Intense volcanism early in Mars history could have aided the emergence of life on the red planet through filtering out harmful ultraviolet (UV) radiation whilst allowing helpful longer wavelength UV to power vital prebiotic chemistry. These are the possibilities raised by a Harvard University study assessing the potential influence of the Martian atmosphere on surface UV exposure billions of years ago.
This artist’s concept depicts the early Martian environment (right) — believed to contain liquid water and a thicker atmosphere — versus the cold, dry environment seen at Mars today (left). Image credit: NASA’s Goddard Space Flight Center.
With a thick, cloud-filled atmosphere above seasonal meltwater lakes and rivers of drinkable water, ancient Mars might not have appeared dissimilar to Earth’s polar regions.
As a result theories abound that not only did life form on the red planet, but because these clement conditions pre-date known life on Earth, Mars might have even seeded our planet through transporting organic material or even basic organisms in meteorites.
And it’s not just its climate that supports this idea. Ancient Martian meteorites show the planet’s rocks and soil contained many raw materials for life.
So did life start up on another planet in our solar system? Is everyone reading this article of Martian descent? One obstacle in answering this is uncertainty around the UV raining down on the red planet billions of years ago.
Able to excite and break molecular bonds, or strip away electrons to give atoms an electronic charge, UV light is thought critical to these pre-life chemical reactions.
“Many recent discoveries of natural pathways for creating complex biomolecules utilize UV,” says Sukrit Ranjan, an astronomer and astrophysicist at Harvard University.
“UV light can provide energy to both drive prebiotic reactions and destroy some molecules, so it’s very relevant to the origin or evolution of life,” agrees planetary scientist Bruce Jakosky, principal investigator on NASA’s MAVEN mission investigating the story of Mars’ lost atmosphere.
Additional evidence for UV’s vital role comes from our genes, in particular the nucleic acids that encode our genetic information.
Out of a great many possible nucleic acids that could have been used, the genetic information for all life on Earth is stored using sequences of just four, all of which display resistance to UV.
And yet we are still uncertain how much UV was reaching the ancient Martian surface.
In his new paper Ranjan modeled Martian UV radiation 3.9 billion years ago, when we know there was surface water.
Their work, to be published in the journal Astrobiology, began by calculating the UV reaching Mars’ atmosphere from models of the evolving output of the Sun.
They then asked how this UV flux would be affected passing through the gas, clouds and dust of the atmosphere. However, whilst we know the atmosphere was thicker than today’s and predominantly carbon dioxide, what made up the rest, and the exact proportions are still debated.
Despite no exact figures the team reviewed the latest scientific work to find likely ranges for atmospheric composition, as well as pressure and temperature.
They then ran simulations showing how these would impact incoming UV levels, and measured their impact on two chemical reactions that could have seriously affected the emergence of life. These were the ability of UV light to destroy RNA and its involvement in producing energy giving sugars that help make it.
“We are making the pre-biology king with very specific reactions pathways strongly linked to the origin of life,” says Ranjan.
Their results showed broadly that UV surface radiation on young Mars is comparable to the young Earth when we first see evidence for life.
On a more detailed level, Ranjan identified the atmospheric factors that would govern whether UV radiation at the surface was supportive or obstructive to life-giving chemistry.
For example, whilst a lot of research has looked at the potential impact of clouds and water in the ancient atmosphere, Ranjan’s team found even thick cloud cover would have little significant impact on UV ray levels.
Far more influential is the amount of UV absorbing dust particles and volcanic gases like sulfur dioxide.
“SO2, like dust, can act as a very effective form of sunscreen,” says Ranjan.
Whether a dustier or more volcanically active ancient Mars would impact prebiotic chemistry negatively or positively, is not just about the amount of UV, but also its wavelength.
With the general principle established from past experiments that life supporting reactions are aided by longer wavelength UV, Ranjan’s team were interested to find it was these that more easily pass through a SO2-rich atmosphere.
“From our reactions a highly volcanic ancient Mars would might actually favor the helpful pro-life chemistry.”
However for dustier Mars models the nature of the impact is less clear.
“We need to see to what extent dust and volcanic gases can build up in our ancient atmosphere models, whilst looking at the effects of different UV wavelengths on these prebiotic pathways to life.”
“This type of approach is valuable in that it tells us what range of possible outcomes might be possible,” says Jakosky.
“As such, it’s an important first step in looking at the problem, though falls short of constrain what might actually have occurred.”
_____
Sukrit Ranjan et al. 2017. Atmospheric Constraints on the Surface UV Environment of Mars at 3.9 Ga Relevant to Prebiotic Chemistry. Astrobiology, under review; arXiv: 1701.01373
