UCLA

Much of the martian surface is covered by dunes, ripples, and other features formed by saltation, the blowing of sand by wind. In addition, Mars’ atmosphere is loaded with large quantities of mineral dust, much of which is likely emitted by saltation occurring in the ubiquitous Martian dust storms and dust devils. Yet, despite the obvious importance of sand transport to shaping the climate and surface of Mars, several enigmatic questions remain regarding the occurrence and properties of martian saltation. Foremost among these is the puzzling finding that both lander measurements and atmospheric circulation models indicate that wind speeds on Mars rarely exceed the threshold wind speed required to initiate saltation. How then does sand transport appear to be so common on Mars? Furthermore, bedforms observed by rovers contain particles so small that they should be easily suspended by turbulence and thus would be unable to form bedforms. Finally, the minimal size of martian dunes is over an order of magnitude smaller than would be expected from scaling up terrestrial analog dunes. Starting from the basic physics of saltation, this talk provides some possible solutions to these varied mysteries surrounding sand movement on Mars.

Up through the early 21^st century, the rate of dune and ripple movement on Mars, or whether activity occurred at all, was an open question. On the one hand, Viking Lander meteorology measurements and climate models indicated that winds sufficient to move sand were rare in the low density Martian atmosphere. In contrast, sand dunes, yardangs, and enigmatic mega-ripples were identified as common features, indicating that, over geologic time, sand activity had occurred. Because Mars has undergone dramatic climate shifts, causing atmospheric densities and wind speeds to change, a related question was whether any activity was current, or was relegated to the past. The previous half decade has seen a revolution in our understanding of Martian aeolian processes due to advances in spacecraft instrumentation, analytical techniques, numerical saltatation models, and studies of applicable terrestrial analogs. These investigations show that both views of Mars are true: Dunes in many regions are active, with mass fluxes comparable to some regions on Earth. In contrast, other dunes and, most notably, large megaripples, are static.

This talk summarizes this story, focusing in on two studies by the presenter, namely the quantification of dune migration rates and fluxes, and the formation of megaripples using a terrestrial analog site in Argentina. It concludes that the two views of aeolian Mars are reconcilable. Active dunes with significant sand fluxes are common where the sand supply is fresh. Megaripples form through accumulation of coarse grains through sand impact creep, commonly on pre-existing topography, and then become static through formation of a desert pavement-like surface. By considering the energetics of saltation on Mars, both processes occur today, although activity has waxed and waned under different climatic conditions. Mars is an active aeolian planet.

 

While organics and water are among the critical components for life on Earth, carbon containing molecules and water co-exist on many cold bodies in our solar system and on the prestellar (interstellar) ice grains as well. How their journey from these tiny grains in the interstellar medium leads them through the solar system, culminating in comet and asteroid impacts on early Earth – that could have delivered prebiotic materials and triggered evolution of life on Earth – is the focus of Murthy’s research activity at JPL.

 

In this talk, recent research activities at the “Ice Spectroscopy Lab, ISL” at JPL will be discussed. These include new photochemical pathways in Titan’s lower atmosphere [Gudipati, Jacovi et al. 2013], survival depths of organic materials beneath Europa’s surface under electron radiation [Barnett, Lignell et al. 2012], and production of functionalized complex organics in interstellar and cometary ice grains subjected to UV and electron radiation [Gudipati and Yang 2012].

 

 

Acknowledgments:

NASA’s funding through Astrobiology Institute (Titan, a model prebiotic system; Icy Worlds, and Early Habitable Environments), Cassini Data Analysis and Planetary Atmospheres Programs enabled this research, which was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.

 

References:

Barnett, I. L., A. Lignell, et al. (2012). “SURVIVAL DEPTH OF ORGANICS IN ICES UNDER LOW-ENERGY ELECTRON RADIATION (<= 2 keV).” Astrophysical Journal 747(1): L24.

Gudipati, M. S., R. Jacovi, et al. (2013). “Photochemical activity of Titan’s low-altitude condensed haze.” Nature Communications 4: 1648.

Gudipati, M. S. and R. Yang (2012). “In-situ Probing of Radiation-induced Processing of Organics in Astrophysical Ice Analogs: Novel Laser Desorption Laser Ionization Time-of-flight Mass Spectroscopic Studies.” The Astrophysical Journal Letters 756(1): L24.

 

Since 1998, under a congressional mandate, NASA has supported several surveys to identify and track Near Earth Objects (NEOs) and the effort continues today to find the smaller potentially hazardous NEOs capable of regional destruction. NEOs are usually found by virtue of their motion against the background stars. All the telescopes currently used for NEO survey were designed for other purposes, and are therefore not optimized for moving objects. We will discuss what makes a survey successful and some of the surprise discoveries by the Catalina Sky Survey.

With Earth-like size and density, Venus should be Earth’s twin. Instead, it lacks plate tectonics and is shrouded by a hot, dense atmosphere with a run away greenhouse. The dramatic divergence of such initially similar terrestrial planets holds lessons for predicting exoplanet behavior. The history of volatiles is clearly central to understanding climate, and also has a major effect on interior evolution. Venus’ atmosphere has lost significant water, yet the interior may have more water than Earth. What are the constraints on water in the interior? Has lithospheric recycling and thus possible volatile recycling occurred? The initiation of subduction is both the gateway to plate tectonics and a key link between interior convection and lithospheric rheology. Numerous potential subduction sites have been identified on Venus. Most of these zones occur in association with corona (possible small-scale mantle upwelling features) and extensional zones. In this talk I will discuss constraints on water in the interior, the evidence for (and against) subduction, and a ‘new’ model for understanding the link between plumes and possible subduction on Venus. This interpretation is based on 1) laboratory fluid dynamics experiments that couple convection and lithospheric deformation and 2) evidence for current volcanism, plumes and possible subduction from gravity, altimetry, radar images and surface emissivity.