Extraterrestrial Atmospheresby Poruri Sai Rahul.

This report on ‘Extraterrestrial Atmospheres’ is to accompany a presentation madeon the same as part of the course ME 5530 ‘Introduction to Atmospheric Science’. Thisreport contains 4 sections. In the Introduction, I mention what my motivation was tochoose this particular topic. In the subsequent sections on Venus and Jupiter, I brieflymention some of the interesting atmospheric phenomenon observed and current researchavenues being pursued to understand said phenomenon. Finally, in the last section onEarth, I look at an interesting and usually neglected side effect of global warming i.erelease of trapped methane.

1 Introduction

Broadly speaking, the study of any extraterrestrial atmosphere is classified as ‘Extrater-restrial Atmospheric Science’, ranging from planetary atmospheres to that of exo-planets,the atmospheres of various moons and in a few cases, the atmospheres of comets and as-teroids. As part of the course on ‘Atmospheric Science’, we’ve studied the vertical profilesof pressure and temperature on Earth, clouds and rainfall, winds and briefly, radiativeheat transfer and climate change. Much of the science we’ve learnt as part of the coursecan be applied in a rudimentary fashion to understand extraterrestrial atmospheres. Inaddition, extraterrestrial atmospheres present exotic phenomenon, a few of which ob-served on Venus and Jupiter will be addressed later in this report. A study of suchphenomenon can lead to a better understanding of the underlying science and in general,a deeper insight into Atmospheric Science in general.

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2 Venus

Venus is the closest planet to Earth, orbiting the Sun at 0.7 AU1. It’s radius is 6051.8kms, roughly 0.952 times that of Earth. It receives Solar Irradiance of 2613.9 W/m2,roughly 1.911 times that of what Earth receives. Assuming a simple radiative heattransfer model to balance the incoming solar radiation and the outgoing radiation fromthe planet’s surface, the black-body temperature of Venus is estimated to be 184.2 K. Butthe surface temperatures on Venus have actually been recorded at 737 K2, far greater thanthe estimated black-body temperature. Multiple reasons contribute to such extremelyhigh temperature conditions on the surface of Venus, the most important of which isthe effect of greenhouse gases. In stark contrast with that on Earth, the atmosphere onVenus is largely comprised of CO2, a small amount of N2 and trace amounts of SO2,Ar and CO. It is also worth mentioning that the concentration of water vapour in theatmosphere of Venus is 20 ppm. CO2 is a common greenhouse gas and being present insuch large quantities lead to an extraordinary greenhouse warming in the atmosphere.

More recently, Ehrenreich et al. (2012) designed a new way to estimate the contributionof CO2 towards atmospheric albedo of Venus. Venus has a Bond albedo3 which is 0.90,2.94 times that of Earth’s Bond albedo! In general, the authors have designed a newway to understand the atmospheric transmission spectrum, ATS here forth, of Venus.Obtaining the ATS of any atmosphere will help us understand the composition of theatmosphere better. For example, in the case of Earth, the ATS is dominated largely byRayleigh scattering at shorter UV wavelength and by water vapour absorption bands inthe longer IR wavelengths.4 Fig. 1 illustrates a qualitative look at the ATS of Earthand Fig. 2 illustrates the contribution of various components in the atmosphere towardsATS. Understanding ATS has further applications in studying exo-planets, specificallytheir atmospheres. This is what motivated the authors to develop this new method. Theauthors intend on observing the atmosphere of Venus during it’s transit of the Sun asobserved from Earth on June 2012. Such a Venus transit has happened earlier on June2004 but will only be happening again in December 21175. Also note that in order tomeasure an ATS, one needs a standard, calibrated radiator or emitter and what betterthan our own Sun. The authors simulated how such an transit ATS will look by usingin-situ measurements of Venus made by the Venus Express6. Assumptions regarding theatmosphere were made such as a variation only along the altitude and ignoring variationsalong the latitudinal and longitudinal directions. The authors concluded that Rayleighscattering by CO2 molecules is a prominent contributor at heights of 70 kms above the

