- Email: [email protected]
Dynamic Mars from long-term observations: Introduction
Icarus 251 (2015) 1–4
Contents lists available at ScienceDirect
Icarus journal homepage: www.elsevier.com/locate/icarus
Editorial
Dynamic Mars from long-term observations: Introduction
Of all the planets in the Solar System besides Earth, Mars stands out as one for which both surface and atmospheric changes occur at decadal, annual, and shorter time scales. Indeed, the seasonal variability of Mars was noted by early astronomers such as Herschel, Schiaparelli, and Lowell. With the advent of the space age, Mars has been of particular focus for exploration. Beginning with Mariner 4 in 1965, no fewer than 20 successful spacecraft have flown by, orbited, landed, and roved on the surface. Although this exploration has been heavily driven by trying to understand the role of present and past water on the planet, and the search for evidence for environments that could support life, data from all these spacecraft has led to numerous discoveries spanning geology, geophysics, atmospheric sciences, and other fields. The period of martian exploration we find ourselves in now is unique, as there has been a continuous record of spacecraft exploration that began with Pathfinder and Mars Global Surveyor in 1997. Now well into the 2nd decade of this campaign, bridged to earlier times by spacecraft observations from the 1960s and 1970s, and continuous telescopic observations, our view of Mars has become one of a planet on which surface and atmospheric changes occur at frequencies of days, years, and decades, a testament to long-term monitoring that continues to this day. Recognizing this accumulation of data and an emerging view of Mars as a dynamic planet, we thought it an appropriate time that this record, with implications for martian geology, climate, atmospheric dynamics, and other processes, be integrated into a single journal special issue, presented here. This volume contains 20 papers each of which make use of one or more datasets and cover >1 Mars year. The papers highlight new results that are a unique outcome of the long-term data acquisition provided by our robust and long-duration program of Mars exploration and telescopic observations. The papers also highlight the long-term implications of processes that are observed and ongoing now, and show the importance of such longevity and consistency in atmospheric and surface observations. These results demonstrate the importance of continuous monitoring of Mars through new and especially extended missions. We thank the authors, reviewers, and the editors, and especially Eva Scalzo at Icarus, without whom this special issue would not be possible. Eva, the editorial office manager for Icarus, was the interface between us and the reviewers and authors, and provided much-needed advice on policies and procedures. Jeff Moersch was the Icarus editor who oversaw this issue. He provided timely and sage advice on several reviews and questions concerning http://dx.doi.org/10.1016/j.icarus.2015.03.001 0019-1035/Ó 2015 Published by Elsevier Inc.
editorial policies. We would finally like to thank Phil Nicholson for granting permission for this special issue to proceed. We summarize the contents of the special issue in the following sections. 1. Ionosphere With the MAVEN spacecraft’s arrival at Mars, our understanding of the martian ionosphere is sure to increase. These data can be compared and contrasted to studies such as two presented in this special issue by Withers et al. (2015) and Zhang et al. (2015), who make use of MGS and MEX observations to gain further insight into the long term (Withers et al., 2015) and transient (Zhang et al., 2015) behavior of the ionosphere. Using both MGS and MEX data, Withers et al. (2015) examine the peak electron densities in the Mars ionosphere and show that they increase smoothly with increasing solar irradiance throughout the >1 solar cycle of observations now available. They also see hints of trends possibly associated with seasonal variations in the martian thermosphere. Zhang et al. (2015) use the MEX/MARSIS data of the martian ionosphere to investigate transient layers that lie above the peak electron density layer studied by Withers et al. (2015). The height and density of the transient layer, typically 60 km above the main density peak, correlate with the height and density of the main peak. They find that in the southern summer, the transient layer is about 10 km higher than other times of year, likely resulting from lower atmospheric heating. They suggest that beam-plasma instabilities are responsible for the transient layers. 2. Lower atmosphere The behavior of the water vapor, water–ice and dust aerosols, and temperatures in the lower atmosphere define the current climate and weather of Mars. This is, of course, critical for accurate modeling, deciphering the controlling physical mechanisms, and for understanding the past climate, when Mars may have been warmer and wetter and a hospitable place for life. Six papers in this special issue discuss these quantities and are summarized here. As highlighted in this special issue, having a long record of observations from a variety of spacecraft with similar, yet different, techniques is very valuable in defining the climate of Mars and understanding differences due to weather. However a challenge is understanding the uncertainties in the different techniques and the cross-calibration between experiments. In 2015, Shirley (2015a) provides a comparison of atmospheric temperatures and aerosols between the MRO/MCS limb sounding experiment and
2
Editorial / Icarus 251 (2015) 1–4
