Gravity of Mars

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https://en.wikipedia.org/wiki/Gravity_of_Mars 

The gravity of Mars is a natural phenomenon, due to the law of gravity, or gravitation, by which all things with mass around the planet Mars are brought towards it. It is weaker than Earth's gravity due to the planet's smaller mass. The average gravitational acceleration on Mars is 3.72076 ms⁻² (about 38% of that of Earth) and it varies. In general, topography-controlled isostasy drives the short wavelength free-air gravity anomalies. At the same time, convective flow and finite strength of the mantle lead to long-wavelength planetary-scale free-air gravity anomalies over the entire planet. Variation in crustal thickness, magmatic and volcanic activities, impact-induced Moho-uplift, seasonal variation of polar ice caps, atmospheric mass variation and variation of porosity of the crust could also correlate to the lateral variations. Over the years models consisting of an increasing but limited number of spherical harmonics have been produced. Maps produced have included free-air gravity anomaly, Bouguer gravity anomaly, and crustal thickness. In some areas of Mars there is a correlation between gravity anomalies and topography. Given the known topography, higher resolution gravity field can be inferred. Tidal deformation of Mars by the Sun or Phobos can be measured by its gravity. This reveals how stiff the interior is, and shows that the core is partially liquid. The study of surface gravity of Mars can therefore yield information about different features and provide beneficial information for future landings.

Measurement

Rotating spherical harmonic, with ${\displaystyle l}$ = 0 to 4 for the vertical, and ${\displaystyle m}$ = 0 to 4 for the horizontal. For the Martian C₂₀ and C₃₀, they vary with time because of the seasonal variation of mass of the polar ice caps through the annual sublimation-condensation cycle of carbon dioxide.

To understand the gravity of Mars, its gravitational field strength g and gravitational potential U are often measured. Simply, if Mars is assumed to be a static perfectly spherical body of radius R_M, provided that there is only one satellite revolving around Mars in a circular orbit and such gravitation interaction is the only force acting in the system, the equation would be,

${\displaystyle {\frac {GMm}{r^{2}}}=mr\omega ^{2}}$,

where G is the universal constant of gravitation (commonly taken as G = 6.674 x 10⁻¹¹ m³ kg⁻¹ s⁻²),^[10] M is the mass of Mars (most updated value: 6.41693 x 10²³ kg), m is the mass of the satellite, r is the distance between Mars and the satellite, and ${\displaystyle \omega }$ is the angular velocity of the satellite, which is also equivalent to ${\displaystyle {\frac {2\pi }{T}}}$ (T is the orbiting period of the satellite).

Therefore, ${\displaystyle g={\frac {GM}{R_{M}^{2}}}={\frac {r^{3}\omega ^{2}}{R_{M}^{2}}}={\frac {4r^{3}\pi ^{2}}{T^{2}R_{M}^{2}}}}$, where R_M is the radius of Mars. With proper measurement, r, T and R_M are obtainable parameters from Earth.

However, as Mars is a generic, non-spherical planetary body and influenced by complex geological processes, more accurately, the gravitational potential is described with spherical harmonic functions, following convention in geodesy, see Geopotential_model.

${\displaystyle U(r,\lambda ,\psi )=-{\frac {GM}{r}}\left(1+\textstyle \sum _{l=2}^{l=L}\displaystyle \left({\frac {R}{r}}\right)^{l}\left(C_{l0}P_{l}^{0}(\sin \ \psi )+\sum _{m=1}^{+l}(C_{lm}\cos \ m\lambda +S_{lm}\sin \ m\lambda )P_{l}^{m}(\sin \ \psi )\right)\right)}$,

where ${\displaystyle r,\psi ,\lambda }$ are spherical coordinates of the test point. ${\displaystyle \lambda }$ is longitude and ${\displaystyle \psi }$ is latitude. ${\displaystyle C_{lm}}$ and ${\displaystyle S_{lm}}$ are dimensionless harmonic coefficients of degree ${\displaystyle l}$ and order ${\displaystyle m}$. ${\displaystyle P_{l}^{m}}$ is the Legendre polynomial of degree ${\displaystyle l}$ with ${\displaystyle m=0}$ and is the associated Legendre polynomial with ${\displaystyle m>0}$. These are used to describe solutions of Laplace's equation. ${\displaystyle R}$ is the mean radius of the planet. The coefficient ${\displaystyle C_{l0}}$ is sometimes written as ${\displaystyle J_{n}}$.

1. The lower the degree ${\displaystyle l}$ and order ${\displaystyle m}$, the longer wavelength of anomaly it represents. In turn, long-wavelength gravity anomaly is influenced by global geophysical structures.
2. The higher the degree ${\displaystyle l}$ and order ${\displaystyle m}$, the shorter wavelength of anomaly it represents. For degree over 50, it has been shown that those variations have high correlation with the topography. Geophysical interpretation of surface features could further help deriving a more complete picture of the Martian gravity field, though misleading results could be produced.

The oldest technique in determining the gravity of Mars is through Earth-based observation. Later with the arrival of unmanned spacecraft, subsequent gravity models were developed from radio tracking data.

Earth-based observation

Before the arrival of the Mariner 9 and Viking orbiter spacecraft at Mars, only an estimate of the Mars gravitational constant GM, i.e. the universal constant of gravitation times the mass of Mars, was available for deducing the properties of the Martian gravity field. GM could be obtained through observations of the motions of the natural satellites of Mars (Phobos and Deimos) and spacecraft flybys of Mars (Mariner 4 and Mariner 6).

Long term Earth-based observations of the motions of Phobos and Deimos provide physical parameters including semi-major axis, eccentricity, inclination angle to the Laplacian plane etc., which allow calculation of the ratio of solar mass to the mass of Mars, moment of inertia and coefficient of the gravitational potential of Mars, and give initial estimates of the gravity field of Mars.

Inferred from radio tracking data

Three-way Doppler, with signal transmitter and receiver separated

Precise tracking of spacecraft is of prime importance for accurate gravity modeling, as gravity models are developed from observing tiny perturbation of spacecraft, i.e. small variation in velocity and altitude. The tracking is done basically by the antennae of the Deep Space Network (DSN), with one-way, two-way and three-way Doppler and range tracking applied. One-way tracking means the data is transmitted in one way to the DSN from the spacecraft, while two-way and three-way involve transmitting signals from Earth to the spacecraft (uplink), and thereafter transponded coherently back to the Earth (downlink). The difference between two-way and three-way tracking is, the former one has the same signal transmitter and receiver on Earth, while the latter one has the transmitter and receiver at different locations on Earth. The use of these three types of tracking data enhances the coverage and quality of the data, as one could fill in the data gap of another.

Doppler tracking is a common technique in tracking the spacecraft, utilizing radial velocity method, which involves detection of Doppler shifts. As the spacecraft moves away from us along line of sight, there would be redshift of signal, while for the reverse, there would be blueshift of signal. Such technique has also been applied for observation of the motion of exoplanets. While for the range tracking, it is done through measurement of round trip propagation time of the signal. Combination of Doppler shift and range observation promotes higher tracking accuracy of the spacecraft.

