How Did Mars Get Such A Thin Atmosphere? New Solar Irradiance Data Promises New Insights

Maven spacecraft collects data for solar irradiance applicable to Mars.

According to a new study by E.M. B. Thiemann et al (Journal of Geophysical Research: Space Physics) a new line of evidence has been found to ascertain the evolution of Mars' early atmosphere. We know it is remarkably thin now and assorted hypotheses have been offered over the years to account for this, ranging from erosion on account of the solar wind, to simple atmospheric outgassing owing to changing planetary conditions.

In respect of the latter, the standard theory for Martian loss of atmosphere had been based on the escape of atmospheric particles  due to reaching escape velocity from thermal effects. Calculations show, in fact, that if the mean molecular speed is as much as one-third of the planet's velocity of escape (or 1.7 km/sec for Mars) the planet will lose one half of its atmospheric gas in only a few weeks.

If the mean molecular speed is even one fifth of escape velocity (1.02 km/sec for Mars) the gas will disperse into space in a few hundred million  years. To hold a gas of sufficient atmospheric density to allow standing water-for billions of years - would necessitate a velocity of escape 6-8 times the mean molecular speed of the gas in question.  This is simply not the case for Mars, where one can easily work out the mean molecular speed of  oxygen, say from:

v =  Ö( (3 k T/m)

where k is Boltzmann's constant (1.38 x 10 ^-23   J/K), T is the absolute temperature applicable in degrees K, and m is the mass of a single gas  molecule.  Then compare it to Mars' escape velocity of ~ 5.1 km/sec, as well as to one -third values and one -fifth values of that velocity.

The new research is based on using data collected from NASA's Mars Atmosphere and Volatile Evolution (MAVEN) mission to calculate the solar irradiance at the planet.  This is the amount of EM power delivered by electromagnetic waves over a given area of the Martian atmosphere.   It can also be thought of as the output of light energy from the entire disk of the Sun, measured at Mars.

A fairly basic equation can in fact be used to get first estimates, and we can compare values for the Earth and Mars.  We use:

L = 4π R² (K)

for the Earth where K is the  solar irradiance (at Earth) we seek and  K' for Mars:

L = 4π R'² (K')

Here, L is he solar luminosity or power delivered, e.g.  L = = 3.9 x 10 ²⁶ W 

R = 1.5 x 10 ¹¹ m   or the Earth - Sun mean distance (semi-major axis)

R' = 2.4 x 10 ¹¹ m, or the Mars-Sun mean distance.

Then the estimated solar irradiance at Earth will be:

K   =   L  / 4π R²     =  ( 3.9 x 10 ²⁶ W )/  4π (1.5 x 10 ¹¹ m)² 

K  =   1360 Wm^-2

And for Mars: 

K'   =   L  / 4π R'²     =  ( 3.9 x 10 ²⁶ W )/  4π (2.4 x 10 ¹¹ m)² 

K  =   539  Wm^-2

As expected the value for Mars is significantly less given its mean distance is 1.6 times greater. Thus the effective luminous radiant sphere at its distance is much larger so the impacting radiation (the "irradiance") is more diffuse. 

Now the Thiemann team, working with data collected by the MAVEN is shedding new insights based on the measurement of solar extreme ultraviolet (EUV) radiation.  These wavelengths ranged from 6 nm to 120 nm, and we know solar EUV heats the upper atmosphere of both Mars and Earth.  The resulting interactions with existing atmospheric gases, e.g. CO2, have an impact on the composition of the planet's atmosphere.

MAVEN's  EUV monitor takes measurements every second that the Sun is in the instrument's field of view, or roughly 60 percent of the time.   Also of use by the team is a mathematical model (the Flare Irradiance Spectral Model- Mars)  or FISM-M, which uses the EUV measurements to calculate the spectral irradiance.  This is the solar irradiance received for a specific wavelength.

Why use a flare-referenced model? Because we already know large solar flares can propel radiance enhancements more than 50 times greater than normal, thereby affecting irradiance. Thus, a means to correct for these extraordinary energetic events needs to be factored in.

