Guest ” If it looks like a delta, walks like a delta and quacks like a delta, it might just be… ” by David Middleton
It looked like a delta from orbit…
Jezero’s Window to the Past
Jezero Crater sits within the Isidis Planitia region of Mars, where an ancient meteorite impact left behind a large crater some 750 miles (1,200 kilometers) across. This event is known as Isidis impact, and it forever changed the rock at the base of the crater. A later, smaller meteorite impact created the Jezero Crater within the Isidis impact basin. Scientists believe that these events likely created environments friendly to life. There is evidence of ancient river flow into Jezero, forming a delta that has long since been dry.
Jezero Crater is thus likely to have been habitable in the distant past. The Mars Reconnaissance Orbiter’s CRISM instrument has revealed that the crater contains clays, which only form in the presence of water. On Earth, scientists have found such clays in the Mississippi river delta, where microbial life has been found embedded in the rock itself. This makes Jezero Crater a great place to fulfill the Mars 2020 mission’s science goal of studying a potentially habitable environment that may still preserve signs of past life.
At Jezero Crater, Perseverance should be able to access rocks that are as old as 3.6 billion years. There are many ideas about what early Mars was like, and how it came to be what it is today. Accessing the ancient rock at Jezero should help answer some of these questions, and tell us more about the formation of rocky planets. It is also a great location for the rover to collect a variety of samples of Martian rock and soil.
How Features are Named at Jezero Crater
Naming things is a great way to remember them. As Perseverance explores the Martian surface, the science team will assign unofficial names to especially interesting regions and features.
The team informally named the rover’s touchdown site “Octavia E. Butler Landing,” after the groundbreaking science fiction author. Butler grew up in Pasadena, California near the Jet Propulsion Laboratory. The first African American woman to win both the Hugo and Nebula awards, and the first science fiction writer to be honored with a MacArthur Fellowship, her writing inspired many in the planetary science community and beyond.
Throughout the mission, the science team will use a naming system similar to the one used to name the locations that the Curiosity rover has explored on Mars. Before Perseverance launched, the team mapped the entire landing site in Jezero Crater, dividing it into squares about 0.75 miles (1.2 kilometers) on each side. These quadrangles were named for various national parks and preserves on Earth. As a nod to the diversity of its international science partners, the team used names from parks in countries that have contributed to the mission.
As the rover explores Jezero Crater, any time the team sees an interesting feature, they will name it for a corresponding location here on Earth. For example, when the Curiosity team named one of its sites “Yellowknife Bay” after a location in Canada, individual rocks and targets within that area were named after features in Canada’s Yellowknife Bay.
The Perseverance team uses a modified version of this approach for some of their earliest exploration. The rover landed in a part of Jezero Crater that the team had named for Canyon de Chelly National Monument (“Tséyi’” in Navajo) in Arizona. Since Canyon de Chelly is the heart of Navajo Nation lands, the mission team worked directly with the Navajo Nation, who is sharing their language to help the team informally name features on Mars. Among the initial list of 50 candidate names are “Máaz” (Mars), “bidziil” (strength), “hoł nilį́” (respect), and “tséwózí bee hazhmeezh” (rolling rows of pebbles, like waves).
Now, it looks like a delta from ground level…
Long distance images of exposed scarps taken by the Perseverance Mastcam-Z now have provided strong evidence that this is indeed a lacustrine deltaic feature…
Perseverance Finds Ancient Delta-Lake System, Flood Deposits in Martian Jezero Crater
Oct 8, 2021 by News Staff / Source
Planetary scientists have analyzed images taken by the Mastcam-Z camera and the Remote Micro-Imager of the SuperCam instrument on NASA’s Perseverance rover — which landed in Lake crater in February 2021 — in the three months after landing. The images show the geologic layers of an ancient river delta, which formed when Jezero crater was filled by a lake. Named Lake Jezero, the paleolake could have been up to 40 km (25 miles) wide and tens of meters deep.
The full text of the paper, Mangold et al., 2021 is available. Here’s the abstract and one of the images of the outcrop:
Perseverance rover reveals an ancient delta-lake system and flood deposits at Jezero crater, Mars
Observations from orbital spacecraft have shown that Jezero crater, Mars, contains a prominent fan-shaped body of sedimentary rock deposited at its western margin. The Perseverance rover landed in Jezero crater in February 2021. We analyze images taken by the rover in the three months after landing. The fan has outcrop faces that were invisible from orbit, which record the hydrological evolution of Jezero crater. We interpret the presence of inclined strata in these outcrops as evidence of deltas that advanced into a lake. In contrast, the uppermost fan strata are composed of boulder conglomerates, which imply deposition by episodic high-energy floods. This sedimentary succession indicates a transition, from a sustained hydrologic activity in a persistent lake environment, to highly energetic short-duration fluvial flows.
For comparison, here is a schematic diagram of a Gilbert-type delta depositional sequence:
Diagram of Gilbert Delta Characteristics.
Cuyahoga Valley National Park
Deltaic deposits are characterized by the presence of topset, foreset, and bottomset beds. Topset and foreset beds develop when high energy flows enter a lower–energy water environment. As the water’s velocity drops, the sand and gravel that were transported along the bed of the channel as bedload come to a stop, resulting in deposition. The deposits accumulate in layers at an incline along the floor of the lake or sea. As time passes, subsequent deposition occurs on top of the previous foreset beds, creating topset beds. Fine-grained material such as mud and silt remains in suspension longer, and settles out in nearly horizontal layers farther from shore in the lower energy environment, creating bottomset beds. At Cuyahoga Valley National Park, glacial lakes accumulated thick deltaic deposits that are exposed in the valley walls today.
The Cuyahoga Valley National Park deltaic features are decent analogies, since they are also lacustrine features.
Now they just need to drive Perseverance over there, pick up a suite of rock samples and drill a few cores and see if it quacks like a delta.
I just love field geology!
Mangold, N. et al. Perseverance rover reveals an ancient delta-lake system and flood deposits at Jezero crater, Mars. Science, published online October 7, 2021; doi: 10.1126/science.abl4051