A mystery on Mars!

When the Trailblazer Mars rover began analyzing the rocks at the base of the Zero crater in the spring of 2021, NASA scientists were surprised: They had expected to find sedimentary rocks because the crater had a lake billions of years ago. This formed when sand and mud settled in what was once a watery environment. Instead, they found that the floor is made up of two types of igneous rocks - one formed by volcanic activity at the surface and the other from magma deep underground.

These findings are described in four new scientific papers published today, Aug. 25, 2022. In the journal Science, one outlines the Trail's investigation of the crater floor before it reached the ancient river delta at Zero in April 2022. A second study in the same journal details unique rocks that appear to have formed from thick magma. Two other papers published in Science Advances document the unique way in which Trail's rock vaporization laser and ground-penetrating radar identified igneous rocks covering the crater floor.

Rock of the Century

Igneous rocks are excellent timekeepers. That's because the crystals inside them record detailed information about the exact moment they formed.

"An important value of the igneous rocks we collected is that they will tell us when the lake appeared at Zero. we know it formed later than the rocks at the bottom of the igneous crater," says Cal tech's Ken Farley, a project scientist at Perseverance and the first new scientific paper's lead author. "This will address some of the main questions: when did Mars' climate favor lakes and rivers on the planet's surface, and when did it turn into the very cold and dry conditions we see today?"

However, igneous rocks are not ideal for preserving the potential signs of ancient microscopic life that Perseverance is looking for because of how it formed. On the other hand, determining the age of a sedimentary rock can be challenging, especially if it contains rock fragments that formed at different times before the rock sediments were deposited. However, sedimentary rocks usually form in water environments that are suitable for life and are better able to preserve ancient signs of life.

That's why the sediment-rich river deltas that Trail has been exploring since April 2022 are so tantalizing to scientists. The rover has begun drilling and collecting core samples of sedimentary rocks there so that the Mars Sample Return Campaign can potentially return them to Earth, where they can be studied with powerful laboratory equipment that is too large to bring to Mars.

Mysterious magma-forming rocks

A second paper published in the journal Science solves a long-standing mystery on Mars. A few years ago, the Mars Orbiter discovered a rock formation filled with the mineral violin. This formation is about 27,000 square miles (70,000 square kilometers) -- almost the size of South Carolina -- and extends from the inner rim of the Zero crater to the surrounding area.

Scientists have come up with a variety of theories as to why periodontists are so abundant over such a large surface area. These include meteorite impacts, volcanic eruptions, and sedimentary processes. Another theory suggests that periodontist was formed deep underground by slowly cooling magma (lava), which was then exposed over time by erosion.

Yang Lou and her co-authors at NASA's Jet Propulsion Laboratory ( PL ) in Southern California have determined that the last explanation is the most likely. Trail wears off a rock to reveal its composition. Scientists studying the exposed patches focused on the large grain size of the periodontist as well as the chemistry and texture of the rock.

Using Perseverance's Planetary Instrument for X-ray Petrochemical (PIX ), they determined that the violin grains in the region measure 1 to 3 millimeters-far larger than would be expected for violin formed in rapidly cooling lava on the planet's surface.

"This large crystal size and its uniform composition in a given rock texture require a very slow cooling environment," Lou said. "So, in all likelihood, Zero's magma did not erupt at the surface."

A unique scientific tool

The discovery of scientific instruments that helped determine the igneous rocks covering the crater floor is detailed in two Science Advances papers. These instruments include Perseverance's Super Cam laser and a ground-penetrating radar called RIM FAX (Radar Image for Mars Subsurface Experiments).

The Super Cam is equipped with a rock vaporization laser that can hit targets as small as the tip of a pencil from distances of up to 20 feet (7 meters). It uses a visible-light spectrometer to analyze the resulting vapor to determine the chemical composition of the rock. During Perseverance's first 10 months on Mars, Super Cam scored 1,450 points, helping scientists conclude the igneous rocks at the base of the crater.

In addition, Super Cam used near-infrared light - the first instrument on Mars with this capability - to find water-altered minerals in the rocks at the crater door. However, based on a combination of laser and infrared observations, these changes are not widespread throughout the crater floor.

"Super Cam's data suggest that these rock layers are either separated from the water in Zero's lake or that the lake has existed for a limited time," said Roger Liens, Super Cam's principal investigator at Purdue University and Los Alamo National Laboratory.

RIM FAX marks another first. Although the Mars orbiter carried ground-penetrating radar, no spacecraft on the Martian surface had it before Trail. On the surface, RIM FAX can provide unparalleled detail and measure the bottom of craters up to 50 feet (15 meters) deep.

Its high-resolution "radar grams" show rock layers unexpectedly tilted 15 degrees downward. Understanding the alignment of these rock layers could help scientists establish a timeline of the formation of Zero Crater.

"As the first such instrument to operate on the surface of Mars, RIM FAX demonstrates the potential value of ground-penetrating radar as a subsurface exploration tool," says Sven-Erik Haman, principal investigator of RIM FAX at the University of Oslo in Norway.

The science team is excited about their findings so far, but they are even more excited about the future of science.