limb profiles taken by the MGS/TES experiment, using MGS RS temperature profiles as a common standard for comparison. He finds good agreement when the opacities are similar, except at the highest TES sampled altitudes, when MCS temperatures are systematically higher, providing confidence in use of these data for long-term studies. Shirley (2015a) finds that there is some interannual variability in latitudinal temperature gradients, thermal tide behavior, and water–ice aerosols during the aphelion season. Understanding the behavior of water vapor in the martian atmosphere is important for characterizing the present global water cycle, and for validating models, which can then be used to gain insight into the past martian climate. In this issue, Trokhimovskiy et al. (2015) provides new retrievals of water vapor from the MEX/SPICAM experiment covering 5 Mars years. His retrieval is improved by accounting for the multiple scattering of both dust and water–ice aerosols. He finds that the annual cycle is largely repeatable with water vapor maxima and minima consistent with other observational datasets. The exception is with MY 28 in which a global dust storm occurred. In this year, near the season of the storm, the water vapor abundances are lower than typical, which cannot be explained by masking by dust. Trokhimovskiy provides maps of water vapor abundance for each year, as well as maps showing retrieved saturation altitudes. Dust in the martian atmosphere is important for controlling the dynamics of the planet, and its optical depth and spatial and temporal variations are needed for accurate modeling of the atmosphere. In particular, dust distribution and amount are needed to calculate the mass mixing ratio and the heating of the atmosphere, both of which are used to estimate the thermal forcing of the atmosphere for the given conditions. Montabone et al. (2015) provide a nearly 8-Mars-year dust climatology using 3 dust datasets: MGS/ TES, ODY/THEMIS, and MRO/MCS (see cover illustration). They provide two types of gridded maps, irregular, which has missing data due to lack of coverage or poor data quality, and complete maps using kriging, an interpolation scheme. Maps showing both data sets are provided as online supplementary material and the datasets are available for download from the Mars Climate Database (MCD) website, referenced in their paper. Further, they have provided an average climatological year. Montabone et al. (2015) compare and quality check their results with dust optical depths from other orbital and landed instruments, including the MER Pancam and Mini-TES, the Phoenix SSI, MRO CRISM and MARCI, and the MGS MOC. Their work identifies and documents various instrument biases (compare to Shirley (2015a)). Examining their resulting climatology, they identify 4 annual phases of dust distribution, for years without a global dust event, consistent with Lemmon et al. (2015), Wang and Richardson (2015), and Kass et al. (2014). To complement global, lower spatial resolution observations of dust optical depth, Lemmon et al. (2015) provides and archives to NASA’s Planetary Data System the extinction optical depth record from both MER rovers (3 MY for Spirit; 5 MY for Opportunity). The data show that, for these two equatorial locations, the dust amount is low for the northern spring through mid-summer, with tens of percent variability year over year. Dust is more variable for the remainder of the year, being characterized by local, regional, and the occasional global dust storm, consistent with Montabone et al. (2015). Comparison with MER mini-TES contemporaneous observations at a different wavelength shows that the mean dust particle size is typically 1–1.4 lm, but becomes 2 lm at the onset of dust storms. This surface view of dust also allows for the study of smaller scale events, such as dust devils. Lemmon finds that insolation received at the surface (from seasonal changes and/or total optical depths) appears to control dust devil frequency at Spirit.
The origin and evolution of large-scale dust events, particularly, global dust events, is not well understood. As such, studies enabled by continuous global imaging covering multiple Mars years to examine many events are key to identifying characteristics and seasons of generation and evolution, as well as for quantifying behaviors that can be compared to models. This has been done in Wang and Richardson (2015), who examined daily global maps of aerosols over portions of 7 Mars years (12 Earth years) to identify and study large, discrete dust events that lasted P5 sols, and in which dust expanded outside the origination region. The daily global maps were created from data taken from the MGS MOC and the MRO MARCI instruments. They studied a total of 65 events, not including dust events that remained confined to the circumpolar regions. They identify that all events occurred between Ls = 135–30°, indicating a short dust-storm free season, consistent with lower optical depths seen in Montabone et al. (2015) and Lemmon et al. (2015). They define types of dust storm development, identify regions of dust storm generation, and provide a figure indicating the routes traveled by these storms. Of the two global events, they propose that both originated with southerly events, although the 2007 storm is somewhat ambiguous. To further understand global dust storm origination, a new hypothesis has been proposed by Shirley (2015b). He provides an updated table of global dust events from 1924 to 2013, and argues that a statistically significant relationship between the storm occurrence and variations in the orbital angular momentum of Mars with respect to the Solar System barycenter exists, meaning that planetary dynamics may influence the interannual variability of the martian atmosphere. Further, Shirley (2015b) provides a prediction for future years that should experience global dust events, if this correlation persists, but cautions that other factors such as availability of dust supply in convenient reservoirs may be important in global dust storm manifestation. 