The tracking data would then be converted to develop global gravity models using the spherical harmonic equation displayed above. However, further elimination of the effects due to affect of solid tide, various relativistic effects due to the Sun, Jupiter and Saturn, non-conservative forces (e.g. angular momentum desaturations (AMD), atmospheric drag and solar radiation pressure) have to be done, otherwise, considerable errors result.

History

The latest gravity model for Mars is the Goddard Mars Model 3 (GMM-3), produced in 2016, with spherical harmonics solution up to degree and order 120. This model is developed from 16 years of radio tracking data from Mars Global Surveyor (MGS), Mars Odyssey and Mars Reconnaissance Orbiter (MRO), as well as the MOLA topography model and provides a global resolution of 115 km. A separate free-air gravity anomaly map, Bouguer gravity anomaly map and a map of crustal thickness were produced along with this model. Compared with MRO110C and other previous models, major improvement of the estimation of the gravity field comes from more careful modeling of the non-conservative forces applied to the spacecraft.

Gravity solutions	Authors	Year	Degree (m) and order (l) of the spherical harmonic solution [Surface resolution (km)]	Data Source
–	JP Gapcynski, RH Tolson and WH Michael Jr	1977	6	Tracking data of Mariner 9, Viking 1 and 2 spacecraft
Geoide martien	G Balmino, B Moynot and N Vales	1982	18 [¬600 km]	Tracking data of Mariner 9, Viking 1 and 2 spacecraft
GMM-1	DE Smith, FJ Lerch, RS Nerem, MT Zuber, GB Patel, SK Fricke and FG Lemoine	1993	50 [200–300 km]	Tracking data of Mariner 9, Viking 1 and 2 spacecraft
Mars50c	AS Konopliv, WL Sjogren	1995	50	Tracking data of Mariner 9, Viking 1 and 2 spacecraft
GMM-2B	FG Lemoine, DE Smith, DD Rowlands, MT Zuber, GA Neumann, DS Chinn, and DE Pavlis	2001	80	Tracking data of Mars Global Surveyor (MGS), and MOLA-derived topography data
GGM1041C	FG Lemoine	2001	90	Tracking data of Mars Global Surveyor (MGS) and Mars Odyssey, and MOLA-derived topography data
MGS95J	AS Konopliv, CF Yoder, EM Standish, DN Yuan, WL Sjogren	2006	95 [~112 km]	Tracking data of Mars Global Surveyor (MGS) and Mars Odyssey, and MOLA-derived topography data
MGGM08A	JC Marty, G Balmino, J Duron, P Rosenblatt, S Le Maistre, A Rivoldini, V Dehant, T. Van Hoolst	2009	95 [~112 km]	Tracking data of Mars Global Surveyor (MGS) and Mars Odyssey, and MOLA-derived topography data
MRO110B2	AS Konopliv, SW Asmar, WM Folkner, Ö Karatekin, DC Nunes, SE Smrekar, CF Yoder, MT Zuber	2011	110	Tracking data of Mars Global Surveyor (MGS), Mars Odyssey and Mars Reconnaissance Orbiter (MRO), and MOLA-derived topography data
MGM2011	C Hirt, SJ Claessens, M Kuhn, WE Featherstone	2012	[3 km (equator) – 125 km]	Gravity solution MRO110B2, and MOLA-derived topography data
GMM-3	A Genova, S Goossens, FG Lemoine, E Mazarico, GA Neumann, DE Smith, MT Zuber	2016	120 [115 km]	Mars Global Surveyor (MGS), Mars Odyssey and Mars Reconnaissance Orbiter (MRO) MGS (SPO-1, SPO-2, GCO, MAP) ODY (ODYT, ODYM) MRO (MROT, MROM)

The techniques in tracking the spacecraft and geophysical interpretation of surface features can affect the resolution of the strength of gravity field. The better technique favors spherical harmonic solutions to higher degrees and orders. Independent analysis on Mariner 9 and Viking Orbiter tracking data yielded a degree and order of 6 spherical harmonic solution., Further combination of the two data sets, along with correlation of anomalies with volcanic features (positive anomaly) and deep-printed depression (negative anomaly) assisted by image data allows a degree and order of 18 spherical harmonic solution produced. Further use of spatial a priori constraint method, which had taken the topography into account in solving the Kaula power law constraint, had favored model of up to degree 50 spherical harmonic solution in global resolution (Goddard Mars Model-1, or GMM-1) then the subsequent models with higher completeness and degree and order up to 120 for the latest GMM-3.

Mars free-air gravity map produced along with the GMM-3 gravity solution (Red: gravity high; Blue: gravity low) (Credit: NASA's Scientific Visualization Studio)

Therefore, gravity models nowadays are not directly produced through transfer of the measured gravity data to any spatial information system because there is difficulty in producing model with sufficiently high resolution. Topography data obtained from the MOLA instrument aboard the Mars Global Surveyor thus becomes a useful tool in producing a more detailed short-scale gravity model, utilizing the gravity-topography correlation in short-wavelength. However, not all regions on Mars show such correlation, notably the northern lowland and the poles. Misleading results could be easily produced, which could lead to wrong geophysics interpretation.

The later modifications of gravity model include taking other non-conservative forces acting on spacecraft into account, including atmospheric drag, solar radiation pressure, Mars reflected solar radiation pressure, Mars thermal emission, and spacecraft thrusting which despins or desaturates the angular moment wheels. In addition, Martian precession and third body attraction due to the Sun, Moon and planets, which could affect the spacecraft orbit, as well as relavistic effects on the measurements should also be corrected. These factors could lead to offset of the true gravity field. Accurate modeling is thus required to eliminate the offset. Such work is still ongoing.

Static gravity field

Many researchers have outlined the correlation between short-wavelength (locally varying) free-air gravity anomalies and topography. For regions with higher correlation, free-air gravity anomalies could be expanded to higher degree strength through geophysical interpretation of surface features, so that the gravity map could offer higher resolution. It has been found that the southern highland has high gravity/topography correlation but not for the northern lowland. Therefore, the resolution of free-air gravity anomaly model typically has higher resolution for the southern hemisphere, as high as over 100 km.

Free-air gravity anomalies are relatively easier to measure than the Bouguer anomalies as long as topography data is available because it does not need to eliminate the gravitational effect due to the effect of mass surplus or deficit of the terrain after the gravity is reduced to sea level. However, to interpret the crustal structure, further elimination of such gravitational effect is necessary so that the reduced gravity would only be the result of the core, mantle and crust below datum. The product after elimination is the Bouguer anomalies. However, density of the material in building up the terrain would be the most important constraint in the calculation, which may vary laterally on the planet and is affected by porosity and geochemistry of the rock. Relevant information could be obtained from Martian meteorites and in-situ analysis.

