There is a small amount ofMethane“It includes.Methane is released not only by natural phenomena, but also by life activities.Therefore, its origin attracts attention. However, there are many mysteries surrounding methane on Mars. One example is measurement results that indicate dramatic changes in concentration.
A research team led by John B. Ortiz of Los Alamos National Laboratory calculates based on the idea that methane concentrations on Mars can fluctuate over short time periods, less than a day, due to changes in the structure of the atmosphere. Although the modeling was relatively simple, the results supported the results of previous measurements. If this research is correct.A sharp rise in methane concentration just before sunriseOrtiz and his colleagues hope that measurements can be made within this time period.
■ There are many mysteries surrounding methane on Mars
It is a simple hydrocarbon.Methane“Emitted by natural phenomena such as volcanic activity and changes in rock composition. On the other hand, it is also known that methane is released through metabolism by microorganisms. On Earth, larger amounts of methane are released from biological activities than from natural phenomena.
The Martian atmosphere also contains methane, albeit at a very small percentage of 1/2.4 billion by volume. In addition to the absence of other particles released associated with volcanic activity, the presence of methane is present.Preliminary evidence that unique life existed on Mars and may still be active todayYou will become. However, instruments currently on Mars or planned for the future cannot evaluate more reliable evidence such as isotope ratio measurements, and so far cannot determine the presence of life.
Therefore, currently, research is being conducted to estimate the source and origin of methane from currently available data. Whatever the reason, methane is thought to have an underground source, but additional details are not well understood and many mysteries remain.
One mystery is that the concentration of methane in the Martian atmosphere fluctuates greatly. Methane in the Martian atmosphere is thought to be around 330 years old, so even if the source is local, it could spread throughout Mars. However, it is known that methane concentrations actually reach a maximum at the end of summer in Mars' northern hemisphere. This is NASA's (National Aeronautics and Space Administration) Mars rover.curiousity(Mars Science Laboratory), but the reason for this is unknown.
In addition to the long-term changes associated with the seasons, it is also known that changes can occur over short periods of time, such as within a single day. As mentioned earlier, Curiosity detected methane in the Martian atmosphere, but the European Space Agency (ESA) and the Roscosmos spacecraft TGO (Trace Gas Orbiter) were able to detect methane in the atmosphere at high altitudes.
This difference is thought to be because TGO takes measurements during the day, while Curiosity takes measurements mainly at night. In fact, Curiosity also failed to detect methane when measurements were taken during the day. These changes in atmospheric methane concentrations suggest that there is an unknown process that efficiently destroys stable methane molecules, or that there is some process that absorbs methane, but so far it has not been well understood.
Related article: One of the mysteries of methane on Mars is close to being solved, announced by NASA (July 10, 2021)
■Can changes in methane concentration be explained by changes in pressure and circulation in the Martian atmosphere?
Ortiz and his team believe that changes in methane concentration are due to changes in pressure and circulation in the Martian atmosphere, and they created a model to verify this. Ortiz and his colleagues believe that the source of methane is located deep underground on Mars, and not near the surface, and that it is released into the atmosphere through cracks, and they are continuing their research.
Ortiz and his colleagues estimate changes in methane concentration on Mars as follows. When atmospheric pressure drops, underground methane rises through cracks. On the other hand, when atmospheric pressure increases, methane is forced into the Earth through cracks, but some of it is absorbed into tiny gaps in the rocks and regolith, so not all of it goes back underground. Because of this difference in intake and outlet, methane deep underground is released from underground into the atmosphere. this processAir pumping(Barometric pumping).
The behavior of methane released into the atmosphere can also be explained by changes in atmospheric pressure. When the pressure is low, methane rises from the ground to the sky, and when the pressure is high, it falls from the sky to the ground. Because the upper atmosphere is larger than the Earth's surface, the concentration of methane that rises is relatively lower.
Mars' atmosphere is more stable than Earth's, so pressure changes are mostly caused by the temperature difference between day and night. During the day, atmospheric pressure drops and updrafts occur, causing methane to spread into the upper atmosphere and reduce its concentration. That's why TGO and Curiosity failed to detect methane during the day. On the other hand, at night, atmospheric pressure increases and downdrafts occur, which facilitates the accumulation of methane on Earth. This explains why Curiosity detects methane gas at night.
On the other hand, it is estimated that seasonal fluctuations are related to changes in atmospheric circulation and temperature as well as changes in atmospheric pressure. Models of Martian atmospheric circulation are not fully understood, but the thickness of the circulation in contact with the surface (the atmospheric boundary layer) is expected to change during summer and winter. In addition, when the temperature is high, methane gas absorbed on rocks and regolith can escape more easily. Taken together, methane concentrations are higher in summer than in winter. It is believed that the reason for recording the highest concentrations during the summer in the Northern Hemisphere is due to a bias in sources between the Northern and Southern Hemispheres.
■ Estimate the appropriate measurement timing to verify the estimate
Ortiz and his colleagues created a simple one-dimensional model and tested whether it could explain measured methane concentrations. As a result, although it is a fairly simple model,Explain well both intraday and seasonal changes.I was able to.
On the other hand, in this study, there were times when there was a large discrepancy between the methane concentration calculated by the model and the actual measured value. It is possible that this is simply because the model is too simple to reproduce, but we cannot rule out other possibilities that cannot be estimated from currently available measurement data.
Curiosity, which can measure methane concentration and measure data over a long period of time, has already been in operation for more than 10 years and may soon reach the end of its life. Curiosity's gas component measuring instrument has a large mechanical load and a large electrical load, so it seems necessary to reduce the number of measurements in order to reduce the load.
Ortiz and his colleagues estimate that if changes in atmospheric methane concentration are caused by atmospheric pressure and atmospheric circulation, there will be distinct changes in methane concentration. In this model,A sharp increase in methane concentration occurs between 4:00 and 7:00 a.m., just before sunrise.He appreciated that. This change is estimated to be particularly noticeable during the Northern Hemisphere summer. Therefore, by measuring methane concentrations during this period, we can validate the results of this study.
When Ortiz and his colleagues were writing this paper, summer had just begun in Mars' northern hemisphere. We are now past that period, and methane concentrations will gradually decline in the fall. If measurements such as those proposed by Ortiz et al are to be made, the appropriate timing remains to be determined.
- J.B. Ortiz, et al. “Subdiurnal methane changes on Mars driven by atmospheric pumping and planetary boundary layer evolution”. (Journal of Geophysical Research: Planets)
- J.B. Ortiz, et al. “Atmospheric pumping through fractured rock: a mechanism for venting methane deep into the Martian atmosphere”. (Geophysical Research Letters)
- Dr.. Viódes-Moreras, et al. “Advective flows in Martian regolith as a mechanism driving methane and other trace gas emissions into the atmosphere”. (Geophysical Research Letters)
- Nick Njegomer. “Atmospheric pressure changes could drive elusive methane pulses on Mars.” (Los Alamos National Laboratory)
- Lisa Ercolano. “Study predicts the best times for a rover to sample Mars' methane in search of life.” (Johns Hopkins Whiting School of Engineering)
Written by Riri Aya
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