In the case of the FISM-M model, the algorithms used incorporate concurrent  solar EUV data  collected in Earth's upper atmosphere by NASA's Solar Dynamics Observatory .  That data from the SDO then helps to calibrate MAVEN data and enable calculation of the solar irradiance at Mars.  This is not only on a daily basis where no exceptional solar events may occur but also after the most explosive solar flares.

In their paper the research team presented solar irradiance measurements calculated using FISM-M between October 2015 and November 2016. These measurements varied due to fluctuations in solar EUV radiation caused by solar flares, the rotation of the Sun, Mars’s elliptical orbit around the Sun, and the progression of the Sun’s 11-year cycle.

The EUV monitor is just one of an array of instruments and sensors that MAVEN uses to study Mars’s upper atmosphere as it seeks clues to the atmospheric history of the Red Planet. The information presented by Thiemann’s team will help inform future research with FISM-M, as well as improvements to the model itself. 

Solar and space physicists definitely look forward to further corroborating results of this work, as well as extending them further - perhaps with the aid of new mathematical models. Solving each clue, say using solar and related spectral irradiance puts us on a more confident path to knowing how Mars' atmosphere evolved - and why it is so very tenuous now.


Interested readers can find an overview of the paper here:

http://onlinelibrary.wiley.com/doi/10.1002/2016JA022677/full

The MAVEN Mars Mission - Few Realize How Incredible It Is

MAVEN in orbit above Mars (Depiction from NASA)

The MAVEN (Mars Atmosphere and Volatile Evolution) spacecraft has been on its Mars orbit mission since Sept. 21, but it is likely few know how remarkable it is. Consider this fist, if you think you know everything about it: It left Earth in November last year with 425 gallons of fuel.  That's correct, you read it right. And further, of this 425 gallons, no more than 25 gallons were allotted for the actual journey to Mars. Part of that allotment designated for 4 trajectory correction maneuvers to keep the craft on course- but only two were actually required consuming 5 gallons- so the extra fuel can be used to extend its mission.

The orbit of the spacecraft will be adjusted over the next 6 weeks or so from its current 35 hour period orbit to a 4 1/2 hour science orbit. Since the period is related to the semi-major axis by Kepler's 3rd law, we infer from this that the orbital dimensions will be significantly smaller, the orbit 'tighter'. A depiction is shown below:



What is MAVEN's mission?

Mars atmosphere "out-gassed" a long time ago and MAVEN' mission is to discover what happened to this early atmosphere. The existing evidence shows that Mars had liquid water flowing over its surface at one point, perhaps 1 billion years ago. This conceivably was an indicator of an environment that could support life. 

The goal of MAVEN then is to determine the history of the loss of atmospheric gases to space, providing answers about Martian climate evolution. By measuring the rate with which the atmosphere is currently escaping to space and gathering enough information about the relevant processes, scientists will be able to infer how the planet's atmosphere evolved over time.

The MAVEN mission has four primary scientific objectives:

1. Determine the role that loss of volatiles to space from the Martian atmosphere has played through time.
2. Determine the current state of the upper atmosphere, ionosphere, and interactions with the solar wind.
3. Determine the current rates of escape of neutral gases and ions to space and the processes controlling them.
4. Determine the ratios of stable isotopes in the Martian atmosphere.^[

These are all incredibly ambitious objectives which, if fulfilled, can enable whole new insights into the Red Planet as well as rewriting many astronomy texts. The dedicated mission (to be conducted in conjunction with India's 'MOM' Mars orbiter - making India the first Asian nation to make a Mars landing) is not to begin until November, but already preliminary analyses have revealed images - released in four colored panels: blue, green, red and composite — constructed from data gathered by MAVEN's Imaging Ultraviolet Spectrograph.

The blue panel shows hydrogen gas extending away from the surface, while the green one depicts oxygen remaining closer to the surface. These two elements are important because they come from the breakdown of water and carbon dioxide in Mars' atmosphere, as well as match what his team thought they would see, said Nick Schneider, who leads the Imaging Ultraviolet Spectrograph science team at the University of Colorado.   According to Schneider:

"The fact that the images matched our simulation means that it exceeded our expectations,"

This news is very encouraging and leads those of us who are Mars junkies to anticipate a ringing success for the rest of MAVEN's one year (planned) exploits.