3. Surface monitoring and dynamic processes High scientific yields using data spanning years and across multiple spacecraft is aptly demonstrated in the 11 papers focusing on martian surface monitoring. These works include those on surface albedo changes, volatiles and ice, gullies, aeolian processes, and surface geology, with overlap in many areas. We give a brief overview of these papers here. Albedo changes are the most obvious and pervasive responses to dynamic processes on Mars, namely the lifting, transport, and deposition of dust. Beginning with telescopic observations, albedo monitoring has continued to the present day with spacecraft data. On this topic, Vincendon et al. (2015) describe observations of surface albedo changes observed by Mars Express’ OMEGA imaging spectrometer from 2004 to 2010. Using a detailed retrieval scheme, they find that bright surfaces are 17% greater than previous measurements, with most changes occurring during the storm season, in particular that of the 2007 global event. Changes over seasonal to decadal timescales are attributed to the removal and deposition of optically thin, bright dust coatings that mask underlying near-IR spectral signatures. The presence, distribution, and dynamics of volatiles and ices control the martian energy budget and the atmospheric circulations, and reflect temperature-dependent phase changes that exchange carbon dioxide and water between the martian surface–subsurface and atmosphere. In this issue Piqueux et al. (2015a) document polar CO2 cap recession/growth over MY 24 to 31 and define a ‘‘climatological’’ cap edge as a function of season. They use thermal IR data, insensitive to lighting conditions, and are therefore able to present for the first time the full growth/recession cycles for all 8 MY. Outside of the two global dust storm periods, they find that the north polar cap behavior is very
Editorial / Icarus 251 (2015) 1–4
repeatable (to 1–2° lat), although south polar cap behavior is less so than the north. During this study period two global dust storms occurred (early- and mid-season) and the cap behaviors were different, suggesting that the timing of the storm and its affect on the atmospheric circulation and temperatures combine to determine the affect on CO2 deposition and sublimation. Calvin et al. (2015) use Mars Color Imager (MARCI) data to document interannual and seasonal changes in the north polar ice deposits over 3 Mars years. In contrast to Piqueux et al. (2015a), their study focuses on the northern cap (not both) within MY 29–31 and uses visual data. The retreating cap edge is very dynamic, with significant variability in the early part of the season. Disappearingreappearing high albedo areas are seasonally cyclical, if large, and variable on multi-year scales, if small. These variabilities are tied to interplays among frost deposition, evolution, and sublimation, along with deposition and removal of dust. Similarly, Piqueux et al. (2015a) find that atmospheric dust is the primary source of seasonal cap variability. Based on orbital and Phoenix surface data, Sizemore et al. (2015) present a model for ice lens initiation to help explain the origin, history, and stratigraphy of shallow ground ice on Mars. This study was motivated by evidence that ground ice exceeds the pore volume of host soils in many areas of the planet. They use a numerical model to account for ice lenses and find that initiation is ubiquitous at high latitudes, with the degree of growth dependent on the properties of the host soil. For example, soils rich in clay-size particles or perchlorates have greater ice segregation in the upper meter of the regolith. Becerra et al. (2015), using multiple spacecraft datasets, examine the enigmatic bright haloes surrounding many of the pits on the south polar residual cap that appear in MY 28. These haloes had not appeared before and have not been documented since. Analyzing reflectance, they find that the haloes were caused by sublimation winds associated with the global dust storm that occurred that year. The lack of halo exhumation in subsequent years indicates a positive mass balance on flat areas that competes against expansion of the residual cap pits and scarps. Gullies have been the focus of intense study since their identification by Malin and Edgett (2000), including evidence that some change over time. Here, Raack et al. (2015) document seasonal gully activity in a south polar pit in which dark flows were deposited on top of a CO2 ice slab overlying a debris apron. Investigating three formation scenarios, the find that dry flows that may be supported by sublimating CO 2 are most consistent with the observations. Dundas et al. (2015) reach a similar conclusion, finding, through a global study, that gully activity occurs only in the presence of sublimating frost. Currently, gully development is more prevalent in the southern hemisphere due to the close timing of perihelion and the summer solstice. Considering variations in obliquity that affect martian climate, Dundas et al. (2015) propose that all gullies can form over time scales of millions of years. With the advent of high-resolution images and long-term observational baselines from HiRISE and other remote sensors, studies of martian dunes and ripples, previously focused on documentation and morphology, now encompass detailed analyses of dynamics and their causes. Along this vein, Hansen et al. (2015) investigate the changes in the north polar dunes over 4 Mars years. They find considerable interannual variability, with most changes occurring between late summer and winter, consistent with CO2 ice changes, strong winds, or a combination of the processes being the driver of the changes. A comparison to analog processes in the southern mid-latitudes (where less haze and greater seasonal coverage results in better observation conditions) suggests that frost emplacement and removal likely play an important role. Moving away from the polar regions, Chojnacki et al. (2015) focus on dunes near Endeavour Crater in Meridiani Planum, an equatorial site that has been monitored both from the surface by the Opportunity
3