11 AU = 1 Astronomical Unit = distance between the Sun and the Earth.2Russian probes Venera 13, 14, 15 and 16 landed on the surface of Venus and recorded surface

characteristics.3Albedo can be quantified using Bond albedo or Geometric albedo. Bond albedo looks at the entire

spectral range of emission, absorption and scattering, it is a more accurate measure of albedo.4Water Vapour, as mentioned in class, is one of the lesser known greenhouse gases. Because of

it’s peculiar electronic and molecular structure, the molecule can absorb emissions in a large range ofwavelengths in the IR regions leading to the ATS having a band structure.

5en.wikipedia.org/wiki/Transit of Venus6Launched in November 2005 by the European Space Agency, Venus Express has been observing

Venus since April 2006 and is considered one of the most successful missions to Venus.

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surface of Venus and Mie scattering as important between heights of 80-100 kms abovethe surface. A complete spectral view is shown in Fig. 3.

An interesting fact about Venus which we haven’t discussed so far is the fact that theplanet takes 243 days to rotate about itself. Just to remind oneself, it takes 24 hours of1 day for the Earth to rotate about itself! It is speculated that Venus collided with anasteroid or comet early on it’s life, a collision that reduced it’s angular momentum to sucha low value and causing retrograde rotation7. We are not interested in why Venus has sucha small rotational angular momentum. Rather, we are interested in it’s consequences.Such a long day-night cycle means that the same face on Venus is facing the Sun for alarge period of time, receiving a large amount of solar radiation! This could in-fact beanother reason for the extremely high temperatures on Venus. In fact, temperatures onVenus is much larger than that on the day-side of Mercury, the closest planet to the Sun!This extreme heating on one side of the atmosphere also leads to a phenomenon referredto as Superrotation. The wind speeds measured in the upper atmospheric layers of Venusare 90 m/s. In contrast, the fastest wind speeds measured on Earth are 113 m/s duringtropical cyclones and 135 m/s in tornadoes. Mind you, these events on Earth are transitphenomenon where as the high wind speeds on Venus persist! The atmosphere is in factmoving so fast that even though it takes 243 days for the planet to rotate about itself,it only takes 4 days for the atmosphere to go around the planet! Hence referred to asSuperrotation. Superrotation is observed in only one other place in the solar system, onTitan, one of Saturn’s moons. Interestingly, while such high wind speeds are observed inthe upper layers of the atmosphere, the surface winds are of the order of . . . m/s, similarto that on Earth. Such unique wind patterns make Venus an interesting case study tounderstand winds in general. In fact, J. Peralta et al. (2014) I and II have in fact usedobservations from the Venus Express mission to categorize the various types of winds onVenus. The authors have been able to broadly distinguish them as

• Acoustic Waves

• Inertia-Gravity waves

• Lamb waves

• Surface waves, similar to those on Earth in the geostrophic regime.

• Centrifugal waves, a special case of the Rossby waves arising from the cyclostrophicapproximation.

Following an understanding of the underlying wind patterns, Yamamoto and Takahashi(2006) suggest meridional flow as a possible cause for Superrotation whereas Durand-Manterola (2010) attributes trans-terminator flow as the cause!

I have tried providing a brief overview of the various exotic atmospheric phenomenonobserved on Venus & I feel that a better understanding of the underlying science willhelp us gain deeper insight into winds and in general, into Atmospheric Science.

7Retrograde rotation refers to motion in the direction opposite to the motion of a reference object,in this case the motion of Venus around the Sun.

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3 Jupiter

Jupiter is the largest planet in the solar system, 371.83 times heavier than Earth andonly 0.09% that of the Sun! In astronomical terms 0.09% is a negligible amount. Infact, it is commonly argued that Jupiter is a failed star. This argument is supported bymultiple observations made about the planet. The first being it’s atmospheric compositionconstituting 89.8% of H2 and 10.2% of He along with a few trace elements, a compositionsimilar to that of our own Sun or in fact any other star in this universe! The secondobservation is the surface temperature of Jupiter, which is much higher than that expectedfrom solving radiative heat transfer! This is attributed to the fact that Jupiter has aninternal source of heat, specifically a source of heat that is gravitational in nature8.

