Ruzi Trisit (JSPS Research Fellow at the time of the research / Currently Research Fellow, Japan Aerospace Exploration Agency)

Kenshi Kawai (Associate Professor, Department of Earth and Planetary Science)

Key points of the presentation

• The formation age of wrinkle ridges on the Martian surface was estimated, and it was found that the Martian interior cooled down to the point that global volcanism stopped 3.7 billion years ago, and that the volcanism that resumed 3.55 billion years ago was only localized, and that the rate of planetary contraction due to cooling was highest between 3.59 and 3.55 billion years ago. The rate of planetary contraction due to cooling is highest at 3.59-3.55 billion years ago.
• The past global thermal history and climate change on Mars has been a mystery until now. By analyzing the latest high-resolution images of Mars using state-of-the-art dating methods, this study succeeded for the first time in the world in estimating the formation age of "wrinkle" structures distributed on the surface of Mars across the globe, and was able to determine when volcanic activity ceased on Mars in the past.
• The absence of wrinkle ridges formed before 3.8 billion years ago suggests that the climate changed from an erosion-inducing "intense" climate to a "moderate" climate at this time. By applying the newly established methods to other surface topography, it is expected that we will be able to understand how and when Mars evolved from a thermally active planet in the past to the cold planet it is today.

Announcement Summary

A research group led by Development Researcher Ruzi Trisit of the Japan Aerospace Exploration Agency (at the time of the research: JSPS Research Fellow of the Japan Society for the Promotion of Science) and Associate Professor Kenshi Kawai of the Graduate School of Science, The University of Tokyo, has used a cutting-edge dating method called Buffered Crater Counting ^{(Note 1)} to determine the age of the Martian surface layer. Using a state-of-the-art dating method called Buffered Crater Counting (Note 1), Prof. Kawai's group estimated the formatio ^{n age of the Wrinkle Ridge (Note 2)}, a "wrinkle" geologic structure found on the Martian surface. However, the methods and data used have been limited, and only a limited number of regions and ages have been understood. The presenters focused on a global feature on Mars known as a crinkle ridge. Wrinkle ridges are thought to have been formed by contraction and distortion of the crust after the cessation of volcanic activity, and are thought to provide clues to understanding when volcanic activity ceased, i.e., when the Martian interior cooled. Global analysis of high-resolution images taken by recent spacecraft has revealed that based on the crater chronology of Mars ^{(Note 3)}, most of Mars' rinkled ridges were formed between 2.5 and 3.8 billion years ago, with the greatest concentration of rinkled ridges between 3.55 and 3.59 billion years ago. Considering the activity ages of the surrounding volcanoes, this result suggests that the Martian interior cooled down about 3.7 billion years ago and large-scale volcanic activity ceased. Based on this study, further research on the thermal evolution of the Martian interior is expected to lead to a better understanding of how and when Mars evolved differently from Earth.

Announcements

Understanding the thermal history is intrinsically important to understanding the evolution of the Earth and terrestrial planets. In particular, on Mars, it is thought that most of the heat was lost by thermal radiation from the surface soon after formation, resulting in the evolution of the dry and cold surface environment that we see today. In reality, however, heat loss due to volcanism and other factors is complex, and the processes and rates of global heat loss are still largely unknown. Unlike the Earth, Mars is less affected by the renewal of surface materials due to plate motions and weathering and erosion. Therefore, it is expected that the geological structure and topography preserved in the surface layer can be observed and the formation process and age of the formation can be clarified to understand the heat loss process and its age on Mars in the past. In addition, elucidation of the thermal history of Mars, which is relatively close to Earth in size and orbital elements, is expected to provide important insights into the geological and thermal evolution on Earth from a comparative planetary perspective.

The landforms formed by globally distributed crustal deformation (especially the stress history associated with volcanism) are one of the most important objects for understanding the past thermal evolution on Mars. Among them, the topography known as crinkle ridge, a "wrinkle" surface topography formed at the surface by impulse faults (blind thrusts; ^{(Note 4)) that} have not appeared at the surface, is considered one of the best indicators for estimating the past stress field. Previous studies that estimated relative ages based on stratigraphic relationships or local absolute ages had large errors in their estimated formation ages due to the low resolution of the images used. Determining the formation age of Wrinkle Ridge with high precision will lead to an understanding of the stress history associated with volcanic activity on Mars at that time, which is key to understanding the thermal history of the planet.