Local gravity anomalies

The crust-mantle boundary variation, intrusion, volcanism and topography can bring effect to the orbit of spacecraft, due to the higher density of mantle and volcanic material and lower density of the crust. (Not in scale) +ve: Positive anomaly; -ve: Negative anomaly

Since Bouguer gravity anomalies have strong links with depth of crust-mantle boundary, one with positive Bouguer anomalies may mean that it has a thinner crust composed of lower density material and is influenced more strongly by the denser mantle, and vice versa. However, it could also be contributed by the difference in density of the erupted volcanic load and sedimentary load, as well as subsurface intrusion and removal of material. Many of these anomalies are associated with either geological or topographic features. Few exception includes the 63°E, 71°N anomaly, which may represent an extensive buried structure as large as over 600 km, predated the early-Noachian buried surface.

Topography anomalies

Strong correlation between topography and short-wavelength free-air gravity anomalies has been shown for both study of the gravity field of the Earth and the Moon, and it can be explained by the wide occurrence of isostasy. High correlation is expected for degree over 50 (short-wavelength anomaly) on Mars. And it could be as high as 0.9 for degrees between 70 and 85. Such correlation could be explained by flexural compensation of topographic loads. It is noted that older regions on Mars are isostatically compensated when the younger region are usually only partially compensated.

Anomalies from volcanic constructs

Mars Bouguer gravity map, produced along with GMM-3 gravity solution in 2016 (Red: gravity high; Blue: gravity low) (Credit: NASA's Scientific Visualization Studio)

Different volcanic constructs could behave differently in terms of gravity anomalies. Volcanoes Olympus Mons and the Tharsis Montes produce the smallest positive free-air gravity anomalies in the solar system. Alba Patera, also a volcanic rise, north of the Tharsis Montes, however, produces negative Bouguer anomaly, though its extension is similar to that of Olympus Mons. And for the Elysium Mons, its center is found to have slight increase in Bouguer anomalies in an overall broad negative anomaly context in the Elysium rise.

The knowledge of anomaly of volcanoes, along with density of the volcanic material, would be useful in determining the lithospheric composition and crustal evolution of different volcanic edifices. It has been suggested that the extruded lava could range from andesite (low density) to basaltic (high density) and the composition could change during the construction of the volcanic shield, which contributes to the anomaly. Another scenario is it is possible for high density material intruded beneath the volcano. Such setting has already been observed over the famous Syrtis major, which has been inferred to have an extinct magma chamber with 3300 kg m³ underlying the volcano, evident from positive Bouguer anomaly.

Anomalies from depressions

Different depressions also behave differently in Bouguer anomaly. Giant impact basins like Argyre, Isidis, Hellas and Utopia basins also exhibit very strong positive Bouguer anomalies in circular manner. These basins have been debated for their impact crater origin. If they are, the positive anomalies may be due to uplift of Moho, crustal thinning and modification events by sedimentary and volcanic surface loads after impacting.

But at the same time there are also some large basins that are not associated with such positive Bouguer anomaly, for example, Daedalia, northern Tharsis and Elysium, which are believed to be underlain by the northern lowland plain.

In addition, certain portions of Coprates, Eos Chasma and Kasei Valles are also found to have positive Bouguer anomalies, though they are topographic depressions. This may suggest that these depressions are underlain by shallow dense intrusion body.

Global gravity anomalies

Global gravity anomalies, also termed as long-wavelength gravity anomalies, are the low-degree harmonics of the gravity field, which cannot be attributed to local isostasy, but rather finite strength of the mantle and density differences in the convection current. For Mars, the largest component of Bouguer anomaly is the degree one harmonic, which represents the mass deficit in the southern hemisphere and excess in the northern hemisphere. The second largest component corresponds to the planet flattening and Tharsis bulge.

Early study of the geoid in the 1950s and 1960s has focused on the low-degree harmonics of the Earth's gravity field in order to understand its interior structure. It has been suggested that such long-wavelength anomalies on Earth could be contributed by the sources located in deep mantle and not in the crust, for example, caused by the density differences in driving the convection current, which has been evolving with time. The correlation between certain topography anomalies and long-wavelength gravity anomalies, for example, the mid-Atlantic ridge and Carlsberg ridge, which are topography high and gravity high on the ocean floor, thus became the argument for the convection current idea on Earth in the 1970s, though such correlations are weak in the global picture.

Another possible explanation for the global scale anomalies is the finite strength of the mantle (in contrast to zero stress), which makes the gravity deviated from hydrostatic equilibrium. For this theory, because of the finite strength, flow may not exist for most of the region that are understressed. And the variations of density of the deep mantle could be the result of chemical inhomogeneities associated with continent separations, and scars left on Earth after the torn away of the moon. These are the cases suggested to work when slow flow is allowed to happen under certain circumstances. However, it has been argued that the theory may not be physically feasible.

Time-variable gravity field

Sublimation-condensation cycle occurs on Mars which results in carbon dioxide exchange between the cryosphere and the atmosphere. In turn, there is exchange in mass between the two spheres, which gives seasonal variation of gravity. (Courtesy NASA/JPL-Caltech)

Seasonal change of gravity field at the poles

The sublimation-condensation cycle of carbon dioxide on Mars between the atmosphere and cryosphere (polar ice cap) operates seasonally. This cycle contributes as almost the only variable accounting for changes in gravity field on Mars. The measured gravitational potential of Mars from orbiters could be generalized as the equation below,

${\displaystyle V(Mars)=V({Solid\,planet})+V(Seasonal\,caps+Atmosphere)}$

In turn, when there is more mass in the seasonal caps due to the more condensation of carbon dioxide from the atmosphere, the mass of the atmosphere would drop. They have inverse relationship with each other. And the change in mass has direct effect towards the measured gravitational potential.

The seasonal mass exchange between the northern polar cap and southern polar cap exhibits long-wavelength gravity variation with time. Long years of continuous observation has found that the determination of even zonal, normalized gravity coefficient C_{l=2, m=0}, and odd zonal, normalized gravity coefficient C_{l=3, m=0} are crucial for outlining the time-variable gravity due to such mass exchange, where ${\displaystyle l}$ is the degree while ${\displaystyle m}$ is the order. More commonly, they are represented in form of C_lm in research papers.

If we regard the two poles as two distinct point masses, then, their masses are defined as,

${\displaystyle M_{NP}={\frac {C_{20}+C_{30}}{2}}\,M_{Mars}}$

${\displaystyle M_{SP}={\frac {C_{20}-C_{30}}{2}}\,M_{Mars}}$

Data has indicated that the maximum mass variation of the southern polar cap is approximately 8.4 x 10¹⁵ kg, occurring near the autumnal equinox, while for that of the northern polar is approximately 6.2 x 10¹⁵ kg, occurring in between the winter solstice and spring equinox.

