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Ion escape from Mars: measurements in the present to understand the past
Umeå University, Faculty of Science and Technology, Department of Physics. Swedish Institute of Space Physics, Kiruna.ORCID iD: 0000-0003-0458-4050
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
##### Abstract [en]

Present-day Mars is a cold and dry planet with a thin CO2-dominated atmosphere comprising only a few ­­­mbar pressure at low altitudes. However, the Martian surface is marked with valley networks, hydrated mineral clays, carbonates and the remains of deltas and meandering rivers, i.e. traces of an active hydrological cycle present early in the planet's geological history. A strong greenhouse effect, and thus a thicker atmosphere, would have been required to sustain a sufficiently warm environment, particularly under the weaker luminosity of the early Sun. The fate of this early atmosphere is currently unknown.

While several mechanisms can remove atmospheric mass over time, a prominent hypothesis suggests that the lack of an intrinsic Earth-like global magnetic dipole has allowed the solar wind to erode the early Martian atmosphere by imparting energy to the planet's ionosphere which subsequently flows out as ion escape, over time depleting the greenhouse gasses and collapsing the ancient hydrological cycle. Previous studies have found insignificant ion escape rates under present-day conditions, however, the young Sun emitted significantly stronger solar wind and photoionizing radiation flux compared to the present. The geological record establishes the time of collapse of the hydrological cycle, estimated to have occurred in the mid-late Hesperian period (~3.3 billion years ago) at latest. To constrain the amount of atmosphere lost through ion escape since, we use the extensive database of ion flux measurements from the Analyzer of Space Plasmas and Energetic Atoms (ASPERA-3) particles package on the Mars Express orbiter (2004-present) to quantify the ion escape rate dependence on upstream solar wind and solar radiation conditions.

The Martian ion escape rate is shown to be insensitive to solar wind parameters with a weak inverse dependence on solar wind dynamic pressure, and linearly dependent on solar ionizing photon flux, indicating efficient screening of the bulk ionosphere by the induced magnetic fields. The impact of an extreme coronal mass ejection is studied and found to have no significant effect on the ion escape rate. Instead, intense solar wind is shown to only increase the escaping energy flux, i.e. total power of escaping ions, without increasing the rate by accelerating already escaping ions. The orientation of the strongest magnetized crustal fields are shown to modulate the ion escape rate, though to have no significant time-averaged effect. We also study the influence of solar wind and solar radiation on the major Martian plasma boundaries and discuss factors that might limit the ion escape rate, including solar wind-ion escape coupling, which is found to be ≲1% and decreasing with increased solar wind dynamic pressure. The significant escape rate dependencies found are extrapolated back in time, considering the evolution of solar wind and ionizing radiation, and shown to account for only 4.8 ± 1.1 mbar equivalent surface pressure loss since the time of collapse of the Martian hydrosphere in the Hesperian, with ~6 mbar as an upper estimate. Extended to the late Noachian period (3.9 billion years ago), the found dependencies can only account for ≲10 mbar removed through ion escape, an insignificant amount compared to the ≳1 bar surface pressure required to sustain a warm climate on early Mars.

##### Place, publisher, year, edition, pages
Umeå: Umeå University , 2017. , p. 66
##### Series
IRF Scientific Report, ISSN 0284-1703 ; 309
##### Keywords [en]
Mars, escape, solar wind, evolution, CME, coupling, plasma, atmosphere
##### National Category
Fusion, Plasma and Space Physics
Space Physics
##### Identifiers
ISBN: 978-91-982951-3-9 (print)ISBN: 978-91-7601-806-4 (electronic)OAI: oai:DiVA.org:umu-141892DiVA, id: diva2:1157328
##### Public defence
2017-12-08, Aulan, Rymdcampus 1, Kiruna, 09:00 (English)
##### Funder
Swedish National Space Board, 172/12Available from: 2017-11-17 Created: 2017-11-15 Last updated: 2018-06-09Bibliographically approved
##### List of papers
1. Phobos 2/ASPERA data revisited: Planetary ion escape rate from Mars near the 1989 solar maximum
Open this publication in new window or tab >>Phobos 2/ASPERA data revisited: Planetary ion escape rate from Mars near the 1989 solar maximum
2017 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 40, no 3, p. 477-481Article in journal (Refereed) Published
##### Abstract [en]