rover and from orbit by HiRISE and other instruments. Here, dome dunes deflate and scatter sand across the plains and into the crater. Activity can be attributed to bi- and possibly tri-modal wind regimes. Probably most significantly, the highest migration rates (4–12 m yr 1) and volumetric sand fluxes (2–13 m3 yr 1) ever found on Mars are recorded, with turnover times much shorter than obliquity cycles, showing that current conditions can fully explain the aeolian dunes. Finally, two papers in this issue rely on large datasets spanning many years that elucidate details of the geologic record in Valles Marineris. Weitz et al. (2015) use CRISM and HiRISE data to study southwestern Melas Chasma, focusing on blocky deposits and layered mounds on the floor, wall rock, and airfall draping materials. They show that the diversity of hydrated minerals and fluvial features within this portion of the chasma and the nearby plateau is best explained by multiple episodes of aqueous activity under distinct environmental conditions from the Hesperian to the Amazonian. Noel et al. (2015) describe a stratigraphic succession in Juventae Chasma. Monohydrated sulfates, dominated by kieserite, are the oldest unit and probably formed from dissolution of mafic minerals followed by precipitation at temperatures of 150– 200 °C. These are overlain by polyhydrated sulfates, many of which have been eroded away. 4. Enumeration of the martian calendar Using a calendar setting martian years beginning in Year 1 (MY 1) at solar longitude = 0° on April 11, 1955, with subsequent incrementing to the present day, as originally proposed by Clancy et al. (2000), has become increasingly common in the literature, as the system decouples annual cycles on Mars from the shorter terrestrial year. However, as described by Piqueux et al. (2015b), many martian telescopic studies, including the investigation of dynamic atmospheric processes, occurred prior to MY 1. They therefore propose extending the calendar back to Galileo’s first telescopic observation in 1610, with a designation of ‘‘MY183.’’ Also in this issue, Montabone et al. slightly adjust the start time of MY 1 from 13:26 to 0:00 local mean solar time to have a sol-based calendar as opposed to one fixed by solar longitude. By introducing these augmented and revised calendar systems in this special issue, the Mars community has the opportunity to consider their use in future studies. 5. Conclusions The 20 papers herein demonstrate the vitality of research into ongoing martian processes and the necessity of maintaining spacecraft capable of making continuous observations of the planet over time. The results are exciting and demonstrative of the high data quality returned by surface and orbital spacecraft at Mars. We therefore expect that the Dynamic Mars Special Issue of Icarus will be a frequently cited volume and a harbinger of future exciting results. References Becerra, P. et al., 2015. Transient bright ‘‘Halos’’ on the south polar residual cap of Mars: Implications for mass-balance. Icarus 251, 211–225. Calvin, W.M. et al., 2015. Interannual and seasonal changes in the north polar ice deposits of Mars: Observations from MY 29–31 using MARCI. Icarus 251, 181– 190. Chojnacki, M. et al., 2015. Persistent aeolian activity at Endeavour crater, Meridiani Planum, Mars; new observations from orbit and the surface. Icarus 251, 275–290. Clancy, R.T. et al., 2000. An intercomparison of ground-based millimeter, MGS TES, and Viking atmospheric temperature measurements: Seasonal and interannual variability of temperatures and dust loading in the global Mars atmosphere. J. Geophys. Res. 105, 9553–9571. Dundas, C.M. et al., 2015. Long-term monitoring of martian gully formation and evolution with MRO/HiRISE. Icarus 251, 244–263.
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Hansen, C.J. et al., 2015. Agents of change on Mars’ northern dunes: CO2 ice and wind. Icarus 251, 264–274. Kass, D.M. et al., 2014. Interannual behavior of large regional dust storms. In: Eighth International Conference on Mars, July 14–18, 2014, Pasadena, CA. Lemmon, M.T. et al., 2015. Dust aerosol, clouds, and the atmospheric optical depth record over 5 Mars years of the Mars Exploration Rover mission. Icarus 251, 96– 111. Malin, M.C., Edgett, K.S., 2000. Evidence for recent groundwater seepage and surface runoff on Mars. Science 288, 2330–2335. Montabone, L. et al., 2015. Eight-year climatology of dust optical depth on Mars. Icarus 251, 65–95. Noel, A. et al., 2015. Mineralogy, morphology and stratigraphy of the light-toned interior layered deposits at Juventae Chasma. Icarus 251, 315–331. Piqueux, S. et al., 2015a. Variability of the martian seasonal CO2 cap extent over eight Mars Years. Icarus 251, 164–180. Piqueux, S. et al., 2015b. Enumeration of Mars years and seasons since the beginning of telescopic exploration. Icarus 251, 332–338. Raack, J. et al., 2015. Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars. Icarus 251, 226–243. Shirley, J.H., 2015a. Temperatures and aerosol opacities of the Mars atmosphere at aphelion: Validation and intercomparison of limb sounding profiles from MRO/ MCS and MGS/TES. Icarus 251, 26–49. Shirley, J.H., 2015b. Solar System dynamics and global-scale dust storms on Mars. Icarus 251, 128–144. Sizemore, H. et al., 2015. Initiation and growth of martian ice lenses. Icarus 251, 191–210.
Trokhimovskiy, A. et al., 2015. Mars’ water vapor mapping by the SPICAM IR spectrometer: Five martian years of observations. Icarus 251, 50–64. Vincendon, M. et al., 2015. Mars Express measurements of surface albedo changes over 2004–2010. Icarus 251, 145–163. Wang, H., Richardson, M.I., 2015. The origin, evolution, and trajectory of large dust storms on Mars during Mars years 24–30 (1999–2011). Icarus 251, 112–127. Weitz, C.N. et al., 2015. Mixtures of clays and sulfates within deposits in western Melas Chasma, Mars. Icarus 251, 291–314. Withers, P., Morgan, D., Gurnett, D., 2015. Variations in peak electron densities in the ionosphere of Mars over a full solar cycle. Icarus 251, 5–11. Zhang, Z. et al., 2015. Topside of the martian ionosphere near the terminator: Variations with season and solar zenith angle and implications for the origin of the transient layers. Icarus 251, 12–25.
