Mars: Ancient And Ongoing Processes That Shape Planet Surface

Professor Alfred McEwen, HiRISE’s Principal Investigator, highlighted some of the most recent results at the European Planetary Science Congress in Münster on September 24th.

A study of the nature and distribution of ancient megabreccia, led by McEwen at the University of Arizona, suggests that this bedrock was formed during the late heavy bombardment period. Megabreccia consists of angular, randomly-orientated blocks that formed suddenly in energetic events such as meteorite impacts. It is thought to contain fragments of the oldest and deepest bedrock exposed on the surface of Mars.

“We think that the megabreccia was formed during a period of heightened meteorite activity about 3.9 billion years ago. This is around the time life appears to have begun on Earth, but we have very little record of that era in our terrestrial geology because ancient rocks are heavily metamorphosed. Mars preserves a much better record of the heavy bombardment and, unlike the dry lunar surface, it shows the environmental effects in a water-rich crust,” said McEwen.

The HiRISE team has identified megabreccia in more than 50 locations consistent with the most ancient terrains on Mars. These include the central uplifts of large craters and deep exposures such as the floor of parts of Valles Marineris. Well-exposed rock outcrops are needed to identify megabreccia, in particular from the diversity of colours and textures indicating diverse rock types.

Megabreccia contains rock fragments from the earliest geological period on Mars, the Noachian era, which is more than 3.8 billion years ago.

The megabreccia blocks vary in size from relatively small (1-5 metres) to larger than 10 metres in diameter. The blocks are cemented in a matrix of finer material. The small blocks were probably formed by post Noachian cratering, particularly when found in material filling crater floors. The large blocks are only found in locations consistent with hard, deep bedrock, such as the central uplifts. McEwen suggests that the blocks are largely cemented by melt from impacts and hydrothermal alteration.

“We are in the midst of a paradigm shift in understanding the Noachian crust of Mars, thanks to the high-resolution data from the Mars Express and MRO missions. The spectrometers on these missions found evidence of alteration due to water in the bedrock in many, if not most Noachian aged places. However, the younger Noachian era may have been relatively dry, so we may need to study the oldest outcrops of megabreccia to understand this era.”

McEwen will also be presenting results of processes that may be shaping the surface of Mars today. A study led by Dr Alexandra Lefort, a researcher in the team of Professor Nicolas Thomas of the University of Bern, has shown that scallop-shaped features found in mid-latitudes on Mars are likely to be formed by the sublimation of shallow ice.

HiRISE imagery shows the scallops have steep polar-facing scaps and gentle equatorial-facing rises. Groups of scallops appear to be separated by areas of knobbly ground. Lefort and colleagues studied scallops in the western part of a region called Utopia Planitia, between 40-55 degrees north.

Lefort said, “We have developed a model where a small hummock on the landscape becomes warmer on the equatorial-facing side. As the fraction of ice in the subsurface sublimates, the ground slumps leaving an asymmetric scallop-shaped hollow. The equatorial-facing slope continues to erode, lengthening out the shape. Near the polar-facing scarp, the depression is deepest and coldest and the underlying ice is most stable. Higher ice concentrations near the scarp leads to the development of a fine network of polygonal-shaped cracks across the floor and this may make the scarp more fragile and prone to landslides. Eventually, neighbouring scallops can coalesce”

Large parts of the mid and high-latitudes of Mars are covered by an erosion-resistant mantle deposit perhaps metres thick in places. This layer consists of ice and dust, with concentrations equivalent to around 4 percent water at 40 degree latitudes, ranging to 20 percent water at 60 degree latitudes. It is unclear as yet whether the scallops form in the ice-rich mantle or in ground that is rich in ice due to some other process

Unlike Earth, Mars has a significant variation in the angle at which its rotational axis is tilted relative to the plane of its orbital axis, known as its obliquity. Over periods of tens of millions of years, this angle can vary from nearly vertical to almost 60 degrees. Models have shown that ice is most stable at lower latitudes during periods of high obliquity. The axis of Mars is currently tilted at an angle of 25.2 degrees, an intermediate obliquity, which means that it is possible that these scallop-forming erosion processes are continuing today.

Dr Kathryn Fishbaugh presented results suggesting that the presence of thick, erosion resistant layers called marker beds, which are found at regular intervals through the north polar layered deposits, are linked to changes caused by periodic fluctuations in the planet’s orbital orientation.

The marker beds are separated by 20-30 metres and are 5-10 metres thick, without any evidence of finer-scale layering within them. This suggests that they have either been deposited quickly or that a coating layer is shrouding evidence of fine layering. Between the marker beds are thinner layers with a thickness of a metre or less.

Fishbaugh and colleagues have found an intriguing resonance between the ratio of marker beds to fine layers (20,30:1) and the ratio of the orbital inclination to the precessional period of Mars’s axis (23:1).

“From our observations, it looks like marker beds are formed on Mars when its orbit is relatively flat with respect to the equatorial plane and its axis is relatively upright. But this makes it hard to explain why these layers are so tough. It’s easier to explain the resistance to erosion of they were formed during periods of high obliquity. With more observations, we hope to answer this question.”