Insights about the near-Mars space environment from Mars Express observations have motivated a revisit of the Phobos 2/ASPERA ion data from 1989. We have expanded the analysis to now include all usable heavy ion(O+, O2+ , CO2+) measurements from the circular orbits of Phobos 2. Phobos 2/ASPERA ion fluxes in the Martian tailare compared with previous results obtained by the instruments on Phobos 2. Further validation of the measurement results is obtained by comparing IMP-8 and Phobos 2/ASPERA solar wind ion fluxes, taking into account the time lag between Earth and Mars. Heavy ion flux measurements from 18 circular equatorial orbits around Mars are bin-averaged to a grid, using the MSE (electric field) frame of reference. The binned data are subsequently integrated to determine the total escape rate of planetary ions. From this we derive a total planetary heavy ion escape rate of (2–3)$\times$1025 s-1 from Mars for the 1989 solar maximum.

##### National Category
Fusion, Plasma and Space Physics
##### Identifiers
urn:nbn:se:umu:diva-141905 (URN)10.1002/grl.50149 (DOI)000317831000003 ()
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2018-06-09
2. The Martian atmospheric ion escape rate dependence on solar wind and solar EUV conditions: 1. Seven years of Mars Express observations
Open this publication in new window or tab >>The Martian atmospheric ion escape rate dependence on solar wind and solar EUV conditions: 1. Seven years of Mars Express observations
2015 (English)In: Journal of Geophysical Research - Planets, ISSN 2169-9097, E-ISSN 2169-9100, Vol. 120, no 7, p. 1298-1309Article in journal (Refereed) Published
##### Abstract [en]

More than 7 years of ion flux measurements in the energy range 10 eV–15 keV have allowed the ASPERA-3/IMA (Analyzer of Space Plasmas and Energetic Ions/Ion Mass Analyzer) instrument on Mars Express to collect a large database of ion measurements in the Mars environment, over a wide range of upstream solar wind (density and velocity) and radiation (solar EUV intensity) conditions. We investigate the influence of these parameters on the Martian atmospheric ion escape rate by integrating IMA heavy ionflux measurements taken in the Martian tail at similar (binned) solar wind density (nsw), velocity (vsw), and solar EUV intensity (IEUV) conditions. For the same solar wind velocity and EUV intensity ranges (vsw and IEUV constrained), we find a statistically significant decrease of up to a factor of 3 in the atmospheric ion escape rate with increased average solar wind density (5.6 × 1024 s−1 to 1.9 × 1024 s−1 for 0.4 cm−3 and 1.4 cm−3, respectively). For low solar wind density (0.1–0.5 cm−3) and low EUV intensity, the escape rate increaseswith increasing solar wind velocity from 2.4 × 1024 s−1 to 5.6 × 1024 s−1. During high solar EUV intensities the escape fluxes are highly variable, leading to large uncertainties in the estimated escape rates; however, a statistically significant increase in the escape rate is found between low/high EUV for similar solar wind conditions. Empirical-analytical models for atmospheric escape are developed by fitting calculated escape rates to all sufficiently sampled upstream conditions.

