Ninth International Conference on Mars 2019 (LPI Contrib. No. 2089) 6433.pdf

RECONSTRUCTUNG THE CLIMATE-DRIVEN EVOLUTION OF PLANUM BOREUM WITH SOUNDING RADAR, VISIBLE IMAGERY AND GENERAL CIRCULATION MODELS. S. Nerozzi1, J. W. Holt2, F. Forget3, A. Spiga3, E. Millour3, 1Institute for Geophysics, Jackson School of Geosciences, The University of Texas at Austin, TX 78757 ([email protected]), 2Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, 3Laboratoire de Météorologie Dynamique, Université Pierre et Marie Curie, Sorbonne Université, Paris, France.

Introduction: The Planum Boreum of Mars is com- posed of two main units: the North Polar Layered De- posits (NPLD), and the underlying basal unit (BU). The NPLD are the second largest water ice reservoir on Mars. Their rich stratigraphic record is regarded as the key for understanding climate evolution of Mars in the last 4 My [1] and its dependency on periodical varia- tions of Mars’ orbital parameters (i.e., orbital forcing) [2-4]. Their initial emplacement represent one of the most significant global-scale migrations of water in the recent history of Mars, likely driven by climate change, yet its dynamics and time scale are still poorly under- stood. Recent studies revealed the composition, stratig- raphy and morphology of the underlying BU (Fig. 1, [5]). These findings depict a history of intertwined polar ice and sediment accumulation in the Middle to Late Amazonian, thus opening a new window into Mars’ past global climate. Here we present a summary of the latest findings on the climate-driven evolution of Planum Boreum, their Figure 1: Topographic map of the BU and surrounding ter- significance in advancing the exploration of Mars, and rains with superimposed shaded relief of the present day new outstanding questions on Mars polar science. Our Planum Boreum topography. The white line delineates the location of the SHARAD profile in Fig. 2. studies are based on the integration of radar profiles and images acquired by the Shallow Radar (SHARAD, [6]) and the High Resolution Imaging Science Experiment (HiRISE, [7]) on the Mars Reconnaissance Orbiter, and the General Circulation Model (GCM [8]) developed by the Laboratoire de Météorologie Dynamique (LMD). Former ice caps preserved with the cavi unit: The cavi unit is an aeolian deposit of basalt sand and water ice making up large portions of the BU. SHARAD sig- nals penetrate through the cavi unit revealing internal Figure 2: (A) SHARAD profile 1294501 depth corrected as- and basal reflectors. We use these detections to recon- suming bulk water ice composition (e=3.1 [10]), and (B) in- struct the general stratigraphic structure of the unit, and terpretation thereof. The putative basal surface of the cavi obtain its bulk composition with an inversion technique. unit is represented in red. Our exercise reveals substantial spatial variability in [9,10], with the thickest accumulation close to the north composition, with average water ice volume fractions pole. In the models, this is soon followed by complete comprised between 62% in Olympia Planum and 88% loss through sublimation. We hypothesize that some of in its northern reaches beneath the NPLD. Similarly, in- this ice has been buried and preserved by aeolian sand ternal reflectors occur more frequently closer to the pole sheets that prevented complete sublimation. Similar and gradually disappear moving south. We hypothesize sand mantles extending for 10s of km have been ob- that the cavi unit is made of alternating ice and sand served on top of thick water ice [11], and within the low- sheets, with water ice becoming prevalent towards the ermost NPLD [5]. Therefore, we argue that ice caps da- north pole (Fig. 2). Water ice accumulation models pre- ting to Middle to Late Amazonian are not necessarily dict substantial ice growth during periods of low spin lost during high obliquity periods, but preserved within axis obliquity before the onset of NPLD deposition sand sheets underneath the NPLD. These ice deposits Ninth International Conference on Mars 2019 (LPI Contrib. No. 2089) 6433.pdf