In long term speaking, it has been found that the mass of ice stored in North Pole would increase by (1.4 ± 0.5) x 10¹¹ kg, while in South Pole it would decrease by (0.8 ± 0.6) x 10¹¹ kg. In addition, the atmosphere would have decrease in term of the mass of carbon dioxide by (0.6 ± 0.6) x 10¹¹ kg in long term as well. Due to existence of uncertainties, it is unclear whether migration of material from the South Pole to the North Pole is ongoing, though such a possibility cannot be ruled out.

Tide

The two major tidal forces acting on Mars are the solar tide and Phobos tide. Love number k₂ is an important proportional dimensionless constant relating the tidal field acting to the body with the multipolar moment resulting from the mass distribution of the body. Usually k₂ can tell quadrupolar deformation. Finding k₂ is helpful in understanding the interior structure on Mars. The most updated k₂ obtained by Genova's team is 0.1697 ± 0.0009. As if k₂ is smaller than 0.10 a solid core would be indicated, this tells that at least the outer core is liquid on Mars, and the predicted core radius is 1520–1840 km.

However, current radio tracking data from MGS, ODY and MRO does not allow the effect of phase lag on the tides to be detected because it is too weak and needs more precise measurement on the perturbation of spacecraft in the future.

Geophysical implications

Crustal thickness

Histogram of percentage area against crustal thickness of Mars: 32 km and 58 km are the two major peaks of the histogram.

 

Comparison of topography, free-air gravity anomaly and crustal density map – Red: gravity high; Blue: gravity low

No direct measurement of crustal thickness on Mars is currently available. Geochemical implications from SNC meteorites and orthopyroxenite meteorite ALH84001 suggested that mean crustal thickness of Mars is 100–250 km. Viscous relaxation analysis suggested that the maximum thickness is 50–100 km. Such thickness is critical in maintaining hemispheric crustal variations and preventing channel flow. Combination studies on geophysics and geochemistry suggested that average crustal thickness could be down to 50 ± 12 km.

Measurement of gravity field by different orbiters allows higher-resolution global Bouguer potential model to be produced. With local shallow density anomalies and effect of core flattening eliminated, the residual Bouguer potential is produced, as indicated by the following equation,

${\displaystyle U_{RB}=U_{B}-U_{core}-U_{local}}$

The residual Bouguer potential is contributed by the mantle. The undulation of the crust-mantle boundary, or the Moho surface, with mass of terrain corrected, should have resulted in varying residual anomaly. In turn, if undulating boundary is observed, there should be changes in crustal thickness.

Global study of residual Bouguer anomaly data indicates that crustal thickness of Mars varies from 5.8 km to 102 km. Two major peaks at 32 km and 58 km are identified from an equal-area histogram of crustal thickness. These two peaks are linked to the crustal dichotomy of Mars. Almost all the crust thicker than 60 km are contributed by the southern highland, with generally uniform thickness. And the northern lowland in general has thinner crust. The crustal thickness of the Arabia Terra region and northern hemisphere are found to be latitude-dependent. The more southward towards the Sinai Planum and Lunae Planum, the more thickened the crust is.

Among all regions, the Thaumasia and Claritis contain the thickest portion of crust on Mars that account for the histogram > 70 km. The Hellas and Argyre basins are observed to have crust thinner than 30 km, which are the exceptionally thin area in the southern hemisphere. Isidis and Utopia are also observed to have significant crustal thinning, with the center of Isidis basins believed to have the thinnest crust on Mars.

Crust redistribution by impacting and viscous relaxation

After the initial impact, high heat flux and high water content would have favored viscous relaxation to take place. The crust becomes more ductile. The basin topography of the craters is thus subjected to greater stress due to self-gravitation, which further drive crustal flow and decay of relief. However, this analysis may not work for giant impact craters such as Hellas, Utopia, Argyre and Isidis basins.

Crustal thinning is believed to have taken place underneath almost all the major impact craters. Crustal excavation, modification through emplacement of volcanic material and crustal flow taking place in the weak lithosphere are the possible causes. With the pre-impact crust excavated, gravitational restoration would take place through central mantle uplift, so that the mass deficit of cavity could be compensated by the mass of the uplifted denser material.

Giant impact basins Utopia, Hellas, Argyre and Isidis are some of the most prominent examples. Utopia, an impact basin located in northern lowland, is filled by light and water-deposited sedimentary material and has slightly thickened crust at the center. This is potentially due to large resurfacing process in the northern lowland. While for Hellas, Argyre and Isidis basins, they have great Moho uplifted relief and exhibit annuli of diffuse thickened crust beyond the crustal rim.

But on the contrary, almost all the Martian basins with diameter of 275 km < D < 1000 km are associated with low amplitude surface and low amplitude Moho relief. Many are even found to have negative free air gravity anomaly, though evidence has shown that all of them should have experienced gravity high (positive free air gravity anomaly). These have been suggested not caused by erosion and burial alone, as the adding of material into the basin would in fact increase the gravity strength rather than decrease it. Thus viscous relaxation should have been taking place. High heat flux and high water content in the early Martian crust favored viscous relaxation. These two factors have made the crust more ductile. The basin topography of the craters would be subjected to greater stress due to self-gravitation. Such stress would drive crustal flow and therefore decay of relief. The giant impact basins are the exceptions that have not experienced viscous relaxation, as crustal thinning has made the crust too thin to sustain sub-solidus crustal flow.

Low bulk crustal density

The most recent crustal density model RM1 developed in 2017 gives the bulk crustal density to be 2582 ± 209 kg m⁻³ for Mars, which represents a global average value. Lateral variation of the crustal density should exist. For example, over the volcanic complexes, local density is expected to be as high as 3231 ± 95 kg m⁻³, which matched the meteorite data and previous estimations. In addition, the density of the northern hemisphere is in general higher than that of the southern hemisphere, which may imply that the latter is more porous than the former.

To achieve the bulk value, porosity could play an important role. If the mineral grain density is chosen to be 3100 kg m⁻³, 10% to 23% porosity could give a 200 kg m⁻³ drop in the bulk density. If the pore spaces are filled with water or ice, bulk density decrease is also expected. A further drop in bulk density could be explained by increasing density with depth, with the surface layer more porous than the deeper Mars, and the increase of density with depth also has geographical variation.

Engineering and scientific applications

Areoid

See also: Geography of Mars § Zero elevation, and Mars § Geography

Further information: Vertical datum

The areoid is a planetary geoid that represents the gravitational and rotational equipotential figure of Mars, analogous to the concept of geoid ("sea level") on Earth. This has been set as the reference frame for developing the MOLA Mission Experiment Gridded Data Records (MEGDRs), which is a global topography model. The topography model is important in mapping the geomorphological features and understanding different kinds of processes on Mars.