##### National Category
Fusion, Plasma and Space Physics
##### Identifiers
urn:nbn:se:umu:diva-141934 (URN)10.1002/2015JE004816 (DOI)000359903800005 ()
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2018-06-09
3. Effects of the crustal magnetic fields on the Martian atmospheric ion escape rate
Open this publication in new window or tab >>Effects of the crustal magnetic fields on the Martian atmospheric ion escape rate
2016 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 43, no 20, p. 10574-10579Article in journal (Refereed) Published
##### Abstract [en]

Eight years (2007–2015) of ion flux measurements from Mars Express are used to statisticallyinvestigate the influence of the Martian magnetic crustal fields on the atmospheric ion escape rate.We combine all Analyzer of Space Plasmas and Energetic Atoms/Ion Mass Analyzer (ASPERA-3/IMA)measurements taken during nominal upstream solar wind and solar extreme ultraviolet conditions tocompute global average ion distribution functions, individually for the north/south hemispheres and forvarying solar zenith angles (SZAs) of the strongest crustal magnetic field. Escape rates are subsequentlycalculated from each of the average distribution functions. The maximum escape rate (4.2 ± 1.2) × 1024 s−1 is found for SZA = 60–80, while the minimum escape rate (1.7±0.6)×1024 s−1 is found for SZA = 28–60,showing that the dayside orientation of the crustal fields significantly affects the global escape rate (p=97%). However, averaged over time, independent of SZA, we find no statistically significant difference inthe escape rates from the two hemispheres (escape from southern hemisphere 46% ± 18% of global rate).

##### National Category
Fusion, Plasma and Space Physics
##### Identifiers
urn:nbn:se:umu:diva-141926 (URN)10.1002/2016GL070135 (DOI)000388293800037 ()
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2018-06-09
4. Solar wind- and EUV-dependent models for the shapes of the Martian plasma boundaries based on Mars Express measurements
Open this publication in new window or tab >>Solar wind- and EUV-dependent models for the shapes of the Martian plasma boundaries based on Mars Express measurements
2017 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, no 7, p. 7279-7290Article in journal (Refereed) Published
##### Abstract [en]

The long operational life (2003-present) of Mars Express (MEX) has allowed the spacecraft tomake plasma measurements in the Martian environment over a wide range of upstream conditions. Wehave analyzed ∼7000 MEX orbits, covering three orders of magnitude in solar wind dynamic pressure, withdata from the on board Analyzer of Space Plasmas and Energetic Particles (ASPERA-3) package, mappingthe locations where MEX crosses the main plasma boundaries, induced magnetosphere boundary (IMB), ionosphere boundary (IB), and bow shock (BS). A coincidence scheme was employed, where data fromthe Ion Mass Analyzer (IMA) and the Electron Spectrometer (ELS) had to agree for a positive boundaryidentification, which resulted in crossings from 1083 orbit segments that were used to create dynamictwo-parameter (solar wind density, nsw, and velocity vsw) dependent global dynamic models for the IMB, IB,and BS. The modeled response is found to be individual to each boundary. The IMB scales mainly dependenton solar wind dynamic pressure and EUV intensity. The BS location closely follows the location of the IMB atthe subsolar point, though under extremely low nsw and vsw the BS assumes a more oblique shape. The IBclosely follows the IMB on the dayside and changes its nightside morphology with different trends for nswand vsw. We also investigate the influence of extreme ultraviolet (EUV) radiation on the IMB and BS, findingthat increased EUV intensity expands both boundaries.

##### National Category
Fusion, Plasma and Space Physics
##### Identifiers
urn:nbn:se:umu:diva-141929 (URN)10.1002/2017JA024098 (DOI)000407627100025 ()
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2018-06-09
5. Global Mars-solar wind coupling and ion escape
Open this publication in new window or tab >>Global Mars-solar wind coupling and ion escape
2017 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, no 8, p. 8051-8062Article in journal (Refereed) Published
##### Abstract [en]

Loss of the early Martian atmosphere is often thought to have occurred due to an effective transfer of the solar wind energy through the Martian induced magnetic barrier to the ionosphere. We have quantified the coupling efficiency by comparing the power of the heavy ion outflow with the available power supplied by the upstream solar wind. Constraining upstream solar wind density nsw, velocity vsw, and EUV intensity IEUV/photoionizing flux FXUV in varying intervals reveals a decrease in coupling efficiency, k,with solar wind dynamic pressure as ∝ pdyn−0.74±0.13 and with FXUV as k ∝ FXUV−2.28±0.30. Despite the decreasein coupling efficiency, higher FXUV enhances the cold ion outflow, increasing the total ion escape rate as Q(FXUV) = 1010(0.82 ± 0.05)FXUV. The discrepancy between coupling and escape suggests that ion escapefrom Mars is primarily production limited in the modern era, though decreased coupling may lead to an energy-limited solar wind interaction under early Sun conditions.

