Laboratory Simulation of CO2 and H2O Frosts on Mars
National Aeronautics and Space Administration Solar System Workings Program
PSI Personnel
Non PSI Personnel: Adam Bruckner (Co-Investigator, University of Washington), Shane Byrne (Collaborator, University of Arizona)
Project Description
In this study, we propose to continue performing realistic laboratory simulations of the thermal and radiative environment at the surface of Mars, producing the first laboratory samples of carbon dioxide frost formed the same way it does on Mars, by radiative cooling in a nearly-pure CO2 atmosphere.
Improving knowledge of the radiative and physical properties of CO2 radiation frost is an important step towards understanding the polar caps, atmosphere, and climate of Mars. Previous Martian frost simulations have used conductive cooling (Kieffer, 1968); condensing CO2 onto a substrate bathed in liquid nitrogen. This technique favors the growth of grains with the best thermal contact, resulting in large grain sizes and a coarse texture. Terrestrial H2O frost formed by radiative cooling (hoarfrost) tends to form fine "fairy-castle" or dendritic structures, but this morphology is driven by the near-surface gradient in water vapor concentration, favoring growth of the tallest crystals. The properties of CO2 radiation frost formed in a CO2 atmosphere can be quite different.
The broad-band solar albedo and infrared emissivity of Martian seasonal frost deposits dominate the annual polar heat balance which, in turn, controls the seasonal variation in atmospheric mass (~25%) (Leighton and Murray, 1966; James et al., 1992; Wood and Paige,1992) as well as the potential for atmospheric collapse at the extremes of Mars epochal obliquity variations (Ward et al., 1974; Toon et al., 1980; Armstrong et al., 2004; Manning et al., 2006).
There are also potentially strong feedbacks between the cycles of CO2 and the cycles of water and dust on Mars (James et al., 1992; Zurek et al., 1992). Scattering model calculations based on laboratory measurements of CO2 optical properties show that the solar albedo and thermal emissivity are extremely sensitive to grain size, and to contamination by dust or water (Warren et al., 1990; Hansen, 1999; Hansen, 2005; Bonev et al., 2008). Recent and ongoing orbital observations have revealed a stunning variety in the apparent physical properties, appearance and behavior of CO2 ice - spawning a growing list of descriptive terms including spots, jets, fans, spiders, swiss cheese, and "cryptic" ice, as shown in Fig. 1. (Kieffer 2000; Titus et al, 2001; Piqueux et al., 2003; Byrne and Ingersoll, 2003; Kieffer et al., 2006). There is spectroscopic and geomorphic evidence that parts of the seasonal CO2 ice deposits are in the form of a solid, semi- transparent slab, while evidence from a combination of gravity and neutron spectrometer data imply a very low density frost (Aharonson, 2004).
To grow radiation frost in the laboratory requires containment of the atmosphere/vapor while simultaneously allowing infrared radiation to escape (to balance the latent heat of condensation). Planets accomplish this using gravity to hold down the atmosphere. The key to our simulation is the use of plastic thin films, e.g. polypropylene, that are largely transparent in the thermal infrared (Aston & Patton, 1973) yet strong enough to maintain the required pressure differential between our Mars-like "atmosphere" and the vacuum-enclosed space simulator (an LN2-cooled plate). As the frost is growing we will measure its broadband thermal infrared emissivity using a thermopile detector through several vacuum windows transparent to different segments of the thermal spectrum. We use fiber optics to briefly illuminate the frost to periodically observe its physical morphology and texture using an in situ fiberscope with an articulated tip. When sufficient frost thickness has developed we will use a full spectrum solar simulator to illuminate the frost and measure the broadband and spectral albedo in the visible using the output light from the fiberscope. The broadband solar radiometry measurement will be done using the thermopile detector, and the spectral albedo using a fiber optic spectrometer. We will also study the radiative and physical effects of contamination by silicate dust and water frost.
In parallel with the experimental work, we are conducting theoretical work using numerical models of Mars’ present and past CO2 cycle. This modeling work enables us to analyze the implications of our experimental results for the past and present behavior of Mars’ polar CO2 ice deposits and their effects on the atmosphere and climate.
