Several recent observations of Mars have hinted that it might presently harbor liquid water, a requirement for life as we know it. However, in a new paper in Nature Astronomy, a team of researchers have shown that stable liquids on present-day Mars are not suitable environments for known terrestrial organisms. (https://astronomycommunity.nature.com/users/394217-edgard-g-rivera-valentin/posts/salty-water-everywhere-nor-any-drop-to-drink).
“Life on Earth, even extreme life, has certain environmental limits that it can withstand,” noted Dr. Edgard G. Rivera-Valentín, a Universities Space Research Association (USRA) scientist at the Lunar and Planetary Institute (LPI) and lead author of the investigation. “We investigated the distribution and chemistry of stable liquids on Mars to understand whether these environments would be suitable to at least extreme life on Earth.”
Due to Mars’ low temperatures and extremely dry conditions, should a liquid water droplet be placed on Mars, it would nearly instantaneously either freeze, boil, or evaporate away. That is unless that droplet had dissolved salts in it. Such salt water, or brine, would have a lower freezing temperature and would evaporate at a slower rate than pure liquid water. Because salts are found across Mars, brines could form there. “We saw evidence of brine droplets forming on the strut of the Phoenix lander, where they would have formed under the warmed spacecraft environment”, noted Dr. Germán Martínez, a USRA scientist at the LPI, co-investigator of the Mars 2020 Perseverance rover, and co-author of the study.
Image of the struts of NASA’s Phoenix lander showing potential brine droplets. Image credit: Marco Di Lorenzo, Kenneth Kremer, Phoenix Mission, NASA, JPL, UA, Max Planck Inst., Spaceflight
Further, some Martian salts can undergo a process called deliquescence. When a salt is at the right temperature and relative humidity, it will take in water from the atmosphere to become a salty liquid. “We’ve been conducting experiments under Martian simulated conditions at the University of Arkansas for many years now to study these types of reactions. Using what we’ve learned in the lab, we can predict what will likely happen on Mars,” says Dr. Vincent Chevrier, co-author of the investigation at the University of Arkansas.
The team of researchers used laboratory measurements of Mars-relevant salts along with Martian climate information from both planetary models and spacecraft measurements. They developed a model to predict where, when, and for how long brines are stable on the surface and shallow subsurface of Mars. They found that brine formation from some salts can lead to liquid water over 40% of the Martian surface but only seasonally, during 2% of the Martian year.
“In our work, we show that the highest temperature a stable brine will experience on Mars is -48°C (-55° F). This is well below the lowest temperature we know life can tolerate,” says Dr. Rivera-Valentín. “For many years we have worried about contaminating Mars with terrestrial life as we have sent spacecraft to explore its surface. These new results reduce some of the risk of exploring Mars,” noted Dr. Alejandro Soto at the Southwest Research Institute and co-author of the study.
“We have shown that on a planetary scale the Martian surface and shallow subsurface would not be suitable for terrestrial organisms because liquids can only form at rare times, and even then, they form under harsh conditions. However, there might be unexplored life on Earth that would be happy under these conditions,” added Dr. Rivera-Valentín. More environmental data from Mars, such as through the upcoming Mars 2020 mission to Jezero crater, along with further exploration of Earth’s biome may shed some light on the potential to finding life on Mars today.
Quelle: Universities Space Research Association (USRA)
Brines on Mars not habitable, study says
They are common, but not ‘special’.
Liquid brines on Mars may be more common and longer lasting than previously thought, but their properties and temperatures make them inhospitable for Earth’s microorganisms, according to a paper published in the journal Nature Astronomy.
And that means, the authors say, that they cannot be classified as Special Regions according to Planetary Protection policies.
Special Regions are defined as environments able to host liquid water that simultaneously meet certain temperature and water activity requirements that allow known terrestrial organisms to replicate.
Stable liquid water can’t persist on the surface of Mars, as the planet’s atmosphere is too thin and cold, but it was known that the presence of salts can create liquid substances, like brines, which can last stably for some time.
To paint a clearer picture, Edgard Rivera-Valentín, from the Universities Space Research Association, Texas, Vincent Chevrier, from the University of Arkansas, and colleagues combined a thermodynamic model with a climate model to investigate where brines could form on Mars and for how long.
“Our results show that metastability expands the locations and duration of brines on Mars, beyond what was previously thought, by including some equatorial regions,” they write.
They found, in fact, that up to 40% of the Martian surface at all latitudes down to the equator could host stable brines, and that these brines could last for up to six consecutive hours and during up to 2% of the entire Martian year.
Brines in the subsurface could last up to 10% of the Martian year at a depth of eight centimetres.
The locations of the stable brines could be targets for future Martian exploration, the authors note, since the risk of biological contamination from Earth is negligible. But they aren’t officially “special”.
“This is because of the hyper-arid conditions of Mars, which require lower temperatures to reach high relative humidities and tolerable water activities,” the paper says.
SwRI SCIENTIST MODELED MARS CLIMATE TO UNDERSTAND HABITABILITY
A Southwest Research Institute scientist modeled the atmosphere of Mars to help determine that salty pockets of water present on the Red Planet are likely not habitable by life as we know it on Earth. A team that also included scientists from Universities Space Research Association (USRA) and the University of Arkansas helped allay planetary protection concerns about contaminating potential Martian ecosystems. These results were published this month in Nature Astronomy.
Due to Mars’ low temperatures and extremely dry conditions, a droplet of liquid water on its surface would instantly freeze, boil or evaporate, unless the droplet had dissolved salts in it. This brine would have a lower freezing temperature and would evaporate more slowly than pure liquid water. Salts are found across Mars, so brines could form there.
“Our team looked at specific regions on Mars — areas where liquid water temperature and accessibility limits could possibly allow known terrestrial organisms to replicate — to understand if they could be habitable,” said SwRI’s Dr. Alejandro Soto, a senior research scientist and co-author of the study. “We used Martian climate information from both atmospheric models and spacecraft measurements. We developed a model to predict where, when and for how long brines are stable on the surface and shallow subsurface of Mars.”
Mars’ hyper-arid conditions require lower temperatures to reach high relative humidities and tolerable water activities, which are measures of how easily the water content may be utilized for hydration. The maximum brine temperature expected is -55 F — at the boundary of the theoretical low temperature limit for life.
“Even extreme life on Earth has its limits, and we found that brine formation from some salts can lead to liquid water over 40% of the Martian surface but only seasonally, during 2% of the Martian year,” Soto continued. “This would preclude life as we know it.”
While pure liquid water is unstable on the Martian surface, models showed that stable brines can form and persist from the equator to high latitudes on the surface of Mars for a few percent of the year for up to six consecutive hours, a broader range than previously thought. However, the temperatures are well below the lowest temperatures to support life.
“These new results reduce some of the risk of exploring the Red Planet while also contributing to future work on the potential for habitable conditions on Mars,” Soto said.
Soto collaborated with co-authors from the Lunar and Planetary Institute (USRA) and the Arkansas Center for Space and Planetary Sciences at the University of Arkansas on the paper “Distribution and Habitability of (Meta)stable Brines on Present-Day Mars,” published in May in Nature Astronomy. The SwRI portion of this research was funded by NASA under the Habitable Worlds program through a grant led by USRA.
Quelle: Southwest Research Institute