Nathan T. Bridges Applied Physics Laboratory, Laurel, MD 20723, USA E-mail address: [email protected] Leslie K. Tamppari Jet Propulsion Laboratory, Pasadena, CA 91109, USA E-mail addresses: [email protected]
Contents lists available at ScienceDirect
Icarus journal homepage: www.elsevier.com/locate/icarus
Editorial
Dynamic Mars from long-term observations: Introduction
Of all the planets in the Solar System besides Earth, Mars stands out as one for which both surface and atmospheric changes occur at decadal, annual, and shorter time scales. Indeed, the seasonal variability of Mars was noted by early astronomers such as Herschel, Schiaparelli, and Lowell. With the advent of the space age, Mars has been of particular focus for exploration. Beginning with Mariner 4 in 1965, no fewer than 20 successful spacecraft have flown by, orbited, landed, and roved on the surface. Although this exploration has been heavily driven by trying to understand the role of present and past water on the planet, and the search for evidence for environments that could support life, data from all these spacecraft has led to numerous discoveries spanning geology, geophysics, atmospheric sciences, and other fields. The period of martian exploration we find ourselves in now is unique, as there has been a continuous record of spacecraft exploration that began with Pathfinder and Mars Global Surveyor in 1997. Now well into the 2nd decade of this campaign, bridged to earlier times by spacecraft observations from the 1960s and 1970s, and continuous telescopic observations, our view of Mars has become one of a planet on which surface and atmospheric changes occur at frequencies of days, years, and decades, a testament to long-term monitoring that continues to this day. Recognizing this accumulation of data and an emerging view of Mars as a dynamic planet, we thought it an appropriate time that this record, with implications for martian geology, climate, atmospheric dynamics, and other processes, be integrated into a single journal special issue, presented here. This volume contains 20 papers each of which make use of one or more datasets and cover >1 Mars year. The papers highlight new results that are a unique outcome of the long-term data acquisition provided by our robust and long-duration program of Mars exploration and telescopic observations. The papers also highlight the long-term implications of processes that are observed and ongoing now, and show the importance of such longevity and consistency in atmospheric and surface observations. These results demonstrate the importance of continuous monitoring of Mars through new and especially extended missions. We thank the authors, reviewers, and the editors, and especially Eva Scalzo at Icarus, without whom this special issue would not be possible. Eva, the editorial office manager for Icarus, was the interface between us and the reviewers and authors, and provided much-needed advice on policies and procedures. Jeff Moersch was the Icarus editor who oversaw this issue. He provided timely and sage advice on several reviews and questions concerning http://dx.doi.org/10.1016/j.icarus.2015.03.001 0019-1035/Ó 2015 Published by Elsevier Inc.
editorial policies. We would finally like to thank Phil Nicholson for granting permission for this special issue to proceed. We summarize the contents of the special issue in the following sections. 1. Ionosphere With the MAVEN spacecraft’s arrival at Mars, our understanding of the martian ionosphere is sure to increase. These data can be compared and contrasted to studies such as two presented in this special issue by Withers et al. (2015) and Zhang et al. (2015), who make use of MGS and MEX observations to gain further insight into the long term (Withers et al., 2015) and transient (Zhang et al., 2015) behavior of the ionosphere. Using both MGS and MEX data, Withers et al. (2015) examine the peak electron densities in the Mars ionosphere and show that they increase smoothly with increasing solar irradiance throughout the >1 solar cycle of observations now available. They also see hints of trends possibly associated with seasonal variations in the martian thermosphere. Zhang et al. (2015) use the MEX/MARSIS data of the martian ionosphere to investigate transient layers that lie above the peak electron density layer studied by Withers et al. (2015). The height and density of the transient layer, typically 60 km above the main density peak, correlate with the height and density of the main peak. They find that in the southern summer, the transient layer is about 10 km higher than other times of year, likely resulting from lower atmospheric heating. They suggest that beam-plasma instabilities are responsible for the transient layers. 2. Lower atmosphere The behavior of the water vapor, water–ice and dust aerosols, and temperatures in the lower atmosphere define the current climate and weather of Mars. This is, of course, critical for accurate modeling, deciphering the controlling physical mechanisms, and for understanding the past climate, when Mars may have been warmer and wetter and a hospitable place for life. Six papers in this special issue discuss these quantities and are summarized here. As highlighted in this special issue, having a long record of observations from a variety of spacecraft with similar, yet different, techniques is very valuable in defining the climate of Mars and understanding differences due to weather. However a challenge is understanding the uncertainties in the different techniques and the cross-calibration between experiments. In 2015, Shirley (2015a) provides a comparison of atmospheric temperatures and aerosols between the MRO/MCS limb sounding experiment and
2
Editorial / Icarus 251 (2015) 1–4
