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Home , Galileo (spacecraft), MAVEN, MESSENGER, Parker Solar Probe

Atmospheric escape at and : past, present and future Shannon Curry (@astrocurry) and the MAVEN + team February 5th 2020 Follow the water

• Habitability is a major question for our terrestrial neighbors and • This heavily relies on the presence of liquid water and atmospheric pressure – Post- hydrodynamic Image courtesy of Darby Dyar, – [email protected] Our terrestrial planets: atmospheres

• Both and Mars Venus have a thick atmosphere capable of sustaining liquid SW Earth water • However, unlike Earth, Venus and Venus Mars lack a dipole SW • A magnetic field can may shield a planet SW from atmospheric erosion from the Our terrestrial planets: atmospheres

• Both Earth and Mars Venus have a thick Martian atmospheric erosion: atmosphere capable - Jeans escape [neutrals] of sustaining liquid - Photochemical escape [neutrals] water SW - escape [charged particles] • However, unlike Earth, Venus and Venus Mars lack a dipole magnetic field • A magnetic field can Venusian atmospheric erosion: may shield a planet SW - Jeans escape [neutrals] from atmospheric - Photochemical escape [neutrals] erosion from the - Ion escape [charged particles] Neutral

Jeans Escape Photochemical escape

• Lighter elements () • Heavier elements (, O2, • Loss of particles from the energetic CO2) tail of the speed distribution” • Also referred to as dissociative + recombination [ O2 + e  O* + O* ] Ion escape

• The neutral upper atmosphere at Mars and Venus gets ionized and those sense electric and magnetic , which accelerate the ions past the escape velocity

• Ions can escape in a number of ways – But all of these escape processes are dependent on Ion escape  Sputtering

(H+ lighter ions)

(O+ • Ions can escape in a number of heavy ions) ways – But all of these escape processes are dependent on The early -4 -3 -2 -1 Present [Billion years] • The early Sun was rotating much faster than it is today, and is believed to have been much more active • XUV: 50 – 100x • Winds: 100 – 200x

• As it loses angular momentum, it spins down Credit: IAU/E. Guinan [email protected] Solar cycle

Solar cycle 24 Solar cycle 25 Past observations at Mars and Venus

Venus Express PVO

Galileo Cassini MGS 15/16 MSL Mariner 9 -2 MAVEN MESSENGER

solar cycle solar cycle solar cycle solar cycle solar cycle solar cycle 19 20 21 22 23 24 MEX and VEX: ion escape MEX- Mars Heavy ion flux [m-2 s-1]

• Similar instruments were able to observe planetary ion escape

• Present day ion escape at Venus (1025) is roughly an order of magnitude higher than at Mars 24 (10 ) Nordström et al [2013] MAVEN: photochemical and sputtered escape

Courtesy of MAVEN • MAVEN found that present day photochemical escape is the dominant atmospheric escape process

• Additionally, measurements showed that 65% of Argon has been removed from the atmosphere through Jakosky et al [2017] sputtering VEX and PVO ion escape rates: implications

• Solar activity levels significantly differed for PVO and VEX, as did ion escape rates.

• What are the limits and the implications for water loss/crustal

oxidation? Figure from Tess McEnulty • Solar storms such as coronal mass ejections can erode the atmosphere at exponentially higher rates

Lee et al 2018, Curry et al. 2020 So how much atmosphere has been lost at Mars and Venus?

An extrapolation back in time with current escape rates , scaled to extreme rates during a more active sun suggests that our total escape rate would be responsible for the loss of a significant amount of water and / or atmosphere.

Assumptions: • Increase in EUV and solar wind • Increase in solar events (CMEs, CIRs) • Early atmospheres are based on GCM models with increased EUV • Post-bombardment • Post-hydrodynamic So how much atmosphere has been lost at Mars and Venus?

At Mars • 412 mbar of atmosphere OR • 11.24 meters of water

At Venus • 1.1 bar of atmosphere OR • 30.5 meters of water

Chassefiere et al 2006 Future missions

EMM

BepiColombo

Parker Solar Probe Past and present observations • (PVO) 1978- 1999 2007 1978-1992 1999 1992

• 2006-2014 (VEX) 2006-2014 View from above • Flybys – 1990 – Cassini 1998, 1999 – Messenger 2007 1998

[email protected] Past and present observations • Pioneer Venus Orbiter (PVO) 1978- 1999 2007 1978-1992 1999 1992

• Venus Express 2006-2014 (VEX) 2006-2014 View from above • Flybys – Galileo 1990 – Cassini 1998, 1999 • Parker– Messenger Solar Probe2007 VGA1 and VGA2 1998 recently completed flybys 1 and 2 [email protected] Parker Solar Probe Venus flybys

Top Down View

3 4 7 6 3 4 7 6

1 2 5 1 2 5

Curry et al. [2020] [email protected] Solar cycle 24-25

• Observing 1 2 3 4 5 6 7 Venus during different parts of the solar cycle significantly enhances the science return

• Solar wind measurements at 0.7 AU provide into the evolution of the [email protected] solar wind over a solar cycle Conclusions

• PVO, VEX, MEX, MGS and MAVEN paved the way in determining atmospheric escape rates during different solar cycles at Venus and Mars

• However, better sampling and energy resolution are needed for future missions to more accurately assess the role of atmospheric erosion and postental water loss at terrestrial planets

• Atmospheric escape at terrestrial planets can inform our understanding of how exoplanetary atmospheres evolve

• Parker Solar Probe (and hopefully BepiColombo and Solar Orbiter) can provide more data to help understand the Venusian plasma environment and atmospheric (ion) escape