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2.6 Martian climate and volatile inventories through time (Paige, Newman, Varadi, Lyons, Young)

	Figure 2.6.2. Oxygen three isotope plot showing the mass-dependent isotope fractionation mechanisms that may have caused negative 17O values for O2 in Earth’s troposphere and positive 17O in Martian carbonates.

In the search for life on Mars, either present or past, it is necessary to develop comprehensive understanding of the inventory and behavior of Martian volatiles, and the Martian climate. Volatiles and climate on Mars are intimately linked to a wide range of dynamical phenomena, including: the dynamics of the solar system and Mars’ spin axis; the dynamics of impactors and their interaction with the Martian surface and atmosphere; the dynamics and chemistry of the Martian crust and interior; the dynamic interaction between the Martian atmosphere and the space environment; the dynamics of the Martian atmosphere and polar caps. During the next five years, the UCLA NAI Mars Volatiles and Climate subgroup will address four focused problems directly relevant to Mars volatiles and climate studies:

2.6.1 Orbital and axial dynamics of Mars

Varadi, Paige, and colleagues proposes to develop new, state-of-the-art models for orbital and rotational dynamics and couple them to low-dimensional climate models to better understand how large-scale quasi-periodic variations in Mars orbital and axial elements are coupled to Martian climate change. Successively refined numerical simulations of the orbits of the planets are expected to provide a more accurate orbital history of the Solar System for the past 70 million years (see §3.5.3). Beyond this time scale, the uncertainties due to chaos make the results of individual simulations unreliable. They are still meaningful, however, in terms of understanding the general features of orbits over hundreds of millions of years. Rotational dynamics, on the other hand, needs to be fundamentally reconsidered.

According to the conventional view, planets rotate around a principal axis of the moments of inertia tensor and the spin axis precesses in space due to torques by the Sun, other planets and also satellites. This is a convenient approximation which is not consistent with the equations of rotational dynamics. The torques responsible for spin axis precession also cause the planet to deviate from exact principal-axis rotation. Our recent results demonstrate that the deviation can be comparable to the amplitude of spin axis precession in the case of synchronously rotating satellites (§3.2.1). Could similar phenomena be important for Mars? Once the body is allowed to deviate from principal axis rotation in our models, there can be a number of resonances between the period of the wobble and short-period orbital forcing from other planets.

We are already developing new analytical machinery, i.e., perturbation theory, to obtain accurate equations for the long-term behavior of obliquity. While rotational variations can affect climate, the reverse is also true. The seasonal deposition and sublimation of large polar caps on Mars perturbs the moments of inertia (as measured by MGS through the gravity field) of Mars, providing a means for feedback between climate and rotation. Other internal processes such as the formation of Tharsis also modify the inertia tensor and therefore influence the rotational state of Mars. We are developing a model for following the time-dependent deformation of Mars under various loads. This model will be coupled to both the climate model (through the polar cap loading/unloading) and the orbital and rotational dynamics through the time-dependent inertia tensor. The dynamics of such a system have not been analyzed, and this feedback may have a significant effect on the obliquity history of Mars.

The orbital and rotational dynamics will be coupled with simplified models of the behavior of the Martian climate system that will calculate the state and mass distribution of the polar caps and atmosphere. Such models will be used to understand potential coupling between the dynamics and of the solar system and the Martian climate, which may be recorded in the Martian layered deposits. We will develop and use sophisticated time series analysis techniques based on Singular Spectrum Analysis to extract potential climate signals that can be compared to observations of Martian layering patterns as demonstrated by Laskar etal, 2002. Our studies will include polar layered deposits as well as mid-latitude layered sedimentary despotis recently identified in Mars Global Surveyor Images (Malin and Edgett, 2000). The analysis will employ a recently developed denoising technique called Random-Lag Singular Cross-Spectrm Analysis. This technique provides not only cleaned-up signals, but also cross-filters with phase information between signals to detect time lags between astronomical forcing and changes in the rates of deposition.

2.6.2 Oxygen isotope fractionation

The stable isotopes of major elements can be used to trace the movement of material in and out of reservoirs in the near surface of both Mars and Earth. Stable isotopes are therefore potentially powerful tools for unraveling the history of climate variations on Mars as well as on Earth. In order to take advantage of this potential tool, it will be necessary to understand fully the factors that affect isotope ratios for the important elements. Among the most important of these elements is oxygen. Oxygen represents a major component of the Martian atmosphere, cryosphere, lithosphere, and possible biosphere if it exists. With respect to the ¹⁶O, ¹⁷O and ¹⁸O isotopic system, data from both Martian meteorites (SNC meteorites) and the terrestrial atmosphere suggest unusual fractionation patterns that have yet to be fully explained. A program of research directed towards understanding what the isotopes of oxygen are telling us about the Martian atmosphere and regolith will be carried out by team members Young and Lyons.

