Today, Mars is a chilly
desert of rock and dust — but 4 billion years ago, the planet had rivers, lakes
and even oceans with sandy beaches. Data from China’s Zhurong rover recently
revealed the first evidence of one of those long-lost Mars beaches, finding
shallow sandy slopes perfectly preserved about 33 feet (10 meters) beneath the
Martian surface. Ground-penetrating radar aboard the rover measured thick
layers of sand, sloping gently upward toward the rocky shore, as if washed up
by ocean waves.
“The structures don’t look like sand
dunes. They don’t look like an impact crater. They don’t look like lava flows.
That’s when we started thinking about oceans,” Michael Manga, a University of
California, Berkeley, professor of earth and planetary science and co-author of
the study, said in a statement. “The orientation of these features are parallel to what the old
shoreline would have been. They have both the right orientation and the right
slope to support the idea that there was an ocean for a long period of time to
accumulate the sand-like beach.”
Zhurong spent a year, from May 2021 to May 2022, trundling along the base
of a steeply-sloped rock outcrop at the edge of a wide, flat plain. From the
rover’s point of view on the ground, it’s hard to tell, but this ancient
shoreline on Mars seemed
to lie inside a 2,050-mile-wide (3,300-kilometer-wide) impact crater called
Utopia Basin. Utopia Basin is, in fact, the largest known crater in the entire solar system. Scientists had speculated Utopia Basin might once have held an ancient
ocean, and they thought the escarpment overlooking Zhurong’s route might once
have been its shoreline.
Along Zhurong’s 1.2-mile (1.9-kilometer)
route, its ground-penetrating radar beamed radio waves 262 feet (80 meters)
down into the Martian ground. The way those radio waves reflected back to the
instrument revealed underground features they came into contact with, like the
boundaries between layers of rock and sediment. Thirty-three feet (10 meters)
beneath the surface, the radar revealed smooth, gently-sloping layers of sand,
several yards thick. Those layers appear to run parallel to the rocky cliff, and
they rise toward that cliff at a shallow 15-degree slope — which is typical of
beaches here on planet Earth.
The buried beach could represent the
first evidence of a true ocean from Mars’ ancient past, and its presence means
the Red Planet must have had an ocean for millions of years — long enough to
leave behind the thick layers of sand Zhurong’s radar measured. And that ocean
must have been fed by rivers, the scientists reason, as those rivers would have
dumped sediment into the ocean. Waves would eventually have spread that
resulting sediment along the shore, forming a beach that’d have been strikingly
familiar to us.
“Shorelines are great locations to look
for evidence of past life,” Benjamin Cardenas, a geoscientist at Pennsylvania
State University and co-author of the study, said in a statement. “It’s thought
that the earliest life on Earth began at locations like this, near the
interface of air and shallow water.” (It’s not known exactly where or how life
on Earth began, but shorelines like this one are a possibility, along with
hydrothermal vents on the deep ocean floor.)
Image: A hypothetical picture of Mars 3.6 billion years ago, when an ocean may have covered nearly half the planet. The blue areas show the depth of the ocean filled to the shoreline level of the ancient, now-gone sea, dubbed Deuteronilus. The orange star represents the landing site of the Chinese rover Zhurong. The yellow star is the site of NASA’s Perseverance rover, which landed a few months before Zhurong. (Image credit: Robert Citron)
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