Looking at Jupiter’s atmosphere, the Hadley circulation cells we observe between vari-ous latitudes on Earth are amplified leading to the presence of bands on Jupiter stretchinglarge a latitude range. Interestingly, these bands also travel in opposite directions, caus-ing eddies and vortices at the boundary layer due to shear between the bands! Anotherobservation on Jupiter, which we will be looking at in further detail, is the Giant redspot on Jupiter. Fig. 4 shows a portion of Jupiter’s band structure across latitudes andthe Giant red spot. It was taken by the Voyager 1 mission as it flew by Jupiter in 1979.

The Great red spot, GRS here forth, is a hurricane of epic proportions! It’s size is24-40,000 kms E-W by 12-14,000 kms N-S9. To put those numbers in context, one couldfit three Earths into that hurricane! The wind speeds observed at the boundary of theGRS are 120 m/s, similar to that observed during the strongest hurricanes on Earth!More interestingly, the GRS has been observed since 1665 i.e for almost 350 years! Incontrast, a hurricane on Earth can last a few weeks, maybe even a month in a few cases.Although it is now observed to be shrinking, it’s longevity is still a mystery. In fact, asimilar problem is faced by scientists trying to predict the longevity of a hurricane onEarth where the the hurricane is observed to decay much slower than what is predictedby theory or simulations! Hassanzadeh and Marcus (2013) have come up with a possibleexplanation for the longevity of the GRS on Jupiter, attributing to meridional flow asthe culprit. Their simulations showed that such a meridional flow helped extract shearenergy from the adjacent bands to sustain the GRS. It is suggested that earlier simulationsdidn’t model the meridional flow to sufficient extent and that their model overcomes thesemisgivings.

And to mention briefly, Tsumura et al. (2014) implemented a new way to study theATS of Jupiter, a method similar to what was explained earlier in the case of Venus. Theauthors observed the Jovian moons Europa, Callisto and Ganymede through Jupiter’sshadow and studied their spectra to infer properties about the planet’s atmosphere! Thisis an ingenious development as it removes the need for satellites to be sent all the way toJupiter to observe it’s atmosphere!

8referred to as the Kevin-Helmholtz mechanism of heating in astronomical objects.9en.wikipedia.org/wiki/Atmosphere of Jupiter#Great Red Spot

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I have barely scratched the surface here, many more interesting features follow fromthe references I’ve mentioned so far.

4 Back to Earth

Coming back to Earth, so far in the course we’ve looked at the causes of climatechange and the various effects it has, the primary of which is a drastic increase in meansea level. While an increase in mean sea level is a significant threat to human life thereis another side effect of the melting of glacial and polar ice that hasn’t been discussed. Looking back, earlier in the course, we discussed the distribution of Carbon on Earth,in the atmosphere, in the Earth’s core and in polar & glacial ice! We also measuredthe rate of melting assuming a constant solar irradiance and a constant surface area forice. In reality, modelling the melting of ice is a tricky question as we need to take intoconsideration the interaction between molten ice and ice! Water has a higher albedo thanice does and therefore absorbs more heat than ice! . Therefore, when one is estimatingthe rate of ice melting, one should consider the extra heat added by this molten ice!

Most of the carbon trapped in glacial and polar ice is in the form of methane. Onceglacial ice starts to melt, this trapped methane starts to leak up to the surface andwill escape into the atmosphere. Methane is a known greenhouse gas and an increasein methane concentration will drive a positive feedback cycle that would cause furtherheating, further melting and further release of harmful methane into the atmosphere!This is an altogether neglected bit of information while discussing climate change.