Oxygen in Earth’s troposphere has ¹⁷O values (¹⁷O = per mil deviation in ¹⁷O/¹⁶O relative to standard mean ocean water) that are ~ 0.3 per mil lower than rocks and waters at the same ¹⁸O value. Differences in ¹⁷O relative to ¹⁸O values are referred to as differences in ¹⁷O where ¹⁷O = ¹⁷O -0.52 ¹⁸O. This definition of ¹⁷O is based on the assumption that the so-called mass-dependent fractionation among the three isotopes of oxygen follows the relation ¹⁷O = 0.52 ¹⁸O. In fact the "slope" factor 0.52 is actually an exponent in a fractionation law and takes on a range of values from about 0.53 to 0.51 depending upon the physicochemical process involved (Young et al. 2002). This factor, referred to as , is predicted to be lower for kinetic processes than it is for equilibrium processes (Young et al. 2002), though only a few direct measurements to confirm or contravene this assertion have been made to date. The convention of assuming that = 0.52 for the purpose of defining ¹⁷O values is well ensconced in the literature and is retained.

Luz et al. (1999) attributed the ¹⁷O value of tropospheric O₂ to photochemistry in the stratosphere. Young et al. (2002) suggested that the ¹⁷O value of tropospheric O₂ is instead a steady state between two processes acting with near constant rates. The two processes are photosynthetic production of O₂ and extraction of O₂ by respiration (Dole 1935). Although the control that these competing biological processes exert on ¹⁸O in atmospheric oxygen is well known, until recently there had been no characterization of values for respiration (photosynthesis results in O₂ with the same ¹⁷O as ocean water because the fractionation associated with the process is small). Young et al. (2002) made the prediction that the value for the kinetic process of respiration should be close to 0.508, and that this value is sufficient to explain the ¹⁷O of tropospheric O₂ with out the need to invoke mixing with photolysis products from the stratosphere. The competing rates hypothesis has gained momentum with new measurements showing that values attending photorespiration and dark respiration are 0.506 and 0.518, respectively (Luz et al. 2002). The measured values are close to the predicted value of 0.508 (Young et al. 2002).

From the preceding discussion it is clear that the cause of the measured___ ¹⁷O of O₂ in the troposphere is uncertain but the existence of the ¹⁷O difference between rocks (¹⁷O ~ 0 similar to waters) and lower atmosphere O₂ means that there are two distinct reservoirs of oxygen maintained by some phenomenon that affects the atmosphere but not the rocks. The phenomenon is either a steady state imposed by the relative rates of O₂ consumption (respiration) and production (photosynthesis) or it is photochemistry that occurs elsewhere in the atmosphere. It is likely that both explanations are valid. In any case, it is clear that before we can use__ ¹⁷O as a tool for tracing material transfer to and from different near-surface reservoirs on Earth, it will be necessary to understand all of the factors that can influence ¹⁷O values, both mass dependent (variable ) and mass independent (photochemistry).

Similar arguments pertain to Mars. The atmosphere of Mars is dominated by CO₂, making the oxygen isotopic compositions of carbonates from the planet especially useful tracers of atmospheric processes on Mars. Carbonates from one SNC meteorite (apparently formed on Mars approximately 3.6 billion years before present near the Hesperian time interval, McKay et al. 1996) have __¹⁷O values ~ 0.5 per mil higher than the igneous minerals from all measured SNCs. The difference between the two mineral types suggests that just as on Earth, there are, or were, at least two oxygen reservoirs on Mars. One in the atmosphere (sampled by SNC carbonate) and the other represented by the regolith (sampled by the SNC igneous minerals). This is an important clue to the way that the Martian atmosphere behaved in Hesperian time, but the cause of the observation is uncertain. Farquhar et al. (1998), the workers that made the measurements of ¹⁷O in carbonate from SNC meteorite ALH 84001, suggested that the difference in ¹⁷O between carbonate and igneous minerals was the result of exchange between CO₂ and electronically excited O liberated by photodecomposition of ozone. Another possibility is that the ¹⁷O in the carbonates reflects an anomalous, partly non-mass dependent fractionation caused by gravitational separation of the isotopes above the homopause and isotopically selective escape of isotopomers at the exobase (Jakosky 1993).

