Summary of the Marsrecent climate change workshop NASA / Ames Research Center , May 15 – 17 , 2012

This note summarizes the results from the Mars recent climate change workshop at NASA/Ames Research Center, May 15–17, 2012. Published by Elsevier Inc. 1. Background and context Climate change on Mars has been a subject of great interest to planetary scientists since the 1970s when orbiting spacecraft first discovered fluvial landforms on its ancient surfaces and layered terrains in its polar regions. By far most of the attention has been directed toward understanding how ‘‘Early Mars’’ (i.e., Mars > 3.5 Ga) could have produced environmental conditions favorable for the flow of liquid water on its surface. Unfortunately, in spite of the considerable body of work performed on this subject, no clear consensus has emerged on the nature of the early martian climate system because of the difficulty in distinguishing between competing ideas given the ambiguities in the available geological, mineralogical, and isotopic records. For several reasons, however, the situation is more tractable for ‘‘Recent Mars’’ (i.e., Mars within 20 Ma). First, the geologic record is better preserved and evidence for climate change on this time scale has been building since the rejuvenation of the Mars Exploration Program in the late 1990s. The extended temporal coverage of the planet from orbit and the surface, coupled with accurate measurements of surface topography, increasing spatial resolution of imagers, improved spectral resolution of infrared sensors, and the ability to probe the subsurface with radar, gamma rays, and neutron spectroscopy, has not only improved the characterization of previously known climate features such as polar layered terrains and glacier-related landforms, but has also revealed the existence of many new features thought to be related to recent climate change such as polygons, gullies, concentric Inc. . Haberle), Francois.Forget@ (J. Head), Melinda.A.Kahre@ avsky), Sandra.J.Owen@nasa. crater fill, internal structure of the polar layered deposits, latitude-dependent, and volatile-rich surface mantling. Second, the likely cause of climate change – spin axis/orbital variations – is more pronounced on Mars compared to Earth. Spin axis/orbital variations alter the seasonal and latitudinal distribution of sunlight, which can mobilize and redistribute volatile reservoirs both on and below the surface. Within 20 Ma, for example, the obliquity is believed to have varied from a low of 15 to a high of 45 with a regular oscillation time scale of 10 years. The amplitude of the corresponding variations for Earth is typically less than 2 . Mars, therefore, offers a natural laboratory for the study of climate change induced by significant spin axis/orbit variations on a terrestrial planet. Finally, general circulation models (GCMs) for Mars have reached a level of sophistication that justifies their application to the study of spin axis/orbitally forced climate change. With recent advances in computer technology the models can run at reasonable spatial resolution for many Mars years with physics packages that include cloud microphysics, radiative transfer in scattering/absorbing atmospheres, surface heat budgets, boundary layer schemes, and a host of other processes. To be sure, the models will undergo continual improvement, but with carefully designed experiments they can now provide insights into mechanisms of climate change in the recent past. Thus, within 20 Ma the geologic record is better preserved, the forcing function is large, and GCMs have become useful tools. While research efforts in each of these areas have progressed considerably over the past several decades, they have proceeded mostly on independent paths occasionally leading to conflicting ideas. To remedy this situation and accelerate progress in the area, the NASA/Ames Research Center’s Mars General Circulation Modeling Group hosted a 3-day workshop on May 15–17, 2012 that brought together the geological and atmospheric science communities to collectively discuss the evidence for recent climate change on Mars, the nature of the change required, and how that change could be brought about. Over 50 researchers, students, and post-docs from the US, Canada, Europe, and Japan attended the meeting. The program and abstracts from the workshop are 416 Note / Icarus 222 (2013) 415–418 available to the public at http://spacescience.arc.nasa.gov/mars-climate-workshop2012/home.html. 2. Key questions and starting point Some specific questions addressed at the workshop were: What constraints does the geologic evidence place on the magnitude, timing, and duration of climate change? How well do we know the orbital history of Mars and the magnitude, timing, and duration of its obliquity, eccentricity, and precessional variations? What is the present nature and distribution of the surface and subsurface volatile reservoirs of water and CO2? And what changes to the climate system result from orbital variations, and how do those changes alter these volatile reservoirs? As the discussion began, there was general agreement among workshop participants on two main points: 2.1. Evidence for geologically recent climate change on Mars is strong The existence of a variety of non-polar ice-related deposits that cannot be produced in today’s climate system provides the strongest evidence for this (see Fig. 1). These include midlatitude glacial features (particularly in the Deuteronilus Mensae region where there is evidence for a large integrated glacial system comparable to Earth’s continental ice sheets), concentric crater fill (where the floors of some midlatitude craters are filled and show parallel ridging), remnant CO2 glaciers (where drop moraines left by cold-based glaciers appear to require soft flowing material), pedestal craters (where the ejecta blanket protects underlying ice while sublimation lowers the surrounding regions), and tropical mountain glacier deposits (on the NW flanks of Olympus Mons and the Tharsis Montes). The ages of these features span the Amazonian epoch (past 2–3 Gy), but many are geologically young. There is also a young, ice-rich, latitude-dependent mantle that is widespread in mid and high-latitudes of both hemispheres. The surface deposits that form this mantle are diverse in their morphology and topography, and include features such as polygons, layers, and gullies. This latitude dependent mantle appears to consist of a succession of meters-thick deposits whose deposition and removal is most likely related to orbitally forced climate change. Fig. 1. Sample evidence for recent climate change on Mars. After Head and Marchant (2 2.2. The main driver for recent climate change is spin axis/orbital variations The lack of a stabilizing Moon, and the proximity to Jupiter lead to large variations in Mars’ spin axis/orbit parameters. The most recent published calculations of these parameters (see Fig. 2) were updated for the workshop using the latest information on planetary and asteroid positions and show that the limits of predictability are 60 My for orbital position, 40 My for eccentricity, and 20 My for obliquity. Beyond these times the solutions become chaotic. However, up until then these quantities can be accurately predicted, and they still show very large variations in eccentricity ( 0–0.12) and obliquity ( 15–45 ) during the time for which they are predictable. This provides the climate modeling community with an accurate history of spin axis/orbit variations, which can be used to assess the consequences on the climate system.