To derive the areoid, two parts of works are required. First, as gravity data is essential for identifying the position of the center of mass of the planet, which is largely affected by the distribution of the mass of the interior, radio tracking data of spacecraft is necessary. This was largely done by the Mars Global Surveyor (MGS). Then, the MOLA 2 instrument aboard the MGS, which operates at 400-km elevation orbit, could measure the range (distance) between the spacecraft and the ground surface through counting the round-trip time of flight of the pulse from the instrument. Combination of these two works allows the areoid as well as the MEGDRs to be constructed. Based on the above, the areoid has taken the radius as the mean radius of the planet at the equator as 3396 km.

The topography model MEDGRs was developed through range (distance) measurement done by MOLA 2 instrument and radio tracking data of the Mars Global Surveyor (MGS). The highest point is located at the Olympus Mons while the deepest point is located within the Hellas Basin. (Brown-Red: Topography high; Green-Blue: Topography low) (Credit: NASA/JPL-Caltech)

Surface landing

As there is a large distance between Mars and Earth, immediate command to the lander is almost impossible and the landing relies highly on its autonomous system. It has been recognized that to avoid failure, precise understanding of the gravity field of Mars is essential for the landing projects, so that offsetting factors and uncertainties of gravitational effects could be minimized, allowing for a smooth landing progress. The first ever man-made object landing on Mars, the Mars 2 lander, crashed for an unknown reason. Since the surface environment of Mars is complex, composed of laterally varying morphological patterns, in order to avoid rock hazard the landing progress should be further assisted by employment of LIDAR on site in determining the exact landing position and other protective measures.

Climate change and invasive species

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Climate_change_and_invasive_species 

 

Buffelgrass (Cenchrus ciliaris) is an invasive species throughout the world that is pushing out native species.

Climate change and invasive species is the destabilization of the environment caused by climate change that is facilitating the spread of invasive species.

Anthropocentric climate change has been found to bring about the increase in temperature and precipitation in a range of ecosystems. The drastic change of these climate factors is predicted to progress leading to the destabilization of ecosystems. Human-caused climate change and the rise in invasive species are directly linked through changing of ecosystems. The destabilization of climate factors in these ecosystems can lead to the creation of a more hospitable habitat for invasive species- species that not historically found in the impacted regions. Thus, invasive species are able to spread beyond their original boundaries. This relationship is notable because climate change and invasive species are also considered by the USDA to be two of the top four causes of global biodiversity loss.

Background

Climate change has a cascading effect on the plants and animals of affected regions and habitats. Impacts may include an increase in CO₂, change in the pH of water, and possibly death of species. These factors often lead to physiological stress and challenges to native organisms in an ecosystem. Measurably warmer or colder conditions create opportunities for non-native terrestrial and marine organisms to migrate to new zones and compete with established native species in the same habitat. Given their remarkable adaptability, non-native plants may then invade and take over the ecosystem in which they were introduced.

Urbanization is the construction of land that ultimately causes death of native species and replacement with non native species, which can affect trophic levels in ecosystems. Global warming can cause droughts in dryland, this later on can kill plants which require heavy water use from soil. It also can shift invasive species into this dryland that require water as well. Which in turn can further deplete water supply for plants of that region. All of these influences can lead to physiological stress of organism, thus increasing invasion and further destroying the native ecosystem.

Contemporary climate change includes both global warming and its impacts on Earth's weather patterns. There have been previous periods of climate change, but the current changes are distinctly more rapid and not due to natural causes. Instead, they are caused by the emission of greenhouse gases, mostly carbon dioxide (CO₂) and methane. Burning fossil fuels for energy use creates most of these emissions. Certain agricultural practices, industrial processes, and forest loss are additional sources. Greenhouse gases are transparent to sunlight, allowing it through to heat the Earth's surface. When the Earth emits that heat as infrared radiation the gases absorb it, trapping the heat near the Earth's surface. As the planet heats up it causes changes like the loss of sunlight-reflecting snow cover, amplifying global warming.

The effects of climate change on agriculture can result in lower crop yields and nutritional quality due to for example drought, heat waves and flooding as well as increases in pests and plant diseases. The effects are unevenly distributed across the world and are caused by changes in temperature, precipitation and atmospheric carbon dioxide levels due to global climate change. In 2019, millions already suffer from food insecurity due to climate change and predicted decline in global crop production of 2% - 6% by decade. It has been predicted in 2019 that food prices will rise by 80% by 2050 which will likely lead to food insecurity, disproportionally affecting poorer communities. A 2021 study estimates that the severity of heatwave and drought impacts on crop production tripled over the last 50 years in Europe – from losses of 2.2% during 1964–1990 to losses of 7.3% in 1991–2015.

Definitions

Invasive Species

According to the International Union for Conservation of Nature (2017), IUCN, invasive species are ”animals, plants or other organisms that are introduced into places outside their natural range, negatively impacting native biodiversity, ecosystem services or human well-being”.

Climate change will also re-define which species are considered as invasive species. Some taxa formerly considered as invasive may become less influential in an ecosystem changing with time, while other species formerly considered as non-invasive may become invasive. At the same time, a considerable amount of native species will undergo a range shift and migrate to new areas.

Shifting ranges, and changing impacts of invasive species, make the definition of the term “invasive species” difficult - it has become an example of a shifting baseline. Considering the changing dynamics mentioned above, Hellmann et al. (2008), concludes that invasive species should be defined as ”those taxa that have been introduced recently” and exert a ”substantial negative impact on native biota, economic values, or human health”. Consequently, a native species gaining a larger range with a changing climate is not considered to be invasive, as long as it does not cause considerable damage.

The taxa that have been introduced by humans throughout history has changed from century to century and decade to decade, and so has the rate of introductions. Studies of global rates of first records of alien species (counted as the amount of first records of established alien species per time unit) show that during the period 1500-1800 the rates stayed at a low level, whether the rates have been increasing constantly after year 1800. 37% of all the first records of alien species have been registered as recently as during the period 1970–2014.

The invasions of alien species is one of the major drivers of biodiversity loss in general, and the second most common threat being related to complete species extinctions since the 16th century. Invasive alien species are also capable of reducing the resilience of natural habitats, and agricultural as well as urban areas, to climate change. Climate change, in turn, also reduces the resilience of habitats to species invasions.

Biological invasions and climate change are both two of the key processes affecting global diversity. Yet, their effects are often looked at separately, as multiple drivers interact in complex and non-additive ways. Some consequences of climate change have been widely acknowledged to accelerate the expansion of alien species, however, among which increasing temperatures is one.

Invasion Pathway

The way in which biological invasions occur is stepwise, and referred to as the invasion pathway. It includes four major stages – the introduction/transport stage, the colonization/casual stage, the establishment stage/naturalization, and the landscape spread/invasion stage. The concept of the invasion pathway describes the environmental filters a certain species need to overcome in each stage in order to become invasive. There is a number of mechanisms affecting the outcome of each step, of which climate change is one.

For the initial transport stage, the filter is of a geographic character. For the second colonization stage, the filter is constituted by abiotic conditions - and for the third establishment stage, by biotic interactions. For the last landscape spread stage, certain factors of the landscape make up the filter the species need to pass through.