Another explanation for the difference in ¹⁷O between ancient Martian carbonate and igneous minerals is that competing processes with different mass-dependent values analogous to the situation that controls ¹⁷O of tropospheric O₂ on Earth are operating near the surface. In this case (Young et al. 2002) a steady state between sublimation of CO₂ ice with a low value for b, consistent with a kinetic process, and condensation of CO₂ with a higher value approaching equilibrium values (since condensation implies partial pressures approaching equilibrium), might be the cause of the disparate carbonate ¹⁷O values.

Young and Lyons intend to test the hypothesis that ¹⁷O can be affected by passage of CO₂ between the cryosphere and the atmosphere on Mars through a program of experiments that will characterize the values attending condensation and sublimation. The first experiments will be on O₂ liquid and vapor (since the three isotopes of oxygen are readily analyzed in this system). Young and Lyons will freeze O₂ in a vacuum line and analyze the residual gaseous O₂ as a function of fraction of gas frozen. Techniques will be similar to those described by Eiler et al. (2000) but with the exception that ¹⁷O as well as ¹⁸O will be measured using the stable isotope laboratory at UCLA. These early studies of O₂ will serve as the basis for new experiments on CO₂ in which fluorination will be used to liberated O₂ from C for isotopic analysis.

Lyons will simultaneously reexamine the suggestion by Jakosky that escape from the top of the Martian atmosphere could have caused significant shifts in ¹⁷O in CO₂. He will evaluate the mass independent component of diffusive escape processes in the Martian homopause. The problem requires examination of how O, CO, and CO₂ behave above the homopause and the physics of escape of O from the exobase. These processes will examined in the context of the competing processes of non-mass dependent photochemical reactions that liberate electronically oxygen.

The product of this research will be a better understanding of the meaning of variable ¹⁷O values (in the ~ 0.5 per mil level) both on Earth and on Mars.

2.6.3 Mars data analysis

NASA’s Mars Program is producing a wide range of exciting new Mars data and there are excellent opportunities for data analysis and synthesis. Key datasets include those produced by the Mars Global Surveyor MOC, MOLA and TES instruments, as well as the Mars Odyssey GRS and Themis instruments. These datasets will soon be augmented by NASA’s Mars Exploration Rovers and Mars Reconnaissance Orbiter missions. Three examples of astrobiologically-relevant issues that can be addressed through data analysis studies include:

The distribution and stratigraphy of layered deposits in the polar regions, and near the equator. What is their basic composition? What are their relative ages and histories? Are they records of past climate variations?

Evidence for past and present water. Where is water in its various forms now on Mars? What environments may have liquid water been stable?

Selection of future landing sites... Where are the best sites for future astrobiological investigation of Mars? To make the most out of future landed opportunities, it is vital that the existing data be understood as rapidly and completely as possible.

We have significant capability and interest in analyzing Mars datasets, including images, gravity, topography, spectral reflectance, thermal emission, and gamma-ray/neutron data. The results of our work in these areas will serve to focus and stimulate or complementary theoretical Mars studies, as well as enhance our participation in NAI Mars Focus Group activities.

2.6.4 Liquid water on Mars

Water is the "elixir of life", but conventional wisdom has dictated that near-surface liquid water should not be stable under present Martian climatic conditions. However, we learn more about Mars, the possibilities for liquid water habitats have greatly expanded, and now it seems possible that under rare circumstances, water could be stable at least for brief periods during the past tens of millions of years. The key problem is that high temperatures are required to sustain liquid water on Mars, yet water will tend to diffuse away from high temperature regions to lower temperature regions, thus making liquid water difficult to sustain. To understand these issues in greater detail, it will be necessary to:

a) Understand the current observations of water-related features such as seepage and runoff features, including their geographic distribution and detailed morphology

b) Better define the thermal and diffusive properties of near-surface materials

c) Define and better understand the current distribution of near-surface adsorbed water and ground ice

d) Create more comprehensive models for the interaction between surface, and subsurface water with the Martian atmosphere

e) Compare model results with observations

We propose to conduct the above-listed investigations of liquid water distribution and behavior, and also consider the potential for localized habitable environments.

2.6.5 Mars impact history and volatile evolution

The evolution of volatiles on Mars depends on a balance between volatile gain and loss mechanisms, which are not fully understood. At the present time, we do not know how Mars, or the other terrestrial planets acquired their oceans, atmospheres and polar caps. We propose to investigate these issues using analytical mathematical tools as well as first-principles computational simulations of the effects of impacts, escape and outgassing on the evolution of the atmospheres of the terrestrial planets through time. These models will be used to track the mass and isotopic composition of volatile reservoirs through time, and better understand the influence impact events, which can result in a net gain or loss of volatiles depending on the size and composition and velocity of the impactor, and the mass of the atmosphere (Newman, Paige).

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