limb profiles taken by the MGS/TES experiment, using MGS RS temperature profiles as a common standard for comparison. He finds good agreement when the opacities are similar, except at the highest TES sampled altitudes, when MCS temperatures are systematically higher, providing confidence in use of these data for long-term studies. Shirley (2015a) finds that there is some interannual variability in latitudinal temperature gradients, thermal tide behavior, and water–ice aerosols during the aphelion season. Understanding the behavior of water vapor in the martian atmosphere is important for characterizing the present global water cycle, and for validating models, which can then be used to gain insight into the past martian climate. In this issue, Trokhimovskiy et al. (2015) provides new retrievals of water vapor from the MEX/SPICAM experiment covering 5 Mars years. His retrieval is improved by accounting for the multiple scattering of both dust and water–ice aerosols. He finds that the annual cycle is largely repeatable with water vapor maxima and minima consistent with other observational datasets. The exception is with MY 28 in which a global dust storm occurred. In this year, near the season of the storm, the water vapor abundances are lower than typical, which cannot be explained by masking by dust. Trokhimovskiy provides maps of water vapor abundance for each year, as well as maps showing retrieved saturation altitudes. Dust in the martian atmosphere is important for controlling the dynamics of the planet, and its optical depth and spatial and temporal variations are needed for accurate modeling of the atmosphere. In particular, dust distribution and amount are needed to calculate the mass mixing ratio and the heating of the atmosphere, both of which are used to estimate the thermal forcing of the atmosphere for the given conditions. Montabone et al. (2015) provide a nearly 8-Mars-year dust climatology using 3 dust datasets: MGS/ TES, ODY/THEMIS, and MRO/MCS (see cover illustration). They provide two types of gridded maps, irregular, which has missing data due to lack of coverage or poor data quality, and complete maps using kriging, an interpolation scheme. Maps showing both data sets are provided as online supplementary material and the datasets are available for download from the Mars Climate Database (MCD) website, referenced in their paper. Further, they have provided an average climatological year. Montabone et al. (2015) compare and quality check their results with dust optical depths from other orbital and landed instruments, including the MER Pancam and Mini-TES, the Phoenix SSI, MRO CRISM and MARCI, and the MGS MOC. Their work identifies and documents various instrument biases (compare to Shirley (2015a)). Examining their resulting climatology, they identify 4 annual phases of dust distribution, for years without a global dust event, consistent with Lemmon et al. (2015), Wang and Richardson (2015), and Kass et al. (2014). To complement global, lower spatial resolution observations of dust optical depth, Lemmon et al. (2015) provides and archives to NASA’s Planetary Data System the extinction optical depth record from both MER rovers (3 MY for Spirit; 5 MY for Opportunity). The data show that, for these two equatorial locations, the dust amount is low for the northern spring through mid-summer, with tens of percent variability year over year. Dust is more variable for the remainder of the year, being characterized by local, regional, and the occasional global dust storm, consistent with Montabone et al. (2015). Comparison with MER mini-TES contemporaneous observations at a different wavelength shows that the mean dust particle size is typically 1–1.4 lm, but becomes 2 lm at the onset of dust storms. This surface view of dust also allows for the study of smaller scale events, such as dust devils. Lemmon finds that insolation received at the surface (from seasonal changes and/or total optical depths) appears to control dust devil frequency at Spirit.
The origin and evolution of large-scale dust events, particularly, global dust events, is not well understood. As such, studies enabled by continuous global imaging covering multiple Mars years to examine many events are key to identifying characteristics and seasons of generation and evolution, as well as for quantifying behaviors that can be compared to models. This has been done in Wang and Richardson (2015), who examined daily global maps of aerosols over portions of 7 Mars years (12 Earth years) to identify and study large, discrete dust events that lasted P5 sols, and in which dust expanded outside the origination region. The daily global maps were created from data taken from the MGS MOC and the MRO MARCI instruments. They studied a total of 65 events, not including dust events that remained confined to the circumpolar regions. They identify that all events occurred between Ls = 135–30°, indicating a short dust-storm free season, consistent with lower optical depths seen in Montabone et al. (2015) and Lemmon et al. (2015). They define types of dust storm development, identify regions of dust storm generation, and provide a figure indicating the routes traveled by these storms. Of the two global events, they propose that both originated with southerly events, although the 2007 storm is somewhat ambiguous. To further understand global dust storm origination, a new hypothesis has been proposed by Shirley (2015b). He provides an updated table of global dust events from 1924 to 2013, and argues that a statistically significant relationship between the storm occurrence and variations in the orbital angular momentum of Mars with respect to the Solar System barycenter exists, meaning that planetary dynamics may influence the interannual variability of the martian atmosphere. Further, Shirley (2015b) provides a prediction for future years that should experience global dust events, if this correlation persists, but cautions that other factors such as availability of dust supply in convenient reservoirs may be important in global dust storm manifestation. 