Interactions

The interaction between climate change and invasive species is complex and not easy to assess. Climate change is likely to favour some invasive species and harm others, but few authors have identified specific consequences of climate change for invasive species.

As early as 1993, a climate/invasive species interaction was speculated for the alien tree species Maesopsis eminii that spread in the East Usambara mountain forests, Tanzania. Temperature changes, extremes of precipitation and decreased mist were cited as potential factors promoting its invasion.

Consequences of climate change for invasive species are distinct from consequences for native species due to different characteristics (traits and qualities associated with invasions), management and abundance and can be direct, through the species survival, or indirect, through other factors such as pest or prey species.

So far, there have been more observations of climate change having a positive or accelerating effect on biological invasions than a negative one. However, most literature focuses on temperature only and due to the complex nature of both climate change and invasive species, outcomes are difficult to predict.

Favorable conditions for the introduction of invasive species

Effects on Invasion Pathway Stages

Climate change will interact with many existing stressors that affect the distribution, spread, abundance and impact of invasive species. Hence, in relevant literature, the impacts of climate change on invasive species are often considered separately per stage of the invasion pathway: (1) introduction/transport, (2) colonization/casual stage, (3) establishment/naturalization, (4) spread/invasion stage. According to those invasion stages there are 5 nonexclusive consequences of climate change for invasive species according to Hellmann:

1. Altered transport and introduction mechanisms
2. Altered climatic constraints on invasive species
3. Altered distribution of existing invasive species
4. Altered impact of existing invasive species
5. Altered effectiveness of management strategies

The first consequence of climate change, altered mechanisms for transport and introduction mechanisms, is given as invasions are often purposefully (e.g. biocontrol, sport fishing, agriculture) or accidentally introduced with the help of humans and climate change could alter the patterns of human transport. Changed recreational and commercial activities will change human transport and increase the propagule pressure of some non-native species from zero, e.g. connecting new regions or above a certain threshold that allows for establishment. Longer shipping seasons can increase the number of transports of non-native species and increase propagule pressure supporting potential invaders as the monkey goby. Additionally, introductions for recreation and conservation purposes could increase.

Changing climatic conditions can reduce native species' ability to compete with non-native species and some currently unsuccessful, non-native species will be able to colonize new areas if conditions change towards their original range. Multiple factors can increase the success of colonization, as described in more detail below in 2.2.

There is a wide range of climatic factors that affect the distribution of existing invasive species. Range limits due to cold or warm temperature constraints will change as a result of global warming, so that cold-temperature constrained species will be less restricted in their upper-elevation and higher-latitude range limits and warm-temperature constrained species will be less restricted in their lower-elevation and lower-latitude range limits. Changing precipitation patterns, the frequency of stream flow and changes in salinity can also affect hydrologic constraints of invasive species. As many invasive species have been selected for traits that facilitate long-distance dispersal it is likely that shifts in suitable climatic zones favor invasive species.

The impact on native species can be altered through population densities of invasive species. Competition interactions and abundance of native species or resources take part in the relative impact of invasive species.

The effectiveness of different management strategies is dependent on climate. For instance, mechanical control of invasive species by cold, hard freezes or ice cover can become less effective with increasing temperatures. Changes in the fate and behaviour of pesticides and their effectiveness in controlling invasive species can also occur. Decoupling of the relationship between some biocontrol agents and their targets can support invasions. On the other hand, the effectiveness of other biocontrol agents could increase due to species range overlaps.

Effects on Climatic Conditions

Another perspective to look at how climate change creates conditions that facilitate invasions is to consider the changes in the environment that have an impact on species survival. These changes in environmental conditions include temperature (terrestrial and marine), precipitation, chemistry (terrestrial and marine), ocean circulation and sea levels.

Most of the available literature on climate-induced biological invasions deals with warming effects, so that there is much more information for temperature effects on invasions than there is for precipitation patterns, extreme events and other climatic conditions.

Temperature

Several researchers found that climate change alters environmental conditions in a way that benefits species’ distribution by enabling them to expand their ranges to areas where they were previously not able to survive or reproduce. Those range shifts are mainly attributed to an increased temperature caused by climate change. Shifts of geographic distributions will also challenge the definition of invasive species as mentioned earlier.

In aquatic ecosystems, cold temperatures and winter hypoxia are currently the limiting factors for the survival of invasive species and global warming will likely cause new species to become invasive.

In each stage of the invasion pathway temperature has potential impacts on the success of an invasive species. They are described in the section about effects of invasion pathway stages. They include facilitating colonization and successful reproduction of invasive species that have not been successful in the respective area before, changed competition interactions between native and invasive species, changed range limits regarding altitude and latitude and changed management effectiveness. Global warming can also modify human activity, like transport, in a way that increases the chances of biological invasions.

Extreme weather events

Climate change can cause increases in extreme weather like cold winters or storms, which can become problematic for the current invasive species. The invasive species that are adapted to a warmer climate or a more stable climate can get a disadvantage when sudden seasonal changes like an especially cold winter. Unpredictable extreme weather can therefore act as a reset mechanism for invasive species, reducing the amount of invasive species in the affected area. More extreme climatic events such as floods may also result in escapes of previously confined aquatic species and the removal of existing vegetation and creation of bare soil, which is then easier to colonize.

Invasive species benefiting from climate change

One important aspect of the success of invasive species under climate change is their advantage over native species. Invasive species often carry a set of traits that make them successful invaders (e.g. ability to survive in adverse conditions, broad environmental tolerances, rapid growth rates and wide dispersal), as those traits are selected for in the invasion process. Those traits will often help them succeed in competition with native species under climate change. However, invasive species do not exclusively, nor do all invasive species carry these traits. Rather there are some species that will benefit from climate change and others will be more negatively affected by it. Invasive species are just more likely than native species to carry suitable traits that favour them in a changing environment as a result of selection processes along the invasion pathway.

Some native species that are dependent on mutualistic relationships will see a reduction in their fitness and competitive ability as a result of climate change effects on the other species in the mutualistic relationship. As non-native species are depending more rarely on mutualistic relationships they will be less affected by this mechanism.

Climate change also challenges the adaptability of native species through changes in the environmental conditions, making it difficult for native species to survive and easy for invasive species to take over empty niches. Changes in the environment can also compromise the native species’ ability to compete with invaders, that are often generalists. Invasive species do not require climate change to damage ecosystems, however, climate change might exacerbate the damage they do cause.

Decoupling of Ecosystems

Food webs and chains are two varying ways to examine energy transfer and predation through a community. While food webs tend to be more realistic and easy to identify in environments, food chains highlight the importance of energy transfer between trophic levels. Air temperature greatly influences not only germination of vegetative species but also the foraging and reproductive habits of animal species. In either way of approaching relationships between populations, it is important to realize that species likely cannot and will not adjust to climate change in the same way or at the same rate. This phenomenon is known as ‘decoupling’ and has detrimental effects on the successful functioning of affected environments. In the Arctic, caribou calves are beginning to largely miss out on food as vegetation begins growing earlier in the season as a result of rising temperatures.