are detected by SHARAD and can be delineated in their spatial extent. Moreover, the high water ice fraction makes cavi an important water ice reservoir, potentially the third largest on Mars after the two PLDs. Newly mapped extent and morphology of the BU: Analysis of SHARAD profiles indicates that the BU ex- tends over a larger area than previously thought. In par- ticular, we detect the presence of cavi unit material ex- tending underneath the NPLD from the western edge of Gemina Lingula to a visible exposure in the eastern end of Olympia Undae, covering an area of over 120,000 km2 (Fig. 1). HiRISE images taken over the outcrop lo- cation reveal sub-horizontal strata forming terraces and characterized by sinuous forms and cross strata. We in- terpret this as a cavi unit outcrop, that we can now place Figure 3: Earliest NPLD water ice accumulation in Planum Boreum as mapped by SHARAD [6]. into the broader stratigraphic context of Planum Boreum based on SHARAD profiles. of ice accumulation, with low obliquity driving thick ice Our radar-based topographic mapping also reveals a growth at ~60°N. Local topography appears to control series of elongated depressions tens to hundreds of me- longitudinal patterns of ice growth in all our simula- ters deep along the edge of the cavi unit. In some cases, tions. We find a strong similarity of the latest GCM out- the base of these depressions are flat and appear to con- puts with the isolated proto-Gemina Lingula deposit tinue as internal reflectors for hundreds of kilometers. (Fig. 3) and present-day icy outliers of Olympia Men- We interpret these findings as further confirmation of sae. This suggests that Olympia Mensae may be rem- the presence of alternating ice and sand layers within the nant of the migration of water ice from low to polar lat- cavi unit, delineating sequences that exhibit different re- itudes that resulted in the initial accumulation of the sistance to erosion. The location of the elongated de- NPLD. Moreover, the formation of proto-Gemina Lin- pressions coincides with the presence and shape of the gula may predate the accumulation of other NPLD units buried chasma observed by [12], suggesting that the closer to the north pole. cavi unit was eroded in the same event that shaped the Outstanding questions: Based on our latest find- chasma. We detect similar features at other locations of ings in Planum Boreum, we delineate the following out- the BU, suggesting that many other erosional events are standing questions. How many episodes of past ice ac- recorded by the unit’s surface morphology. cumulation are recorded within the cavi unit? What is Reconstructing the initial NPLD accumulation: their precise age? What is the nature of western half of SHARAD-based analysis of the lowermost NPLD stra- the BU, which is dominated by the Rupes Tenuis unit? tigraphy reveals that initial water ice accumulation was Does this unit also record past polar ice accumulation not uniform, but limited in extent and confined into two events? areas centered around the north pole and in the proto- Acknowledgments: This work was supported by Gemina Lingula region. Likewise, subsequent ice accu- NASA MDAP grant NNX15AM52G. SHARAD is mulation was variable in extent. We use the newly ac- provided and operated by the Italian Space Agency. The quired information on BU composition, topography and GCM was kindly provided by the Laboratoire de lateral extent to accurately constrain the initial condi- Météorologie Dynamique, UPMC, Paris. tions and parameter space for sensitivity experiments References: [1] Cutts J.A. (1973) JGR, 78, 4231- with the LMD GCM aimed at understanding the driving 4249. [2] Murray B.C. et al. (1973) Science, 180, 638- forces responsible of the initial accumulation of the 640. [3] Laskar J. et al. (2002) Nature, 419, 375-377. [4] NPLD, and its temporal evolution. In particular, we de- Laskar J. et al. (2004) Icarus, 170, 343-364. [5] Nerozzi, fined parameter sets of spin axis obliquity (15-40°, 5° S. and Holt, J.W. (2019) GRL. [6] Seu R. et al. (2007b) steps), orbital eccentricity (0-0.12, 0.03 steps), perihe- JGR, 112, E05S05. [7] McEwen, A.S. et al. (2007) JGR, lion precession (0-270°, 90° steps), and atmospheric 112, E05S02. [8] Forget F. et al. (1999) JGR, 104, 155- pressure (current, +106% based on ref. [13]). 176. [9] Levrard B. et al. (2007) JGR, 112, E06012. [10] The GCM output reveals that both obliquity and ec- Greve R. et al. (2010) PSS, 58, 931-940. [11] Conway centricity play key roles in driving the amount of water S.J. et al. (2012) Icarus, 220, 174-193. [12] Holt J.W. et ice accumulation in Planum Boreum, with low obliquity al. (2010), Nature, 465, 446-449. [13] Bierson C.J. et al. and high eccentricity scenarios resulting in the largest (2016) GRL, 43, 4172-4179. ice growth. Obliquity also appears to control the latitude