Geology topics

O. Aharonson

Publications and source records attributed to O. Aharonson.

7 recordsLinked to original sources

USGS2021

Solar-system-wide significance of Mars polar science

1. The North Polar Layered Deposits contain thousands of ice layers that record accumulation and climate history for at least several million years, making the most accessible and most complete climate record aside from the Earth’s – and the only one to record the impact of large obliquity shifts. 2. Mars Polar Science is a diverse and integrated system spanning much of the planet, above and below the surface: geographic poles, mid- and high-latitudes, numerous active volatile processes that interact with the near-surface, and the lower and middle atmosphere circulations, even influencing the upper atmosphere and atmospheric escape. 3. Mars polar processes serves as an analogue and scientific link to numerous solar system bodies. Mars has two volatile cycles that influence surface processes, atmospheric processes, and climate cycles. Mars, with many data sets and observations, serves as a proxy for outer planets and moons that undergo the same physics but with other volatile species. Additionally, technology designed for Mars polar exploration benefits exploration elsewhere. 4. Mars Polar and Atmospheric Science results are critical for future human exploration and in situ resources and the study of planetary habitability.

Bulletin of the AAS↗

USGS2011

Transient surface liquid in Titan's south polar region from Cassini

Cassini RADAR images of Titan’s south polar region acquired during southern summer contain lake features which disappear between observations. These features show a tenfold increases in backscatter cross-section between images acquired one year apart, which is inconsistent with common scattering models without invoking temporal variability. The morphologic boundaries are transient, further supporting changes in lake level. These observations are consistent with the exposure of diffusely scattering lakebeds that were previously hidden by an attenuating liquid medium. We use a two-layer model to explain backscatter variations and estimate a drop in liquid depth of approximately 1-m-per-year. On larger scales, we observe shoreline recession between ISS and RADAR images of Ontario Lacus, the largest lake in Titan’s south polar region. The recession, occurring between June 2005 and July 2009, is inversely proportional to slopes estimated from altimetric profiles and the exponential decay of near-shore backscatter, consistent with a uniform reduction of 4 ± 1.3 m in lake depth. Of the potential explanations for observed surface changes, we favor evaporation and infiltration. The disappearance of dark features and the recession of Ontario’s shoreline represents volatile transport in an active methane-based hydrologic cycle. Observed loss rates are compared and shown to be consistent with available global circulation models. To date, no unambiguous changes in lake level have been observed between repeat images in the north polar region, although further investigation is warranted. These observations constrain volatile flux rates in Titan’s hydrologic system and demonstrate that the surface plays an active role in its evolution. Constraining these seasonal changes represents the first step toward our understanding of longer climate cycles that may determine liquid distribution on Titan over orbital time periods.

Icarus↗

USGS2010

Bathymetry and absorptivity of Titan's Ontario Lacus

Ontario Lacus is the largest and best characterized lake in Titan's south polar region. In June and July 2009, the Cassini RADAR acquired its first Synthetic Aperture Radar (SAR) images of the area. Together with closest approach altimetry acquired in December 2008, these observations provide a unique opportunity to study the lake's nearshore bathymetry and complex refractive properties. Average radar backscatter is observed to decrease exponentially with distance from the local shoreline. This behavior is consistent with attenuation through a deepening layer of liquid and, if local topography is known, can be used to derive absorptive dielectric properties. Accordingly, we estimate nearshore topography from a radar altimetry profile that intersects the shoreline on the East and West sides of the lake. We then analyze SAR backscatter in these regions to determine the imaginary component of the liquid's complex index of refraction (κ). The derived value, κ = (6.1 −1.3 +1.7 ) × 10 −4 , corresponds to a loss tangent of tan Δ = (9.2 −2.0 +2.5 ) × 10 −4 and is consistent with a composition dominated by liquid hydrocarbons. This value can be used to test compositional models once the microwave optical properties of candidate materials have been measured. In areas that do not intersect altimetry profiles, relative slopes can be calculated assuming the index of refraction is constant throughout the liquid. Accordingly, we construct a coarse bathymetry map for the nearshore region by measuring bathymetric slopes for eleven additional areas around the lake. These slopes vary by a factor of ∼5 and correlate well with observed shoreline morphologies.

Journal of Geophysical Research E: Planets↗

USGS2010

Active shoreline of Ontario Lacus, Titan: A morphological study of the lake and its surroundings

Of more than 400 filled lakes now identified on Titan, the first and largest reported in the southern latitudes is Ontario Lacus, which is dark in both infrared and microwave. Here we describe recent observations including synthetic aperture radar (SAR) images by Cassini's radar instrument (??= 2 cm) and show morphological evidence for active material transport and erosion. Ontario Lacus lies in a shallow depression, with greater relief on the southwestern shore and a gently sloping, possibly wave-generated beach to the northeast. The lake has a closed internal drainage system fed by Earth-like rivers, deltas and alluvial fans. Evidence for active shoreline processes, including the wave-modified lakefront and deltaic deposition, indicates that Ontario is a dynamic feature undergoing typical terrestrial forms of littoral modification. Copyright ?? 2010 by the American Geophysical Union.

Geophysical Research Letters↗

USGS2008

Hydrocarbon lakes on Titan: Distribution and interaction with a porous regolith

Synthetic Aperture Radar (SAR) images of Titan's north polar region reveal quasi‐circular to complex features which are interpreted to be liquid hydrocarbon lakes. We investigate methane transport in Titan's hydrologic cycle using the global distribution of lake features. As of May 2007, the SAR data set covers ∼22% of the surface and indicates multiple lake morphologies which are correlated across the polar region. Lakes are limited to latitudes above 55°N and vary from <10 to more than 100,000 km 2 . The size and location of lakes provide constraints on parameters associated with subsurface transport. Using porous media properties inferred from Huygens probe observations, timescales for flow into and out of observed lakes are shown to be in the tens of years, similar to seasonal cycles. Derived timescales are compared to the time between collocated SAR observations in order to consider the role of subsurface transport in Titan's hydrologic cycle.

Geophysical Research Letters↗

USGS2007

Pyroclastic activity at home plate in Gusev crater, Mars

Home Plate is a layered plateau in Gusev crater on Mars. It is composed of clastic rocks of moderately altered alkali basalt composition, enriched in some highly volatile elements. A coarse-grained lower unit lies under a finer-grained upper unit. Textural observations indicate that the lower strata were emplaced in an explosive event, and geochemical considerations favor an explosive volcanic origin over an impact origin. The lower unit likely represents accumulation of pyroclastic materials, whereas the upper unit may represent eolian reworking of the same pyroclastic materials.

Science↗

USGS2005

New Perspectives on Ancient Mars

Mars was most active during its first billion years. The core, mantle, and crust formed within ∼50 million years of solar system formation. A magnetic dynamo in a convecting fluid core magnetized the crust, and the global field shielded a more massive early atmosphere against solar wind stripping. The Tharsis province became a focus for volcanism, deformation, and outgassing of water and carbon dioxide in quantities possibly sufficient to induce episodes of climate warming. Surficial and near-surface water contributed to regionally extensive erosion, sediment transport, and chemical alteration. Deep hydrothermal circulation accelerated crustal cooling, preserved variations in crustal thickness, and modified patterns of crustal magnetization.

Science↗