Specific examples of decoupling within an environment include the time lag between air warming and soil warming and the relationship between temperature (as well as photoperiod) and heterotrophic organisms. The former example results from the ability of soil to hold its temperature. Similar to how water has a higher specific heat than air, which results in ocean temperatures being warmest at the close of the summer season, soil temperature lags behind that of air. This results in a decoupling of above and below ground subsystems.

This affects invasion because it increases growth rates and distribution of invasive species. Invasive species typically have better tolerance to different environmental conditions increasing their survival rate when climate changes. This later translates to when species die because they can not live in that ecosystem any more. The new organisms that move in can take over that ecosystem.

Other effects

The current climate in many areas will change drastically, this can both effect current native species and invasive species. Current invasive coldwater species that are adapted to the current climate may be unable to persist under new climate conditions. This shows that the interaction between climate change and invasive species doesn't need to be in favour for the invader.

If a specific habitat changes drastically due to climate change, can the native species become an invader in its native habitat. Such changes in the habitat can inhibit the native species from completing its life cycle or forcing range shift. Another result from the changed habitat is local extinction of the native species when its unable to migrate.

Migration

Higher temperatures also mean longer growing seasons for plants and animals, which allows them to shift they ranges toward Nord. Poleward migration also changes the migration patterns of many species. Longer growing seasons mean the time of arrival for species changes, which changes the amount of food supply available at the time of arrival altering the species reproductive success and survival. There is also secondary effects global warming has on species such as changes in habitat, food source, and predators of that ecosystem. Which later could lead to the local extinction of species or migration to a new area suitable for that species.

Examples

Insect Pests

Insect pests have always been viewed as a nuisance, most often for their damaging effects on agriculture, parasitism of livestock, and impacts on human health. Influenced heavily by climate change and invasions, they have recently been looked at as a significant threat to both biodiversity and ecosystem functionality. Forestry industries are also at risk for being affected. There are a plethora of factors that contribute to existing concerns regarding the spread of insect pests: all of which stem from increasing air temperatures. Phenological changes, overwintering, increase in atmospheric carbon dioxide concentration, migration, and increasing rates of population growth all impact pests’ presence, spread, and impact both directly and indirectly. Diabrotica virgifera virgifera, western corn rootworm, migrated from North America to Europe. In both continents, western corn rootworm has had significant impacts on corn production and therefore economic costs. Phenological changes and warming of air temperature have allowed this pests’ upper boundary to expand further northward. In a similar sense of decoupling, the upper and lower limits of a species’ spread is not always paired neatly with one another. Mahalanobis distance and multidimensional envelope analysis performed by Pedro Aragon and Jorge M. Lobo predict that even as the pests’ range expands northward, currently invaded European communities will remain within the pests’ favored range.

In general, it is expected that global distribution of crop pests will increase as an effect of climate change. This is expected for all kinds of crops creating a threat for both agriculture and other commercial use of crops.

When the climate gets warmer is the crop pest predicted to spread towards the poles in latitude and in altitude. Dry or cold areas with a current mean temperature around 7,5 ̊C and a current precipitation below 1100 mm/year could potentially be more affected than other areas. The present climate in these areas are often unfavourable for the crop pest that currently lives there, therefore will an increase in the temperature bring advantages to the pests. With increased temperatures will the life-cycle for the crop pests be faster and with winters above freezing temperatures will new crop pests species be able to inhabit these areas. Precipitation has a lesser effect on crop pests than temperatures but it can still impact the crop pests. Drought and dry plants makes host plants more attractive for insects and therefore increases the crop pest during droughts. For example, is the confused flour beetle predicted to increase in the South America austral region with an increased temperature. A higher temperature decreased the mortality and development time for the confused flour beetle. The confused flour beetle population is expected to increase the most in higher latitudes. 

Areas with a warmer climate or lower altitudes are predicted to experience and decrease in crop pests. The largest decline in crop pests is expected to occur in areas with a mean temperature of 27 ̊C or a precipitation above 1100 mm/year. Despite the decline in crop pests it is unlikely that climate change will result in the complete removal of the existing crop pest species in the area. With a higher amount of precipitation can flush away eggs and larvae that is a potential crop pest. 

Pathogen Impacts

While still limited in research scope, it is known that climate change and invasive species impact the presence of pathogens and there is evidence that global warming will increase the abundance of plant pathogens specifically. While certain weather changes will affect species differently, increased air moisture plays a significant role in the rapid outbreaks of pathogens. In the little amount of research that has been completed regarding the incidence of plant pathogens in response to climate change, the majority of the completed work focuses on above-ground pathogens. This does not mean that soil-borne pathogens are exempt from experiencing the effects of climate change. Pythium cinnmomi, a pathogen that causes oak tree decline, is a soil-borne pathogen that increased in activity in response to climate change.

Freshwater and Marine Environments

Barriers between marine ecosystems are typically physiological in nature as opposed to geographic (i.e. mountain ranges). These physiological barriers may be seen as changes in pH, water temperature, water turbidity, or more. Climate change and global warming have begun to affect these barriers- the most significant of which being water temperature. The warming of sea water has allowed crabs to invade Antarctica, and other durophagous predators are not far behind. As these invaders move in, species endemic to the benthic zone will have to adjust and begin to compete for resources, destroying the existing ecosystem.

Freshwater systems are significantly affected by climate change. Extinction rates within freshwater bodies of water tend to be equitable or even higher than some terrestrial organisms. While species may experience range-shifts in response to physiologic changes, outcomes are species-specific and not reliable in all organisms. As water temperatures increase, it is organisms that inhibit warmer waters that are positively affected, while cold-water organisms are negatively affected. Warmer temperature also leads to the melting of arctic ice which increase the sea level. Most photosynthesizing species because of the rise in sea water are not able to get the right amount of light to sustain living.

Compared to terrestrial environments, freshwater ecosystems have very few geographical and allosteric barriers between different areas. The increased temperature and shorter duration of cold temperature will increase the probability of invasive species in the ecosystem, due to that the winter hypoxia that prevents the species survival will be eliminated. This is the case with the brook trout that is an invasive species in lakes and streams in Canada.

The invasive brook trout has the capacity to eliminate the native bull trout and other native species in Canadian streams. The temperature of the water plays a big part in the brook trout capacity to inhabit the stream, but other factors like the stream flow and geology are also important factors in how well established the brook trout is. Today has the bull trout a positive population growth or hold a competitive advantage only in the streams that doesn't exceed 4 – 7 ̊ C in the warmest months. The brook trout has a competitive and a physiological advantage over bull trout in warmer water 15 – 16 ̊C. The winter period is also an important factor for brook trout's capacity to inhabit a stream. Brook trout may reduce its survival rate if it is exposed to especially long and harsh winter periods. Due to the observations that the range of brook trout is dependent on the temperature is there an increasing concern that the brook trout will eliminate the bull trout even further in colder water due to increasing temperature because of climate change. Climate change not only influences the temperature in the lake but also the stream flow and therefore other factors in the stream. This unknown factor makes it hard to predict how the brook trout and bull trout will react to climate change.

