—that previously measured at near-infrar...
We present an atmospheric transmission spectrum for the ultra-hot Jupiter WASP-121b, measured using the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST). Across the 0.47-1 micron wavelength range, the data imply an atmospheric opacity comparable to - and in some spectroscopic channels exceeding - that previously...
Atmospheric xenon is strongly mass fractionated, the result of a process that apparently continued through the Archean and perhaps beyond. Previous models that explain Xe fractionation by hydrodynamic hydrogen escape cannot gracefully explain how Xe escaped when Ar and Kr did not, nor allow Xe to escape in the Archean. Here we show that Xe is the o...
Atmospheric xenon is strongly mass fractionated, the result of a process that apparently continued through the Archean and perhaps beyond. Previous models that explain Xe fractionation by hydrodynamic hydrogen escape cannot gracefully explain how Xe escaped when Ar and Kr did not, nor allow Xe to escape in the Archean. Here we show that Xe is the o...
We have analyzed ancient atmospheric gases trapped in fluid inclusions contained in minerals of Archean (3.3 Ga) to Paleozoic (404 Ma) rocks in an attempt to document the evolution of the elemental composition and isotopic signature of the atmosphere with time. Doing so, we aimed at understanding how physical and chemical processes acted over geolo...
We perform a suite of smoothed particle hydrodynamics simulations to investigate in detail the results of a giant impact on the young Uranus. We study the internal structure, rotation rate, and atmospheric retention of the post-impact planet, as well as the composition of material ejected into orbit. Most of the material from the impactor's rocky c...
Humanity has long been fascinated by the planet Mars. Was its climate ever conducive to life? What is the atmosphere like today and why did it change so dramatically over time? Eleven spacecraft have successfully flown to Mars since the Viking mission of the 1970s and early 1980s. These orbiters, landers and rovers have generated vast amounts of da...
The gas giant planets in the Solar System have a retinue of icy moons, and we expect giant exoplanets to have similar satellite systems. If a Jupiter-like planet were to migrate toward its parent star the icy moons orbiting it would evaporate, creating atmospheres and possible habitable surface oceans. Here, we examine how long the surface ice and...
Mars today has no active volcanism and its atmosphere is oxidizing, dominated by the photochemistry of CO2 and H2O. Mars experienced widespread volcanism in the past and volcanic emissions should have included reducing gases, such as H2 and CO, as well as sulfur-bearing gases. Using a one-dimensional photochemical model, we consider whether plausib...
The planets of the Solar System divide neatly between those with atmospheres and those without when arranged by insolation ($I$) and escape velocity ($v_{\mathrm{esc}}$). The dividing line goes as $I \propto v_{\mathrm{esc}}^4$. Exoplanets with reported masses and radii are shown to crowd against the extrapolation of the Solar System trend, making...
Recent work has shown that sulfur hazes may arise in the atmospheres of giant exoplanets due to the photolysis of H$_{2}$S. We investigate the impact such a haze would have on an exoplanet's geometric albedo spectrum using a suite of established radiative-convective, cloud, and albedo models, and how it may affect the direct imaging results of WFIR...
We assess the fates of ejecta from the large craters Aeneas on Dione and Ali Baba on Enceladus (161 and 39 km in diameter, respectively), as well as that from Herschel (130 km in diameter) on Mimas. The ejecta are treated either as ‘spalls’ launched from hard surfaces, or as ‘rubble’ launched from a weak rubble pile regolith. Once in orbit we consi...
With the advent of exoplanetary astronomy and the ongoing discovery of terrestrial-type planets around other stars, our own solar system becomes a key testing ground for ideas about what factors control planetary evolution. In particular, what allows a planet to be both within a potentially habitable zone and sustain habitability over long geologic...
The past decade has seen significant progress on the direct detection and characterization of young, self-luminous giant planets at wide orbital separations from their host stars. Some of these planets show evidence for disequilibrium processes like transport-induced quenching in their atmospheres; photochemistry may also be important, despite the...
Ashes of ancient meteors recovered from a 2.7-billion-year-old lake bed imply that the upper atmosphere was rich in oxygen at a time when all other evidence implies that the atmosphere was oxygen-free. See Letter p.235
We use a 1D model to address photochemistry and possible haze formation in the irradiated warm Jupiter, 51 Eridani b. The intended focus was to be carbon, but sulfur photochemistry turns out to be important. The case for organic photochemical hazes is intriguing but falls short of being compelling. If organic hazes form, they are likeliest to do so...
