NASA: Mars May Have Always Been Cold and Dry

A new study led by University of Colorado at Boulder researchers
indicates Mars has been primarily a cold, dry planet following its
formation some 4 billion years ago, making the possibility of the
evolution of life there challenging at best.

Led by CU-Boulder doctoral candidate Teresa Segura and her adviser,
Professor Owen B. Toon, the team used Mars photos and computer models
to show that large asteroids or comets hit the planet some 3.5
billion years ago. These impacts apparently occurred about the time
major river channels were formed on the Red Planet, said Segura.

According to the available evidence, roughly 25 huge impactors, each
about 60 miles to 150 miles in diameter, slammed into Mars roughly
every 10 million to 20 million years during the period, blowing a
volume of debris equivalent to a global blanket hundreds of yards
thick into the atmosphere. The material is believed to have melted
portions of subsurface and polar ice, creating steam and scalding
water that rained back on Mars at some six feet per year for decades
or centuries, causing rivers to form and flow, according to the study.

But the study belies the warm, wet, Mars theory of rivers and oceans
embraced by many planetary scientists, since such impactors were so
infrequent. “There apparently were some brief warm and wet periods
on Mars, but we believe that through most of its history, Mars has
been a cold, dry planet,” said Segura, currently a visiting
researcher at NASA-Ames in California.

A paper by Segura, Toon, CU-Boulder graduate Anthony Colaprete — now
at NASA-Ames — and Kevin Zahnle of NASA-Ames, will appear in the
Dec. 6 issue of Science.

“When the river valleys on Mars were confirmed in the 1970s, many
scientists believed there once was an Earth-like period with warmth,
rivers and oceans,” said Toon, director of CU-Boulder’s Program in
Oceanic and Atmospheric Sciences and a professor at the University’s
Laboratory for Astrophysics and Space Physics. “What sparked our
interest was that the large craters and river valleys appeared to be
about the same age.”

In between such catastrophic events, the planet was likely very cold,
dry and inhospitable to any life forms, said Toon. “We definitely
see river valleys but not tributaries, indicating the rivers were not
as mature as those on Earth.”

The rare, hot rains pelting Mars that likely came from water in
asteroids and comets hitting the planets and the evaporation of some
ice from polar caps and ice beneath the impacts would have been
spectacular, said Segura. “We believe these events caused short
periods of a warm and wet climate, but overall, we think Mars has
been cold and dry for the majority of its history.”

According to Toon, previous theories that carbon dioxide gas and
clouds warmed Mars during its early history “just have not worked out
quantitatively.” There is no evidence on Mars of large limestone
deposits from the first billion years, which would be directly linked
to large amounts of C02, a greenhouse gas, he said.

There also is no evidence that another greenhouse gas, methane —
which can be created naturally by volcanic eruptions or produced by
primitive life — was present in the Martian atmosphere. But even
CO2 and methane combined would not be enough to warm the planet as
greenhouse gases did on Earth and Venus in their early histories,
Toon said.

“Hypotheses of a warm, wet Mars, based on the presumption that the
valley networks formed in a long-lasting greenhouse climate, imply
that Mars may once have been teeming with life,” wrote the authors in
Science. “In contrast, we envision a cold and dry planet, an almost
endless winter broken by episodes of scalding rains followed by flash
floods.

“Only during the brief years or decades after the impact events would
Mars have been temperate, and only then might it have bloomed with
life as we know it,” they wrote. Although temperatures in the
subsurface of Martian soil may have exceeded the boiling point during
the impact period and provided a possible refuge for life
underground, the short duration of warm periods predicted by the
researchers would have made it difficult for life to ever establish
itself on Mars, the team concluded.