3. Surface monitoring and dynamic processes High scientific yields using data spanning years and across multiple spacecraft is aptly demonstrated in the 11 papers focusing on martian surface monitoring. These works include those on surface albedo changes, volatiles and ice, gullies, aeolian processes, and surface geology, with overlap in many areas. We give a brief overview of these papers here. Albedo changes are the most obvious and pervasive responses to dynamic processes on Mars, namely the lifting, transport, and deposition of dust. Beginning with telescopic observations, albedo monitoring has continued to the present day with spacecraft data. On this topic, Vincendon et al. (2015) describe observations of surface albedo changes observed by Mars Express’ OMEGA imaging spectrometer from 2004 to 2010. Using a detailed retrieval scheme, they find that bright surfaces are 17% greater than previous measurements, with most changes occurring during the storm season, in particular that of the 2007 global event. Changes over seasonal to decadal timescales are attributed to the removal and deposition of optically thin, bright dust coatings that mask underlying near-IR spectral signatures. The presence, distribution, and dynamics of volatiles and ices control the martian energy budget and the atmospheric circulations, and reflect temperature-dependent phase changes that exchange carbon dioxide and water between the martian surface–subsurface and atmosphere. In this issue Piqueux et al. (2015a) document polar CO2 cap recession/growth over MY 24 to 31 and define a ‘‘climatological’’ cap edge as a function of season. They use thermal IR data, insensitive to lighting conditions, and are therefore able to present for the first time the full growth/recession cycles for all 8 MY. Outside of the two global dust storm periods, they find that the north polar cap behavior is very
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repeatable (to 1–2° lat), although south polar cap behavior is less so than the north. During this study period two global dust storms occurred (early- and mid-season) and the cap behaviors were different, suggesting that the timing of the storm and its affect on the atmospheric circulation and temperatures combine to determine the affect on CO2 deposition and sublimation. Calvin et al. (2015) use Mars Color Imager (MARCI) data to document interannual and seasonal changes in the north polar ice deposits over 3 Mars years. In contrast to Piqueux et al. (2015a), their study focuses on the northern cap (not both) within MY 29–31 and uses visual data. The retreating cap edge is very dynamic, with significant variability in the early part of the season. Disappearingreappearing high albedo areas are seasonally cyclical, if large, and variable on multi-year scales, if small. These variabilities are tied to interplays among frost deposition, evolution, and sublimation, along with deposition and removal of dust. Similarly, Piqueux et al. (2015a) find that atmospheric dust is the primary source of seasonal cap variability. Based on orbital and Phoenix surface data, Sizemore et al. (2015) present a model for ice lens initiation to help explain the origin, history, and stratigraphy of shallow ground ice on Mars. This study was motivated by evidence that ground ice exceeds the pore volume of host soils in many areas of the planet. They use a numerical model to account for ice lenses and find that initiation is ubiquitous at high latitudes, with the degree of growth dependent on the properties of the host soil. For example, soils rich in clay-size particles or perchlorates have greater ice segregation in the upper meter of the regolith. Becerra et al. (2015), using multiple spacecraft datasets, examine the enigmatic bright haloes surrounding many of the pits on the south polar residual cap that appear in MY 28. These haloes had not appeared before and have not been documented since. Analyzing reflectance, they find that the haloes were caused by sublimation winds associated with the global dust storm that occurred that year. The lack of halo exhumation in subsequent years indicates a positive mass balance on flat areas that competes against expansion of the residual cap pits and scarps. Gullies have been the focus of intense study since their identification by Malin and Edgett (2000), including evidence that some change over time. Here, Raack et al. (2015) document seasonal gully activity in a south polar pit in which dark flows were deposited on top of a CO2 ice slab overlying a debris apron. Investigating three formation scenarios, the find that dry flows that may be supported by sublimating CO 2 are most consistent with the observations. Dundas et al. (2015) reach a similar conclusion, finding, through a global study, that gully activity occurs only in the presence of sublimating frost. Currently, gully development is more prevalent in the southern hemisphere due to the close timing of perihelion and the summer solstice. Considering variations in obliquity that affect martian climate, Dundas et al. (2015) propose that all gullies can form over time scales of millions of years. With the advent of high-resolution images and long-term observational baselines from HiRISE and other remote sensors, studies of martian dunes and ripples, previously focused on documentation and morphology, now encompass detailed analyses of dynamics and their causes. Along this vein, Hansen et al. (2015) investigate the changes in the north polar dunes over 4 Mars years. They find considerable interannual variability, with most changes occurring between late summer and winter, consistent with CO2 ice changes, strong winds, or a combination of the processes being the driver of the changes. A comparison to analog processes in the southern mid-latitudes (where less haze and greater seasonal coverage results in better observation conditions) suggests that frost emplacement and removal likely play an important role. Moving away from the polar regions, Chojnacki et al. (2015) focus on dunes near Endeavour Crater in Meridiani Planum, an equatorial site that has been monitored both from the surface by the Opportunity