Reflected light spectroscopy and photometry of cool, directly imaged extrasolar giant planets are expected to be performed in the next decade by space-based telescopes equipped with optical wavelength coronagraphs and integral field spectrographs, such as the Wide-Field Infrared Survey Telescope (WFIRST). We are developing a new atmospheric retriev...
Planets larger than Earth and smaller than Neptune are some of the most numerous in the galaxy, but observational efforts to understand this population have proved challenging because optically thick clouds or hazes at high altitudes obscure molecular features. We present models of super Earths that include thick clouds and hazes and predict their...
Planets larger than Earth and smaller than Neptune are some of the most
numerous in the galaxy, but observational efforts to understand this population
have proved challenging because optically thick clouds or hazes at high
altitudes obscure molecular features (Kreidberg et al. 2014b). We present
models of super Earths that include thick clouds and...
Two habitable planetary states are proposed: an aqua planet like the Earth
and a land planet that has a small amount of water. Land planets keep liquid
water under larger solar radiation compared to aqua planets. Water loss may
change an aqua planet into a land planet, and the planet can remain habitable
for a longer time than had it stayed an aqua...
Here, we report on the findings obtained from our initial suite of 35 SPH simulations of the impact origin of the obliquity of Uranus.
Lasers shine new light on the drying of Mars while reviving the mystery of methane [Also see Reports by Mahaffy et al. and Webster et al. ]
We report Hubble Space Telescope optical to near-infrared transmission spectroscopy of the hot-Jupiter WASP-6b, measured with the Space Telescope Imaging
Spectrograph and Spitzer's InfraRed Array Camera. The resulting spectrum covers the range 0.29–4.5 μm. We find evidence for modest stellar activity
of WASP-6 and take it into account in the transm...
Much of the Earth's mantle was melted in the Moon-forming impact. Gases that were not partially soluble in the melt, such as water and CO2, formed a thick, deep atmosphere surrounding the post-impact Earth. This atmosphere was opaque to thermal radiation, allowing heat to escape to space only at the runaway greenhouse threshold of approximately 100...
We address disequilibrum abundances of some simple molecules in the
atmospheres of solar composition brown dwarfs and self-luminous extrasolar
giant planets using a kinetics-based 1D atmospheric chemistry model. We employ
cloudless atmospheres of approximately solar metallicity. Our approach is to
use the complete model to survey the parameter spac...
Oxygenic photosynthesis appears to be necessary for an oxygen-rich atmosphere like Earth's. However, available geological and geochemical evidence suggests that at least 200 m.y., and possibly as many as 700 m.y., elapsed between the advent of oxygenic photosynthesis and the establishment of an oxygen atmosphere. The interregnum implies that at lea...
Existence of liquid water on the planetary surface is thought to be an important condition for the origin and evolution of life. Planets with oceans (or lakes) are classified in two types: Earth-like `aqua planets' and less water `land planets'. The latter shows stronger resistance than the former to the runaway greenhouse caused by the increase of...
We present a new set of simulations of the origin of Uranus obliquity using the same method as Slattery et al. (1992) with mass resolution 1000 times better.
In extremely arid regions on Earth, such as the Atacama Desert, nitrate, sulfate and perchlorate salts form in the atmosphere and accumulate on the surface from dry deposition according to diagnostic evidence in their oxygen isotopes. Salts of similar oxyanions should have formed in the atmosphere of Mars because of comparable photochemical reactio...
We address the thermal history of the Earth after the Moon-forming impact, taking tidal heating and thermal blanketing by the atmosphere into account. The atmosphere sets an upper bound of ∼100 W/m2 on how quickly the Earth can cool. The liquid magma ocean cools over 2-10 Myr, with longer times corresponding to high angular-momentum events. Tidal h...
We ask what happened to Titan after the impacts came. A nominal Menrva heats the surface to ~170 K; it takes heroic assumptions to reach 273 K. Bigger impacts (e.g., putative Hotei impact) produce meltwater oceans that last for decades or centuries.
It is now understood that the accretion of terrestrial planets naturally
involves giant collisions, the moon-forming impact being a well known example.
In the aftermath of such collisions the surface of the surviving planet is very
hot and potentially detectable. Here we explore the atmospheric chemistry,
photochemistry, and spectral signatures of...
Oxygenic photosynthesis appears to be necessary for an oxygen-rich atmosphere like Earth's. But available geological and geochemical evidence suggest that at least 200 Myr, and possibly more than 700 Myr, elapsed between the advent of oxygenic photosynthesis and the establishment of an oxygen atmosphere. The interregnum implies that at least one ot...