The team analyzed Context Camera (CTX; ^{(Note 5))} images, which have the highest resolution currently available globally on Mars, and described the crinkle ridge globally (Figure 1).

Figure 1: Thermal Emission Imaging System (THEMIS) mosaic image (Christensen et al., 2004) with Mars Orbital Laser Altimeter (MOLA) - High-Resolution Stereo Camera (HRSC) blended DEM (Smith et al., 2001; Fergason et al., 2017) projected onto the Mars Orbital Laser Altimeter (MOLA) - High-Resolution Stereo Altimeter (HRSC) mosaic image (Christensen et al., 2004). The terrain extending in a linear northeast-southwest direction is the Wrinkle Ridge.

We first identified the rinkled ridges based on their elongation direction and morphology (length, height, width, and spacing) and found that rinkled ridges are distributed in 27 regions. These regions surround major volcanic regions on Mars. The identified rinkled ridges were dated using Buffered Crater Counting (BCC), a powerful dating method that can accurately and precisely determine the formation age of even linear geological structures such as rinkled ridges. The formation ages of all identified rinkle ridges were determined using BCC and were found to be concentrated between 2.5 and 3.8 billion years, with the largest number of rinkle ridges formed between 3.55 and 3.59 billion years ago (Figure 2).

Figure 2: Distribution and formation ages of crinkled ridges throughout Mars described by this study. Blue lines are the identified crinkle ridges. Numbers refer to ages (in Ga) determined based on the BCC. Background color indicates elevation of Mars global based on MOLA-HRSC blended DEM (Smith et al., 2001; Fergason et al., 2017).

Considering the formation age of the lava plains, which are thought to have been formed by surrounding volcanoes, the volcanic activity that formed the lava plains subsided about 3.7 billion years ago, followed by the formation of the crinkle ridge. After that, some volcanoes are thought to have temporarily resumed activity around 3.5 billion years ago, but the distribution of the rinkled ridges after 3.5 billion years ago suggests that they were localized. This suggests that heat loss from the interior due to volcanic activity was not constant on Mars, but was most rapid around 3.7 billion years ago. In addition, the absence of the wrinkle ridge formed before 3.8 billion years ago on the surface suggests that the planet had a humid climate before 3.8 billion years ago, when severe erosion occurred, and that the wrinkle ridge has been preserved to the present because of the large-scale climate change that occurred around 3.8 billion years ago.

The age of formation of the rinkle ridge elucidated in this study is much older than the age of the tectonically active surface landforms on the Earth's surface, and is relatively close to the age at which life on Earth was born. In other words, Mars may have already evolved into a thermally "dying" state by the time life emerged on Earth, and the surface environment may have been dry and cold, as it is today. In the future, we intend to apply our methods to other surface features found on Mars, such as rifts and normal faults, to accurately determine their age. This study is a first step toward understanding the dynamics of the Martian interior and will lead to a better understanding of the thermal history on Mars.

Journal

Journal name	Icarus
Title of paper	A global investigation of wrinkle ridge formation events; Implications towards the thermal evolution of Mars
Author(s)	Trishit Ruj*, Kenji Kawai*
DOI number	10.1016/j.icarus.2021.114625
Abstract URL	https://doi.org/10.1016/j.icarus.2021.114625

Terminology

Note 1 Buffered Crater Counting (BCC).

A method of age estimation that is a development of crater geochronology. It is capable of determining the formation ages of linear and curvilinear landforms, which is difficult with conventional crater geochronology. ↑up

Note 2 Wrinkle Ridge

Wrinkle ridges are wrinkle-like linear structures seen on the Moon and Mars. They are thought to be formed in association with lava ejected by volcanic activity. ↑up

Note 3 Crater Dendrochronology on Mars

Crater geochronology is a method to determine the formation age of specific regions and landforms based on the number density of craters on the surface of solid bodies, and is the most widely used method to determine the age of bodies for which sample returns have not been conducted. For Mars, it is possible to determine the age with a precision of several tens of percent by assuming a crater formation rate based on observations and simulations. ↑up

Note 4: Blind thrust fault

A reverse fault formed by compressive stress in the earth's crust that causes the strata on one side to slide up on top of the strata on the other side. The compressive stress generated in the crust at the time of formation can be estimated from the angle and size of the fault. ↑up

5 Context Camera (CTX )

A camera onboard the Mars Reconnaissance Orbiter (MRO) with a maximum resolution of ~6m/pix. It has imaged more than 99% of Mars and has the highest resolution of all cameras that have imaged the entire planet. ↑ up