Coming to the main point, glacial ice, specifically in Greenland, is melting faster thanever before thanks to soot! We’ve discussed the positive, cooling effects of soot in theEarth’s atmosphere but soot only has a residence time of the order of a few weeks in theatmosphere! It will therefore settle on the earth’s surface eventually. It has now beenobserved that large portions of ice on Greenland are now covered with soot! Soot, as weknow, has a high albedo and soot covered ice will therefore heat and melt faster. TheDark Snow project led by Jason Box, is one of the teams studying the effects of soot onglacial melting and mapping Greenland’s surface to understand the extent of soot cover!North-American and Russian forest fires, industrial exhaust and bio-waste managementare the major sources of soot in the atmosphere. Keegan et al. (2014) have studied,independently, the melting of glacial ice on Greenland and looked for a correlation withforest fires in the North American mainland. While anomalies do exist, there is sufficientevidence to suggest that soot deposits increase the rate of melting, lead to a longer periodof glacial melting, one that begins earlier & ends later and in an overall larger loss ofice cover! Bacteria have also been observed on the glacial ice surface, bacteria that werepreviously unobserved and that increase the albedo of ice!Technological advancementssuch as the use of unmanned aerial drones have helped survey a larger region of Greenlandand have helped survey it faster!

Climate change is one of the most pressing topics of our generation and technologi-cal advancements & expertise in various fields need to be brought together in order to

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understand it better and tackle it!

References

• Ehrenreich et al. (2012)

• J. Peralta et al. (2014) I and II

• Yamamoto and Takahashi (2006)

• Durand-Manterola (2010)

• Tsumura et al. (2014)

• Keegan et al. (2014)

• http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html

• http://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html

• http://nssdc.gsfc.nasa.gov/planetary/factsheet/jupiterfact.html

• http://csep10.phys.utk.edu/astr161/lect/venus/features.html

• http://csep10.phys.utk.edu/astr161/lect/jupiter/features.html

• http://darksnow.org/

• http://www.meltfactor.org/blog/?p=1329

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Venus Earth RatioRadius(km) 6051.8 6356.8 0.952Bond albedo 0.90 0.306 2.94

Solar irradiance (W/m2) 2613.9 1367.6 1.911Black-body temperature (K) 184.2 254.3 0.724Sidereal Orbit period (days) 224.701 365.256 0.615Sidereal rotation period (hrs) -5832.6 23.9345 243.690

Surface pressure(bars) 92 1 92Average temperature (K) 737 288 2.599

Table 1: Venus fact sheet

(by vol) Venus EarthCO2 96.5% 400 ppmN2 3.5% 78.08%O2 - 20.95%SO2 150 ppm -Ar 70 ppm 9340 ppmH2O 20 ppm -CO 17 ppm -He 12 ppm 5.24 ppmNe 7 ppm 18.18 ppm

Table 2: Atmospheric composition of Venus.

Jupiter Earth RatioMass (1024 kg) 1898.3 5.9726 371.83Radius(km)* 66,854 6356.8 10.517Bond albedo 0.343 0.306 1.12

Solar irradiance (W/m2) 50.50 1367.6 0.037Black-body temperature (K) 110.0 254.3 0.433Sidereal Orbit period (days) 4332.589 365.256 11.862Sidereal rotation period (hrs) 9.9250 23.9345 0.415

Surface pressure(bars) � 1000 1 � 1000Temperature (at 1 bar) (K) 165 288 0.572

Table 3: Jupiter fact sheet.*Jupiter is an oblate planet and the polar - equatorial radii differ by a significant amount.

(by vol) Jupiter EarthH2 89.8% 0.55 ppmHe 10.2% 5.24 ppmCH4 3000 ppm 1.7 ppmNH3 260 ppm -O2 - 20.95%H2O 4 ppm -

Table 4: Atmospheric Composition of Jupiter.

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Figure 1: Atmospheric transmission of Earth - a qualitative look.

Figure 2: Atmospheric transmission of Earth - component wise contributions.

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Figure 3: Atmospheric transmission of Venus

Figure 4: Atmospheric bands and the Great red spot on Jupiter

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