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rover and from orbit by HiRISE and other instruments. Here, dome dunes deflate and scatter sand across the plains and into the crater. Activity can be attributed to bi- and possibly tri-modal wind regimes. Probably most significantly, the highest migration rates (4–12 m yr 1) and volumetric sand fluxes (2–13 m3 yr 1) ever found on Mars are recorded, with turnover times much shorter than obliquity cycles, showing that current conditions can fully explain the aeolian dunes. Finally, two papers in this issue rely on large datasets spanning many years that elucidate details of the geologic record in Valles Marineris. Weitz et al. (2015) use CRISM and HiRISE data to study southwestern Melas Chasma, focusing on blocky deposits and layered mounds on the floor, wall rock, and airfall draping materials. They show that the diversity of hydrated minerals and fluvial features within this portion of the chasma and the nearby plateau is best explained by multiple episodes of aqueous activity under distinct environmental conditions from the Hesperian to the Amazonian. Noel et al. (2015) describe a stratigraphic succession in Juventae Chasma. Monohydrated sulfates, dominated by kieserite, are the oldest unit and probably formed from dissolution of mafic minerals followed by precipitation at temperatures of 150– 200 °C. These are overlain by polyhydrated sulfates, many of which have been eroded away. 4. Enumeration of the martian calendar Using a calendar setting martian years beginning in Year 1 (MY 1) at solar longitude = 0° on April 11, 1955, with subsequent incrementing to the present day, as originally proposed by Clancy et al. (2000), has become increasingly common in the literature, as the system decouples annual cycles on Mars from the shorter terrestrial year. However, as described by Piqueux et al. (2015b), many martian telescopic studies, including the investigation of dynamic atmospheric processes, occurred prior to MY 1. They therefore propose extending the calendar back to Galileo’s first telescopic observation in 1610, with a designation of ‘‘MY183.’’ Also in this issue, Montabone et al. slightly adjust the start time of MY 1 from 13:26 to 0:00 local mean solar time to have a sol-based calendar as opposed to one fixed by solar longitude. By introducing these augmented and revised calendar systems in this special issue, the Mars community has the opportunity to consider their use in future studies. 5. Conclusions The 20 papers herein demonstrate the vitality of research into ongoing martian processes and the necessity of maintaining spacecraft capable of making continuous observations of the planet over time. The results are exciting and demonstrative of the high data quality returned by surface and orbital spacecraft at Mars. We therefore expect that the Dynamic Mars Special Issue of Icarus will be a frequently cited volume and a harbinger of future exciting results. References Becerra, P. et al., 2015. Transient bright ‘‘Halos’’ on the south polar residual cap of Mars: Implications for mass-balance. Icarus 251, 211–225. Calvin, W.M. et al., 2015. Interannual and seasonal changes in the north polar ice deposits of Mars: Observations from MY 29–31 using MARCI. Icarus 251, 181– 190. Chojnacki, M. et al., 2015. Persistent aeolian activity at Endeavour crater, Meridiani Planum, Mars; new observations from orbit and the surface. Icarus 251, 275–290. Clancy, R.T. et al., 2000. An intercomparison of ground-based millimeter, MGS TES, and Viking atmospheric temperature measurements: Seasonal and interannual variability of temperatures and dust loading in the global Mars atmosphere. J. Geophys. Res. 105, 9553–9571. Dundas, C.M. et al., 2015. Long-term monitoring of martian gully formation and evolution with MRO/HiRISE. Icarus 251, 244–263.
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Hansen, C.J. et al., 2015. Agents of change on Mars’ northern dunes: CO2 ice and wind. Icarus 251, 264–274. Kass, D.M. et al., 2014. Interannual behavior of large regional dust storms. In: Eighth International Conference on Mars, July 14–18, 2014, Pasadena, CA. Lemmon, M.T. et al., 2015. Dust aerosol, clouds, and the atmospheric optical depth record over 5 Mars years of the Mars Exploration Rover mission. Icarus 251, 96– 111. Malin, M.C., Edgett, K.S., 2000. Evidence for recent groundwater seepage and surface runoff on Mars. Science 288, 2330–2335. Montabone, L. et al., 2015. Eight-year climatology of dust optical depth on Mars. Icarus 251, 65–95. Noel, A. et al., 2015. Mineralogy, morphology and stratigraphy of the light-toned interior layered deposits at Juventae Chasma. Icarus 251, 315–331. Piqueux, S. et al., 2015a. Variability of the martian seasonal CO2 cap extent over eight Mars Years. Icarus 251, 164–180. Piqueux, S. et al., 2015b. Enumeration of Mars years and seasons since the beginning of telescopic exploration. Icarus 251, 332–338. Raack, J. et al., 2015. Present-day seasonal gully activity in a south polar pit (Sisyphi Cavi) on Mars. Icarus 251, 226–243. Shirley, J.H., 2015a. Temperatures and aerosol opacities of the Mars atmosphere at aphelion: Validation and intercomparison of limb sounding profiles from MRO/ MCS and MGS/TES. Icarus 251, 26–49. Shirley, J.H., 2015b. Solar System dynamics and global-scale dust storms on Mars. Icarus 251, 128–144. Sizemore, H. et al., 2015. Initiation and growth of martian ice lenses. Icarus 251, 191–210.
Trokhimovskiy, A. et al., 2015. Mars’ water vapor mapping by the SPICAM IR spectrometer: Five martian years of observations. Icarus 251, 50–64. Vincendon, M. et al., 2015. Mars Express measurements of surface albedo changes over 2004–2010. Icarus 251, 145–163. Wang, H., Richardson, M.I., 2015. The origin, evolution, and trajectory of large dust storms on Mars during Mars years 24–30 (1999–2011). Icarus 251, 112–127. Weitz, C.N. et al., 2015. Mixtures of clays and sulfates within deposits in western Melas Chasma, Mars. Icarus 251, 291–314. Withers, P., Morgan, D., Gurnett, D., 2015. Variations in peak electron densities in the ionosphere of Mars over a full solar cycle. Icarus 251, 5–11. Zhang, Z. et al., 2015. Topside of the martian ionosphere near the terminator: Variations with season and solar zenith angle and implications for the origin of the transient layers. Icarus 251, 12–25.
Nathan T. Bridges Applied Physics Laboratory, Laurel, MD 20723, USA E-mail address: [email protected] Leslie K. Tamppari Jet Propulsion Laboratory, Pasadena, CA 91109, USA E-mail addresses: [email protected]