Management and Prevention

Mechanical/manual control of invasive species

Management strategies generally have a different approach regarding invasive species compared to most native species. In terms of climate change and native species, the most fundamental strategy is conservation. The strategy for invasive species is, however, majorly about control management. There are several different types of management and prevention strategies, such as following.

Approaches

1. Prevention: is generally the more environmentally desirable approach, but is difficult to practice due to the issues with separating invasive from non-invasive species. Border control and quarantine measures are normally the first prevention mechanisms. Preventative measures include exchanging ballast water in the middle of the ocean, which is the main tool accessible for ships to limit the introduction of invasive species. Another method of prevention is public education to increase the understanding of individual actions on furthering the spread of invasive species and promote awareness about strategies to reduce the introduction and spread of invasive species. Invasion risk assessment can also aid in preventative management since it allows for early identification. Invasion risk is done by the identification of a potentially invasive species through comparison of common traits.
2. Monitoring and early detection: samples can be taken in specific areas to see if any new species are present. These samples are then run through a database in order to see if the species are invasive. This can be done using genetic tools such as environmental DNA (eDNA). These eDNA-samples, taken in ecosystems, are then run through a database that contains bioinformatics of species DNA. When the database matches a sequence from the sample's DNA, information about what species that are or have been present in the studied area can be obtained. If the species are confirmed to be invasive, the managers can then take precaution in form of a rapid response eradication method. The eDNA method is majorly used in marine environments, but there are ongoing studies about how to use it in terrestrial environments as well.
3. Rapid response: several methods of eradication are used to prevent distribution and irreversible introduction of invasive species into new areas and habitats. There are several types of rapid response:
   • Mechanical/manual control: often done through human labor, such as hand pulling, mowing, cutting, mulching, flooding, digging and burning of invasive species. Burning often takes place mid spring, to prevent further damage to the area's ecosystem and harm to the managers that administer the fires. Manual control methods can kill or reduce the populations of non-native species. Mechanical controls are sometimes effective and generally doesn't engender public criticism. Instead, it can often bring awareness and public interest and support for controlling invasive species.
   • Chemical control: chemicals such as pesticides (e.g. DDT) and herbicides can be used to eradicate invasive species. Though it might be effective to eliminate target species, it often creates health hazards for both non-target species and humans. It is therefore generally a problematic method when, for example, rare species are present in the area.
   • Biological control: a method where organisms are used to control invasive species. One common strategy is to introduce natural enemy species of invasive species in an area, with the aim to establish the enemy which will drive the invasive species's population to a contracted range. One major complication with the biological method is that introduction of enemy species, which itself in a sense is an invasion as well, sometimes can affect non-target species negatively as well. There has been criticism regarding this method, for example when species in conservation areas have been affected or even driven to extinction by biocontrol species.
4. Restoration of ecosystems: restoration of ecosystems after eradication of invasive species can build resilience against future introductions. To some degree, ecological niche models predict contraction of some specie's ranges. If the models are somewhat accurate, this creates opportunities for managers to alter the composition of native species to build resilience against future invasions.
5. Forecasting: climate models can somewhat be used to project future range shifts of invasive species. Since the future climate itself can't be determined, though, these models are often very limited. However, the models can still be used as indicators of general range shifts by managers to plan management strategies.
6. Genetic control: new technology has presented a potential solution for invasive species management: genetic control. A form of genetic pest management has been developed that targets the mating behavior of pests to introduce harm-reducing genetically engineered DNA into wild populations. Though not widely implemented yet for invasive species specifically, there is an expanding interest in using genetic pest management for invasive species control. Triploidy also exists to manage invasive species through the production of sterile males to biologically control insect pests. Similar to triploidy, another form of genetic control is Trojan Y which serves as a sex-marker identification and aims to bias the sex ratio of populations, typically fish, towards males in order to drive the population to extinction. Trojan Y specifically uses sex-reversed females containing two Y chromosomes, known as Trojan Y, to reduce the success of breeding in the population. A counterpart to the Trojan Y technique, the Trojan Female technique aims to release "Trojan females" carrying mitochondrial DNA mutations that lead to a reduction in female, rather than male, fertility. Gene drive is also another technique to suppress pest populations.

Predictions

The geographical range of invasive alien species threaten to alter due to climate change, such as the brook trout (Salvelinus fontinalis). To forecast future impact of climate change on distribution of invasive species, there is ongoing research in modelling. These bioclimatic models, also known as ecological niche models or climate envelope models, are developed with the aim to predict changes in species ranges and are an essential tool for the development of effective management strategies and actions (e.g. eradication of invasive species of prevention of introduction) to reduce the future impact of invasive species on ecosystems and biodiversity. The models generally simulate current distributions of species together with predicted changes in climate to forecast future range shifts.

Many species ranges are predicted to expand. Yet, studies also predict contractions of many species future range, especially regarding vertebrates and plants at a large spatial scale. One reason for range contractions could possibly be that species ranges due to climate change generally move poleward and that they therefore at some point will reach the sea which acts as a barrier for further spread. This is, however, the case when some phases of the invasion pathway, e.g. transport and introduction, are not considered in the models. Studies majorly investigate predicted range shifts in terms of the actual spread and establishment phases of the invasive pathway, excluding the phases of transportation and introduction.

These models are useful for making predictions but are yet very limited. Range shifts of invasive species are very complex and difficult to make predictions about, due to the multiple variables affecting the invasion pathway. This causes complications with simulating future predictions. Climate change, which is the most fundamental parameter in these models, can't be determined since the future level of the greenhouse emissions are uncertain. Additionally, climate variables that are directly linked to greenhouse emissions, such as alterations in temperature and precipitations, are likewise difficult to predict with certainty. How species range shifts will react to changes in climate, e.g. temperature and precipitation, is therefore largely unknown and very complex to understand and predict. Other factors that can limit range shifts, but models often don't consider, are for example presence of the right habitat for the invader species and if there are resources available.

The level of accuracy is thus unknown for these models, but they can to some extent be used as indicators that highlight and identify future hotspots for invasions at a larger scale. These hotspots could for example be summarized into risk maps that highlight areas with high suitability for invaders. This would be a beneficial tool for management development and help to construct prevention strategies and to control spreading.

Research

Numerous studies are ongoing to create pro-active management strategies to prevent the introduction of invasive species which are expanding their range due to climate change. One such center of study is the Northeast Climate Adaptation Science Center (NE CASC) at University of Massachusetts Amherst. "Scientists affiliated with the center provide federal, state and other agencies with region-specific results of targeted research on the effects of climate change on ecosystems, wildlife, water and other resources."