A. Mission Overview
The preliminary mission concept envisions that the MarsCAT mission will address these challenges by placing two or more identical CubeSats into orbit around Mars [Bering et al., 2016]. Each CubeSat will carry a magnetometer to measure the magnetic field, a plasma analyzer to measure the thermal ion density, ion temperature and ion velocity, and a Langmuir probe to measure electron density and temperature. Radio equipment to remotely measure electron densities between two spacecraft will also be evaluated.
Figure 2. An example of observation of the Rayleigh-Taylor instability by MAVEN [Fowler et al., 2016]. Panel 1 shows the perturbed density enhancements. Panel 3 is the altitude. As can be seen, MAVEN is under-sampling these phenomena. MarsCat will have the data cadence to resolve similar observations.
The greatest leverage for such an opportunity lies in the ability to host such a mission as a secondary payload to a major mission. If possible, having the host mission provide the Mars orbit insertion (MOI) will greatly increase science options. However, each spacecraft will carry a Phase Four RF Thruster (RFT) that will be evaluated to provide autonomous capture and insertion capabilities. Each satellite will have a 3-axis attitude determination and control system capable of attitude solutions in interplanetary space and near Mars, and an embedded radio navigation system utilizing the navigation capabilities of the Mars Relay’s CCSDS protocol. Thus a full capability to establish a stable orbit with periapis below 180 km with an inclination near 60° will be developed.
This concept study will include the trades between available propulsion, mission lifetime, orbit eccentricity, orbit periapsis, and station keeping. This study will evaluate the impact of orbit choice on our investigation of the solar wind response, ionospheric structure, and the impact of crustal fields. All of these topics can be investigated with different levels of clarity depending on altitude. For example, solar wind response studies require an eccentric orbit to be in the solar wind at apoapsis. At the same time, peak ionospheric density lies near 150 km, the crustal fields fall off rapidly with altitude, and instability driven plasma transport has been observed by MAVEN below 200 km (Figure 2).
These issues suggest that periapsis should be at or below 180 km for some period of time, in order to take data in this critical region.
Figure 3. Strawman orbital configuration of MarsCAT
The study will also include the trades between the available carry mass, ability to resolve space and time variations or at least recognize them, and maximizing science return from the Total Electron Content (TEC) system. For example, variation disambiguation is easiest if one places two or more platforms in a string-of-pearls configuration, whereas the TEC system may perform better if we have two platforms flying at the same altitude in non-coplanar but otherwise identical orbits. Crustal field observations are optimized by having periapsis at ~60° S, since that is where the strongest crustal fields are located. The RFT reaction mass budget may allow for up to ~4 km s-1 of Δv, which means the concept study will be able to plan a sequence of orbital configurations designed to study separate questions at different times during the mission.
A possible configuration is shown in Figure 3. This figure shows four platforms, two each in a pair of string-of-pearls orbits that are separated by 15° in Right Ascension of the Ascending Node (RAAN). The spacecraft will have elliptical orbits with periapsis at or below 180 km and apoapsis at 7000 km such that they sample the solar wind, magnetosphere, and ionosphere. As the first ever constellation mission at Mars, precise station keeping is not required to accomplish the mission goals.
Each satellite is anticipated to be an equivalent 6-U Cubesat structure compatible with the system deployers that may be carried by the host mission. The greatest leverage for such an opportunity lies in the ability to host such a mission as a secondary payload to a major mission. If possible, having the host mission provide the Mars orbit insertion (MOI) will greatly increase science options. However, each spacecraft will carry a Radio Frequency Thruster (RFT) that will be evaluated to provide autonomous capture and insertion capabilities. Each satellite will have a 3-axis attitude determination and control system capable of attitude solutions in interplanetary space and near Mars, and an embedded radio navigation system utilizing the navigation capabilities of the Mars Relay’s CCSDS protocol. Thus a full capability to establish a stable orbit with periapis below 180 km with an inclination near 60° will be developed.
Measurements To Be Taken
The two MarsCAT spacecraft have identical instrumentation. They carry an induction Magnetometer (iMAGS) to measure the DC and ULF magnetic wave field (provided by UM), a Faraday Cup Analyzer to measure the electron and ion density, temperature and flow velocity (provided by UM), double-Langmuir Probes (LP) to measure the cold electron plasma density (provided by UH) and possibly two Medipix radiation detectors, each with dual orthogonal sensor assemblies to measure the charged particle radiation field and characterize the X-ray bremsstrahlung from below (provide by UH as part of the SEO options). In addition to these instruments, MarsCAT carries a dual frequency radio package to provide total electron content information between the two spacecraft (provided by UM). Both satellites make continuous measurements and return data at various temporal resolutions, although the primary data cadence is 1 second. The quality, temporal resolution and quantity of data from each instrument is discussed in Table 1.
Data To Be Returned
The Instrument DPU combines and packetizes the data from the different instruments on board the MarsCAT spacecraft, as shown in Table 1. These data, along with other instrument housekeeping and state of health data are then passed to the spacecraft telemetry system.
Quality Of Data To Be Returned
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|Instrument Requirement||iMags||Faraday Cup (FC)||Langmuir Probe (LP)||Radio Sounder (RS)||Radiation Detector (MP) (SEO Option)|
|Energy Range||n/a||0.1 eV to 500 eV||thermal||thermal||3 keV to GeV|
|Energy Resolution||n/a||18%||±100 K||thermal||< 1 KeV for e- < 500 KeV Variable LET-based for heavy charged particles|
|Mag Field Res (nT)||1||n/a||n/a||n/a||n/a|
|Mag Field Range (nT)||0-10,000||n/a||n/a||n/a||n/a|
|Freq Range (vector/sec or samples/sec)||16||64 samples/s. 32 step sweep||64 samples/s. 256 step sweep||1||Integral fluences (aka light curves) 16/s; energy spectra 1e-3/s (4 min/ spectrum)|
|Time Resolution (sec)||0.0625||5||4||1||0.0625 (light curve) 240 (spectra)|
|Avg Data Rate (kbps)||1||0.04||0.5||0.25||1|
Cause of density anomalies in the Martian ionosphere
The structure and variability of the Martian ionosphere can only be well determined with simultaneous in situ and remotely sensed observations at different altitudes. MarsCAT will test current models for the Martian Ionosphere. Specifically, the MarsCAT pair will provide the location of the Martian ionopause with concurrent solar wind/magnetosheath density and velocity information throughout the mission. Multipoint MarsCAT observations within the Mars ionosphere will enable us to separate spatial from temporal effects during close spacecraft separation intervals.
The two most likely causes of the observed density anomalies are variations in the neutral atmosphere source and plasma transport. The Faraday cups on both MarsCAT CubeSats will provide the high resolution plasma densities needed to quantify the anomalies, while MarsCAT velocity measurements and density profiles will enable us to assess the role of transport. As described below, modeling will be used to estimate possible changes in background neutral density.
The ~1/16 s cadence of the MarsCAT magnetometer suffices to resolve the structure Martian ionopause [e.g., 31], while the ~1 s cadences of all the other instruments suffices to resolve the structure of the broader magnetic pile up region.
Quantify the energy balance of the Mars ionosphere
MarsCAT will measure the plasma density, ion and electron temperature, and ion flow velocity profiles at multiple locations in the Martian ionosphere. Other NASA spacecraft at Mars and elsewhere in the solar system will provide solar EUV flux data. If needed, Langmuir Probe photoemission currents can be used to assess the actual EUV flux at MarsCAT. MarsCAT and MAVEN (if still operating) observations will be used to estimate solar wind plasma and magnetic field forcing. Neutral wind input from the lower atmosphere will be estimated using the models discussed below.
These data and model inputs represent a nearly complete assessment of most of the terms in the fluid model energy equation. The balance or lack thereof in this equation will indicate if there are additional sources or sinks to be considered.
Determine the response of the ionospheric plasma temperature to solar activity and the solar cycle
By making continuous measurements of the thermal plasma populations in the Martian ionosphere over the lifetime of the mission for a broad range of altitudes, seasons, and solar activity levels, MarsCAT will determine the response of the Martian ionosphere to solar forcing.
Quantify the role of residual magnetism upon the structure and dynamics of the Martian ionosphere
MarsCAT will make simultaneous two point measurements of the vector magnetic field along with the density, temperature, and flow velocities within the Martian ionosphere at cadences greater than or equal to 1 s. We will use a combination of direct mapping and assimilative modeling to render and visualize the plasma structures and flow patterns in the vicinity of residual magnetic enhancements. In particular, the data will be examined for evidence of closed flow patterns and regions of temperature and density anomalies.
Separate spatial from temporal effects to understand the dynamics of mini-magnetospheres in the Martian system
The separation of spatial from temporal effects is one of the classic problems in experimental space physics. The solution is equally well known, simultaneous observations from multiple spacecraft. MarsCAT is a dual spacecraft mission. The spacecraft orbits have been chosen to provide many simultaneous nearby passes through the likely mini-magnetosphere regions in the Southern Hemisphere of Mars. Spatial structures will recur from orbit to orbit, whereas temporal variations do not. On the other hand, any fluctuations that are seen by both spacecraft in any of the plasma or field parameters are probably signatures of temporal variations.
Per , as cited by , potential crustal reconnection regions cover about 7% of the Martian surface and extend upwards more than 1300 km. Consequently encounters will be relatively common. At 400 km, the maximum radial magnetic field strength associated with any crustal feature is on the order of 220 nT. By contrast, fields as large as 1600 nT have been observed at 100 km13. Both values lie comfortably within the range observable by the magnetometers on MarsCAT. Encounters have durations on the order of 5-20 min [e.g., 34] and are therefore readily resolvable by all instruments.
Quantify the physics of reconnection in the weak magnetic field environment of mini-magnetospheres
Mini-magnetosphere reconnection events are expected to have two possible micro-scale signatures, parallel plasma flows moving at or near the Alfvén speed, and shock accelerated energetic electrons. The Faraday cup will detect high-speed plasma flow in the boundary layers of mini-magnetosphere regions. Reconnection may also result in the detachment of anti-sunward-moving flux ropes or blobs from mini-magnetospheres. MarsCAT will employ its two-point measurements to identify and track these anti-sunward-moving features.
Some of the energetic electron flux coming from reconnection acceleration processes will precipitate into the neutral atmosphere. This precipitation will, among other things, result in the emission of X-Ray bremsstrahlung. The Medipix radiation detectors (part of the Science Enhancement Opportunity options), have a detection threshold for all types of ionizing radiation including X-rays of about 3 KeV. These devices can determine the traversal axis of energetic charged particles and the actual direction for stopping particles such as incident electrons <500 KeV. Spectral information for X-Rays, as well as the directional information that can be determined by comparison of the relative fluxes seen by the 4 separate sensors, will enable the inference of locations of X-Rays due to It will be possible, therefore, to remotely sense energetic electron precipitation taking place 100’s of km from the MarsCAT field lines. The X-Ray images delineate the footprints of the region in which reconnection is occurring. The X-Ray energy spectra can be used to infer the spectra of the precipitating electrons that caused the emissions, providing crucial information concerning reconnection.
An encounter with a magnetotail current sheet reconnection region similar to those expected at mini-magnetospheres was reported by  and lasted some 30-60 s. The ~5 s time resolution of the MarsCAT plasma and magnetometer instruments will suffice to resolve these regions. Enhanced fluxes of electrons were seen at energies ranging from 10-120 eV. Reconnection is expected to generate outflows with velocities on the order of the Alfvén speed. Taking a mini-magnetosphere magnetic field strength of 40 nT and a magnetosheath density of ~5 cm-3, outflows on the order of 400 km s-1 are expected, within the range of proton velocities observable by the Faraday cup.
Determine the role of plasma disturbances such as flux ropes and Kelvin-Helmholtz waves in the solar wind-Martian interaction
Instabilities at the Martian ionopause can result in the occurrence of Kelvin-Helmholtz waves and flux ropes. Because they carry away the Martian ionosphere, both may play an important role in the interaction of the solar wind with Mars. The significance of each depends upon the occurrence rates and dimensions of the events they generate as a function of solar wind conditions. When widely separated, the two MarsCAT spacecraft can be used to monitor solar wind drivers and survey ionopause responses simultaneously. When closely-separated, the two-point measurements will be used to measure the boundary conditions needed to test instability criteria, observe the resulting outflow, and determine event velocities and dimensions. For example, they can be used to test the predicted effects of density gradients30 and magnetosheath magnetic field orientations. Minimum variance algorithms applied to the magnetic field measurements will permit determination of boundary orientations31. Particle and plasma observations within the structures will aid in the determination of magnetic topologies.
In particular, we will use MarsCAT observations to test the criteria governing the Kelvin-Helmholtz instability when wavelike structures are observed, and the likelihood of reconnection when flux ropes are observed. We will seek evidence for the giant plasma flow vortices reported by. Finally, we will test the prediction that the occurrence of the Kelvin-Helmholtz instability is favored in the hemisphere with the outward pointing magnetosheath convection electric field.
Event durations are on the order of 1 to several minutes29, consequently they will be readily resolved by MarsCAT instrumentation. MarsCAT will make wave observations with the magnetometers, and with the plasma instruments. When the plasma instruments are sweeping, the sweep rate will set the upper frequency limit of the observations. Higher frequency modes will be observed by setting the double-Langmuir probe and Faraday cup to fixed bias and measuring current collection.
Understand the causes of the infrequent Martian aurora
The recent results from MAVEN have confirmed the occurrence of Martian aurora. Nonetheless, his science objective is an exploratory, relatively high risk goal and is part of the Science Enhancement Opportunity options. As described above, the Medipix radiation detectors have a detection threshold for all types of ionizing radiation including X-rays of about 3 keV. The combination of 4 separately oriented sensors will allow inference of the arrival direction of incoming X-Rays, which in turn will enable the localization of bremsstrahlung emissions from the underlying ionosphere. It should thus be possible to remote sense energetic electron precipitation taking place 100’s of km from the MarsCAT field lines. The X-Ray energy spectra can then be used to infer the spectra of the precipitating electrons that caused the emissions.
Science Products And Data
The MarsCAT instruments provide a variety of data products to be used for scientific analysis. The MarsCAT team includes co-investigators who use the data for both case study and statistical analysis of the controlling factors for ionosphere, ionopause and mini-magnetosphere variability; ingest into models of the Martian atmosphere; and for testing theories of wave-induced particle acceleration and loss. The results of these studies will be presented at scientific meetings and published in the peer-reviewed scientific literature.
Modeling the Lower Boundary
Three-dimensional simulation of Mars’ ionosphere is a rapidly evolving research area,. As part of the MarsCAT effort, we will couple a photochemistry model with a general circulation model (GCM) to conduct 3-D simulations of the Martian ionosphere. For the photochemistry model, we will use the Caltech/JPL multi-dimensional model (KINETICS), that was built to answer fundamental questions concerning the chemical composition and dynamical processes responsible for the photochemistry and the transport/mixing of chemical species in planetary atmospheres [e.g. 9„„]. The Caltech/JPL KINETICS model is generally run in one-dimensional (1-D)41 and two-dimensional (2-D) modes42, to save computer power and time. However, the model has the structure to run 3-D simulations. For 3-D simulations, the model needs dynamic output from a GCM simulation. Therefore, dynamic variables (e.g., winds and mixing coefficients) from the GCM simulation will be used to drive the 3-D photochemistry model.