We present Hubble Space Telescope near-infrared transmission spectroscopy of the transiting hot-Jupiter HAT-P-1b. We observed one transit with Wide Field Camera
3 using the G141 low-resolution grism to cover the wavelength range 1.087–1.678 μm. These time series observations were taken
with the newly available spatial-scan mode that increases the d...
Cosmic collisions between terrestrial planets resemble somewhat the life
cycle of the phoenix: worlds collide, are consumed in flame, and after
the debris has cleared, shiny new worlds emerge aglow with
possibilities. And glow they do, for they are molten. How brightly they
glow, and for how long, is determined by their atmospheres, and by their
mo...
We present an optical to near-infrared transmission spectrum of the hot
Jupiter HAT-P-1b, based on HST observations, covering the spectral regime from
0.29 to 1.027{\mu}m with STIS, which is coupled with a recent WFC3 transit
(1.087 to 1.687{\mu}m). We derive refined physical parameters of the HAT-P-1
system, including an improved orbital ephemeris...
We present HST optical transmission spectra of the transiting hot Jupiter
WASP-12b, taken with the STIS instrument. From the transmission spectra, we are
able to decisively rule out prominent absorption by TiO in the exoplanet's
atmosphere. Strong pressure-broadened Na and K absorption signatures are also
excluded, as are significant metal-hydride...
The atmospheres of terrestrial planets are expected to be in long-term radiation balance: an increase in the absorption of solar radiation warms the surface and troposphere, which leads to a matching increase in the emission of thermal radiation. Warming a wet planet such as Earth would make the atmosphere moist and optically thick such that only t...
We measure the transmission spectrum of WASP-19b from 3 transits using
low-resolution optical spectroscopy from the HST Space Telescope Imaging
Spectrograph (STIS). The STIS spectra cover a wavelength range of 0.29-1.03
microns with resolving power R=500. The optical data are combined with archival
near-IR data from the HST Wide Field Camera 3 (WFC...
We perform modeling investigations to aid in understanding the atmospheres
and composition of small planets of ~2-4 Earth radii, which are now known to be
common in our galaxy. GJ 1214b is a well studied example whose atmospheric
transmission spectrum has been observed by many investigators. Here we take a
step back from GJ 1214b to investigate the...
Recent observations of the super-Earth GJ 1214b show that it has a relatively
featureless transmission spectrum. One suggestion is that these observations
indicate that the planet's atmosphere is vertically compact, perhaps due to a
water-rich composition that yields a large mean molecular weight. Another
suggestion is that the atmosphere is hydrog...
We propose an impact erosion stability limit that determines the
presence or absence of atmospheres on exoplanets and solar system
bodies.
Titan’s thick atmosphere and volatile-rich surface cause it to respond to big impacts in a somewhat Earth-like manner. Here we construct a simple globally-averaged model that tracks the flow of energy through the environment in the weeks, years, and millenia after a big comet strikes Titan. The model Titan is endowed with 1.4 bars of N2N2 and 0.07...
Arising from F. Gaillard, B. Scaillet & N. T. Arndt Nature 478, 229–232(2011)
Around 2.5 billion years ago, Earth’s atmosphere turned from anoxic to oxic in what is known as the Great Oxidation Event. Gaillard et al.1 suggest that this oxygenation was caused by the emergence of the continents and a shift in volcanism from predominantly submarine t...
Venus’s descent into hellish heat must have been caused by its
proximity to the Sun. The story of how Venus lost its water becomes: too
much sunlight caused a runaway greenhouse effect, any water evaporated,
and the hydrogen escaped into space.
The ultimate and final end for Earth as a habitable planet will be the
transition to a "runaway greenhouse," an apocalypse triggered by failure
of the planet to maintain energy balance.
In 2008, NASA's Mars Phoenix Lander measured perchlorate (salts
containing ClO4-) concentrations of 0.6 wt%; an
amount that is larger than the highest natural concentrations found on
Earth [1,2]. However, the origin of Martian perchlorate is unknown.
Perchlorates are interesting because some forms are highly deliquescent
with very low eutectic temp...
Oxygenic photosynthesis appears to be a necessary condition for the
creation of an oxygen-rich atmosphere like Earth's. But available
geological and geochemical evidence suggests that oxygenic
photosynthesis appeared hundreds of millions of years before the
oxygen-rich atmosphere was fully established. The interregnum implies
that at least one othe...
Through the greenhouse effect, the presence and composition of an
atmosphere is critical for defining a (conventional) circumstellar
habitable zone in terms of planetary surface temperatures suitable for
liquid water. Lack of knowledge of planetary atmospheres is likely to
frustrate attempts to say with any certainty whether detected
terrestrial-si...