Monday, August 7, 2017

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

Image may contain: outdoor
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

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:

No comments: