Conversations with Tim Kusky: On the discovery of Mars’ ancient ocean in Utopia Planitia.

A team led by Professor Long Xiao from China University of Geosciences discovered marine sedimentary rocks on Mars, a first in Martian exploration. This was achieved by analyzing data from the multispectral camera on China’s Zhurong rover, part of the Tianwen-1 Mars mission. Their findings, published in “National Science Review,” support the existence of ancient oceans in Mars’ northern plains, specifically in the Utopia Planitia region.This discovery is crucial for understanding Mars’ early history and potential for life, as further exploration and sample return missions could provide more insights into Mars’ habitability. The research involved a joint team from Chinese and American universities, with Professors Long Xiao, Jun Huang, and Timothy Kusky playing key roles.
Timothy Kusky discussed the groups findings with SCINQ.

Dr. Timothy Kusky.

Can you provide more context on the significance of the bidirectional current orientations observed in the Zhurong Member of the VBF?

Bidirectional current indicators in sedimentary rocks are typically produced in shallow-water tidal systems on Earth, where the ebb and flood tides flow in opposite directions.  Typically, the flood current (moving landward) is stronger, and carries sand particles landward, forming layers called cross-strata.  When the tide recedes towards the ocean, called an ebb tide, it is typically slower and has less energy, so it transports and deposits slightly finer grained sands, silts, or mud with cross laminae dipping in the opposite direction. 

The composite bedform formed in the sands deposited by these tidal currents is often called herringbone cross-stratification, the type of structure described from the VBF by Long Xiao and team, based on their observations from the Zhurong Rover. This bi-directional current system is fairly unique to tidal system on Earth, so we assume that the similar structures were also deposited by a tidal system on Mars.

The grain size of the layers in the herringbone cross-stratification is rather small, showing that the tidal energy was low, which is consistent with Mars having lower tidal energy than Earth, since Mars has two small moons, and Earth has one large close moon. Further, since the rocks imaged by Zhurong Rover, and interpreted by the Chinese-American team led by Prof Long Xiao, are about 282 km south of the proposed Deuteronilus shoreline (> 3.3 billion years old), the authors suggest that the tidal sediments were deposited when the Deuteronilus Ocean was receding, or drying up and shrinking into the northern lowlands during the Hesperian (~3.7-3.0 billion years ago).

Utopia Planitia (CREDIT: ESA/DLR/FU Berlin).

It was mentioned that the structures in the discovered rocks indicated bidirectional flow characteristics consistent with low-energy tidal currents in Earth’s littoral-shallow marine environment. Could you elaborate on the significance of this bidirectional flow in the context of ancient Mars?

The tidal energy on Mars should have been much lower than that on Earth 3 or 3.3 billion years ago, as then Earth’s moon was closer, and induced strong tidal current.  Mars has always (as far as we know) had two, much smaller moons, that cause much lower tidal forces. 

Therefore, we would expect the tidal currents to be weaker on Mars than Earth.  Consistent with this idea is the observation reported by Long Xiao’s team, that the inferred tidal deposits on Mars suggest a low-energy tidal system, as the sedimentary particles are fine-grained, and the cross-lamination bed forms are small in size.  Similar relationships are found in low-energy tidal system on Earth.

You mention that these sedimentary structures are not indicative of aeolian wind deposits, but rather water-laid deposits. What are some of the distinctive features that led your team to this conclusion?

Geologists use different groupings of sedimentary structures and minerals to help determine the sedimentary environment in which they formed. The authors provided a supplementary data figure explaining the features of some common sedimentary environments (Figure S9, reproduced below), showing that fluvial (river system) deposits typically include gravels at the base of the river channel, cross-bedded sands with the cross-laminae dipping in one direction (not two as in the case of the VBF), and muds deposited in the overbank environments.

Wind-blown (aeolian) deposits typically have giant bedforms, with huge cross lamina dipping in one direction, but in the case of longitudinal dunes, they may have large-scale laminae dipping in two directions, but with different geometries and scale than the authors document from the VBF.

Marine environments, particularly low-energy tidal environments have fine-grained sands and silts and muds, with cross-laminae dipping in two directions, and small-scale tidal channels cutting these deposits.  This is precisely what Long Xiao and team document from the VBF, and therefore suggest that they are examining an ancient recessional shoreline of the Deuteronilus Ocean.

Figure S9. Summary of representative sedimentary environments and their sedimentary structures on Earth. (a) Typical fluvial deposit sequence contains a basal sandy gravel layer, middle sandstone layers with medium-scale trough bedding, wedge-shaped cross-bedding, and parallel bedding, upper fine sandstone and mudstone layers [15]. (b) Typical sedimentary structures in aeolian linear dunes, large-scale. Note the different lamination pattern in different parts of the dune. Particularly, trough cross bedding may form large-scale cross strata dipping away from the crest in opposite directions, therefore having a bimodal dip pattern [16]. Martian aeolian dunes are similar with the terrestrial analogue [15]. (c) Typical sedimentary structures in marine environment. Bidirectional cross bedding + though cross bedding + planar cross bedding in the foreshore, current/wave ripple bedding, hummocky cross-bedding and contorted bedding and in the shoreface, and increased mudstone from shoreface to offshore in low-energy condition, but in the high-energy conditions (e.g., storm and tsunami), tempestites turbidites and tsunamiites can be formed. Modified from [17] and SEPM Stratigraphy Web (,#).

