A key pacemaker of ice ages on the Earth is climatic forcing due to variations in planetary orbital parameters. Recent Mars exploration has revealed dusty, water-ice-rich mantling deposits that are layered, metres thick and latitude dependent, occurring in both hemispheres from mid-latitudes to the poles. Here we show evidence that these deposits formed during a geologically recent ice age that occurred from about 2.1 to 0.4 Myr ago. The deposits were emplaced symmetrically down to latitudes of approximately 30 degrees--equivalent to Saudi Arabia and the southern United States on the Earth--in response to the changing stability of water ice and dust during variations in obliquity (the angle between Mars' pole of rotation and the ecliptic plane) reaching 30-35 degrees. Mars is at present in an 'interglacial' period, and the ice-rich deposits are undergoing reworking, degradation and retreat in response to the current instability of near-surface ice. Unlike the Earth, martian ice ages are characterized by warmer polar climates and enhanced equatorward transport of atmospheric water and dust to produce widespread smooth deposits down to mid-latitudes.

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A major obstacle in understanding the evolution of Cenozoic climate has been the lack of well dated terrestrial evidence from high-latitude, glaciated regions. Here, we report the discovery of exceptionally well preserved fossils of lacustrine and terrestrial organisms from the McMurdo Dry Valleys sector of the Transantarctic Mountains for which we have established a precise radiometric chronology. The fossils, which include diatoms, palynomorphs, mosses, ostracodes, and insects, represent the last vestige of a tundra community that inhabited the mountains before stepped cooling that first brought a full polar climate to Antarctica. Paleoecological analyses, 40 Ar/ 39 Ar analyses of associated ash fall, and climate inferences from glaciological modeling together suggest that mean summer temperatures in the region cooled by at least 8°C between 14.07 ؎ 0.05 Ma and 13.85 ؎ 0.03 Ma. These results provide novel constraints for the timing and amplitude of middle-Miocene cooling in Antarctica and reveal the ecological legacy of this global climate transition.climate change ͉ tundra biota ͉ Dry Valleys ͉ diatoms ͉ ostracods

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A radiocarbon chronology shows that piedmont glacier lobes in the Chilean Andes achieved maxima during the last glaciation at 13,900 to 14,890, 21,000, 23,060, 26,940, 29,600, and >/=33,500 carbon-14 years before present ((14)C yr B.P.) in a cold and wet Subantarctic Parkland environment. The last glaciation ended with massive collapse of ice lobes close to 14,000(14)C yr B.P., accompanied by an influx of North Patagonian Rain Forest species. In the Southern Alps of New Zealand, additional glacial maxima are registered at 17,720(14)C yr B.P., and at the beginning of the Younger Dryas at 11,050 (14)C yr B. P. These glacial maxima in mid-latitude mountains rimming the South Pacific were coeval with ice-rafting pulses in the North Atlantic Ocean. Furthermore, the last termination began suddenly and simultaneously in both polar hemispheres before the resumption of the modern mode of deep-water production in the Nordic Seas. Such interhemispheric coupling implies a global atmospheric signal rather than regional climatic changes caused by North Atlantic thermohaline switches or Laurentide ice surges.

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Documenting the morphology and distribution of polygonally patterned ground on Mars is critical for understanding the age and origin of the Martian latitude‐dependent mantle. Polygonally patterned ground on Mars is analyzed using High Resolution Imaging Science Experiment image data in order to document the variation of polygon morphology within latitude bands 30–80° in both northern and southern hemispheres. Small‐scale (<∼25 m diameter) polygons are classified on the basis of morphological characteristics into seven groups, which are present in both northern and southern hemispheres. Polygon morphology is shown to be consistent with thermal contraction cracking of an ice‐rich mantling unit, consistent with observations of sediment wedge thermal contraction crack polygons forming in ice‐cemented sediment at the Phoenix landing site. Polygon groups are distributed symmetrically in both northern and southern hemispheres, suggesting strong climate controls on polygon morphology. Northern hemisphere polygonally patterned surfaces are found to decrease in age from low to high latitude, spanning surface ages from ∼1 to <0.1 Ma, suggesting more recent deposition of ice‐rich material at high latitudes than at low latitudes. Six of the seven classes of polygons are interpreted to be capable of forming because of the combined effects of thermal contraction cracking and differential sublimation, suggesting that sublimation and sand wedge polygons dominate Martian high latitudes. Gully polygon systems present at midlatitudes suggest that small amounts of liquid water may have been involved in thermal contraction crack polygon processes, producing composite wedge polygons. No evidence is found for the presence of pervasive small‐scale ice wedge polygons.

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