The planetWRF Mars GCM, will be used in the coupling. PlanetWRF is a flexible planetary atmospheric numerical model which has the ability to simulate the atmospheric dynamics of different planets at multiple scales. This model, which is an enhancement and generalization of the terrestrial Weather Research and Forecasting (WRF) model developed by the National Center for Atmospheric Research (NCAR), has been converted for use on Mars by the addition of Mars-specific parameterizations, such as radiation, orbital mechanics, and a CO2 sublimation cycle.
Coupling between the Caltech/JPL KINETICS model and the planetWRF model will make it possible to simulate the time-independent 3-D structure of Mars ionosphere, investigate the roles of solar activities in reshaping Mars ionosphere, and explore the interaction between Mars ionosphere and the lower neutral atmosphere. Comparative study of the 3-D simulation results and the proposed global observations from this mission will further advance our understanding of Mars ionosphere. In particular, the coupled models will be used to study the possible roles of atmospheric dynamics in the variable, structured, electron density in Mars ionosphere revealed by the observations from the Mars Global Surveyor11.
Ion pickup may be a major loss process for the Martian atmosphere. Pickup occurs when and where charges solar wind ions encounter neutral exospheric atoms. Because the density of neutral atoms falls off rapidly with distance from the planet, ion pickup increases greatly when the ionopause moves inward toward the planet. Determining the location, structure, and motion of the ionopause is consequently an important task, and one that is greatly facilitated by the presence of two identically-equipped spacecraft.
According to , the ionopause transition between plasmas and magnetic fields of solar wind versus planetary origin generally occurs some 300 to 600 km above the surface of the planet and can be either gradual or sharp (for reasons that remain unknown). Two-point MARSCAT plasma and magnetic field measurements will be used to map the locations of the ionopause. When the spacecraft are nearby, the two point measurements can be used to determine ionopause velocities and (assuming simple radial advection) the fine structure of this boundary. For example, there is evidence indicating lumpy or permeable structures near radial crustal remnant magnetic fields18. The simple advection hypothesis can be tested by checking whether both spacecraft observe corresponding structures.
Increases in the solar wind dynamic pressure may18„ or may not move the magnetic pile-up and photoelectron boundaries at the ionopause towards the planet. The presence of hot flow anomalies upstream from the planet may create transient enhancements and decreases in the pressure applied to the ionopause, resulting in large amplitude motion [e.g. ]. When interspacecraft separation distances between the spacecraft are large, one will be used to monitor incoming magnetosheath conditions and the other to determine the location, motion, and structure of the ionopause as a function of these conditions. We will test whether18 or not31, the boundary moves significantly in response to variations in solar EUV. We will inspect the evidence indicating that crustal magnetic fields corresponding to minimagnetospheres raise the altitude of the magnetic pileup boundary (i.e., ionopause)27 or even replace it, as predicted by simulations15,16.
With appropriate models for charge exchange and the exosphere, we will then determine atmospheric loss rates as a function of solar wind conditions, e.g. during the space weather storms when it is expected to increase„ solar EUV, and in the presence of crustal structures.
Data Analysis Plan
The MarsCAT data management plan integrates the in situ plasma and magnetometer measurements and the remote sensing radio observations – from both spacecraft – into a single coherent web-based database that provides not only quick-look plotting but also detailed analysis capabilities directly over the Internet. This data management model allows scientists from both within the MarsCAT team and from the broader Mars scientific community access to all the high-resolution data. We will use the THEMIS Space Physics Environment Data Analysis Software, which is written in IDL. The web interface is customizable allowing for quick look plots of specific intervals from specific instruments and spacecraft, analysis of correlation between different spacecraft measurements, and the ability to download the data in various formats. The open data architecture and innovative integrated database allows for the maximum scientific return by providing meaningful access to the data to the entire solar-terrestrial physics community. It is being used by a variety of NASA missions.
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