The physical character of these sedimentary structures is reported to suggest deposition in a medium- to low-energy marine environment. Could you discuss the processes or methods by which your team made this determination?

In simple terms, we can estimate the energy that it took to deposit a sedimentary particle by the size of the particle.  If there is a huge boulder in a stream, and a very weak current, it will not move.  Following this, the maximum grain size of particles we have in our system is sand, with other fine silt and mud.  The amount of energy it takes to deposit these grain sizes in water is medium- to low-energy.  If the energy was higher, also, the finer-grained particles would be carried away and deposited somewhere else, where the energy was lower.

Crater in Utopia Planitia (CREDIT: NASA/JPL/University of Arizona/USGS)

Considering the landing site’s distance from the proposed Deuteronilus level, you suggest that these sedimentary rock deposits occurred during the regressional period of a Hesperian ocean. Could you expand on how this geographical context further supports your conclusions?

Sure, we are happy to do so. The northern lowlands are like a giant salad bowl, deeper in the middle, dipping shallowly inwards.  The Deuteronilus shoreline wraps around the upper and outer edge of this giant salad bowl for thousands of kilometers (see our Supplementary Figure S1, reproduced below).

At the height of the ocean’s realm, when water was pouring into the bowl in mega-floods from the circum-Chryse region outflow channels, the water level was stable long enough to form distinctive shoreline features including deltas, and other features typically associated with shorelines. While the Zhurong Rover’s images were obtained 282 km south of the proposed shoreline, the very shallow dip of the sides of the “salad bowl” means that the observations were made only ~500 meters below the maximum height of the water in the ocean. 

Since the water is now all gone, at some point (in the Hesperian) the ocean evaporated, and the shoreline receded into the salad bowl, at some point passing Zhurong’s present position, where the tidal currents at that time deposited the rocks we are examining.

Figure S1. Topographic map of the northern hemisphere of Mars. Previously proposed shorelines collected by ref. [14] are shown in solid color lines. The red star denotes the location of the landing site of the Zhurong rover, ~282 km to the north of the Deuteronilus shorelines. The data are color-coded MOLA elevation over MOLA shaded relief centered at the North Pole with the Lambert azimuthal equal-area projection.

You’ve stated that the findings provide new insights into the early history of Mars. Could you discuss some potential hypotheses or theories about Mars’ early history that these findings could support or challenge?

Whether or not there have ever been large oceans on early Mars has been one of the most controversial subjects in Martian studies for decades.  This is important for understanding the early paleoclimate of ancient Mars, and for the search for suitable environments for possible primitive life forms. Previous studies have only been able to use satellite or orbital observations, which have shown features including fluvial valley networks, outflow channels, deltas, and possible shorelines, but the observations reported by Long Xiao and team are the first to document marine sedimentary conditions on Mars, supporting the interpretation that the VBF is a marine deposit from an Hesperian (~3.3 billion years) ocean on Mars. This represents the most positive evidence yet supporting an early ocean on the red planet.

You describe the discoveries made by the Tianwen-1 Zhurong rover as the first in situ observations of a unit many believe to represent the deposits of an ancient Hesperian-aged sea. What are the implications of this for our understanding of Mars?

Yes, the Zhurong Rover, and the sensors on board provided us with the opportunity to collect the data, process the images to the maximum resolution possible, revealing unprecedented detail about the small scale structures of the rocks.  After scrutinizing the images in every way possible, we are quite confident in our interpretation that they are marine sedimentary rocks, deposited in a shallow tidal environment, under low energy tidal conditions, during the retreat of an Hesperian-aged Deuteronilus Ocean.

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Given the successful results of this mission so far, can we anticipate additional exploratory missions to Mars, perhaps with a focus on sample return? How could this further advance our understanding of the habitability of Mars and the preservation of microbial life traces?

We anticipate that the many successful results of China’s Tianwen-1/Zhurong rover mission will lead to new missions, with the aim of sample return to Earth for further study, and even whether or not humans may some day inhabit the planet.  Our documentation and interpretation of ancient marine sedimentary environments offers hope for finding traces of past microbial life on Mars, as shallow marine settings are one of the most favored habitats for developing and proliferating life on Earth, so they may well have been on Mars.

Many scientific questions can be addressed by future missions, such as searching for evidence of beach deposits, evidence for bio- or cryoturbation of sediments.  What were the climate conditions during the retreat and loss of a northern ocean, and what led to this ancient climate change on Mars? What was the timescale and ultimate fate of this water loss and what are the implications for the history of Mars? Is there any evidence for organics, bioturbation or fossils? These results show the importance of in situ measurements (e.g., Zhurong rover), complementary orbital observations (e.g., Tianwen-1, Mars Reconnaissance Orbiter and Exomars Trace Gas Orbiter), and their combined usefulness in preparation for optimizing Mars sample return missions and preparing for future human exploration. 


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