##### Place, publisher, year, edition, pages
Washington: American Geophysical Union (AGU), 2017
##### National Category
Fusion, Plasma and Space Physics
##### Identifiers
urn:nbn:se:umu:diva-141930 (URN)10.1002/2017JA024306 (DOI)000411788800015 ()
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2018-06-09Bibliographically approved
6. Mars Under Primordial Solar Wind Conditions: Mars Express Observations of the Strongest CME Detected at Mars Under Solar Cycle #24 and its Impact on Atmospheric Ion Escape
Open this publication in new window or tab >>Mars Under Primordial Solar Wind Conditions: Mars Express Observations of the Strongest CME Detected at Mars Under Solar Cycle #24 and its Impact on Atmospheric Ion Escape
2017 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007Article in journal (Refereed) Epub ahead of print
##### Abstract [en]

An extremely strong Coronal Mass Ejection (CME) impacted Mars on 12 July 2011, while theMars Express spacecraft was present inside the nightside ionosphere. Estimated solar wind density andspeed during the event are 39 particles cm−3 and 730 km/s, corresponding to nominal solar wind fluxat Mars when the solar system was ∼1.1 Ga old. Comparing with expected average atmospheric heavy ionfluxes under similar XUV conditions, the CME impact is found to have no significant effect on the escaperate 3.3 × 1024 s−1, with an upper limit at 1025 s−1 if the observed tail contraction is not taken into account.On the subsequent orbit, 7 h later after magnetosphere response, fluxes were only 2.4% of average. As such,even under primordial solar wind conditions we are unable to find support for a strong solar wind-driven ion escape, rather the main effect appears to be acceleration of the escaping ions by ×10–×20 typicalcharacteristic energy.

##### National Category
Fusion, Plasma and Space Physics
##### Identifiers
urn:nbn:se:umu:diva-141931 (URN)10.1002/2017GL075446 (DOI)
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2018-06-09
7. Ion escape from Mars through time: An extrapolation of atmospheric loss based on 10 years of Mars Express measurements
Open this publication in new window or tab >>Ion escape from Mars through time: An extrapolation of atmospheric loss based on 10 years of Mars Express measurements
##### Abstract [en]

Solar wind driven atmospheric ion escape has long been hypothesized as a major influence on the evolution of the Martian atmosphere due to the lack of a Martian global dipole magnetic field. We use 10 years (2007-2017) of Mars Express data to quantify the ion escape rate over the full sampled upstream solar wind dynamic pressure, pdyn, and solar photoionizing flux, FXUV, parameter space. The modeled dependence on the upstream parameters indicates a near-linear dependence on FXUV and weak negative correlation with pdyn. Integrating total heavy ion escape back through time, considering the evolution of the upstream parameters and the modeled trends, can only account for an estimated 4.8 ± 1.1 mbar of atmosphere lost as ions since the mid-late Hesperian (3.3 Ga ago). Accounting for the recently reported stability of ion escape through the energetic oxygen ion plume provides an upper estimate of 6 mbar lost. Extending the extrapolation to the late Noachian (3.9 Ga ago) accounts for 6.3 ± 1.9 mbar, and analogously up to 9 mbar, lost through ion escape since. Thus the ion escape trends observed by Mars Express indicate that atmospheric ion escape contributed only a minor role in the evolution of the Martian atmosphere. We also report solar wind control of the cold ion outflow channel, providing a tentative explanation for the low response of the ion escape rate to upstream solar wind.

##### National Category
Fusion, Plasma and Space Physics
##### Identifiers
urn:nbn:se:umu:diva-141932 (URN)
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2018-06-09

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Cite
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